JPH021214B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for controlled rolling and controlled cooling of hot rolled wire rods. [Conventional technology and its problems] In general, carbon steel wire rods and low alloy steel wire rods for machine structures are subjected to spheroidizing annealing before cold casting in order to improve the deformability of the wire rod and prolong tool life in the secondary processing process. is being carried out. This process is based on the technical phenomenon that a so-called spheroidized structure in which spherical cementite is finely dispersed within a ferrite matrix has excellent cold forgeability. Several methods are known for spheroidizing annealing, and the core process is heat treatment near the _A1 transformation point after reheating the steel material.
A ₁ method of repeatedly heating and cooling above and below the transformation point temperature,
A ₁ Method of heating and holding above and below the transformation point,
There is also a method of maintaining the temperature just below the _A1 transformation point. The above conventional heat treatments are all carried out in batches over a long period of 7 to 24 hours in a tunnel type annealing furnace, a bell type annealing furnace, or the like. Therefore, the spheroidizing annealing process occupies a large proportion of the process of manufacturing various wire rod products, but it is an inefficient process.It is therefore desirable to save labor and energy by eliminating this heat treatment in the secondary wire processing process. There is a strong need to promote In order to meet this need, each wire manufacturer
For wire rods that have a quality equal to or superior to that of spheroidized annealed material in the secondary processing process by so-called in-line heat treatment using rolling processing heat in the wire rod production line, that is, wire rods produced by conventional methods,
We are competing to develop methods for manufacturing wire rods that have lower tensile strength and higher toughness. As an in-line heat treatment method characterized by lowering the tensile strength of wire rods, as shown in Japanese Unexamined Patent Publication No. 56-41325, the method involves controlling the cooling rate in the step of cooling the wire rods following hot rolling. Controlled cooling is known in which the wire is softened by heating. This method allows a wire rod with lower tensile strength to be obtained compared to conventional methods, and it is possible to omit softening annealing, but in order to meet the increasingly strict demands of customers, lower tensile strength and higher toughness are desired. It is rare. In addition, in addition to controlled cooling in the cooling stage that follows hot rolling, Japanese Patent Application Laid-Open No. 1973-1997 describes a method using controlled rolling and controlled cooling, which goes back to the previous process and controls the temperature of the rolled steel material during hot rolling and then gradually cools it.
143716, and a controlled rolling method for controlling the temperature of rolled steel was published in Japanese Patent Publication No. 14371-1.
This is shown in Publication No. 13364. However, among the processes for producing hot rolled wire rods using these methods, in the process of producing hot rolled wire rods using a finishing block mill, the desired low tensile strength of the wire rods cannot be achieved for the following reasons. In the first place, the metallurgical mechanism for reducing the tensile strength of wire rods by controlled rolling cooling is, as elucidated by the present inventors in JP-A No. 58-27926, the first step being the dispersion of a large number of cementite generation nuclei and their subsequent dispersion. The subsequent second step is the agglomeration and spheroidization _of each cementite.
The temperature must be within the range of 30°C or higher and _A3 transformation point -20°C. That is, in controlled rolling, it is essential to specify the final finishing temperature.
That is, when rolling with a finishing block mill, the finishing outlet temperature becomes high due to heat generation during processing, the effect of controlled rolling is small, and sufficient softening cannot be achieved. However, the regulation regarding controlled rolling in JP-A No. 54-143716 simply refers to the finish rolling inlet temperature.
