Materials Science Forum Online: 2016-11-15 ISSN ...

5 downloads 929 Views 4MB Size Report
Materials Science Forum. Online: 2016-11-15. ISSN: 1662-9752, Vol. 879, pp 1651-1655 doi:10.4028/www.scientific.net/MSF.879.1651. © 2017 Trans Tech ...
Materials Science Forum ISSN: 1662-9752, Vol. 879, pp 1651-1655 doi:10.4028/www.scientific.net/MSF.879.1651 © 2017 Trans Tech Publications, Switzerland

Online: 2016-11-15

Comparison of Microstructure and Mechanical Behavior of the Ferritic Stainless Steels ASTM 430 Stabilized with Niobium and ASTM 439 Stabilized with Niobium and Titanium. Leandro Paulo de Almeida Reis Tanure1, Cláudio Moreira de Alcântara2, Tarcísio Reis de Oliveira2, Dagoberto Brandão Santos1 and Berenice Mendonça Gonzalez1* 1

Department of Metallurgical and Materials Engineering, Universidade Federal de Minas Gerais UFMG, Escola de Engenharia - Bloco 2, Av. Antônio Carlos, 6627, Pampulha, CEP 31270-901, Belo Horizonte, MG, Brazil. 2

Research Department, Aperam South America, Praça 1º de Maio, 09, Centro, CEP 35180-018 Timóteo, MG, Brazil.

Keywords: Ferritic stainless steel, mechanical behavior, anisotropy.

Abstract: The use of Ferritic Stainless Steels has become indispensable due its lower cost and the possibility to replace austenitic stainless steels in many applications. In this study, cold rolled sheets of two stabilized ferritic stainless steels with 85% thickness reduction were annealed by applying a heating rate of 24 ºC/s and a soaking time of 24 s. The niobium stabilized ferritic stainless steel type ASTM 430 (430Nb) was annealed at 880 ºC while the niobium and titanium bi-stabilized steel ASTM 439 was annealed at 925 ºC. The annealed samples were tensile tested and due to the smaller grain size, steel 430Nb, showed a higher yield stress and a higher total elongation. Concerning drawability the steel ASTM 439 presented a better performance with higher average R-value, lower planar anisotropy coefficient and a greater value for Limit Drawing Ratio (LDR). These results are explained in terms of the differences in size and volume fraction of precipitates between the two steels. Introduction Ferritic Stainless Steels (FSS) are used around the world in many different applications. Due to a lower cost, a lower thermal expansion, a higher temperature oxidation resistance and a higher thermal conductivity compared to the austenitic stainless steels, its use becomes practically indispensable [1, 2]. High levels of chromium are important to stabilize the ferritic structure while carbon and nitrogen, precipitate on the grain boundary leaving a depleted chromium zone nearby promoting intergranular corrosion (problem known as sensitization). To minimize the effects of sensitization, which is one of the most important factors that limit the applications of the FSS, some specific alloying elements such titanium, vanadium, zirconium and niobium have been used to form precipitates that form faster and are more stable than those ones which would be formed with chromium[3, 4]. Titanium and niobium have proved to be the best options to reduce the effect of sensitization and also improve mechanical resistance without considerable reduction of toughness and ductility because they promote the formation of carbides and nitrides that reduce the recrystallized grain size [3, 4, 5]. The lower plasticity compared to plain carbon steels and austenitic stainless steels leads to different attempts to develop new metallurgical routes capable of improving the formability of FSS[6] specially which concerns deep drawability because it is the main requirement for the major applications[1]. The influence of microstructure, such as grain size and amount and size of precipitates are important to evaluate and predict the most suitable FSS for a specific use and best performance under the required requests. In this study, a comparison of microstructure and mechanical properties of the FSS ASTM 430 stabilized with niobium and ASTM 439 stabilized with niobium and titanium was performed. The materials were provided by Aperam South America and the experimental work tried to reproduce the industrial conditions.

