The method of thermal hardening of the reinforcing bars and the device for its implementation

 

(57) Abstract:

The invention relates to metallurgy, and in particular to thermal processing for hire, and can be used for thermal hardening of the reinforcing carbon steel bars. The technical result of the invention is to improve the quality of the reinforcing bars by increasing strength, plastic and viscous properties. This technical result is achieved by the fact that the speed is regulated cooling set equal to 200-300C/s, cooling leads to an intermediate temperature of 500-600C, which stand for 0.2-0.8 seconds, after which the cooling complete with an arbitrary velocity. The water in the cooling chamber served with speed, 1.1-1.2 times higher than the rate of movement of the reinforcing rod, and at a pressure of 0.5 to 0.9 kgf/mm2. The device comprises a flow nozzle with a cooling chamber. The ratio of the area of the slit to the expiration of the nozzle to the outlet cross section of a cooling chamber is 1.5-1,15, while the cross-sectional area cooling chamber exceeds the cross-sectional area of reinforcing bar 9-15 times. 2 S. and 1 C.p. f-crystals, 4 Il., table 2.

The invention relates to metallurgy, namely termitidae steel.

According to the standard of the HUNDRED ASCM 7-93, reinforcing bars for concrete structures must have the following complex of mechanical properties:

t500 N/mm2;55%; cold bend 180o- stand up.

A known method of thermal hardening steel, mainly reinforcing bars, including austenitization and the cooling water flow in a closed chamber with excess static pressure of 5-10 MPa, according to which the car is cooled with speed 680-880oC/s to achieve the average temperature over the cross section 575-630oC the water flow is directed relative to the surface of the car with a speed of 9-14 m/s [1].

The disadvantage of this method is that the high cooling rate leads to thermal stresses and cracks, reducing plastic and viscous properties of reinforcing bars.

There is also known a method of heat strengthening rebar, mainly with a diameter of 18 mm and more, including austenitization blanks, two-stage cooling to achieve average section temperature 550-625oC with a cooling rate in the second stage 400-600oC/s, and a final cooling, according to the Rina on their surface below 500oC, reducing the overall temperature of the rods to 750-800oC, and then carry out a General cooling of the rods [2].

In this way, the heat strengthening in the reinforcement rods is formed acicular microstructure, reduced plastic and viscous properties of carbon steel. This degrades the quality of heat-treated steel.

The closest in technical essence and the achieved results of the present invention is a method of thermal hardening steel, mainly reinforcing bars. The method includes austenitization billet, hot rolling and cooling with a regulated speed to an intermediate temperature 575-630oC moving rebar in a closed cooling chamber, with once-through water with speed 9-14 m/s, with the excess static pressure of 5-10 MPa. This regulated cooling rate support equal 100-550oC/s consistently cameras with direct-flow and counter-flow water supply, and a counter-current water flow carried out with the speed of 10-25 m/s [3].

A device for implementing this method contains a set of sequentially alternating straight and action is allows to obtain simultaneously high strength, the ductility and toughness of the reinforcing bars. This impairs their quality. In addition, the device for implementing the method does not allow to realize optimum thermal hardening and is characterized by structural complexity, because a large number of direct-flow and counter-flow nozzle with cooling chambers.

The technical problem solved by the present invention is to improve the quality of the reinforcing bars by increasing strength, plastic and viscous properties. In addition, achieved a side effect, which is expressed in simplifying the structure of the device.

The specified technical task is solved in that in the known method of thermal hardening of the reinforcing carbon steel bars, including austenitization billet, rolling and cooling with a regulated speed to an intermediate temperature moving rebar in a closed cooling chamber, with direct-flow water supply, according to the proposal, speed regulated cooling set equal to 200-300oC/s, cooling leads to an intermediate temperature of 500-600oC, which stand for 0.2-0.8 seconds, after which the cooling complete with proizvolstva with speed, 1.1-1.2 times higher than the rate of movement of the reinforcing rod, and at a pressure of 0.5 to 0.9 kgf/cm2.

The proposed mode of thermal hardening can be implemented using a device containing a flow nozzle with a cooling chamber in which the ratio of the area of the slit to the expiration of the nozzle to the outlet cross section of the chamber is of 1.05 to 1.15, and the cross-sectional area cooling chamber exceeds the cross-sectional area of reinforcing bar 9-15 times.

The essence of the invention consists in the following. Cooling the laminated reinforcing rod of carbon steel temperature end of rolling up to 500-600oC with a speed of 200-300oC/s eliminates the formation of high thermal stresses and cracks, at the same time increases the strength of steel. During isothermal holding at 500-600oC for 0.2 to 0.8 with a complete transformation of undercooled austenite in equiaxial sorbitol (banic), with even higher strength, plastic and viscous properties. This improves the quality of the reinforcing bars.

