Existing titanium alloy materials have high strength levels and can be cold-rolled (coiled) well, and also possess good machinability. The titanium alloy material of this invention does not require expensive alloying elements, thus reducing costs. The titanium alloy material of this invention satisfies the following: Al equivalent (Al + 10O(oxygen)): 3.5–7.2% (mass %); Al: more than 1.0% and less than 4.5%; O: less than 0.60%; and Fe equivalent (Fe + 0.5Cr + 0.5Ni + 0.67Co + 0.67Mn): more than 0.8% and less than 2.0%, and contains one or more elements selected from the group consisting of Cu: 0.4–3.0% and Sn: 0.4–10%, with the balance being Ti and unavoidable impurities.
Titanium alloys are widely used in aerospace equipment components, chemical plant components, automotive components, and other fields due to their high specific strength and excellent corrosion resistance. A representative titanium alloy is Ti-6Al-4V. This Ti-6Al-4V alloy, as standardized in ASTM Gr.5 as having a yield strength of 0.2% above 828 MPa, exhibits excellent strength properties. However, it contains a large amount of Al as an additive element, resulting in poor cold rollability. Therefore, it is difficult to manufacture thin sheets by coil rolling, and it is generally processed into thin sheets using a method called patch rolling. Patches rolling involves overlapping layers of hot-rolled titanium sheets, covering them with a mild steel cap, and rolling them while holding them at a temperature no lower than a specified level. Compared to cold rolling, this method is extremely complex and very expensive. Furthermore, the temperature range suitable for hot rolling is limited, thus imposing many processing constraints.
In contrast, a general-purpose titanium alloy that can be rolled in coils is, for example, Ti-3Al-2.5V alloy (ASTM Gr.9). However, the 0.2% yield strength of this alloy is approximately 500 MPa, which is significantly lower than that of the aforementioned Ti-6Al-4V alloy. Additionally, Japanese Patent Application Publication No. 02-57136 discloses a heat-resistant Ti alloy sheet with excellent cold workability. This alloy sheet was developed primarily to improve cold workability, and the concentrations of either the α-phase stabilizing element or the β-phase stabilizing element are low. Therefore, the strength increase from solid solution strengthening is small, making it unsuitable for applications requiring high strength.
A titanium alloy with equivalent strength to the Ti-6Al-4V alloy, which can be rolled into coils, is Ti-4.5Al-2Mo-1.6V-0.5Fe-0.3Si-0.05C (ASTM Gr.35), and is actually manufactured in mass production as cold-rolled coils. In KSTi-9, similar to the Ti-6Al-4V alloy, Mo and V are used as β-phase strengthening elements. Additionally, a high-strength Ti alloy is Ti-4Al-2.5V-1.5Fe-0.25O (ATI425 (US registered trademark)). In this Ti alloy, V is used as the main β-phase stabilizing element (β-phase strengthening element).
Furthermore, Japanese Patent Application Publication No. 01-111835 discloses an alloy developed for improving cold workability. The Ti alloy shown therein exhibits high workability due to the retention of the β-phase, thus requiring a high concentration of the β-phase stabilizing element.
As mentioned above, titanium alloys used in aerospace equipment components, etc., require high strength and excellent cold rollability (the ability to be rolled into coils). When cold rollability is significantly low, cracks enter from the ends of the titanium alloy sheet during cold rolling, and these cracks propagate, causing fracture. Even if cold rolling (coil rolling) is possible, if cold rollability is significantly low, repeated cold rolling-annealing is required, leading to increased costs. Furthermore, when titanium alloys have low machinability, even if cold rolling is possible, there are situations where processing at the current product level (such as bending) is difficult. The goal is to achieve a titanium alloy with higher strength than existing materials, good coil rolling (cold rolling), and machinability (elongation, ductility) while controlling costs, without relying on expensive alloying elements (Mo, V, Nb, etc.).
The titanium alloy material satisfies the following conditions: Al equivalent (Al+10O(oxygen)) : 3.5-7.2% (mass %), Al: more than 1.0% and less than 4.5%, O: less than 0.60%, and Fe equivalent (Fe+0.5Cr+0.5Ni+0.67Co+0.67Mn) : more than 0.8% and less than 2.0%, and contains one or more elements selected from the group consisting of Cu: 0.4-3.0% and Sn: 0.4-10%, with the balance being Ti and unavoidable impurities. This allows for a titanium alloy with higher strength than the existing Ti-3Al-2.5V alloy, which can be rolled in coils, while also possessing good cold-rolling properties suitable for coil rolling and good machinability (a certain elongation). The titanium alloy of this invention can achieve a strength level equivalent to that of Ti-6Al-4V alloy, thus enabling the high-strength production of components for aerospace equipment, chemical plants, automobiles, etc., at high productivity and low cost.
Furthermore, the titanium alloy can achieve a strength level higher than that of the aforementioned Ti-3Al-2.5V alloy, which can be rolled into coils, and is equivalent to the strength of Ti-6Al-4V alloy.
