JPS6410567B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing steel bars and wire rods with excellent cold workability. In recent years, cold forging has become increasingly important as an energy-saving alternative to hot forging and as a forming method for steel parts due to its good processing accuracy. However, on the other hand, when the ductility of the material is low, cold forging cracks occur due to poor workability, and when the material is hard, the deformation resistance increases, leading to wear and tear on the die. In order to solve these problems, conventional efforts have been made to improve workability by subjecting the material to spheroidization treatment to change the steel structure, thereby increasing ductility and softening the hardness. However, since the above-mentioned spheroidizing treatment is usually performed by annealing and requires a long operation time, as a countermeasure, by adjusting the cooling conditions after hot working, it is possible to create an intermediate stage structure in the steel that leads to the spheroidal structure. Attempts have been made to shorten the time between spheroidization processes, but no satisfactory results have been achieved, and measures to further shorten the process time are desired. was. SUMMARY OF THE INVENTION In response to the above-mentioned needs, the present invention aims to provide a manufacturing method that allows steel wire rods with excellent cold workability to be manufactured easily and in a relatively short time. That is, in hot finish rolling of a steel bar wire rod in which a carbon steel material or a low alloy steel material for machine structures is heated to a temperature of Ac ₃ or higher and then rolled, only the surface layer of the heated steel material is forcedly cooled to reduce Ar ₃ Rolling is carried out at a temperature below 500 m
By repeating the controlled rolling pattern at least once in which only the surface layer is forcedly cooled after reheating to a temperature range of ₁ point + 50°C from ℃ to A, and rolling with a total processing rate of 15% or more. A method for producing a steel bar wire rod with excellent cold workability, and a method for repeating the controlled rolling at least once to achieve a total processing rate of 15%.
The gist of the present invention is a method for manufacturing a steel bar wire rod in which cold workability is further improved by performing the above rolling and then reheating to a temperature range of 650° C. to _one point A. As a result of intensive research into shortening processing time and improving cold workability in spheroidizing treatment, the inventors of the present invention found that it is not possible to change the steel structure by adding a step such as annealing after hot working as in the past. Controlled rolling is achieved by adding a device to the hot working process itself, that is, by providing a process of forced cooling, rolling reheating, and forced cooling during hot working, and skillfully adjusting these process temperatures and processing conditions. By carrying out this process, it was possible to make the steel structure in the surface layer of the steel material spherical and to refine the structure in the center of the steel material in a short time, thereby successfully improving cold workability. The present invention is particularly applicable to cold forging, particularly for upset processing such as bolt head forming, which requires ductility in the surface area, and also to multi-stage combinations of the above-mentioned upset processing and axial drawing processing, which requires workability in the center area. This is especially effective for cold working. The present invention will be explained in detail below based on the drawings. FIG. 1 is an explanatory diagram of an example of a hot finishing rolling line for steel bars that implements the method of the present invention as set forth in claim 1. In the figure, 1 is a heating furnace, 2 is a steel material, 3 ₁ to 3 ₆ are rolling rolls, and 4 ₁ to 4 ₃ are cooling devices. The steel material 2 heated to the Ac ₃ transformation point (hereinafter referred to as the Ac ₃ point) or higher in the heating furnace 1 and passed through the rolling rolls 3 ₁ and 3 ₂ is forcibly cooled only in the surface layer by water sprinkling in the cooling device 4 ₁ . Ar ₃ transformation point (hereinafter referred to as Ar ₃ point)
Rolling is performed with rolling rolls ₃ at the following temperature, and the surface layer of the steel material 2 is reheated to a temperature range of 500℃ to _A1 transformation point (hereinafter referred to as _A1 point) + 50℃ by the rolling process, and then reheated again. Controlled rolling is performed in which only the surface layer is forcibly cooled in the same manner as above by water sprinkling in the cooling device ₄₂ . In the illustrated example, the controlled rolling is successively carried out repeatedly through the rolling rolls 3 ₄ and the cooling device 4 ₃ , and the total processing rate by the rolling rolls 3 ₃ and 3 ₄ is 15.
% or more, rolling rolls 3 ₅ and 3 ₆
Through this, the steel material 2 is rolled to a desired size (diameter) and sent to a subsequent process to be manufactured into a steel bar product. FIG. 2 is a chart showing an example of the heat pattern of the surface layer portion of the steel material 2 as a characteristic curve P while passing through the cooling device 4 ₃ from the cooling device 4 ₁ via the rolling rolls 3 ₃ and ₃ 4 . FIG. 3 is a plan view of the steel material 2 taken along line A--A in FIG. 1, showing the thickness of the surface layer of the steel material which has been cooled by the forced cooling and then reheated by rolling. The thickness W of the surface layer 5 shown in the figure is 1 to 1 from the surface of the steel material.