When the regulated speed of the cooling water, which is 1.1-1.2 times higher than the SC the con rod is formed a steam layer, whereby the cooling rate of the rod in the temperature range from 1000-1050oC up to 500-600oC is the optimum value of 200-300oC/s

To implement the proposed thermal hardening of the reinforcing carbon steel bars is possible using a device containing a flow nozzle with a cooling chamber in which the ratio of the area of the slit to the expiration of the nozzle to the outlet cross section of a cooling chamber is equal to 1,05-1,15, and the ratio of the cross sectional area of the cooling chamber to the cross sectional area of the rebar is 9-15. When performing correlations of these parameters, the thickness of the steam layer provides the speed of the cooling flow in the cooling chamber 200-300oC/s to an intermediate temperature of 500-600oC and subsequent exposure of 0.2 to 0.8 C. In contrast to the device described in the prototype, thermal hardening ensures the use of only one closed cooling chamber proposed design.

It is found experimentally that when the speed regulated cooling below 200oC/s is the decrease of the strength characteristics of the bars. Increasing the cooling rate in excess of 300oC/s leads to p the properties.

Intermediate temperature of 500-600oC determined from the conditions obtaining sorbitol (Benita), which combines high strength, ductility and toughness, and exclusions of education sites (needle troostite). The temperature decrease of less than 500oC leads to uneven formation of the microstructure, growth of structural stress (voltage conversion), the formation of troostite needle shape, which affects the rheological properties. Increasing the temperature above 600oC causes a decrease in strength properties, which is unacceptable.

When the exposure time of undercooled austenite in the range of 500-600oC less than 0.2 is not achieved with the full process of phase transformation, residual austenite worsen the properties of the reinforcing bars. Increasing the exposure time of more than 0.8 seconds leads to the weakening of the reinforcing bars due to smoothback.

If the water velocity in the cooling chamber will exceed the speed of movement of the rebar less than 1.1 times, the intensity of cooling will decrease, as the reinforcing rods will deteriorate. Increasing this ratio of speeds of more than 1.2 leads to the disruption of steam layer, to increase the cooling rate and ochre which consists of the transportation of rebar, worsens the conditions of cooling. Increasing excess pressure in excess of 0.9 kgf/cm2does not improve the quality of rebar, but only increases energy consumption, resulting in inappropriate.

When the ratio of the area of the slit to the expiration of the nozzle to the outlet cross section of the camera is less of 1.05, a steam layer is unstable or is not formed at all, which degrades the quality of hire. The increase of this ratio more than 1,15 increases the cooling rate and leads to the formation in carbon steel sections with needle-like microstructure, which is unacceptable.

If the area of the slit to the expiration of the nozzle exceeds the sectional area of the cooling chamber less than 9 times in a cooling chamber is steam separation layer, the cooling rate will be more than acceptable. When these values more than 15 increase the size of the device and the flow rate of cooling water.

Examples of implementation of the method

The blank square section of carbon steel subjected to austenitization - heating and aging at a temperature of 1250oC. Then the blank is rolled on the mill 250 in the system calibers in reinforcing rod with a diameter of 18 mm, the temperature of the rebar at the output of polaromonas water supply, established for the last stand of the mill. The water in the cooling chamber serves excessive pressure P = 0.7 kgf/cm2when the ratio of the velocity of water flow to the speed rebar = 1,15. In the process of moving along a cooling chamber surface layer of reinforcing rod is cooled from a temperature of 1000oC to the intermediate temperature Tp= 550oC with velocity Vabout= 250oC/S. Then the rod is maintained at a temperature Tp= 550oC over a period of time = 0.5 s, then finish cooling on a random speed in the water.

Embodiments of the method and indicators of efficiency are presented in table 1.

From table. 1 shows that when implementing the proposed method (options 2-4) achieves the improvement of the quality of the reinforcing bars by increasing the range of mechanical properties. In cases of transcendent values of the declared options (options 1 and 5) quality of reinforcement bars is reduced. Also lower quality rods obtained by the implementation of the prototype method (option 6).

The proposed method can be implemented using the device for thermal hardening of the reinforcing bars. In Fig. 1 image which consists of the casing 1, within which is coaxially fixed to the input funnel 2. In the housing 1 is also fixed pipe 3 for supplying cooling water. The input funnel 2 forms with the inner surface of the housing 1 an annular gap within 4 cooling water, which represent direct-flow nozzle. The cross-sectional area of the slit to the expiration of 4 (cross section a-a) is equal to Fand.

On the housing 1 is fixed to the cooling chamber which is formed by series-connected branch pipes 5, 6 and 7. The cross-sectional area of the pipe 6 (section b-B) is Faboutand the outlet pipe 7 (section b-b) is Fwith.

The value of Faboutdetermined by the relation: Fabout/Fp= 9-15,

where Fp- the cross-sectional area a hardening of the reinforcing rod.