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (#71347940, University of Toronto Library, Toronto, Canada-16/11/16,11:39:26)

1652

THERMEC 2016

Experimental Work Materials and Manufacturing Process. The investigated materials were the niobium stabilized FSS type ASTM 430 (430Nb) and ASTM 439 stabilized with niobium and titanium. Their chemical composition (wt%) are shown in Table 1 Table 1 – Chemical compositions [wt%] of the investigated steels. Steel C Mn Si P Cr N Ni Nb 0.017 0.17 0.34 0.031 16.5 0.022 0.23 0.31 430Nb 0.19 0.50 17.2 0.011 0.20 0.19 ASTM439 0.010

Ti 0.15

The samples came from hot rolled coils (samples HR) with 4.0 mm thick and then were cold rolled to 0.60 mm of thickness. The annealing was carried out with a heating rate of 24 °C/s and a soaking time of 24 s (samples A). The soaking temperature for the 430Nb and ASTM 439 steels were 880 °C and 925 °C, respectively (the standard temperatures for each steel). Microstructure Analysis. The specimens for microstructural analysis were taken from the middle of sheets and observed through the longitudinal sections using optical microscopy and Scanning Electron Microscopy (SEM). The samples were cut, mounted, grinded and polished according to the standard metallographic procedures and then etched in the etchant Villela for 50 s. The grain sizes of the annealed specimens (samples A) were measured using the Abrams concentric circles method following the standard ASTM E112-13[7]. Mechanical Properties. Tensile tests were performed at room temperature on the annealed specimens (samples A) using an INSTROM 5583 equipment. The determination of yield strength (YS) and ultimate tensile strength (UTS) followed the standard ASTM A370-10[8]. To evaluate the anisotropy of the sheets, the average R-value, , was calculated according to Eq. 1, .

(1)

where , and correspond to the R-value in the directions of 0°, 45° and 90° to the rolling direction, respectively. Also the planar anisotropy coefficient, ΔR, was evaluated using the Eq. 2.

.

(2)

The samples were strained by 15% and the anisotropy tests followed the standard ASTM E51710[9]. The deep drawability capacity of the annealed specimens was evaluated trough a Swift test using a Erichsen 14240 equipment. Blanks of different diameters were tested to determine the Limit Drawing Ratio (LDR). Results Microstructure Analysis. The samples of the two steels taken from the hot rolled coils (HR), examined by optic microscopy, are shown in Fig. 1.

Materials Science Forum Vol. 879

1653

(a) 430Nb HR (b) 439 HR Figure 1 – Microstructure of the samples in hot rolled conditions (HR). The microstructures of the hot rolled condition show a uniform slightly elongated grain size distribution. The smaller grain size in 430Nb is a consequence of the higher content of interstitial elements (C+N~400ppm) that precipitate and stimulate the nucleation of new grains. This effect is also noticed at the final recrystallization on the annealed specimens as can be seen on SEM images on Fig. 2.

(a) 430Nb annealed sample (b) 439 annealed sample Figure 2 – Precipitates of the annealed specimens. The Fig. 2 shows for the 430Nb steel a greater amount of precipitates. In both steels, coarse precipitates are located preferentially in grain boundaries while the fine precipitates are both at grain boundaries and the matrix. The results of grain size measurement are shown in Table 2 and present smaller grains for the steel with higher amount of precipitates, which work as preferential sites for nucleation of new grains since heterogeneous nucleation occurs faster. Another possible factor was the lower annealing temperature. Table 2 – Ferrite grain size. Material 430Nb ASTM 439

