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10./s-022--2

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Effect of Copper Addition on the Formability of 304L Austenitic Stainless Steel

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Ali Huang

1School of Materials Science and Engineering, Shanghai University, Shanghai, People&#;s Republic of China

2State Key Laboratory of Advanced Special Steel, Shanghai University, Rm 425 Material Building C, 333 Nanchen Road, Shanghai, People&#;s Republic of China

Find articles by Ali Huang

Keping Wang

1School of Materials Science and Engineering, Shanghai University, Shanghai, People&#;s Republic of China

2State Key Laboratory of Advanced Special Steel, Shanghai University, Rm 425 Material Building C, 333 Nanchen Road, Shanghai, People&#;s Republic of China

Find articles by Keping Wang

Yangyang Zhao

1School of Materials Science and Engineering, Shanghai University, Shanghai, People&#;s Republic of China

2State Key Laboratory of Advanced Special Steel, Shanghai University, Rm 425 Material Building C, 333 Nanchen Road, Shanghai, People&#;s Republic of China

Find articles by Yangyang Zhao

Wurong Wang

1School of Materials Science and Engineering, Shanghai University, Shanghai, People&#;s Republic of China

2State Key Laboratory of Advanced Special Steel, Shanghai University, Rm 425 Material Building C, 333 Nanchen Road, Shanghai, People&#;s Republic of China

Find articles by Wurong Wang

Xicheng Wei

1School of Materials Science and Engineering, Shanghai University, Shanghai, People&#;s Republic of China

2State Key Laboratory of Advanced Special Steel, Shanghai University, Rm 425 Material Building C, 333 Nanchen Road, Shanghai, People&#;s Republic of China

Find articles by Xicheng Wei

Jingguang Peng

1School of Materials Science and Engineering, Shanghai University, Shanghai, People&#;s Republic of China

2State Key Laboratory of Advanced Special Steel, Shanghai University, Rm 425 Material Building C, 333 Nanchen Road, Shanghai, People&#;s Republic of China

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Author information Article notes Copyright and License information PMC Disclaimer

1School of Materials Science and Engineering, Shanghai University, Shanghai, People&#;s Republic of China

2State Key Laboratory of Advanced Special Steel, Shanghai University, Rm 425 Material Building C, 333 Nanchen Road, Shanghai, People&#;s Republic of China

Jingguang Peng,

:

nc.ude.uhs@gjgnepCorresponding author.

Corresponding author.

Copyright © ASM International

This article is made available via the PMC Open Access Subset for unrestricted research re-use and secondary analysis in any form or by any means with acknowledgement of the original source. These permissions are granted for the duration of the World Health Organization (WHO) declaration of COVID-19 as a global pandemic.

Abstract

To improve the antibacterial properties of 304L austenitic stainless steel, copper is often added as an antibacterial agent, but the forming performance of the resulting material is poor, impacting its actual production and use. Therefore, this study investigated the influence of copper addition on the formability of 304L austenitic stainless steel with drawing, cupping and conical cup forming tests. Mechanical properties were determined with tensile and hardness tests. The microstructure and phase transformation were further characterized by metallographic microscopy, scanning electron microscopy and x-ray diffraction analysis. It was found that the addition of copper impaired the mechanical properties of 304L austenitic stainless steel, increased the stacking fault energy of the material and inhibited the occurrence of strain-induced martensite transformation, leading to a decrease in the formability of 304L austenitic stainless steel.

Keywords:

304L austenitic stainless steel, copper alloying, formability, stacking fault energy (SFE), strain-induced martensite

