NOVELTY - Thermal cracking process of high severity type is used to process a charge comprising at least 80 wt.% of 2-4C hydrocarbons and at least 20 wt.% of hydrocarbons of ethane and propane group.    USE - In oil processing and petrochemical industry, as steam cracking process of highly severe regime for charge containing ethane and/or propane.    ADVANTAGE - High degree of conversion is obtained.    DETAILED DESCRIPTION - The charge is circulated, with addition of steam, through the radiation zone of steam cracker, in at least one pipe having length L equal at least 16 m and hydraulic diameter DH higher than or equal to 34 mm at least in its end part. The residence time T of charge in steam cracker is at least 150 minutes and up to 2800 minutes, and the temperature at the exit is at least 858 degrees C and up to 1025 degrees C. The ratio R (equal L/DH) and the residence time T are sufficiently high to produce at least 77% conversion of ethane present in charge and at least 96% conversion of propane present in charge. Thermal flow FT within the radiation zone is sufficiently weak to ensure that parameter S1, equal (FT1)2/R, (where R is as above and FT1 is FTxf, where FT is the average thermal flow transmitted through the pipe in radiation zone, expressed in KW per m2 of external surface of pipe, and f is a) 0.85 if the end part of pipe comprises at least one internal rim or anti-coking coating on at least one part of its internal surface, b) 0.72 if the end part of pipe comprises at least one internal rim and anti-coking coating on at least one part of its internal surface and c) 1 if the end part of pipe comprises neither of them) is up to 11, being sufficiently low to maintain the degree of conversion as above over duration of technological cycle equal at least 8 days. For the charge comprising at least 20 wt.% of ethane, the conversion of ethane is at least 77%, preferably at least 85%, and especially at least 90%, while the conversion of propane is at least 97%, especially 98%. Tubular steam cracker has, in its radiation zone, at least one metallic pipe whose end part is made of material having critical temperature at least 1060 degrees C, preferably 1120-1220 degrees C. Duration of technological cycle is at least 15 days, preferably at least 30 days. The wt. ratio of dilution of charge with steam is preferably 0.05-2, especially 0.15-1. The effluents from the tempering zone have ethane content up to 24 wt.%, preferably 15 wt.% of the ethane initially contained in charge.    An INDEPENDENT CLAIM is also included for the high severity steam cracker for the process as claimed, comprising convection zone for preheating the charge as claimed enriched with steam, and radiation zone connected to convection zone above and tempering zone below, with radiation zone comprising number of pipes with L/DH ratio at least 680 (preferably 760-2000) and means for heating of charge enriched with steam to temperature at least 876 degrees C, delivering thermal flow 20-79 (preferably 25-64) KW/m2.    DESCRIPTION OF DRAWING(S) - The drawing represents schematic view of steam cracker according to invention.    entry to convection zone (1)    steam supply (2)    convection zone (3)    pipe (4)    radiation zone (5)    burners (6)    primary heat exchanger (7)    effluent pipe leading to second heat exchanger (8)    pyrolysis pipes (9)    steam cracker (B)    entry (I)    entry to primary heat exchanger (M)    exit of radiation zone (O)