The crystal grains of the "H-SR3" (left) and conventional material (right)
The crystal grains of the "H-SR3" (left) and conventional material (right)
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Nippon Steel & Sumitomo Metal Corp (NSSMC) developed a stainless steel plate that can be used for ultra-slim springs and is suited for precision machining.

The steel plate, "SUS304H-SR3 (H-SR3)," is SUS304 or a stainless steel containing chromium (18% percent by mass) and nickel (8% by mass). And it has a smaller size of ultra-fine crystal grain than any other SUS304 in the world that can be mass-produced, NSSMC said.

As a result, it prevents deformations caused by etching or laser machining. The thickness of the H-SR3 is about 0.1mm, and it can be used for various electronic devices.

In general, a high suitability for precision machining (no warpage or torsion caused by a process) is required for stainless steel plates used for precision machining, in addition to an appropriate thickness and flatness (no warpage or undulation).

In recent years, as the size of electronic devices such as smartphones and tablet computers is decreasing while their component density is increasing, demands made on stainless steel plates are becoming stricter. Among them, the key to improving suitability for precision machining is the reduction of the size of materials' crystal grains.

For the H-SR3, NSSMC succeeded in reducing average grain size to 2μ or smaller, which is more than 90% smaller than the average grain size of conventional SUS304, by making changes to material components and controls of cold rolling and heat treatment. This is the first time in the world that SUS304 with such a small grain size has been mass-produced, the company said.

The H-SR3 has the following characteristics. First, because of the reduced size of crystal grain, a wall surface with an opening that is produced by etching or laser machining becomes smooth. Second, because of reduced residual stress, it becomes possible to prevent deformations caused by half-etching or local elevation of temperature. Third, the amount of smut (carbide particles contained in materials) is small, making etching easy.