JPH0559966B2 - - Google Patents

Description

[Detailed description of the invention]

Industrial Application Field The present invention relates to a method for producing wire rods with excellent cold workability, and in particular, directly heat-treats wire rods made of alloy steel with high hardenability and ideal critical diameter (DI) of 100 mm or more to form cold workable wire rods. This invention relates to a method for obtaining a wire coil with excellent workability. Prior art and its problems In the production of wire rods, if the rolled wire rod is large in diameter, it is wound up on a polling reel and left to cool in a coiled form, but if it is small in diameter, it is wound up in a laying cone. It is spread out on a conveyor and cooled either by air cooling or by blowing air. For this reason, the cooling rate of the small-diameter coil is 2°C/sec to 10°C/sec even when it is left to cool, and the cooling rate may be 10°C/sec or more at the portion in contact with the reel or conveyor. Therefore, when manufacturing wire rods with high hardenability and an ideal critical diameter (DI) of 100 mm or more, a bainite or martensite structure is generated, and if as-rolled, peeling,
Processing such as wire drawing, profile processing, forging, cutting, and drilling becomes difficult. For this reason, the following methods are conventionally known for steel having high hardenability. A method of annealing rolled wire rod coils in a heat treatment furnace on a separate line for 10 to 100 hours, giving up on wire rods that require a high cooling rate, and forging and forging from steel ingots.
A method of annealing, cutting, etc., and a method of softening the wire by using the sensible heat of the wire after rolling and gradually cooling or retaining the wire immediately after rolling (JP-A-56-
133445, JP 58-27926, JP 58-58235, JP 58-107416, JP 59-13024, etc.). However, in all of these methods, the temperature is reduced only on the surface of the coil because it is wound outside the furnace after rolling, and only the part where the coil contacts the winding device is locally cooled, causing the internal temperature of the coil to decrease. The disadvantage is that the distribution is non-uniform and the quality of the coil varies widely. Furthermore, since the transformation rate varies widely depending on the steel type, continuous processing of different steel types is difficult. Purpose of the Invention This invention was made in view of the above-mentioned conventional problems, and the ideal critical diameter (DI) is 100 mm.
The object of the present invention is to propose a method of directly heat-treating a wire rod having high hardenability as described above to obtain a wire rod coil with excellent cold workability. Structure of the Invention The method of manufacturing a wire rod with excellent cold workability according to the present invention is to heat a highly hardenable steel having an ideal critical diameter (DI) of 100 mm or more to an Ac point of _{1 or more, and then heat the steel to an Ac point of 1 or} less. It is characterized in that it is rolled into a wire rod with a processing degree of 10% or more in a temperature range of ₁ or more Ar points, and then wound in a heat retention furnace or a slow cooling furnace. Here, the ideal critical diameter (DI) is a value calculated from the chemical composition values of steel using the following formula. DI (mm) = 7.95√% × {1 + 0.64 [Si%]} × {1 + 4.10 [Mn%]} × {1 + 2.87 [P%]} × {1-0.69 [S%]} × { 1 + 2.33 [Cr%]} × {1 + 0.52 [NI%]} × {1 + 3.14 [Mo%]} × {1 + 0.27 [Cu%]} × {1 + 1.5 (0.9 - [C%]) )} However, {1+1.5(0.9-[C%])} has a C amount of 0.9
Applicable only to steels with B added below %. That is, the present invention is based on the ideal critical diameter (DI)
_A steel wire rod with a diameter of 100 mm or more, preferably 130 mm _or more is targeted, and after heating to Ac _{1 point} or more, untransformed This is a method in which the raw material is given a working degree of 10% or more, preferably 20% or more, and after rolling, it is rolled up in a heat retention furnace or a slow cooling furnace installed close to the rolling line. In this invention, the ideal critical diameter (DI) is 100
The target pearlite structure wire with a diameter of mm or more is
This is due to the reasons shown below. In conventional hot-rolled wire manufacturing equipment, the hot-rolled wire is rolled out on a conveyor, cooled, and then bundled into a coil, or immediately wound into a coil to form a coil bundle. Either it is cooled by Although the cooling rate is lower in the latter case, steel wire rods with high hardenability will still have a structure containing bainite and martensite at this time. Normally, for steel types with an ideal critical diameter (DI) of 100 mm, if the wire diameter of the hot rolled wire rod is 9.0 mmφ or less, bainite and martensitic structures will be present, making cold working impossible. On the other hand, as the wire diameter increases, the cooling rate decreases, making it difficult to generate bainite and martensite, but as the hardenability of steel increases, bainite and martensite will form even if the wire diameter increases. Steels with an ideal critical diameter (DI) of 130 mm or more have high hardenability, resulting in a structure with a mixture of bainite and martensite in all sizes within the range that can be manufactured using a wire rolling mill.
Subsequent cold working becomes impossible. Hot-rolled wire rods made of steel with such high hardenability cannot be processed by coil unwinding, straightening, wire drawing, etc., so the hot-rolled coils had to be reheated and annealed on a separate line. . The purpose of this invention is to solve the above-mentioned problems of hot-rolled wire rods of steel types with high hardenability. Critical diameter (DI)
This applies to steel with a thickness of 100 mm or more. The present invention provides a method for producing a wire rod that can be processed into a wire rod having a pearlite structure without bainite and martensite structures while hot-rolled, and that can be subjected to secondary processing without reheating and annealing on a separate line. It provides: In addition, 7.95√ is used in the formula for calculating the ideal critical diameter (DI) because it is based on the C content and austenite grain size in the formula for calculating DI = DIc x MF%Mn x MF%Si + MF%Ni... DIc, which is a term indicating the basic hardenability of the related iron-carbon binary, is assumed as follows. In other words, considering that the austenite grain size of the hot rolled material is approximately No. 8, the grain size is determined by the constant k when the austenite grain size number is No. 8 in the generally known DIc = k × √%. (k=0.32 inch=7.95 mm). Further, the reason why the pre-rolling heating temperature of the steel is set to Ac ₁ point or more is as follows. In order to make the structure after rolling into a pearlite structure, it is necessary to decompose pearlite into a solid solution and disappear before rolling. For this reason, it is necessary to set the heating temperature to Ac ₁ point or higher. If the Ac temperature is lower than ₁ point, cementite remains undecomposed in the structure and is rolled, resulting in a pearlite structure with excellent green workability. This is because it becomes impossible to do so. Further, the reason why the finish rolling start temperature is set to Ac ₁ point or more is as follows. Since austenite is metastable at Ae _{of 1} point or less, pearlite transformation can be induced by applying working strain, and even highly hardenable steel can easily obtain a pearlite structure. However, in the temperature range of Ae ₁ or higher, austenite is in a stable range, so the above effects become extremely small. Further, when the Ar is ₁ point or less, austenite precipitates ferrite and cementite, so even if rolling is performed after such a structure appears, the desired pearlite structure cannot be obtained. In addition, it takes a long time to reach the Ar point of ₁ point in the target high-hardenable steel, and it is often difficult to roll the Ar point below ₁ point on a continuous rolling line. Note that some of the steels targeted in this invention have a temperature lower than 500°C at the Ar ₁ point, but in order to obtain the desired pearlite structure, the austenite transformation temperature (Ar ₁ ) must be lower than 500°C. It is desirable that the temperature is 500℃ or higher. 500
