FASTER SMARTER THINNER BETTER. Evolution of Microstructures and Product Opportunities in Low Carbon Steel Strip Casting

Transcription

1 FASTER SMARTER THINNER BETTER Evolution of Microstructures and Product Opportunities in Low Carbon Steel Strip Casting By K Mukunthan, L Strezov, R Mahapatra, and W Blejde BlueScope Steel, Port Kembla, Australia The Brimacombe Memorial Symposium Vancouver, Canada October 1-4, Rexford Road, Charlotte NC Telephone: Facsimile:

2 Evolution of microstructures and product opportunities in low carbon steel strip casting by K. Mukunthan and L. Strezov BHP Minerals Development R. Mahapatra and W. Blejde BHP Steel, Flat Product Division ABSTRACT One of the greatest challenges in the direct casting of low carbon steel strip is the production of material with mechanical properties similar to those obtained via conventional strip production routes. A fundamental understanding of the microstructure evolution from solidification through to the final product is of critical importance in this endeavour. Experimental work was carried out using a special apparatus to establish links between heat transfer, nucleation, solidification, strip microstructures and mechanical properties. This information was used in conjunction with the actual microstructures and properties of strip obtained from a full scale development plant to define the breadth of product opportunities possible from the direct casting of low carbon steel strip. Brimacombe Symposium, Vancouver, British Columbia, Canada, 1-4 October,

3 INTRODUCTION BHP and IHI began collaborating on the development of strip casting technology in 1989 and results obtained from the full scale development plant have been summarised elsewhere (1,2). Initial development efforts were directed towards solving process issues relating to metal delivery, edge containment, solidification and gauge control (3). From a metallurgical context, a fundamental challenge has been to overcome issues imposed by the rapid solidification regime associated with strip casting. Significant progress has been made in expanding fundamental knowledge in the areas of initial nucleation, interfacial heat transfer and subsequent solidification behaviour (4,5). This knowledge has been valuable in defining solidification strategies to cast strip with good surface quality. From a product perspective, results from the product processing trials have been encouraging. As-cast coils (low carbon Si-killed) have been successfully cold rolled, metal coated and roll formed into a number of roofing and walling profiles (1). Mechanical properties of the cold rolled/annealed material compared favourably with hot strip mill (HSM) products (1). As-cast material has also been directly converted to piping and tubing products (1,2). This result has surprised metallurgists who hold the view that the ductility of cast material will be inherently low due to the coarser austenitic grain structure, which is an inevitable outcome of the solidification process. These notions need to be reassessed in light of the strip casting regime, which is accompanied by much higher heat fluxes, and solidification rates compared to conventional casting processes (3). Microstructure evolution from solidification to the final ferrite structure has to be understood at a fundamental level to determine the true product potential of strip casting. This paper focuses on defining the product opportunities possible from strip casting based on the actual analysis of the results obtained from a full scale plant and also the fundamentals developed from laboratory studies on microstructure evolution. AS-CAST STRIP MATERIAL PROPERTIES AND MICROSTRUCTURES A comparison of typical properties of Si-killed low carbon steel strip obtained from strip casting and Al-killed steel from the conventional hot strip mill route (6) is presented in Figure 1. Although the strength levels are slightly higher and elongation levels lower, overall material properties of the as-cast strip compare favourably with the strip produced via the hot strip mill route. Yield Strength (MPa) Strip Casting Hot Strip Mill (a) (b) Figure 1 - Comparison of mechanical properties of low carbon steel strip produced via strip casting and hot strip mill routes (a) yield strength (b) elongation Elongation (%) Strip Casting Hot Strip Mill Brimacombe Symposium, Vancouver, British Columbia, Canada, 1-4 October,

4 There are however significant differences in the final microstructures of the strip produced by the two routes as illustrated in Figure 2. (a) (b) Figure 2: Comparison of the final strip microstructures produced via (a) strip casting (b) hot strip mill Key differences between the two microstructures are summarised in Table I. Hot strip mill products exhibit a fine equiaxed ferrite microstructure whilst cast strip microstructure is predominantly a mixture of coarse polygonal and Widmanstatten/acicular ferrite. Evolution of microstructure in the HSM route is well understood. These products are an outcome of large reductions which break up the original cast structure resulting in significant refinement of the austenite grains, which upon further transformation produce a fine equiaxed ferrite grain structure. Thus, product metallurgy knowledge developed for HSM route is not directly applicable to strip cast material. Table I - Key differences between the strip microstructures Prior austenite grain morphology, size Final microstructure constituents Ferrite grain morphology, size Strip Casting Columnar shape 100 to 250 µm wide, 300 to 700 µm long 30 to 60 % polygonal ferrite 70 to 40 % Widmanstatten and acicular ferrite Polygonal 10 to 50 µm wide, 50 to 250 µm long Hot Strip Mill Equiaxed 25 µm 100 % equiaxed ferrite Equiaxed 10 µm Brimacombe Symposium, Vancouver, British Columbia, Canada, 1-4 October,