Grain size [μm] 15 ± 1 19 ± 1

ASTM grain size 9 8

1654

THERMEC 2016

Tensile Properties. The results provided by tensile test for both steels are shown in Table 3. Table 3 – Tensile properties of the steels 430Nb and ASTM 439 with its mean errors. Material YS [MPa] UTS [MPa] Uniform elongation [%] Total elongation [%] 430Nb 291 ± 4 460 ± 3 22 ± 3 36.4 ± 0.4 ASTM 439 281 ± 3 465 ± 2 22.8 ± 0.2 33.2 ± 0.7 The values for yield stress and total elongation were slightly higher for the 430Nb steel due to the smaller grain size once it is the only strengthening mechanism capable of increasing mechanical resistance without considerable loss of toughness. On the other hand, the ultimate tensile strength and the uniform elongation remains approximately the same considering the mean error. Drawability. The drawability was evaluated through the average normal anisotropy coefficient value, , the planar anisotropy coefficient, ΔR, and LDR values. The results are shown in Table 4. Table 4 – Parameters related to drawability for the studied steels. Material ΔR 430Nb 1.45±0.01 1.09±0.04 1.36±0.18 1.25±0.03 0.31±0.13 ASTM 439 1.86±0.02 1.73±0.11 2.15±0.05 1.87±0.05 0.28±0.14

LDR 2.18 ≥2.30

The higher average R-value, which means a better resistance regarding thickness thinning, and the lower value for the planar anisotropy coefficient, which represents a lower level of "earing", lead to choose ASTM 439 when the drawability performance is the major requirement. In addition, the LDR value indicates that it is possible to draw deepest blanks using this steel. According to Gao and colleagues[10], for FSS, a fine and dense precipitates distribution promotes the nucleation of randomly oriented grains weakening the γ-fiber recrystallization texture which leads to a worse performance concerning drawability and justifies the observed behavior. Texture comparison will be investigated in a future study. Conclusions The followed annealing heat treatment for both steels showed a smaller recrystallized grain size for the steel 430Nb, due to the greater volume fraction of precipitates and lower annealing temperature, and an elevated yield stress and total elongation compared to ASTM 439. On the other hand the niobium titanium bi-stabilized steel ASTM 439 presented a better performance under deep drawability requirements through higher average normal anisotropy coefficient, higher LDR and lower planar anisotropy coefficient mainly because of smaller amount of precipitates that could affect the recrystallized texture. Acknowledgement The authors are grateful for financial support by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG), Pró-Reitoria de Extensão da Universidade Federal de Minas Gerais (PROEX/UFMG) and technical support by Aperam South America.

Materials Science Forum Vol. 879

1655

References [1] International Stainless Steel Forum. The Ferritic Solution - Properties, Advantages and Applications: The Essential Guide to Ferritic Stainless Steels. Brussels, Belgium, 2007. [2] K. H. Lo; C. H. Shek; J. K. L. Lai, Recent Developments in Stainless Steels. Materials Science and Engineering R, v. 65, p. 39-104, 2009. [3] J. L. Cavazos, Characterization of Precipitates Formed in a Ferritic Stainless Steel Stabilized with Zr and Ti Additions. Materials Characterization, v. 56, p. 96-101, 2006. [4] X.-M. You; Z.-H. Jiang; H.-B. Li, Ultra-Pure Ferritic Stainless Steels - Grade, Refining, Operation and Application. Journal of Iron and Steel Research, International, v. 14, n. 4, p. 24-30, 2007. [5] H. Yan et al. Microstructure and Texture of Nb+Ti Stabilized Ferritic Stainless Steel. Materials Characterization, v. 59, p. 1741-1746, 2008. [6] W. Du; X. Jhiang, Microstructure, Texture and Formability of Nb+Ti Stabilized High Purity Ferritic Stainless Steel. Journal of Iron and Steel Research, International, v. 17, n. 6, p. 47-52, 2010. [7] ASTM E112-13. Standard Test Methods for Determining Average Grain Size, ASTM International, West Conshohocken, 2013. [8] ASTM A370-10. Standard Test Methods and Definitions for Mechanical Testing of Steel Products, ASTM International, West Conshohocken, 2010. [9] ASTM E517-10. Standard Test Method for Plastic Strain Ratio r for Sheet Metal, ASTM International, West Conshohocken, 2010. [10] F. Gao; Z-Y Liu; G-D Wang. Effect of The Size and Dispersion of Precipitates Formed in Hot Rolling on Recrystallization Texture in Ferritic Stainless Steels. Journal of Materials Science, v. 48, p. 2404-2415, 2013.