High quality bearing steel manufacturing process heat treatment

Bearing steel is steel class quality requirement is very strict. Current requirement for bearing steel is put forward: the user free machining and inspection, improve the quality and specifications refine and improve the dimensional precision, and more and more importance on these requirements. To meet these requirements, JFE steel used various guarantee product quality and finish machining equipment production of bearing steel. These facilities and improve the quality of the development of the new technology, combining can produce wide size range, high quality and high added value of heat treatment and hot rolling bearing steel. JFE steel bearing manufacturing technology is characterized by: 1) Surface quality of the fine processing and quality inspection system with the billet scarfing and rolling into a small round billet continuous casting billet, the method of uniform to remove surface defects, inclusions and decarburization layer under the skin. For the materials, particularly high quality billet skinned homework high removal of defects. In order to ensure the small round billet surface quality and using automatic eddy current flaw detector, and magnetic powder flaw detector check; For internal defect, use the round billet whole section ultrasonic flaw detector to detect internal porosity and inclusion. 2) Bearing steel precision manufacturing technology and quality assurance in the wire - Bar factory, on the bar rolling line add wire rod rolling line, conduct joint rolling. For bar and wire rod finishing with 4 roll finishing mill, and the size precision of bar steel at 0. 01 mm below, the user can omit skinned and drawing processing. Rolling of wire rod can be free size, and can produce Φ 4. 2 mm small size of the wire. Due to the size of the wire rod has been rolling into forging, so users can omit wire drawing, heat treatment and surface treatment process. 3) Improving steel cleanliness in recent years, JFE steel in order to improve the cleanliness of steel, adopted a PERM, Add pressure refining) 低频( Outside the furnace refining furnace) The production process of steel was improved. PERM is in converter smelting, nitrogen, hydrogen and other gases dissolved in the steel, and then, with RH furnace ( Vacuum degassing) Rapid decompression, the produced gas in steel, the use of this gas capture and remove inclusions in molten steel. JFE steel also build the LF furnace in 2008, has greatly increased the inclusion removal ability. USES the effect of process and equipment is: compared with the original process, inclusion number prediction index decreased by 34%, the maximum diameter of inclusions reduced 29%, the maximum diameter of inclusions index distribution of standard deviation to reduce by 73%. Bearing steel heat treatment process such as normalizing, annealing heat treatment and final heat treatment are two major link in advance. ( 1) Heat treatment in advance (1) is the fire: chromium bearing steel quenching process, the workpiece through 40 ~ 60 min after thermal insulation, cooling needs faster, immediately after normalizing spheroidizing annealing. (2) the spheroidizing annealing: GCr15 chrome steel bearing, isothermal spheroidizing annealing process is often used to 790 ℃ is considered to be the best spheroidizing heating temperature. Heating to 900 ~ 920 ℃ before annealing, normalizing after insulation 2/3 ~ 1 h. Holding time on the workpiece size, the uniformity of the heating furnace, furnace charging method and the furnace charging amount, annealing the original organization uniformity of the former. Spheroidizing annealing at low temperature is mainly suitable for cold punching, cold extrusion ring of recrystallization annealing. Ordinary spheroidizing annealing, isothermal spheroidizing annealing is mainly suitable for forging ring, thermal shock and horizontal forging ball annealing. Chromium bearing steel ball annealing process. ( 2) The final heat treatment (1) bearing parts: general by quenching and low temperature tempering, its purpose is to improve the strength of steel, hardness, wear resistance and fatigue resistance. GCr15 steel quenching temperature at 820 ~ 860 ℃, oil quenching critical diameter is 25 mm. Commonly used oil cooling quenching. Heat preservation time is longer than alloy tool steel, salt bath heating coefficient values 0. 8 to 1. 5min/mm。 Air furnace heating coefficient of 1. 5 ~ 2分钟/毫米。 160 ℃ + 10 ℃ low temperature tempering, the tempering time is usually 2 ~ 4 h. To stabilize the size precision bearing parts, should be - after quenching 60 ~ 80 ℃ cold treatment, heat preservation time is 2 ~ 4 h, after cold treatment parts return to room temperature, tempering within 4 h, in order to prevent the parts cracking. Low temperature tempering failed to completely eliminate the residual stress will redistribute after grinding. These two kinds of stress will result in changes of parts size, even cracks. Therefore, we should again complement tempering, the tempering temperature is 120 ~ 160 ℃, the heat preservation 5 ~ 10 h or longer. (2) advanced GCr15 heat treatment: because the steel is easy to produce white point defects, large work mold heat treatment is easy craze, the slow heating or 690 ℃ for a long time, Greater than 5 h) Segmented isothermal can reduce the probability of cracking, choose the austenitizing temperature 810 ℃ plus or minus 10 ℃, coefficient of heat preservation a = 1. 6 ~ 0. 9min/mm。 Greater than 60 mm diameter workpiece need water double liquid quenching oil. For GCr15 work die steel, to temper many times, fully eliminate the residual stress, reduce the residual austenite volume, reduce the cracking phenomenon, prevent the workpiece during the process of using surface cracks.