It is preferable to set the range to 30 mm (however, it should be 40% or less of the steel radius R). In addition, the temperature of forced cooling in the above controlled rolling is
The temperature of the surface layer 5 varies from 500°C to A point +50°C due to the rolling applied by the rolling rolls 3 ₃ and 3 ₄ .
A temperature of ₃ points or less of Ar that can be recuperated to a temperature range of 3 is selected as appropriate depending on the material. FIG. 4 is an explanatory diagram of an example of a hot finishing rolling line for steel bars that implements the method of the present invention as set forth in claim 2, in which 1 is a heating furnace, 2 is a steel material, and 3 ₁ to 3 ₆
4 is a rolling roll, 4 ₁ to 4 ₃ are cooling devices, 6 is a reheating furnace, and 7 is an air cooling device. The rolling roll 3 is heated to Ac ₃ points or higher in the heating furnace 1.
The steel material 2 that has passed through the rollers ₁ and 3 ₂ passes through the steps of cooling device 4 ₁ - rolling roll 3 ₃ - cooling device 4 ₂ - rolling roll 3 ₄ - cooling device 4 ₃ in the same manner as shown in FIG.
After repeating the controlled rolling of recuperation and forced cooling twice and performing rolling with a total processing rate of 15% or more using the rolling rolls 3 ₃ and 3 ₄ , the temperature is increased to 650°C in the reheating furnace 6.
A: Tempering is performed by reheating to a temperature range of _one point and air cooling with air cooling device 7, and then passing through rolling rolls 3 ₅ and 3 ₆ as shown in Fig. 1, to produce a steel bar product of the desired size (diameter). . Next, in the present invention, heating temperature, forced cooling temperature,
The reason why the tempering temperature, rolling rate, etc. are limited as described above will be explained in detail. First, the heating temperature of the steel material before hot finish rolling was set to Ac ₃ or higher because it is necessary to make the steel structure austenite in order to facilitate rolling. Next, only the surface layer of the steel material 2 passed through the rolling rolls 3 ₁ , 3 ₂ at a temperature of Ac ₃ points or higher is forcibly cooled to Ar ₃ points or lower by water sprinkling from the cooling device 4 ₁ to change the steel structure to austenite. This is to transform from ferrite to pearlite. In the surface layer of the steel material 2, which has a ferrite + pearlite structure, the granulation of pearlite carbide (cementite) precipitated during the transformation is promoted by the rolling process of the rolling rolls ₃₃ , and the center part undergoes transformation. At the same time, the precipitated ferrite + pearlite structure is refined. In addition, the temperature of the surface layer was reduced to 500% by the above rolling process.
The reason for reheating to the temperature range of ₁ point + 50 °C from ℃ to A is to make the carbide granulation of the pearlite in the surface layer into a more stable and good state by reheating to the temperature range. , below 500℃, the fragmentation of pearlite granulated by processing is not promoted;
Furthermore, heating above point _A1 +50°C is undesirable because solid solution of cementite in pearlite will proceed and the effect of rolling will disappear. Next, the steel material 2 that has been reheated by rolling with the rolling rolls ₃₃ is subjected to forced cooling again in the cooling device ₄₂ , rolling with the rolling rolls ₃₄ , reheating, and controlled rolling with forced cooling in the cooling device ₄₃ . The reason why this was applied repeatedly is that the carbide granulation in the surface layer is more likely to be formed when the number of rolling passes is increased, and by repeating cooling and reheating, the carbide granulation structure in the surface layer is more easily formed. This is because it becomes more stable and good. Next, the total working rate of rolling rolls 3 ₃ and 3 ₄ was set to 15% or more, because if the working degree of one rolling pass becomes extremely small, the effect of accumulating strain energy due to hot working becomes weaker. This is because if the amount is less than 15%, the effect of granulating pearlite carbides due to the accumulated strain energy will not be sufficiently exhibited. In the method of the present invention described in item 2, the steel material 2 subjected to the controlled rolling is further reheated to 650°C to ₁ point A and tempered. This is because the oxidation is further promoted and becomes even more stable and good, and the ductility and softening of the steel material are further improved. Also, the tempering temperature
This is because if the temperature is lower than 650°C, softening is insufficient and the tempering effect is poor, and if the temperature exceeds the _A1 point, the granulated structure of the carbide is re-dissolved and the effect in rolling disappears. Next, the effects of the present invention will be explained with reference to Examples. Table 1 shows the components of the test steels A, B, C, D, and E and the temperatures at their respective transformation points. The test steels are 45C, 20C,
These are 20C-Cr-Mo steels, which are all steel types commonly used for cold forging. A, B, and C were used as materials for carrying out the method of the present invention, and D and E were used as materials for carrying out the comparative method (Johei method). Steel bars of various diameters were manufactured.