Introduction

Stainless steel has been widely used in schools, restaurants, transportations and other public areas because of its excellent combination of good mechanical properties, reliable chemical stability and outstanding decorative functions (Ref 1-3). With increasing attention given by society to health and hygiene awareness, more stringent requirements for the performance of stainless steel materials have been put forward. Copper ions released from the surface of the copper-containing materials endow the material with a broad spectrum of antibacterial activities. It shows strong inactivation of common infectious bacteria such as Escherichia coli and Staphylococcus aureus (Ref 4). Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) shows strong stability on the surface of conventional stainless steel, and even after three days, infectious virus can be detected, posing a high risk of virus transmission through surface contact in public places (Ref 5). The developed anti-pathogen stainless steel containing 20% copper can significantly reduce its surface-active SARS-CoV-2 by 99.99% within 6 h (Ref 6). In addition, copper is a trace element needed by human body, which can promote the synthesis of human hemoglobin and effectively reduce the apoptosis rate of cells (Ref 7, 8). Therefore, the research and development of new stainless steel materials with antibacterial and antiviral functions has become the next important goal of the developers of antibacterial stainless steel containing copper.

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In metastable austenitic steels, deformation mechanisms, including deformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP), are generally determined by the stacking fault energy (SFE), which is sensitively affected by its composition (Ref 9). Copper is an SFE-increasing element (Ref 10). Choi and coworkers (Ref 11) added 1 wt.% copper into an austenitic high-Mn TRIP steel, and the tensile properties were improved when TRIP and TWIP were well homogenized. However, a high addition of copper could suppress the formation of martensite and improve the amount of deformation twins after tensile deformation, in accordance with the SFE (Ref 12). Gonzalez et al. (Ref 13) investigated the effect of copper on the formability of austenitic stainless steel. The results showed that the addition of copper suppressed ε-martensite formation and decreased the kinetics of α&#;-martensite formation. Both uniform elongation and total elongation were improved by the decrease in the strain hardening rate due to the addition of copper. The ultimate tensile strength increases with higher copper concentrations until alloying with 5.5% copper, at which point it remains constant (Ref 14). The mechanical properties of austenitic steels also depend strongly on the stability of the matrix. Copper is also an austenite stabilizer (Ref 10). According to the research conducted by Kim et al. (Ref 15), the volume of retained austenite increased with the addition of copper, as the increased austenite stability resulted in both a higher strength and a higher ductility.

Copper usually remains either in solution or as nanoprecipitates in steels. Precipitation of the copper-rich phase will occur upon aging treatment, and copper-rich precipitation is one of the most effective intermetallic strengthening precipitates (Ref 16). Because of the slower diffusion kinetics of copper, low interfacial energy and high strain energy of copper-rich precipitates in the austenite matrix, copper-rich precipitates grow and coarsen slowly, which is consistent with the modest change in hardness and yield strength with extended aging (Ref 17). Yang et al. (Ref 18) thought that this fine precipitate improved the strength and maintained the plasticity of the material. Small additions of copper will not have adverse effects on the fracture and fatigue behavior of the stainless resulting steel (Ref 19).

In this paper, 304L austenitic stainless steel was treated with more than 3.5% copper, and only solution treatment was used; this resulted in an excellent antibacterial effect. However, this kind of material easily cracks during the forming process and has poor forming performance, which greatly limits its application range. Since copper affects the microstructural and mechanical properties of steel, which may further affect its formability, this study started from practical problems, conducted an in-depth study on the poor formability of this copper-containing austenitic antibacterial stainless steel and determined the influence of copper addition on the formability of 304L austenitic stainless steel.

Materials and Methods

The billet was heated to  °C in a walking beam furnace and then rolled into a steel strip with a thickness of 3.0 mm. First, the steel strip was held at  °C for 10 min for solution treatment and then cold rolled into a 0.8 mm cold rolled steel strip many times. Finally, the cold rolled steel strip was held at  °C for 10 min and then pickled to obtain the experimental steel sheet. The chemical compositions of the two experimental steels used in this work are listed in Table . The contents of Cr, Ni and other elements in the two materials were similar, while the content of Cu in 304L-Cu was 4.01%.

Table 1

SteelCSiMnPSNCrNiCuFe304L0....018&#;<&#;0...997.850.18Bal.304L-Cu0.......777.854.01Bal.Open in a separate window

At room temperature, deep drawing, cupping and conical cup forming tests were performed with a Zwick BUP-600 sheet metal forming tester according to GB/T.3- &#;Sheet metal formability and test methods&#;, and the rising speed of the punch was 1 mm/s.

