Influence of Hot Band Annealing and Cold Rolling on Texture and Ridging of 430 Stainless Steel Containing Aluminum ()
1. Introduction
Ferritic stainless steel 430 grade is extensively used in various applications like kitchen wares, electrical appliances, automobile parts and white goods on account of its good corrosion resistance, high thermal conductivity and beautiful surface gloss as a low cost alternative to nickel containing austenitic stainless steel. Occurrence of ridging after cold forming operation can hamper the surface finish of the material necessitating considerable effort in polishing of the surface. Wright [1] has pointed out that ridging is associated with anisotropic plasticity of bands of contrasting textures. Shin et al. [2] established that differences in the deformation behavior of {001} <110> and {111} <110> grain colonies arises due to difference in their plastic strain ratio during cold forming operation thereby causing ridging. Extensive efforts have been made to minimize ridging. Kim et al. [3] suggested refinement of solidification structure by using EMS to avoid columnar grains with <001>//ND orientation is utilized for improving ridging resistance. This entails significant capital cost. Kimura et al. [4] advocated high interpass time during roughing in conjunction with high reduction per pass to facilitate recrystallization in the colony structures. High interpass time during rough rolling can adversely affect the productivity of the hot mill. Since 430 stainless steel has two phase α + γ structure during hot rolling, some martensite forms on cooling of the hot coils. Such coils are generally subjected to bell annealing to transform the martensite to ferrite to facilitate cold rolling. Jha et al. [5] concluded that continuously hot band annealed material exhibits superior roping characteristic. In this study, they also noticed that more than 25% martensite in the microstructure can cause breakage of the coil during cold rolling. Huh et al. [6] observed pronounced through thickness texture gradient in hot bands of this grade which had an impact on appearance of ridging in the final recrystallised sheet. Huh and Engler [7] suggested that introducetion of intermediate anneal during cold rolling weaken the texture gradients and minimize the extent of ridging compared to single stage cold rolled product. Avila & Zapata [8] annealed the hot bands of 430 grade in 2 phase region and followed by controlled cooling to obtain dual phase structure. These authors noted that grain refinement due to martensite dispersion during cold rolling resulted in significant improvement in ridging. Mola et al. [9] found that samples without hot band annealing when subjected to cold rolling and subsequent bell annealing exhibited superior ridging resistance compared to conventional bell annealed product which was continuously annealed after cold reduction. Batch annealing is an energy intensive operation which increases cost of the finished product. The present paper highlights the beneficial use of the intrinsic heat of the hot rolled coil under an insulated hood to enable the coil to cool slowly and thereby facilitate transformation of austenite to ferrite and reduce the hardness of martensite to permit extensive cold reduction. In this manner 430 grade with superior ridging resistance has been successfully produced at a lower processing cost. The paper describes plant trials with different processing routes with variations in mode of annealing and amount of cold reduction. The study was done on coils made from continuously cast slabs through EAF-AOD-LF-CC route without using EMS. Mechanical properties, texture, ridging characteristics of product in inner and outer wrap as well as in the middle of the coils are presented.
2. Experimental Detail
The chemical composition of AISI 430 ferritic stainless steel used in the current study is shown in Table 1. Four Continuously casted slabs of 200 mm thickness were hot rolled in a roughing and steckel mill to 2.5 mm thick coil. Processing schedule of these four slabs is given in Figures 1(a)-(d). Coil-A was soaked at 1170˚C and then hot rolled in roughing and finishing at steckle mill upto 2.5 mm thickness. The coil was cooled in air to room temperature and then bell annealed at 820˚C for 8 hrs. Thereafter it was cold rolled to 0.6 mm and finally continuously annealed at 820˚C. Coil B was soaked at 1210˚C and then it followed the same route of coil A, just to investigate the effect of soaking temperature. Coil C and D were kept in an insulated box just after hot rolling and allowed to cool slowly. Coil C was then directly cold rolled to 0.6 mm and finally continuous annealed at 820˚C. Coil D was subjected to 33% cold reduction and continuously annealed at 820˚C and thereafter cold rolled to 0.6 mm and finally continuously annealed at 820˚C. Samples from the centre as well as end portions were cut from the coils for microstructural characterization and mechanical property evaluation. The specimens were etched chemically by glyceregia reagent to reveal the microstructural features in optical microscope. Microstructures were examined by Carl Zeiss optical microscope and Carl Zeiss EVO-40 scanning electron microscope. Micro-texture was studied by Zeiss EVO-40 SEM attached with Oxford Inca EBSD system and analysed with HKL software. Ridging characteristics were measured after giving 20% tensile elongation by instrument manufactured by Mitutoyo. Hardness was evaluated by Vickers hardness test under 10 kg load. Tensile testing was done in a universal testing machine and Ericsson cupping test was performed for investigating the stretchability of the sheets.