It is in the range of 600 to 900℃, and
In Publication No. 13364, the finishing rolling inlet temperature is 954.4°C or less, and there are no other regulations regarding the final finishing temperature. Also, in particular, Special Publication No. 57-13364 is 12.7
It is limited to mmφ or more. When the above-mentioned method is applied to a manufacturing process using a finishing block mill, the finishing block mill has narrow stand intervals and high-speed high-area reduction processing, which generates a large amount of heat during processing and increases the temperature of the rolled steel material. Therefore, even if the finish rolling inlet temperature is
Even if it is cooled to 600℃, the final finishing temperature is ``A <sub>3</sub> points -
In some cases, the temperature exceeds 20°C, and the ferrite precipitated by cooling before finishing block milling becomes austenite again, and many cementite nuclei are not dispersed. Therefore, the crystal grain refinement effect is reduced, and the subsequent transformation promotion effect is reduced, so that it does not contribute to lower tensile strength. In particular, since slow cooling is not disclosed in Japanese Patent Publication No. 57-13364, softening cannot be achieved. In addition, Belgian Patent No. 856255 (promulgated on August 14, 1976) limits the temperature after finish rolling to 870-970°C with an average cooling rate of 8°C or less, but the temperature is too high and controlled rolling is not possible. The effect of this is small and there is no slow cooling, so softening cannot be achieved. Also, it is not applied to block mills with short distances between stands. In addition, Japanese Patent Publication No. 46-21292 also discloses a method of cooling the wire to a set finishing temperature during the rolling period during which the wire rises. This refers to processing heat generated within the finishing mill. However, since there is no slow cooling in this case, as shown above, softening cannot be achieved. The present invention relates to an in-line heat treatment method for manufacturing carbon steel wire rods and low-alloy steel wire rods for machine structures, with the aim of lowering tensile strength and increasing toughness, and eliminating the need for spheroidizing annealing in the secondary processing process. The purpose of this invention is to provide a production method that can be implemented in a process using a block mill. (Means and effects for solving the problems) In order to achieve the above object, in the present invention, in the method for manufacturing hot rolled wire rod, one or more cooling devices disposed before the finishing block mill are used to cool the finishing block mill at the inlet of the finishing block mill. A temperature-controlled rolling process in which the temperature of the rolled steel material is lowered and the heat generated during finish rolling is suppressed by a cooling device placed after the caliber in the finishing block mill, so that the final finishing temperature is ``Ar ₃ -20℃'' or less, and 640 ℃ to 2.0℃/
A method for producing a low tensile strength high toughness wire rod characterized by comprising a slow cooling step of cooling at a cooling rate of sec or less, and in the method, cooling in the finishing block mill is performed only on the exit side of the round caliber. Adopt the manufacturing method of tough wire. In the present invention, in order to prevent cracking, the cooling water is divided into cooling before the finishing block mill and cooling inside the finishing block mill. For steel types that are particularly prone to cracking, such as those containing Cr, Mo, etc., only the round caliber is used for cooling in the finishing block mill. As a result, processing heat generation is suppressed, and the final finishing temperature is "A ₃ point - 20℃"
By making the following finish rolling possible and cooling to 640°C at a cooling rate of 2.0°C/sec or less in the subsequent cooling step, a soft, low tensile strength, high toughness wire rod can be produced. If rolled steel is cooled only before finishing block milling, the final finishing temperature may exceed the " _A3 transformation point -20℃" due to the heat generated during finish rolling, and the cementite generation nuclei will disappear, contributing to softening. do not. On the other hand, if a cooling device is configured only in the finishing block mill to cool the rolled steel, it is possible to carry out finishing rolling so that the final finishing temperature is below the " _A3 transformation point - 20°C", but Since the distance between the stands of a block mill is short, the rolled steel material is rolled at the next stand without time for reheating, and a temperature difference occurs between the surface layer and the center of the rolled steel material, resulting in surface cracking. In other words, there is a cooling limit to the cooling inside the finishing block mill, and surface flaws will occur if the block is cooled more than the processing heat generated by finish rolling can be suppressed. Therefore, by combining cooling before the finishing block mill and cooling inside the finishing block mill, and by limiting the cooling inside the finishing block mill to suppressing processing heat to prevent cracking, cracking can be prevented and the final finishing temperature can be reduced to "A". _3. Finish rolling with a transformation point below -20°C is possible, and during subsequent cooling,
By cooling to 640℃ at a rate of 2.0℃/sec or less, we achieved lower tensile strength and higher toughness than wire rods made using conventional methods. Furthermore, in the process of carrying out factory experiments, we discovered the advantage of cooling only the exit side of the round caliber, especially in cooling inside the finishing block mill, from the perspective of uniform cooling within the cross section of the steel material and prevention of cracking. discovered. The caliber arrangement in finishing block mills usually consists of an oval-round combination. When cooling inside the finishing block mill, if cooling was applied to the exit side of the round caliber and the exit side of the oval caliber so that the same finishing temperature was applied, cracks could occur depending on the steel type. No cracking occurred when cooling was applied only to the exit side. This is because when cooling on the exit side of the oval caliber, supercooling occurs at the so-called oval edges, creating a temperature difference between the oval edge and the center, resulting in a large difference in deformability within the cross section of the rolled steel material, which induces cracking. This is because supercooling did not occur when only the exit side of the round caliber was cooled. Next, regarding the slow cooling process,
As is also known from the method for producing a wire rod having a spheroidized structure described in Publication No. 27926, from experimental results, cementite spheroidization is promoted when the cooling rate after rolling to 640°C is 2.0°C/sec or less. Softening is improved. If it exceeds 2.0°C/sec, there is little effect on spheroidization. On the other hand, since the lower limit of the cooling rate is more preferable as gradual cooling is achieved, cooling rate of 0, 0,
That is, it also includes a case where the temperature is maintained at an isothermal temperature. The above cooling rate is preferably short. Among the direct heat treatment methods for hot-rolled wire rods described in Japanese Patent Publication No. 59-31573, heat treatment of 0.025℃/sec to 0.25℃/sec from 600 to 675℃ is included.
