Abstract: The 19CrNi5 steel with hardness higher than 200 HRB after rolling was selected as the research object, the steel specimens were heated to 800, 850, 900, 950, 980, 1050 and 1100 ℃ in the same carbon-silicon rod box-type high temperature furnace. The specimens were hold at the selected temperature for 40 min, then air cooled at 15 ℃,the cooling rate of the steel was measured by infrared thermometer, the hardness was tested by hardness testing device, and the metallographic microstructure of the steel was tested by means of scanning electron microscope(SEM). According to the research results, the main reason is the high proportion of bainite structure that affects the hardness of the steel. When the heating temperature ≥1000 ℃, and the cooling rate ≥2.5 ℃/s, the higher the heating temperature is, the quicker the cooling rate is, the larger the proportion of bainite in the steel is,the higher the hardness of the steel is. When the heating temperature <1000 ℃, cooling rate <1.0 ℃/s, the austenite is completely transformed into pearlite, and finally pearlite and ferrite structure are formed during the cooling process with low hardness of the steel. In actual production, the final rolling temperature is controlled below 1000 ℃, and the cooling rate <1.0 ℃/s, which can slow down or eliminate the formation of bainite structure in the steel.

Key words: 19CrNi5 steel, heating temperature, microstructure, hardness