Due to the poor tempering stability, when the carbon steel is quenched and tempered, it is necessary to adopt a lower tempering temperature in order to ensure higher strength, so the toughness of the steel is low; In order to ensure better toughness, the strength is low when high tempering temperature is adopted, so the comprehensive mechanical property level of carbon steel is not high. Due to the poor tempering stability, when the carbon steel is quenched and tempered, it is necessary to adopt a lower tempering temperature in order to ensure higher strength, so the toughness of the steel is low; In order to ensure better toughness, the strength is low when high tempering temperature is adopted, so the comprehensive mechanical property level of carbon steel is not high.