Slow cooling is performed at a cooling rate of sec. Judging from these, the temperature was limited to 2.0°C/sec or less up to 640°C. Next, the configuration of the present invention will be explained in detail with reference to FIGS. 1 and 2. In the process of manufacturing hot rolled wire rods, one or more finishing block mill pre-cooling devices 1 are arranged in front of the finishing block mill 2. Further, in the finishing block mill 2 which is composed of a combination of an oval caliber 9 and a round caliber 10, an internal finishing block mill cooling device 3 is disposed on the outlet side of each caliber. A cooling water supply device 8 is connected to the finishing block mill internal cooling device 3 via a pipe 13 and a valve 12. Figure 2 shows a case where the cooling device is placed only on the exit side of the round caliber. Finishing block mill 2
In front of the finishing block mill is one or more pre-cooling devices 1
is located. Further, in the finishing block mill 2 which is composed of a combination of an oval caliber 9 and a round caliber 10, an internal finishing block mill cooling device 3 is disposed only on the outlet side of the round caliber 10. The finishing block mill internal cooling device 3 includes:
A cooling water supply device 8 is connected via piping 13 and a valve 12 . Finishing block mill post-cooling device 4 for finishing rolling the wire rod after the finishing block mill 2
is arranged, followed by a laying head 5 and a slow cooling cover 6 in this order. The wire rod 11 is passed through a cooling device 1 before a finishing block mill to lower the temperature of the rolled steel material. Thereafter, cooling water is sprayed through the cooling device 3 in the finishing block mill after each caliber arranged in the finishing block mill 2. This spraying is Oval Calibur 9,
Cool the wire alternately after round caliber 10. This cooling is performed by opening each valve 2 and supplying cooling water from the cooling water supply device 8 through the piping 13 after the oval caliber 9 and the round caliber 10. The above operation is as follows. The valves 12 are set to respective opening degrees to match the cooling water required for the rear stage of each stand. Next, the cooling water supply device 8 is operated to supply water to the cooling device 3 in the finishing block mill via the piping 13 and the valve 12.
Cooling water is supplied to the surface of the wire to cool it by spraying it on the surface of the wire. In this case, the valve 12 may be an electric valve, electrically connected to the cooling water supply device 8, and configured for remote automatic operation.Also, the amount of cooling water to the rear stage of each stand can be adjusted as necessary when viewed in the direction of wire movement. In order to adjust
It is also possible to install a flow meter between each valve 12 and the cooling device 3 in the finishing block mill to check and adjust the flow rate. These cooling devices make it possible to control the temperature of the rolled steel material without any inconvenience and achieve a predetermined final finishing temperature. The wire rod subjected to controlled rolling is cooled to a predetermined temperature in a finishing block mill post-cooling device 4, and then formed into a spiral ring shape by a rolling head 5, spread on a conveyor, and conveyed. These spiral ring-shaped wire rods are slowly cooled by a slow cooling cover 6 that controls the ambient temperature. The case where only the exit side of the round caliber shown in Fig. 2 is cooled is also omitted because the explanation is almost the same as above. (Example) The method of the present invention was carried out under the following conditions. (1) Example 1 Rolling speed: 60 m/sec The steel type, size, and composition of the wire rod are as shown in Table 1.

[Table] Operating conditions Conditions such as cooling before and inside the block mill, temperature at the block mill inlet, etc. are shown in Table 2 for each type of test material.
Shown below.

[Table] Cooling water (a) Amount of cooling water: 2 to 2.5 m ³ /h/place, 12℃ (b) Cooling water: Industrial water Cooling device (a) Cooling nozzle: Nozzle for uniform cooling of wire rod (b) Cooling water supply Equipment pump capacity: 50Nm ³ /min (c) Piping Main pipe 200A Branch pipe 75A (2) Example 2 Rolling speed 60m/sec The steel type, size and composition of the wire rod are as shown in Table 3.

[Table] Operating conditions Conditions such as cooling before and during block milling, temperature at block mill inlet, etc. are shown in Table 4 for each type of test material.
Shown below.

[Table] Cooling water (a) Amount of cooling water: 2.5 to 3.0 m ³ /H/place, 12°C (b) Cooling water: Industrial water Cooling device Same as Example 1. Example 1 is carbon steel wire rod SWRCH45K for cold rolling.