Table 1. Chemical composition of the steel used in this study.
3. Results and Discussion
3.1. Ridging Resistance
Ridging height after 20% elongation under tension is shown in Figure 2. Ridging height is highest for Coil A and then it has decreased in coil B, C and D. In all cases coil end sample is showing higher ridging resistance than coil centre sample. As coil A and coil B had similar processing schedule and only difference is soaking temperature, better ridging resistance in coil B can be explained by the high soaking temperature. Hood annealed coils are showing better ridging resistance than bell annealed coils due to the randomization of initial texture by deformation in dual phase microstructure.
3.2. Microstructure and Texture of Hot Bands
Microstructure of as hot rolled coil B is shown in Figure 3. It contains approximately 35% martensite in ferrite. Microstructure of entire cross section of coil B after bell annealing is shown in Figure 4. Ferrite grains are finer
Figure 2. Roughness measurement of different coils after 20% tensile elongation.
Figure 3. Cross section microstructure of as hot rolled coil B.
Figure 4. Collage of microstructure throughout the thickness coil B just after bell annealing along with Inverse pole figure map of subsurface and center region taken in RD.
near surface and relatively coarser in the center. Interspersed carbide particles are also visible. Surface region consists of equiaxed grain of size of 30 - 35 µm but center layer has elongated grains with 80 - 100 µm. The finer equiaxed grains constitute only 30 volume percent of the microstructure. Texture study revealed random texture in the subsurface layer and mostly α fiber component in the center region of the band. Wei et al. [10] studied the hot band annealing of a ferritic stainless steel and concluded that elongated ferrite grains with α fibre orientation had not recrystallized during bell annealing.
Microstructure of the coils C&D after hood annealing exhibits banded structure with lamella of ferrite and tempered martensite (Figure 5(a)). Hardness after hood annealing decreased from nearly 200 HV to 175 HV. Hood annealed coils could be given very heavy deformation without any breakage of the coil with beneficial effect on ridging resistance. Microstructure of coil D after 33% cold reduction followed by continuous annealing is shown in Figure 5(b).
3.3. Microstructure and Mechanical Properties of the Cold Rolled and Annealed Samples
In coil A&B, equiaxed ferrite grains are present in both coil center and edge in the cold rolled and annealed
Figure 5. (a) Microstructure of coil C just after hood annealing, (b) Microstructure after 33% cold reduction and subsequent annealing of hood annealed coil D.
recondition (Figure 6). Recrystallized grains of ASTM 8.5 to 9 (Table 2) were observed in the center samples of the coils whereas coil end samples are slightly finer (ASTM 9 - 9.5) in size.
In the hood annealed coils, while recrystallization occurred after cold rolling and annealing, the grains are pancake shaped (Figure 7). The grains are also slightly finer in size as shown in Table 2. Occurrence of deformation in the presence of martensite can be the reason of finer grains in hood annealed coils.
Due to decrease in the grain size, increase in the yield strength and marginal decrease in the ductility can be observed (Table 2) in the hood annealed coils.
3.4. Texture of the Cold Rolled and Annealed Coils
Figure 8 shows φ2 = 45˚ section of the ODF measured in the cold rolled and annealed coils. A weak <111>