This is because bainite is mixed at temperatures below ℃. Therefore, the finishing start temperature is set to Ae ₁ to _{Ar 1} . Preferably, the temperature range is from Ae ₁ point to 500°C. The Ae ₁ point is determined by the steel composition, and is widely distributed from around 730°C for low alloy steels to around 850°C for some alloy tool steels. Note that when the rolling speed is high, the wire temperature increases due to heat generation during rolling, and the rolling end temperature may be higher than the finish rolling start temperature. There are many factors that affect this rolling end temperature, and strict control is extremely difficult. Therefore, in the present invention, the finish rolling start temperature is limited as described above. However, the rolling end temperature is Ac ₃
If the temperature exceeds the Ac point, the wire returns to the austenitic structure, so it is necessary to set the rolling end temperature to ₃ Ac points or lower. In addition, the reason why processing of 10% or more is applied in this invention is that when processing is performed in the above temperature range, the higher the processing degree, the greater the effect of promoting pearlite transformation.
This is because if it is less than 10%, this effect will not be fully exhibited. Practically speaking, 20% or more is desirable. Further, the present invention is characterized in that after rolling, the coil is wound in a heat retention furnace or a slow cooling furnace, and this is to ensure uniformity of temperature in the height direction and radial direction of the coil, thereby stabilizing quality. That is, since the wire remains in the heat retention furnace or the slow cooling furnace from the start to the end of winding, there is no difference in temperature from the start to the end of winding. Therefore, the temperature in the coil height direction is made uniform, and temperature variations in the coil radial direction due to heat dissipation from the coil surface are also eliminated. Furthermore, since Ar ₁ transformation occurs in the heat retention furnace or slow cooling furnace, extremely stable quality can be obtained. Here, slow cooling is 2
Heat retention means cooling at a cooling rate of ℃/sec or less, and isothermal maintenance. Specific Examples FIG. 1, FIG. 2, FIG. 3, FIG. 4, and FIG. 5 show an example of a direct softening heat treatment apparatus for carrying out the method of the present invention. That is, FIG. 1 shows a method of heat-treating a wire rod M that has been hot or warm rolled by a rolling mill 1 in a pot furnace 2. It is thus formed into a spiral shape and immediately wound up in a pot furnace 2 located close to the laying head. The pot furnace 2 is maintained at a predetermined temperature in advance by a heating element 4 provided inside. Wire M is pot furnace 2
Once the coil M' is wound inside, it is immediately sealed with the furnace lid 5. The coils housed in the pot furnace are conveyed onto the conveyor 6, subjected to predetermined slow cooling or heat retention during conveyance, and when the direct softening process is completed after reaching a predetermined temperature or a predetermined time, the coils are transferred to the furnace lid 5.
removed and taken out. The emptied pot furnace is returned to a predetermined temperature in the vicinity of the laying head 3 through a separate line, and then directly used for the softening heat treatment. FIG. 2 shows a method in which the softening heat treatment is performed directly in a continuous furnace, in which the wire M is hot or warm rolled by a rolling mill 1 and then wound up by a rolling head 3 in a continuous furnace 12. The continuous furnace 12 also has a heating element 14 attached inside the furnace body, but in the case of a continuous furnace, a conveyor 16 is installed inside the furnace, and a door 13 for taking out the coil is provided on the exit side. The wire M coming out of the rolling mill 1 is wound by a rolling head 3 in a continuous furnace 12 which is set in advance to a predetermined temperature or slow cooling heat pattern, and the furnace lid 15 is immediately closed when it becomes a coil M'. When the preceding coil M' in the continuous furnace moves and becomes ready to be carried into the furnace, the entrance door 17 opens and the conveyor 16 passes through the entrance door 17 and is conveyed one by one, and is heated or slowly cooled during conveyance. will be applied. The coil M′ that has been subjected to the prescribed heat treatment is installed at the outlet door 18 and the carry-out door 13.
is passed through and taken out, and the direct softening heat treatment is completed. FIG. 3 shows an example of a method for winding a wire without using a rolling head. A winding reel 28 driven by a motor 27 is installed in a pot furnace 22 equipped with a heating element 24, and a rolling head is used to wind the wire. This is a method in which the wire rod M that has come out of the wire rod 1 is sequentially wound onto a take-up reel 28. When the winding is completed to form the coil M', the furnace cover 25 is closed and the coil M' is sequentially conveyed by the conveyor 26. FIGS. 4 and 5 show examples of apparatus in which the laying head is of a horizontal type and an incline type, respectively. In this case, since the laying head is open in the direction of movement of the wire rod, the wire rod formed into a loop shape is transferred to the heat retention or slow cooling conveyor 6-1, 1.
After moving on 6-1, it is wound up in the furnace. In addition, in order to prevent participatory decarburization from occurring on the surface layer of the wire during heat treatment, the pot furnaces 2, 2
2 or the continuous furnace 12 is preferably equipped with equipment that can seal in an inert gas or a reducing gas. Therefore, in this case, in a continuous furnace, it is necessary to provide an entrance door 17 and an exit door 18 as shown by broken lines at the entrance and exit so that the atmosphere inside the furnace is not disturbed when the coil is carried in and out. In addition, as a means for raising the temperature of the furnace atmosphere and retaining heat, we have shown here a case in which a heating element installed inside the furnace is used, but the heat source does not necessarily have to be inside the furnace, and even if there is a heat source outside the furnace. good. Furthermore, if a predetermined heat retention or slow cooling can be carried out, a heat source is not necessarily required. Example 1 Melting in a vacuum in an electric potential furnace and forming into a 2-ton steel ingot
Among the nine types of test steel having the composition and ideal critical diameter (DI) shown in Table 1, which were obtained by hot forging into a steel billet with a cross section of 130 mm x 130 mm and then hot rolling at a wire rod factory, steel type A , B, E, F, and G, and wire rods were manufactured under the manufacturing conditions shown in the second example. At that time, the heat pattern for slow cooling was such that the furnace temperature was set at 750°C during coil winding, and after winding was completed, slow cooling was carried out to 650°C over 1 hour, and the coil was taken out.
The wire rod is 2.5mmφ after peeling using the reverse die method.
The wire was drawn using a continuous wire drawing machine in the cold to investigate the workability. In addition, for comparison, wire rods were manufactured using the same steel types A, B, E, F, and G shown in Table 1 as above, using the conventional method shown in Table 2, and using the same method as above. Processability was investigated. The results of this example are shown in Table 3.