[Table] Table 2 shows the Hv values showing the hardness of the cut surface at different depths from the outer surface of the steel bars produced by the method of the present invention and the comparative method. The above-mentioned Hv value indicating hardness is a test value conducted based on the JIS Witzkers hardness test method.

[Table] As shown in Table 2, the hardness (Hv) of the surface up to 1.5 mm from the surface of all products manufactured by the method of the present invention
It was found that the cold forging cracking resistance was significantly lower than that obtained by the comparative method, and that the cold forging cracking resistance was excellent. Table 3 is a table of mechanical properties and critical compressibility comparing the method of the present invention and the comparative method. Mechanical properties are shown by tensile test values using a JIS No. 4 test piece taken from the center of a 32 mm diameter steel bar, and critical compression ratio is shown by a compression test using a 32 mm diameter x 48 mm length test piece taken from a 32 mm diameter steel bar. The specimen was compressed using a machine, and the difference between the original height of the specimen and the height at which cracks appeared due to compression was expressed as a percentage (%) of the original height. As shown in Table 3, carbon steel (A
-1), (A-2), (B-1), and (B-2) are carbon steel (D) made by the comparative method and low alloy steel made by the present invention method.
(C) has lower tensile strength (TS) at the center of the steel bar, as well as improved elongation (El.) and reduction of area (RA), compared to low alloy steel (E) produced by the comparative method. It was observed that the ductility of the specimens was improved compared to those produced by the comparative method. In addition, (A-2) (B
-2) is due to tempering treatment.

【table】

[Table] A further decrease in tensile strength and improvement in ductility were observed compared to (A-1) and (B-1). In addition, as for the limit compressibility, all the products produced by the method of the present invention showed much higher values than those produced by the comparative method, and it was revealed that they had excellent cold workability. Table 4 is a table comparing the method of the present invention and the comparative method and showing the extrusion process limit test values, which are indicators of the workability of the central part of the steel bar. The above test value is the difference between the cross-sectional area at the time when a crack appears on the surface of the test piece after extrusion and the original cross-sectional area, expressed as a percentage of the original cross-sectional area, after an extrusion test was performed using a 32 mm diameter steel bar test piece. .
The extrusion conditions for the extrusion test above are: die half-width: 15°, lubrication: Bonde Bondaryube +
MoS was used, and the extrusion speed was 0.5 mm/S. As shown in Table 4, carbon steel (A-
1) and (A-2) are compared with those made of carbon steel (D) of the comparative method, and those made of low alloy steel (C) of the present invention are made of low alloy steel (E) of the comparative method. In all cases, the extrusion limit value was significantly improved compared to the above, and the processability improved by the method of the present invention.

[Table] As explained above, the method for manufacturing long steel according to the present invention is to form a granular carbide structure in the surface layer of the steel material and a fine ferrite + pearlite structure in the center by controlled rolling during hot finishing rolling, thereby forming a steel bar in the cold rolling process. processing,
In particular, it not only has a great effect on upset processing such as bolt head forming, or a combination of the above-mentioned upset processing and axial drawing processing, but also on hot-rolled materials as they are, or on the hot-rolled materials. which has been tempered for a short period of time,
Since it can be used as a material for cold forging, it is possible to significantly shorten the processing time for cold forging, which greatly contributes to reducing the manufacturing cost of cold forging materials.

[Brief explanation of drawings]

1 and 4 are explanatory diagrams of an example of a hot finishing rolling line for steel bars that implements the method of the present invention, and FIG. 2 is a chart showing an example of the heat pattern of the surface layer of steel in the hot finishing rolling line. FIG. 3 is a cross-sectional plan view showing the thickness of the treated surface layer of the steel material. DESCRIPTION OF SYMBOLS 1: Heating furnace, 2: Steel material, _31-36 : Rolling roll, _41-43 : Cooling device, 5: _Surface layer part, ₆ : Reheating furnace, 7: Air cooling device.

Claims (1)

[Scope of Claims] 1. In hot finish rolling of a steel bar wire rod in which a carbon steel material or a low alloy material for machine structures is heated to a temperature of Ac ₃ or higher and rolled, only the surface layer of the heated steel material is forcedly cooled. At least one controlled rolling pattern in which rolling is performed at a temperature of Ar ₃ or lower, the surface layer is reheated to a temperature range of 500°C to _{A 1} point + 50°C, and then only the surface layer is forcibly cooled again. A method for manufacturing a steel bar wire rod with excellent cold workability, characterized in that rolling is repeated several times at a total working rate of 15% or more. 2 Ac ₃ carbon steel or low alloy steel for machine structures
In hot finish rolling of a steel wire rod that is heated to a temperature of Ar 3 or higher and rolled, only the surface layer of the heated steel material is forcedly cooled and rolled at a temperature of Ar ₃ or lower; After reheating to a temperature range of 500℃~A ₁ point + 50℃, a controlled rolling pattern in which only the surface layer part is forcibly cooled is repeated at least once, and rolling with a total processing rate of 15% or more is performed.
A method for producing a steel bar wire rod with excellent cold workability, which comprises then reheating and tempering it to a temperature range of 650°C to ₁ point A.