Can carbon steel safety valves be used for LNG?

As far as I know. No, stainless steel has to be used.

It probably won’t work; at low temperatures, low-carbon manganese steel (C: 0.05–0.28%, Mn: 0.6–2%). The Mn/C ratio is set to around 10, and harmful impurities such as oxygen, nitrogen, sulfur, and phosphorus are reduced; in some cases, small amounts of elements like aluminum, niobium, titanium, and vanadium are added to refine the grain structure. The minimum operating temperature for this type of steel is around –60°C. 2. Low-alloy steel Low-alloy steel. The main types are low-nickel steel (Ni 2–4%), manganese-nickel-molybdenum steel (Mn 0.6–1.5%, Ni 0.2–1.0%, Mo 0.4–0.6%, C ≤ 0.25%), and nickel-chromium-molybdenum steel (Ni 0.7–3.0%, Cr 0.4–2.0%, Mo 0.2–0.6%, C ≤ 0.25%). The strength of these steel grades is higher than that of low-carbon steel, with a minimum operating temperature of around -110°C. China has developed several nickel-saving low-alloy steels for low-temperature applications, such as 09Mn2V. 3. Medium (high) alloy steel Medium (high) alloy steel. The main types are 6% Ni steel, 9% Ni steel, and 36% Ni steel, among which 9% Ni steel is the most widely used steel for cryogenic applications. The operating temperature of such high-nickel steels can go as low as -196°C. 4. Austenitic stainless steels are generally used at temperatures above -196°C; conditions below this temperature are considered low-temperature conditions, and in such cases a low-temperature impact test is required.

The low-temperature impact energy test isn’t passing – what on earth are you thinking?

If it is in gaseous state at room temperature, the temperature should not be lower than that of the pipe material