Bearing steel is very strict quality requirements of the steel. At present to bearing steel of the request: users from processing and examination, improve the quality and specifications refined and improved dimensional accuracy etc, and, to the requirements of the important degree more and more high. To meet these requirements, JFE system of steel used to guarantee the quality of the product and finish machining equipment production bearing steel. The equipment and the development of new technology to improve the quality of the combination, can produce the size range wide, high quality, and high added value of hot rolling bearing steel and heat treatment.
JFE bearing steel manufacturing technology of the features are:
1) surface quality fine processing and quality check system
Use of the slab to clean up and continuous casting slab flame will be rolling into small round billet method, uniform removing surface defects, subcutaneous inclusion and decarburization layer. To the quality requirements of particularly high material, the implementation of the slab height splite clear defects. To ensure that small round billet surface quality, with automatic eddy current testing and magnetic powder flaw detector check; To the internal defects, with round billet, whole section ultrasonic detectors detection internal pore and inclusion.
2) bearing steel fine manufacturing technology and quality assurance
In the wire-steel rolling mill, in the bar rolling line add wire rolling line, a joint rolling. Bar and wire to use 4 roller fine finishing rolling, the dimensional accuracy of the steel bar in 0.01 mm the following, the user can omit onion and pull out processing. Can be free to wire size rolling, and can produce Φ 4.2 mm small size of the wire.
By putting the wire to the size of the rolling has been forging, so users can omit wire drawing, heat treatment and surface treatment working procedure.
3) improve steel cleanliness
In recent years, JFE steel in order to improve the system of steel cleanliness, using a PERM (add pressure refined), LF (outside the furnace finer) of steel production technology was improved. PERM method is in con-verter steelmaking, making the nitrogen and hydrogen gas, such as dissolved in steel, then, use RH furnace (the vacuum takes off the gas) rapid decompression, make steel produce gas, use the gas capture and ruled out of inclusions liquid steel.
JFE system steel in 2008 new LF furnace, greatly improving the inclusion remove ability. Using the above process and equipment effect is: compared with the original technology, inclusions forecast 34%, reduce the number of index inclusions index decreased by 29%, the biggest diameter of the inclusions in the biggest diameter index distribution of standard deviation by 73%.
Bearing steel heat treatment process including is fire, annealing, heat treatment and final heat treatment in two main link.
(1) heat treatment in advance
(2) is the fire: chrome bearing steel is fire craft, the heat penetrating warm after 40 to 60 min, cooling need faster, is right after the fire to ball annealing.
(3) the ball annealing: GCr15 chromium bearing steel often USES the isothermal ball annealing process, 790 ℃ is considered to be the best spheroidizing heating temperature. Annealing need before heated to 900 ~ 920 ℃, heat preservation two-thirds ~ 1 h is after the fire.
Holding time with the size and the uniformity of the heating furnace, the furnace and furnace with method with quantity, annealing of original organization before uniformity and decide.
Low temperature ball annealing mainly used in cold blunt ball, cold extrusion of collar recrystallization annealing.
Ordinary ball annealing, isothermal ball annealing is mainly applied in forging rings, hot rushed the ball and horizontal forging of ball annealing. Chromium bearing steel ball annealing process.
(2) the final heat treatment
(1) bearing parts: general use of the quenching and tempering at low temperature, its purpose is to improve the strength of steel, hardness, wear resistance and corrosion fatigue performance. GCr15 steel quenching temperature in 820 ~ 860 ℃, the oil quenching critical for 25 mm in diameter. General use of the oil cold hardening. Heat insulation than alloy tool steel time long, salt bath heating coefficient 0.8 ~ 1.5 min/mm. Air furnace heating coefficient 1.5 ~ 2 min/mm.
160 ℃ ± 10 ℃ low-temperature tempering, tempering time general for 2 ~ 4 h.
Precision bearing parts to the stable size, harden should undertake-60 ~ 80 ℃ cold treatment, heat preservation time is 2 ~ 4 h, cold treatment after restore parts to room temperature, in 4 h within the tempering, in order to prevent the parts craze.
Low temperature to completely eliminate when tempering of the residual stress in after grinding to distribution. The two kinds of stress can lead to parts size change, even cracks. Therefore, we should another added tempering, tempering temperature of 120 ~ 160 ℃, heat preservation 5 ~ 10 h or longer.
(2) the work GCr15 mold heat treatment: as this steel easy to produce white point defects, large workers die &mould heat treatment easy craze, the slow heating or 690 ℃ long time (more than 5 h) subsection isothermal can reduce cracking probability, austenitic transition temperature of 810 ℃ ± choose 10 ℃, heat preservation coefficient a = 1.6 ~ 0.9 min/mm. More than 60 mm diameter workpiece need water oil dual fluid quenching.
Work for GCr15 steel in many tempering, fully eliminate the residual stress, reduce austenitic residual volume, reduce the cracking phenomena, and prevent the use process produce surface cracking.
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