JPS6345449B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a method for directly improving surface layer structure (surface hardening) of steel bars and wires.

(Conventional technology) Direct surface structure improvement of steel bars (surface hardening)
Following hot rolling, the surface layer of the rod and wire rod is cooled by water jet cooling from above the Ar ₁ transformation temperature to below the bainite transformation temperature, preferably from above the Ar ₃ transformation temperature to below the martensitic transformation temperature. (quick cooling). That is, cooling is performed by increasing the heat transfer coefficient on the surface of the rod or wire so that the radial thermal conductivity of the rod or wire becomes rate-determining.

Here, thermal conductivity rate-limiting refers to a situation in which when the heat transfer coefficient αkcal/m ² h℃ on the surface of the rod and wire increases, the heat conduction (thermal diffusion) in the radial direction of the rod and wire cannot keep up. It is a state in which λ/r<α, where the thermal conductivity of the wire is λkcal/m ² h°C and the radius of the wire rod is r.

After quenching so that thermal conductivity is the limiting factor, the wire rods are blown with high-pressure air to drain water. Large-diameter rods and wires are drained and subsequently cooled by water injection. It is known that the cooled rods and wires are transported in the atmosphere between a water cooling device and a cooling bed or a winder, and the surface layer is recuperated by heat conduction from the high temperature core. (JP-A No. 49-134513, JP-A No. 51-90912, JP-A-Sho
No. 51-99619) A water cooling system consists of a plurality of cooling units (cooling boxes) arranged in tandem. Each cooling box is equipped with an annular forward injection nozzle or reverse injection nozzle, and the cooling water is injected from the nozzle to the rods and wires at high pressure. Further, the above-mentioned draining device is arranged in the middle of the cooling box row.

In order to give the product the required mechanical properties, the cooling rate and cooling time must be adjusted according to the diameter of the rod and wire rod. For this purpose, in the past, the surface temperature of the rod and wire rod was measured at a position sufficiently distant from the outlet side of the cooling device, and the cooling rate was controlled by adjusting the amount or water pressure of the cooling water based on this measurement.

(Problems to be Solved by the Invention) In the conventional method, the cooling rate during quenching or the quenching end temperature is determined based on the surface temperature after reheating measured at a location sufficiently far from the exit side of the cooling device, and is determined by simulation. It had to be estimated by Here, the quenching end temperature refers to the surface temperature when a desired depth from the surface of the rod or wire is cooled to a predetermined temperature or lower. However, since the cooling rate varies in a complicated manner depending on the operating conditions, it is difficult to accurately predict whether or not the material has been hardened to the desired depth based only on the measurement results at the exit side of the cooling device. Therefore, it has been difficult to quench at the required cooling rate and to stably produce products with the required mechanical properties.

Furthermore, in the case of using the above-mentioned simulation, whenever operating conditions change, simulation must be performed in accordance with the operating conditions, making it impossible to adequately respond in terms of operational management.

(Means for Solving the Problems) In the present invention, the surface temperature of the rod and wire is measured immediately after draining, and the surface temperature of the rod and wire is measured while being transported in the atmosphere. Based on these two measured temperatures, the amount of cooling water and the cooling time are adjusted so that the rod and wire are cooled at a required cooling rate.

Since the steel material surface temperature cooling curve during quenching is determined by the thermal conductivity of the rod and wire rod, the influence of the rod and wire rod diameter at the quenching end temperature measurement point (immediately after draining) is small.
Further, the surface temperature of the steel material on the atmospheric conveyance line can be kept almost constant by increasing the cooling time as the diameter becomes larger.

After rapid cooling, the surface temperature increases due to reheating, so
The quenching end temperature must be measured immediately after draining.

In addition, considering quality variations, temperature measurement accuracy, etc., the recuperation temperature is approximately equal to the temperature difference between the surface and center of the steel material.
It is preferable to measure at a position where the temperature is 10°C or less.

Based on these two measured temperatures, the amount of cooling water and cooling time are adjusted to obtain the required cooling curve. For example, if the temperature measurement results show that cooling is insufficient, that is, the quenching end temperature or recuperation temperature is too high, the amount of cooling water is increased. Here, if the target temperature cannot be satisfied only by increasing the amount of water, the number of cooling units used will be increased.