This is an example manufactured under the conditions of the present invention using. This steel type is normally subjected to spheroidizing annealing prior to cold forging in the secondary processing process, and in the case of this steel, spheroidizing annealing provides mechanical properties such as a tensile strength of 58 kg/mm ² or less and a reduction of area of 55% or more. is obtained. In Table 2 and Figure 3, test material B is
This is an example in which the method described in Publication No. 41325 is applied, and although a decrease in tensile strength is observed compared to test material A prepared by the conventional method, both tensile strength and reduction of area are insufficient to the level of spheroidizing annealing. Test material C is an example in which the method described in JP-A-54-143716 is applied, and although an improvement in toughness is recognized compared to the conventional method, the decrease in tensile strength is smaller than test material B. This is because the final finishing temperature is lower than that of test materials A and B, so the crystal grains are finer and the toughness is improved. However, the final finishing temperature exceeds 900℃, which is 20℃ at point _A3 , and the ferrite nuclei that have been subjected to stress and strain disappear and become an austenite structure, resulting in a large number of cementite generation nuclei becoming dispersed. This is due to the fact that the tensile strength is lowered by accelerating transformation through grain refinement and slow cooling, and at the same time, the tensile strength is improved by grain refinement. Test material D was cooled only in the finishing block mill in order to bring the final finishing temperature below ``A ₃ point - 20℃'', but cracks occurred on the surface due to cooling only in the finishing block mill. did. This is because the rolled steel is cooled more than absorbing the processing heat generated during finish rolling, so it is rolled without reheating, creating a temperature difference in the cross section of the rolled steel, which causes a large difference in the deformability of the steel within the cross section. This is to induce cracks. Test material E is an example of the present invention, and the same quality as that of the spheroidized annealed material was obtained. Example 2 is an example in which the present invention was implemented on SCM435 alloy steel wire for machine structures. This steel type has a tensile strength of 60 due to spheroidizing annealing.
Mechanical properties with an aperture value of 60% or more can be obtained at Kg/mm ² or less. Steel type SCM435 also shows almost the same tendency as steel type SWRCH45K (Fig. 4), but as shown in Table 4, cracks occurred in test material E produced by the method of the present invention.
On the other hand, test material F was able to prevent cracking by cooling the finishing block mill only on the exit side of the round caliber. This is because when cooling on the exit side of the oval caliber, supercooling occurs on the so-called oval sides, and a large temperature difference occurs within the cross section of the rolled steel material, which causes a large difference in deformability within the cross section of the rolled steel material, which can induce cracking. On the other hand, this is because supercooling did not occur when only the exit side of the round caliber was cooled. (Effects of the Invention) As mentioned above, in the process of manufacturing rolled steel materials in a finishing block mill, cracking can be prevented by dividing the process into cooling before the finishing block mill and cooling inside the finishing block mill, and the final finishing temperature can be kept at "A". It has become possible to control the temperature of rolled steel to below ₃ points -20°C, and in combination with slow cooling after finishing block milling, it has become possible to manufacture low tensile strength and high toughness wire rods.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram of the method described in claim 1 of the present application, Fig. 2 is an explanatory diagram of the method of claim 2 of the present patent application, and Fig. 3 is a test of steel type SWRCH45K of Example 1. Figure showing the results, 4th
The figure shows the test results for steel type SCM435 of Example 2. 1... Cooling device before finishing block mill, 2... Finishing block mill, 3... Cooling device inside finishing block mill,
4... Cooling device after finishing block mill, 5... Laying head, 6... Annealing cover, 7... Conveyor, 8... Cooling water supply device, 9... Oval caliber, 10... Round caliber, 11... Wire rod, 12... Valve, 1
3...Piping.

Claims (1)

[Scope of Claims] 1. In a method for producing a hot rolled wire rod, the temperature of the rolled steel material at the entrance of the finishing block mill is lowered by one or more cooling devices placed before the finishing block mill, and a cooling device placed after the caliber in the finishing block mill is used. A temperature-controlled rolling process in which heat generation during finish rolling is suppressed using equipment to keep the final finishing temperature below ``Ar ₃ -20℃'', and a temperature control rolling process of 2.0℃/2.0℃ up to 640℃.
A method for producing a low tensile strength and high toughness wire, comprising a slow cooling step in which the wire is cooled at a cooling rate of sec or less. 2. A method for producing a low tensile strength and high toughness wire rod according to claim 1, wherein cooling in the finishing block mill is performed only on the exit side of the round caliber.