Development of High Strength Gas Cylinder-purposed Seamless Pipe by Baosteel and Study on its Microstructure and Mechanical Properties

It is regarded that development of the gas cylinder-purposed seamless pipe with tensile strength equal to or higher than 1 100 MPa and high strength/high toughness is the important foundation for producing the lightweight gas cylinder. Based on the conventionally-used 34CrMo4 steel for cylinder-making，and owing to taking such measures as increasing Mo and adding Ni to assist the micro-alloying control，controlling the inclusion level and N content，etc.，the high strength/high toughness gas cylinder-purposed pipe is successfully developed. The pipe as developed features such mechanical properties i.e.，the tensile strength≥1 100 MPa，elongation≥14%，and -50℃ low temperature transversal impact toughness≥60 J/cmm². Moreover the pipe has been put into massive commercial application，being the first case in the country. The heat treatment result of the pipe shows that along with the rise of the temper temperature，the pipe’s strength is getting down，whereas its plasticity and low-temperature toughness are getting better，which is resulted from the microstructure evolution，such as coarsening of the martensite lath，and the coarsening and aggregation of the precipitation phase. Appropriate heat-treatment procedure for good strength and toughness balance is recommended，i.e.，quenching at 860℃ and tempering at 520~560℃.