5 FUNDAMENTAL STUDIES OF MICROSTRUCTURE EVOLUTION IN STRIP CASTING Microstructure development in strip casting is fundamentally coupled to the solidification process. Considerable attention has therefore been devoted towards establishing links between heat transfer, nucleation, solidification, strip microstructures and mechanical properties. Experimental Simulation of Microstructure Evolution A special apparatus was employed to simulate the initial solidification conditions encountered in the twin-roll strip casting regime (4). Strip samples were cast by immersing a paddle mounted with substrates (representing the mould surface) into a furnace containing molten steel. After withdrawal of the paddle from the furnace, the temperature of the solidified sample was continuously monitored using an optical pyrometer to understand the solid state phase transformation kinetics. Provisions were made to quench the solidified samples using helium gas to freeze the microstructures at a desired temperature. This practice was effective in defining the austenite grain size at different temperatures. The arrangement of the experimental apparatus including the rapid cooling and the concurrent temperature measurement facilities is shown in Figure 3. High volume gas flow valve Data acquisition and quench control Helium gas supply Immersion / Withdrawal Substrate Pyrometer Strip sample Steel Bath Induction Furnace Figure 3: Schematic representation of the experimental apparatus employed to study microstructure evolution in strip casting Strip Microstructures The different stages of the phase transformation occurring during solidification and subsequent cooling of low carbon steel are illustrated in Figure 4. Solidification commences around 1500 C with the formation of a dendritic structure in the delta ferrite phase. This structure then transforms to austenite phase at around 1400 C. Finally, austenite transforms into ferrite phase at a temperature below 900 C. Figure 4 - Phase transformations occurring during solidification and subsequent cooling of low carbon steel. Liquid Steel 1500 C Dendritic structure (δ-ferrite) 1400 C Austenite 900 C Ferrite Brimacombe Symposium, Vancouver, British Columbia, Canada, 1-4 October,

6 Resolution of the structures associated with each transformation step (see Figure 4) is critical to understanding the microstructure evolution process. This was achieved through development of appropriate etching techniques. The different microstructures obtained from the same position in the sample during solidification and subsequent solid state phase transformations are shown in Figure 5 which includes micrographs of dendritic, austenitic and ferritic structures. The rapid solidification rate encountered in strip casting produces a fine dendritic structure (see Figure 5a). Secondary dendrite arm spacings were measured to be in the range of 5 to 15 µm which was consistent with measurements obtained by other groups (7,8). These values were an order of magnitude lower than the typical secondary arm spacings reported for thick slab casting (100 to 250 µm) and thin slab casting processes (50 to 100 µm) (9). Substrate side Substrate side Substrate side (a) (b) (c) Figure 5 - Different strip microstructures obtained from the same position in the sample (a) Dendritic (b) Austenitic (c) Ferritic The austenite grain structure (Figure 5b) is the vital link between the dendritic solidification structure (Figure 5a) and the final ferrite microstructure (Figure 5c). Characterisation of the austenite grain size is critical to understanding the final ferrite microstructure, which subsequently controls the final product mechanical properties. Austenite grain size was determined from the width of the austenite grains measured perpendicular to the solidification direction. Impact of Solidification Behaviour on Austenite Grain Size Links between initial solidification rate and austenite grain size were established by conducting controlled immersion experiments using the apparatus shown in Figure 3. Solidification experiments were carried out using a low carbon steel melt with different sulphur levels (0.01 and 0.04 wt%) on different substrate textures. Substrate texture was used as a variable in this study due to its strong influence on the initial heat transfer rate (4). Sulphur, owing to its surface active nature, can dramatically impact the melt/substrate contacting conditions and subsequent initial heat transfer and solidification rates (10). Results from these experiments which are summarised in Figure 6, show that austenite grain size is significantly influenced by changes in the melt sulphur and also the substrate texture. It should also be noted that the measured austenite grain sizes of 50 to 200 µm are Brimacombe Symposium, Vancouver, British Columbia, Canada, 1-4 October,