(Function) Since the quenching end temperature of surface-hardened rods and wires is measured immediately after draining, the increase in surface temperature due to reheating is within a range that hardly poses a problem in terms of manufacturing control. You can know exactly. Furthermore, by adjusting the amount of cooling water based on such measured temperatures, the rod and wire rods are cooled in accordance with a required cooling curve.

(Example) FIG. 1 schematically shows a cooling device for carrying out the method of the present invention. As shown in the drawings, a cooling device 3 and a winding machine 21 are disposed following the finishing mill 1.

The cooling device 3 is divided into a quenching zone 4 and a dual-purpose zone 5 which serves as a small-diameter recuperation zone and a large-diameter quenching zone, and is made up of a plurality of cooling boxes 7 arranged in a row. Each cooling box 7 is equipped with an annular forward injection nozzle or a reverse injection nozzle (none of which is shown). The rods and wires M running through the cooling box 7 are cooled by jetting cooling water from around them through nozzles.

A draining device 1 is installed approximately in the center of the cooling box 7, that is, between the quenching zone 4 and the quenching recuperation zone 5.
1 is placed. The draining device 11 consists of a cooling water reverse injection device 12 and an air blowing device 13 adjacent to the outlet side thereof. The wire rod M that has been cooled in the quenching zone 4 and entered the draining device 11 is first sent to the cooling water reverse injection device 12, where the cooling water attached to the wire rod M and entrained therein is blown off by reverse injection.
Subsequently, the remaining adhering water is blown off with high pressure air in the air blowing device 13, and the surface of the rod and wire rod is dried.

A quenching end temperature detector and a recuperation temperature detector 17 are disposed on the outlet side of the drainer 11 and the inlet side of the winder 21, respectively. Both temperature detectors 15, 17 are non-contact thermometers, for example radiation thermometers.

Next, a method for surface hardening a rod or wire rod using the apparatus configured as described above will be described.

When cooling water is injected onto the rod or wire, the rod or wire is cooled by heat transfer from the rod or wire to the cooling water, but the cooling due to heat transfer in this case is rate-limiting by the thermal conductivity of the rod or wire. Therefore, as shown in FIG. 2, the surface temperature drops rapidly from the start of cooling. In the example shown in Figure 2, the temperature reaches 250℃ approximately 0.7 seconds after the start of cooling.
The following is true. Furthermore, the heat transfer coefficient on the rod and wire surface is a function of the rod and wire surface temperature, and increases as the temperature decreases. This is because the rod and wire rods are cooled by boiling heat transfer of the cooling water, and during cooling, heat transfer changes from film boiling to nucleate boiling. Equation (1) shows an example of the influence coefficient k on the heat transfer coefficient depending on the rod and wire surface temperature Ts.

Influence coefficient k = exp (3.28−0.0049・Ts) ...(1) As is clear from equation (1), the rod and wire surface temperature Ts
The heat transfer coefficient increases exponentially as the value decreases. Therefore, as the rod and wire temperature decreases, even if the amount of cooling water is reduced in inverse proportion to the influence coefficient k obtained by equation (1), the average heat transfer required for surface direct quenching cooling will decrease. A rate of 10 ⁴ to 2×10 ⁴ kcal/m ² h°C can be obtained. FIG. 3 shows an example in which the amount of cooling water is reduced as the rod and wire surface temperature Ts decreases based on equation (1). According to FIG. 3, the amount of cooling water when the rod and wire surface temperature is 500°C is about 1/5 of the amount of cooling water when it is 800°C.

From the above, a large amount of cooling water is supplied to the cooling box 7 at the front stage of the cooling device 3, and a small amount of cooling water is supplied to the cooling box 7 at the rear stage as the surface temperature decreases, that is, according to the straight line in FIG. supply Cooling water is supplied to the cooling box 7 in which the surface temperature of the rod and wire M is about 250° C. or lower in an amount sufficient to suppress surface recuperation due to heat conduction from the center of the rod and wire.

