Research Of Microstructure Controlling Mechanism And Ultra-Fast Cooling Process Of Bearing Steel After Hot Rolling

Againsting the problem of precipitation of network carbide and over-proof of network level in bearing steel delivery state after conversion of continuous casting and continuous rolling in some domestic special steel mill, structure and property of GCr15 bearing steel were researched through thermo-mechanical simulations、hot rolling test and different controlled cooling test. Influence of different rolling processes and cooling processes to the precipitation of hypereutectoid secondary carbide and transformation of pearlite were analyzed. The technique, in which fine laminar pearlite without net-work carbide would be obtained were study. Ultra-fast cooling industrialized test and multitudinous industrialized manufacture after high temperature final rolling were processed through the installation of ultra-fast cooling system in the condition of keeping to the inhere continuous rolling production line of special mill. The main works involved as follows:1 Thermal simulation was performed by MMS-300 thermo-mechanical simulator in GCr15 bearing steel.High temperature deformation advanced the precipitation of secondary carbide and transformation of pearlite in continuous cooling process. With the increase of distortion, initial precipitation temperature of secondary carbide and pearlite rised, the diameter of pearlite grain decreased, but the influence of variety of distortion to secondary carbide thickness was minute; Low temperature deformation advanced the crushing of inter-granular secondary carbide, the laminar and diameter of pearlite grain decreased.In the process of continuous cooling of GCr15 bearing steel, the phase transition staple were secondary carbide、pearlite and martensite. Along with reduce of cooling rate, the right side of martensite transformation curve raised. Secondary carbide in grain boundary was (Fe·Cr)3 C, the critical cooling rate of restraining the precipitation of network carbide was 8℃/s; the critical cooling rate of completely transformation of pearlite was 5℃/s, residual austentite can transformed to pearlite absolutely in the condition of thermal insulation 20-200s in 756-510℃. Along with the increased of cooling rate, initial precipitation temperature of secondary carbide and pearlite decreased, the pattern of inter-granular secondary carbide convert from compact network to dispersive scatter, thickness of secondary carbide and content of C、Cr in inter-granular secondary carbide reduced, the pearlite laminar and the diameter of pearlite grain decreased, and degenerate pearlite appeared along with the increase of micro-hardness.2 Controlled cooling process was simulated by thermo-mechanical simulator in GCrl5 after high-temperature final rolling.Content of pearlite increased and the inclination of inter-granular carbide precipitation enlarged along with extension of isothermal time in isothermal procedure after cooling to different temperature in a given cooling rate. Along with the increase of cooling rate and reduction of isothermal temperature in fast cooling period, carbide precipitation in grain boundary weakened, the diameter of pearlite grain decreased, lamellar structure of pearlite become thin and short, some degenerate pearlite appeared. In slow cooling period after cooling to different temperature in a given cooling rate, the diameter of pearlite grain reduced and precipitation of inter-granular secondary carbide weakened along with the increase of slow cooling rate. Quenching martensite appeared when slow cooling rate was oversized, the precipitation of inter-granular secondary carbide was nonexistent, majority pearlte structure was lamellar with ferrite and cementite and little degenerate pearlite appeared. The target of obtaining fine laminar pearlite without net-work carbide could accomplish through the technique, in which cooling process was formulated in different temperature province and increase the cooling rate in fast cooling period cooperating with rational slow cooling rate.3 New style cooling technique was proceeded on bearing steel plate after high temperature final rolling in laboratory condition.In the process of conventional cooling on steel plate after high temperature final rolling, pearlite structure without net-work carbide would be obtained in surface because of the major cooing rate, but room-temperature structure was coarse pearlite and network carbide precipitating in grain boundary in center due to the deficiency of cooling intensity; expected cooling rate of restraining precipitation of network carbide was reached and fine pearlite without net-work carbide was obtained in the whole cross-section of bearing steel plate through ultra-fast cooling with surfaced cooling rate being greater than 100℃/s; lamellar interval of center pearlite was greater than that of surface pearlite because of the weaker cooling intensity in center than in surface; along with the reduce of final cooling temperature, the pearlite laminar and the diameter of pearlite grain decreased, the level of network carbide reduced, impact ductility and hardnss after spheroidal annealing enlarged.4 The variety of temperature and cooling rate in different site of bearing steel bar was analysed, through simulating the temperature pattern in ultra-fast cooling process after high temperature insulation.In the process of simulating temperature pattern with ANSYS software, the accurate coefficient of ultra-fast cooling heat transfer was defined in the calculate procedure, which calculation was end while final cooling temperature being same with the measure value and the heat transfer mode was modified continuous. The coefficient of air cooling heat transfer was defined by empirical equation.AgainstΦ<60mm bearing bar, once ultra-fast cooling was proceeded in the principle of which, surface temperature after cooling was above martensite and redding temperature was low than 700℃, cooling rate in different site of bearing bar was reach to the cooling rate require of restraining network carbide and residual austensite transforming to pearlite absolutely; AgainstΦ≥60mm bearing bar, the puzzle of internal cooling difficulty was settled and internal cooling rate was increased through segmented secondary ultra-fast cooling, cooling rate in different site of cross section was reach to the cooling rate require of restraining network carbide and residual austensite transforming to pearlite absolutely; in the condition of invariant ultra-fast cooling total time, internal cooling intensity was higher through segmented secondary ultra-fast cooling, internal cooing rate was increased. 5 Three-group ultra-fast cooling system was installed at the back of continuous rolling mill in the condition of keep to the inhere continuous rolling production line of special mill. Its instant cooling rate can reached to 400℃/s through adjusting water pressure、size of nozzle hole and number of opened water tank. cooling rate in different specs bar was reach to the cooling rate require of restraining network carbide and residual austensite transforming to pearlite absolutely, the level of network carbide was≤2, which was measured up to the bearing professional standard.Apply of ultra-fast cooling process settled the problem of network carbide level over-proof in different specs bearing steel after high temperature final rolling. Not only fine laminar pearlite without net-work carbide was obtained, which was beneficial to next spheroidal annealing process, but also the rolling rate in continuous rolling production line was assurance, which increasing the efficiency and obtaining a rather large economic benefits. Subject about "Research of Structure Controlled Mechanism and Ultra-Fast Cooling Process of Bearing Steel afer Hot Rolling" would have guiding significant for increasing quality of domestic bearing steel bar and accelerating the development of economy and society.