7 significantly lower than the austenite grain size values (600 to 1400 µm) reported for continuously cast thin slabs (11,12). Austenite Grain Width (µm) Texture A Texture B wt% S 0.04 wt% S Figure 6 - Effect of melt sulphur and substrate surface texture on austenite grain size in low carbon steel strip casting The effect of the initial nucleation behaviour on austenite grain size could not be established due to difficulties in resolving the surface microstructures in the low carbon steel samples. Experiments were therefore carried out with stainless steel (304 austenitic) where the surface nucleation pattern and cross-sectional austenite grain structure could be revealed with sufficient contrast. Results from Stainless Steel Studies Melt sulphur content has a dramatic effect on the nucleation behaviour as evident from the surface microstructures shown in Figure 7. The observed nucleation pattern was random and the nuclei count was considerably enhanced with the higher sulphur level (see Figure 7b) which was effective in improving the melt/substrate contact by reducing the melt surface tension (10). (a) (b) Figure 7 Effect of melt sulphur on surface solidification structures showing nucleation pattern (a) wt% (b) 0.03 wt% Brimacombe Symposium, Vancouver, British Columbia, Canada, 1-4 October,

8 The meniscus heat fluxes measured during solidification showed a strong correlation with the surface nucleation density (see Figure 8). An increase in nucleation density from 50 to 1000 nuclei/mm 2 increased the meniscus heat flux from 10 to 40 MW/m Meniscus Heat Flux (MW/m 2 ) Nucleation Density (Nuclei/mm 2 ) Figure 8 - Correlation between measured meniscus heat fluxes and solidification nucleation density The relationship between the austenite grain width and surface nucleation density is presented in Figure 9. The increase in nucleation density from 50 to 1000 nuclei/mm 2 led to a corresponding reduction in austenite grain width from 140 to 30 µm. Austenite Grain Width (µm) Nucleation Density (Nuclei/mm 2 ) Figure 9: Effect of surface nucleation density on austenite grain width measured perpendicular to the solidification direction The controlled laboratory experiments have illustrated that the microstructure development in strip casting is fundamentally coupled to the solidification process and that the nature of Brimacombe Symposium, Vancouver, British Columbia, Canada, 1-4 October,

9 the initial melt/substrate contact, as influenced through wetting by chemistry effects or as imposed by the substrate surface texture effects, has a profound influence on the size of the austenite grains. FERRITE MICROSTRUCTURE CONTROL IN STRIP CASTING Control of the ferrite microstructure is critical as it determines the mechanical properties of the final product. A number of strategies can be potentially employed to control the final strip microstructure. Metallurgical fundamentals associated with these options are discussed in this section. Solidification Control Changes in the solidification parameters such as melt chemistry, substrate texture and casting speed can strongly influence the final ferrite microstructure of the as-cast strip. This is illustrated in Figure 10 which shows the strong effect of the melt sulphur content. High sulphur samples produced a microstructure which was dominated by polygonal ferrite (see Figure 10a). On the other hand, the low sulphur samples were predominantly bainitic (see Figure 10b). The differences in the ferrite microstructures are a direct outcome of austenite grain size differences. A decrease in sulphur level from 0.04 to 0.01 wt% increased the austenite grain size from 60 to 200 µm. Substrate side Substrate side (a) (b) Figure 10 - Effect of melt sulphur content on final ferrite microstructures of low carbon steel samples (a) 0.04 wt% (b) 0.01 wt% Brimacombe Symposium, Vancouver, British Columbia, Canada, 1-4 October,

10 Strip Cooling Control Strip cooling rates encountered during the transformation of austenite can influence the ferrite microstructure. The immersion facility (see Figure 3) was used to examine the effect of different cooling regimes during the austenite transformation for constant initial austenite grain size. Cooling intensity had a dramatic effect on the final strip microstructures as reflected in the hardness values which are shown in Figure 11. The water quenched sample was associated with a martensitic microstructure with a hardness value of around 360 VHN (Vickers Hardness Number) as opposed to the air cooled sample which exhibited a mixed polygonal/widmanstatten/bainite microstructure with a correspondingly lower hardness value of 190 VHN. Similar tests were carried out with samples which were re-austenitised prior to cooling. Reaustenitisation produced austenite microstructures similar to that of a hot strip mill product. Figure 11 clearly shows that the response of the hot strip mill austenite structure was significantly lower in comparison to strip cast microstructures. Unlike hot strip mill products, coarser austenite grains inherent to strip cast material can be transformed into a variety of ferrite microstructures through appropriate cooling treatment during the austenite transformation process. It should also be noted that through strip casting it is possible to achieve a remarkable degree of hardenability on a typical low carbon steel chemistry which is usually considered to be non-hardenable. Hardness (VHN) Austenite grain size =150 X 400 µm Strip Casting Hot Strip Mill Water Quenched Air Cooled Austenite grain size = 25 µm Figure 11 - Effect of cooling rates during austenite transformation on final strip hardness Brimacombe Symposium, Vancouver, British Columbia, Canada, 1-4 October,