Following the above rapid cooling, drain the water. The timing of draining or the installation position of the draining device 11 is determined by the rod wire M
This is near (immediately before or immediately after) the cooling end position determined by the smallest diameter of (therefore, the required cooling length is the shortest). When the amount of cooling water is sequentially reduced as described above, the length of the water-wetted edge of the rod and wire introduced into the draining device 11 becomes extremely short. As a result, the load on the draining device 11 is small, and water can be drained sufficiently.

Following draining, measure the surface temperature. This quenching end temperature measurement is carried out immediately after draining, and in this example, approximately 0.04 seconds after draining completely.
For this purpose, the quenching end temperature detector 15 is placed approximately 100 mm downstream from the draining device 11.

Following the measurement of the quenching end temperature, the large diameter rod or wire is cooled by water injection in a recuperation zone 5 which also serves as quenching recuperation. Note that when the amount of cooling water (water density) decreases, it becomes difficult to supply cooling water uniformly in the circumferential direction of the rod and wire rods, so a method in which water is directly jetted against the rod and wire rods is not preferable. Therefore, it is desirable that the several cooling boxes 7 before the end of quenching be of a cooling type having a swirling flow in the inner tube, an immersion cooling type, or a mist cooling type.

The wire rod M cooled as described above is transported in the atmosphere from the cooling device 3 to the winder 21. Then, the recuperation temperature is measured during the conveyance, ie, at a point where the temperature difference between the surface and the center of the rod or wire becomes about 10° C. or less (in this example, about 17 seconds after the start of reheating).

Based on the result of the temperature measurement, the amount of cooling water to be supplied to each cooling box 7 is set, and the rods and wires M are cooled at a required cooling rate.

In this example, due to layout constraints, the distance between the final rolling mill 1 and the cooling device 3 is long, so it takes about 7 seconds or more to start quenching after rolling, which prevents coarsening of austenite grain size and generation of ferrite. In order to suppress this, it is necessary to start cooling as soon as the rod and wire rod enters the cooling device. A draining device 11 was installed near the quenching end position of the smallest diameter.
Therefore, it was necessary for large diameter rods and wires to be cooled by water injection following the measurement of the quenching end temperature. However, in the case of a layout where rapid cooling can be started in a short time, as shown in FIG.
Needless to say, the quenching start position can be changed depending on the diameter of the rod and wire rod, so that the quenching end position is always immediately before the draining device.

Note that when direct surface hardening is not performed, there is no need to measure the hardening end temperature.

(Effect of the invention) Since the amount of cooling water is set based on the quenching end temperature directly measured immediately after draining and the recuperation temperature measured during transportation in the atmosphere, the rod and wire rod can be cooled at a high temperature according to the required cooling rate. Can be cooled with precision.
Therefore, it is possible to produce rods and wires having the required mechanical properties. Further, the conventional simulation is unnecessary, and the amount of cooling water can be easily adjusted based on the temperature measurement results.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram of a cooling device that implements the method of the present invention, Fig. 2 is a diagram showing an example of the cooling curve of a rod and wire, and Fig. 3 is a relationship between the surface temperature of the rod and wire and the cooling water amount reduction ratio. FIG. 4 is a diagram showing another example of the cooling curve of a wire rod. 1... Finishing rolling mill, 3... Cooling device, 7... Cooling box, 11... Draining device, 12... Cooling water reverse injection device, 13... Air blowing device, 15, 17...
...Temperature detector, 21... Winder.

Claims (1)

[Claims] 1 Following hot rolling, the surface layer of the rod and wire rod is cooled by water jet cooling to control the cooling pattern from above the Ar ₁ transformation temperature to below the bainite transformation temperature, and then, while being conveyed in the atmosphere, the heat from the high temperature core is removed. In the method of reheating the surface layer by conduction to improve the surface layer structure, water is drained following the control of the cooling pattern, and the surface temperature of the rod and wire rod is measured immediately after draining, and the surface temperature of the rod and wire rod is measured while being transported in the atmosphere. A method for directly improving the surface layer structure of a steel bar or wire material, the method comprising measuring the surface temperature and adjusting the amount of cooling water to achieve a required cooling rate based on both measured temperatures.