Research on Large-size Pre-hardened Mould Blocks of Plastic Mould Steels

Abstract

Plastic mould steels are the most versatile alloy steels whose consumption amount is the largest among mould and die steels. Pre-hardened plastic mould blocks are mainly used for manufacturing large moulds of automobile interior trimming products and shells of large-size household appliance, etc. The metallurgical processes for large-size pre-hardened mould blocks of steel 718 was developed by Central Iron & Steel Research Institute (CISRI) and Dongbei Special Steel Group Co., Ltd. (DSSC) together and the effects of relative processes, such as LF + VD melting process, ingot mold design, argon protective ingot casting process, high temperature homogenizing process and forging process for heavy ingots, quenching process and high temperature tempering process for mould blocks on material purity and hardness uniformity were systematically tested. Results show that sulfur content can be controlled as low as 0.005%, and total oxygen content as low as 12 ppm if slag basicity of LF and VD were kept within 3.5–4.0 and 3.0–3.5 respectively. Rejection rate caused by inclusions at ingot bottom was reduced from 6.81 to 1.55% by optimizing the design of the 28 t ingot mold bottom taper shape and ingate chamfering. It was confirmed by argon protective casting test that argon flow rate should be controlled at 4–8 m³/h. The macro-segregation and hardness deviation were improved from ≤grade 3.0 and ≤5.0 HRC to ≤grade 2.0 and ≤3.5 HRC respectively by adopting high temperature homogenizing process. The macro-porosity and the ultrasonic testing result was improved from ≤grade 3.0 and C/d to ≤grade 2.0 and D/d respectively by applying FM forging method instead of drawing by flat anvil. The hardness deviation on the cross section of 650 mm × 1,080 mm mould blocks was ≤3.5 HRC with pre-hardening treatment i.e. water–air alternatively timed quenching + high temperature tempering by using electrical heating furnace.