11 Figures 12 a-d show the final microstructures for strip samples obtained from the plant which were cast under similar solidification conditions but subjected to different cooling rates through austenite transformation. Table II shows that cooling rates had a dramatic impact on the strip microstructure and mechanical properties. (a) (b) (c) (d) Figure 12 - Effect of cooling rates during austenite transformation on final ferrite microstructures for samples obtained from strip casting plant: (a) 0.1; (b) 13; (c) 25; and (d) 100 C/s Table II - Effect of cooling conditions on microstructures and properties of strips cast in the plant Cooling Rate ( C/s) Coiling Temperature ( C) Microstructure Constituents Yield Strength (MPa) 0.1 > 800 Polygonal ferrite, 210 Pearlite Polygonal ferrite, 320 Widmanstatten ferrite Polygonal ferrite, Bainite < 400 Polygonal ferrite, Bainite, Martensite 490 Brimacombe Symposium, Vancouver, British Columbia, Canada, 1-4 October,

12 In-line Hot Rolling In-line hot rolling was found to be effective in refining the as-cast microstructure. However it was found that less than 15 % hot reduction was not sufficient to modify the microstructure produced during the casting process (2). For similar hot reduction levels, rolling temperature had a profound impact on the microstructure refinement (see Figures 13a-b). Fine ferrite microstructure was observed when rolling at around 860 C (see Figure 13a). Ultra fine ferrite grains in the range of 2 to 4 µm were observed near the surface and this region extended up to 30% of the thickness. The rest of the sample consisted of fine polygonal ferrite grains in the range of 10 to 20 µm. Material with high yield strength (400 MPa) and a total elongation in excess of 30 % can be produced with this microstructure. Hot rolling temperatures of around 1050 C on the other hand produced microstructures with polygonal ferrite content estimated to be more than 80%, with grains in the size range of 10 to 40 µm. Thus, wide range of strip microstructures can be produced through adequate control of the degree of hot reduction and the rolling temperature. Strip surface Strip surface (a) (b) Figure 13 - Effect of in-line hot rolling temperature on microstructure refinement observed in strips produced from full scale plant: (a) 36% reduction at 860 C; and (b) 31% reduction at 1050 C Residuals Strip casting can tolerate a significant amount of residuals such as Cu and Sn compared to conventional casting processes. The presence of residuals had a significant effect on the microstructure of the strip cast products. Under similar strip cooling conditions, the presence of residuals enhanced the proportion of low temperature transformation products (particularly the bainites), producing material with higher strength (see Figure 14). Although the austenite grain size was not influenced, the presence of residuals lowered the transformation temperatures and slowed the kinetics of polygonal ferrite formation. Brimacombe Symposium, Vancouver, British Columbia, Canada, 1-4 October,

13 Yield Strength (MPa) Without Residuals With Residuals Figure 14 - Effect of residuals on yield strength measured on strips produced in the full scale plant: (Cu=0.4, Sn=0.2, Cr=0.2, Ni= 0.2, Mo=0.2 - in wt%) PRODUCT OPPORTUNITIES IN STRIP CASTING The evolution of microstructure in strip casting is fundamentally different to strip products produced via conventional hot strip mills. A variety of ferrite microstructures can be produced from the strip casting route through control of solidification, strip cooling (during austenite transformation), in-line hot rolling and residuals. Combinations of material strength and elongation levels that can be potentially obtained from the strip casting process using single low carbon steel chemistry are summarised in Figure 15. In the conventional strip production processes, significant chemistry changes are necessary to produce a broad range of properties. Figure 16 summarises a typical range of steel chemistries used to produce a variety of hot rolled structural products (6). Generally, higher C, Mn and micro-alloys (Ti, Nb) are necessary to produce high strength material. Strip casting on the other hand has the potential to achieve a broad range of properties with a single chemistry because of its unique coarse austenite grain structure, thus avoiding the difficulties encountered with casting peretectic carbon steel and also avoiding the additional costs associated with micro-alloys. Brimacombe Symposium, Vancouver, British Columbia, Canada, 1-4 October,