The limiting drawing ratio (LDR) was measured with a deep drawing test. The diameter difference of two adjacent samples was 1.25 mm, and there were six valid samples in each group. The method of increasing the sample diameter step by step was used to measure the maximum allowable diameter of the wall near the bottom fillet of the drawing cup without breakage. In a group of samples, when three samples were broken, three samples were not broken, or when the number of broken samples of a certain grade was less than 3 and the diameter increased by one grade, the number of broken samples was greater than or equal to 4, and the experiment was stopped.

LDR=D0maxdp

1

where (D0)max is the maximum diameter that can be punched out of a cup shape without damage and dp is the punch diameter of 50 mm. The higher the LDR is, the better the drawability of the sheet metal.

The samples used for the cupping tests were 100&#;×&#;100 mm thin square sheets. The Erichsen index (IE) is the dent depth measured by displacement of the punch, which was measured six times and averaged. The larger the IE was, the better the bulging performance of the sheet metal.

The diameter of the conical cup test sample was 36 mm, and the conical cup was formed by a spherical punch until the sidewall at the bottom of the cup broke. The distance between the peaks of two opposite lugs at the mouth of the conical cup was measured as the maximum outer diameter (D¯ max), and the distance between the valleys was measured as the minimum outer diameter (D¯ min). The conical cup value (CCV) was obtained according to Eq 2. The smaller the CCV is, the better the &#;drawing&#;+&#;bulging&#; composite performance of the sheet metal.

CCV=D¯max+D¯min

2

According to GB/T 228.1-, an MTS tensile test machine was used to carry out tensile tests on the 304L and 304L-Cu samples at 0, 45 and 90° to the rolling direction. The tensile rate was 3 mm/min.

The microstructures of the samples before and after tensile testing were observed with a Nikon LV150NL vertical metallographic microscope. The hardness was measured on a Hengyi MH-3 L Micro Vickers Hardness Tester at six different points on the sample surface, and the average was used for the hardness value.

Phases were examined with a Japan D/max- x-ray diffractometer (Cu -Kα radiation). The maximum rated power was 18 kW, and the speed was 6°/min. The volume fraction of martensite was quantified with the following equation:

Vα&#;=1n&#;j=1nIα&#;jRα&#;j1n&#;j=1nIα&#;jRα&#;j+1n&#;j=1nIγjRγj

3

where n is the number of peaks examined, I is the integrated intensity and R is the material scattering factor. The integrated intensities of the (200)γ, (220)γ, (311)γ, (200)α&#; and (211)α&#; peaks were used to calculate the volume fraction of the phase.

The tensile fracture morphology was observed with a ZEISS Sigma 300 SEM to determine the fracture mechanism.

Analysis and Discussion

Effect of Copper Addition on Strain-Induced Martensitic Transformation

The microstructures of 304L and 304L-Cu after solution treatment indicated metastable austenite, which transforms to martensite during plastic deformation at room temperature, namely, strain-induced martensite. Strain-induced martensite has two different crystalline structures. The first is ε-martensite with a hexagonal close-packed structure, which is paramagnetic. The other is α&#;-martensite with a body-centered cubic structure, which is magnetic (Ref 20). No obvious diffraction peaks for the ε phase were detected in any of the samples, indicating that little ε-martensite existed in both steels. These results agree with those of Huang et al. (Ref 21) and Choi et al. (Ref 22).