The values of Fandand Fwithin turn, connected by the relation: Fand/Fwith= 1,05-1,15.

The above ratio is determined experimentally. Their implementation allows thermal hardening of the reinforcing bars on the optimal mode and to eliminate the need to use additional means for cooling.

If Fabout/Fp< 9, the process is s cooling rate above is valid. When Fabout/Fp> 15 increase size and metal devices, worsening conditions of transportation of refrigerated car.

When Fand/Fwith< 1,05, the device does not provide the required cooling rate of reinforcement bars equal to 200-300oC/sec. At Fand/Fwith> 1,15, due to an excessively high cooling rate in the reinforcement bars of carbon steel microstructure is formed with portions of the needle shape. This reduces the quality of the reinforcing bars.

The device works in the following way

For thermal hardening of the reinforcing carbon steel bars having a cross sectional area Fp= 254 mm2select the device whose cross-sectional area cooling chamber 12 times more and is Fabout= 3084 mm2, i.e., Fabout/Fp= 3048 mm2/254 mm2= 12.

Through the housing 1 of the device is fixed in the last stand of the mill along the axis of rolling. In the pipe 3 serves under the pressure of the cooling water that passes through the slit to the expiration of 4 (nozzle) having a cross sectional area Fand= 1837 mm2. The flow of water fills the cooling chamber, obrazovannuyu>with= 1670 mm2. While Fand/Fwith= 1837 mm2/1670 mm2= 1,10.

Reinforcing rod with cross-sectional area Fp= 254 mm2emerging from the last stand of a rolling mill with a temperature of 1000oC falls into the input funnel 2, which directs it to the cooling chamber formed by the nozzles 5, 6 and 7, where flows the cooling water flow. The cooling chamber reinforcing rod is moving in the same direction as the flow of water to the outlet. Due to the fact that Fabout/Fp= 12 and Fand/Fwith= 1,10, near the surface of the rebar to form a stable steam layer that reduces heat transfer from the heated metal to the cooling water. Due to this, the cooling rate of the rebar temperature 1000oC to an intermediate temperature of 550oC has an optimal value equal to 250oC/s in a time interval 0.5 s after the release of rebar from the outlet pipe 7, when the metal is complete martensite transformation, the rod gets into the water and cooled arbitrarily. The microstructure of the rebar does not contain troostite needle shape, which improves its quality. In table. 2 shows the VA is proposed correlations device settings (options 2-4) provides the maximum yield reinforcing bars. In cases beyond the values of the parameters (options 1 and 5) the efficiency of the device decreases.

Technical appraisal and economic benefits of the proposed invention consists in that the cooling reinforcing rods initially with a speed of 200-300oC/s to a temperature of 500-600oC, holding at this temperature 0.2 to 0.8 seconds and a final cooling with an arbitrary velocity allow us to form a martensitic microstructure of carbon steel and prevent needle troostite. Ultimately, this enhances the complex of mechanical properties and improving the quality of the reinforcing bars.

Optimum modes of heat strengthening can be achieved when the water in the cooling chamber serves direct-flow speed, 1.1-1.2 times higher than the rate of movement of the reinforcing rod, and at a pressure of 0.5 to 0.9 kgf/mm2. The method can be implemented using a device, in which the ratio of the area of the slit to the expiration of the nozzle to the outlet cross section of a cooling chamber is of 1.05 to 1.15, and the cross-sectional area cooling chamber exceeds the cross-sectional area of reinforcing bar 9-15 times.

For the base object adopted way-Prov of carbon steel by 15-20%.

Literature sources used in the preparation of the description:

1. Auth. St. CCCP N 635144, IPC C 21 D 1/02, C 21 D 7/14, 1978

2. Auth. St. USSR N 1216220, IPC C 21 D 1/02, 1/78, 1986

3. Auth. St. USSR N 1520112, IPC C 21 D 1/02, 1989 - prototype.

1. The method of thermal hardening of the reinforcing carbon steel bars, including austenitization billet, rolling and cooling with a regulated speed to an intermediate temperature moving rebar in a closed cooling chamber, with direct-flow water supply, characterized in that the speed is regulated cooling set equal to 200 - 300oC/s, cooling leads to an intermediate temperature of 500 - 600oC, which stand for 0.2 - 0.8 seconds, after which the cooling complete with an arbitrary velocity.

2. The method according to p. 1, characterized in that the water in the cooling chamber served with speed, 1.1 - 1.2 times higher than the cooling rate of the rebar, and at a pressure of 0.5 to 0.9 kgf/cm2.

3. Device for thermal hardening of the reinforcing rods containing flow nozzle with a cooling chamber, characterized in that the ratio of the area of the slit expiration forgenie exceeds the cross-sectional area of the reinforcing bar 9 - 15 times.

 

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