14 600 Residuals or Fast Cooling Yield Strength (MPa) Moderate Cooling Low Temperature Hot Rolling Typical Products Slow Cooling Elongation (%) Figure 15 - Product opportunities in strip casting from a single low carbon steel chemistry Yield Strength (MPa) C = 0.08 / 0.1% Mn = 1.4 / 1.5% Ti = 0.01 / 0.03% Nb = 0.04 / 0.05% C = 0.08 / 0.11% Mn = 0.5 / 0.6% Nb = 0.03 / 0.04% C = 0.14 / 0.17% Mn = 0.7 / 0.8% C = 0.09 / 0.17% Mn = 0.5 / 0.75% Elongation (%) Figure 16 - Chemistry requirements in conventional hot strip mill route to produce range of hot rolled structural products (6) Brimacombe Symposium, Vancouver, British Columbia, Canada, 1-4 October,

15 The decisive advantage of the strip casting process lies in its capability to produce high strength material (e.g., Grade 80, ASTM specification) directly from the caster without the need for a cold rolling step. Currently these products can only be produced through a cold rolling operation. CONCLUDING REMARKS A fundamental understanding of microstructure evolution in strip casting has been obtained using a laboratory facility. Links between initial heat transfer, nucleation, solidification and subsequent microstructures have been established. This has provided an understanding of the origin of the strip microstructures and properties obtained on a full-scale strip casting plant. In particular, it was demonstrated that coarse austenite grains inherent in strip casting can be transformed into a variety of ferrite microstructures. Strip casting by virtue of its unique metallurgical regime offers the potential to achieve a range of mechanical properties with a single steel chemistry. ACKNOWLEDGMENTS We would like to thank our many colleagues for technical assistance, discussions and support. In particular, we wish to acknowledge F. DeSylva, T. Pham, H. Kaul, J. Browne, A. Horti and Y. Durandet. REFERENCES 1. W. Blejde, H. Fukase and R. Mahapatra, Recent Developments in Project M The Joint Development of Low Carbon Steel Strip Casting by BHP and IHI, International Conference on New Development on Metallurgical Process Technology, METEC Congress, June 1999, pp W. Blejde, R. Mahapatra and H. Fukase, Development of Low Carbon Thin Strip Production Capability at Project M, Iron and Steelmaker, Vol. 27, No. 4, 2000, pp W. Blejde, R. Mahapatra and H. Fukase, Application of Fundamental Research at Project M, The Belton Memorial Symposium Proceedings, 2000, pp L. Strezov and J. Herbertson, Experimental Studies of Interfacial Heat Transfer and Initial Solidification Pertinent to Strip Casting, ISIJ, Vol. 38, 1998, pp L. Strezov, J. Herbertson and G.R. Belton, Mechanisms of Initial Melt/Substrate Heat Transfer Pertinent to Strip Casting, The Belton Memorial Symposium Proceedings, 2000, pp Product Data Manual, Hot-Rolled Products, Flat Products Division - BHP Steel, L. T. Shiang and P.J. Wray, The Microstructures of Strip-Cast Low-Carbon Steels and their Response to Thermal Processing Metallurgical Transactions, Vol. 20A, 1989, pp A. Cramb, Strip Casting of Steels: Current Status and Fundamental Aspects, Proceedings of the International Symposium on Near-Net-Shape Casting in the Mini-Mills, 1995, pp Brimacombe Symposium, Vancouver, British Columbia, Canada, 1-4 October,

16 J.K. Brimacombe and I.V. Samarasekera, The Challenges in Thin Slab Casting, Proceedings of the International Symposium on Near-Net-Shape Casting in the Mini-Mills, 1995, pp T. Evans and L. Strezov, Interfacial Heat Transfer and Nucleation of Steel on Metallic Substrates, The Belton Memorial Symposium Proceedings, 2000, pp G. Backmann, A. Kothe, W. Loster, J. Richter, T. Schubert, S. Thiem and L. Hering, Investigation of the Solidification Behaviour and Mechanical Properties of Thin Slabs, Stahl u. Eisen, Vol. 113, No. 2, 1993, pp C.A. Muojekwu, D.Q. Jin, I.V. Samarasekera and J.K. Brimacombe, Thermomechanical History of Steel Strip during Hot Rolling - A Comparison of Conventional Cold-Charge Rolling and Hot-Direct Rolling of Thin Slabs, 37 th Mechanical Working and Steel Processing Conference Proceedings, ISS, Vol. XXXIII, 1996, pp Brimacombe Symposium, Vancouver, British Columbia, Canada, 1-4 October,