SFE is an important parameter that influences the amount of martensite formed (Ref 23). According to the method reported by Curtze et al. (Ref 24), the SFEs of 304L and 304L-Cu were 23.69 and 27.41 MJ/m2, respectively. The SFE value for 304L-Cu was higher than that for 304L by approximately 4 MJ/m2 due to the addition of copper. The volume fractions of transformed martensite for 304L and 304L-Cu after tensile deformation were 28.75% and 9.43%, respectively. There were larger quantities of α&#;-martensite formed in 304L than in 304L-Cu, and the value for 304L was about three times that for 304L-Cu. SFE affects the frequency of intersection between mechanical twins, which act as nucleation sites for α&#;-martensite (Ref 22). Additionally, as reported by Venables (Ref 25), the stress required for twinning deformation is parabolically proportional to the SFE. Therefore, 304L with a lower SFE more easily formed mechanical twins than 304L-Cu, so 304L formed a larger amount of α&#;-martensite than 304L-Cu. A high copper content promoted an increase in SFE, thus inhibiting the formation of magnetic martensite nucleation sites (Ref 26). Therefore, little martensite was found on the 304L-Cu matrix after plastic deformation (Fig.  d).

A higher SFE for 304L-Cu means that the austenite was more stable; therefore, it should have a lower Md30 (the temperature at which 50% martensite is formed at a plastic strain of 30%) than 304L. According to the following empirical equation proposed by Nohara et al. (Ref 27), Md30(°C)&#;=&#;552-462(C%&#;+&#;N%)-9.2Si%-8.1Mn%-13.7Cr%-29.0(Ni%&#;+&#;Cu%)-18.5Mo%-68.0Nb%-1.42(GS-8), where GS is the ASTM grain size and the Md30 values of 304L and 304L-Cu were 18.2 and &#;&#;80.5 °C, respectively. Copper alloying increased the stability of the steel by increasing the SFE and decreasing Md30.

The strain hardening behavior of metastable austenite depends on the state of the strain-induced martensite (Ref 28), which is strongly related to the SFE. The hardness of martensite is much higher than that of austenite. Rapid accumulation in a short time leads to a great increase in the hardness of a material, resulting in strain hardening. The hardness values of 304L and 304L-Cu were increased by 75% and 30%, respectively. The increased hardness was consistent with the martensite transformation.

Effect of Copper Addition on Mechanical Properties

With the addition of copper, the grain sizes of 304L were reduced by 28%. The grains were obviously refined, which is consistent with the results of previous studies (Ref 29, 30). According to the Hall&#;Petch formula, the finer the polycrystalline grains of metal are, the higher the yield strength of the material, so the yield strength of 304L-Cu was higher than that of 304L. The formation of a smaller amount of martensite led to a decrease in the strain hardening rate of the steel containing copper, resulting in a decrease in the maximum uniform elongation of 304L-Cu and deterioration of the UTS values. The yield ratio increased, and the uniform deformation ability decreased, from which we concluded that adding copper impaired the mechanical properties of the steel.

Effect of Copper Addition on Formability

The enhanced formability of steels undergoing strain-induced martensitic transformation is associated with high uniform elongation. When copper is added, the martensite transformation is inhibited and cannot promote deformation dispersion. This led to a lower uniform strain and decreases in the n value and r value. The greater the value of n is, the better the uniform deformation and formability of the material. The larger the value of r is, the stronger the thinning resistance in the thickness direction and the better the drawing performance of the material. Consequently, the 304L-Cu exhibited a worse stretch formability than the 304L, as measured with the LDR, IE and CCV metrics.

Conclusions

1. The addition of copper improved the yield strength and yield ratio, increased the deformation resistance and decreased the uniform elongation of 304L austenitic stainless steel. During the forming process, the material easily cracked, which is not conducive to forming.

2. Adding copper increased the stability of 304L by increasing its SFE and decreasing the Md30 temperature, which led to inhibition of strain-induced martensitic transformation and decreases in the strain hardening index n and the plastic strain ratio r.

3. After copper was added, the LDR and IE decreased, the CCV increased, and the &#;drawing&#;+&#;bulging&#; performance decreased, which indicated that the formability of 304L was diminished.

Acknowledgments

Joint support from the National Natural Science Foundation of China [Grant No. ], Shanghai Pujiang Talent Program [Grant No. 19PJ], Baoshan Transformation and Development Science and Technology Special Project [Grant No. 21SQBS] and the Program of Shanghai Technology Research Leader (19XD) is gratefully acknowledged.

Conflict of interest

The authors declare that they have no conflict of interest.

Footnotes

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