【table】

【table】

【table】

[Table] From the results in Table 3, it can be seen that in the conventional steel type A, bainite is mixed in the structure, and it does not become a pearlite structure or a softened structure, so that the objective is not achieved. Similarly, steel type B had a mixed structure of bainite and martensite, and could not be processed. Similarly, in steel types E, F, and G, bainite and martensite were partially formed in the cross section of the wire rod and in the longitudinal direction, and the structure was nonuniform. Although peeling was possible, wire drawing was not possible. On the other hand, in all the examples of the present invention, there was no structural defect, the hardness was low, and peeling and wire drawing were extremely easy and there were no troubles. Example 2 Steel types C, D, H, and I shown in Table 1 were used, and after rolling, they were wound up in a pot furnace shown in FIG. 1 and kept at a constant temperature of 730° C. for 3 hours. Finish rolling temperature is 490℃~950℃,
The degree of finish rolling was 9% to 64%, and the products were manufactured under the manufacturing conditions shown in Table 4. Note that N ₂ gas was injected into the atmosphere inside the pot furnace to prevent oxidation and decarburization. After keeping the temperature constant, it was allowed to cool inside the furnace. The results of this example are shown in Table 5.

【table】

[Table] From the results in Table 5, steel types C, D, H, and I, which cannot be processed into bainite or martensitic structures using conventional manufacturing methods, can be processed using the method of this invention. It was found that it can be manufactured as a wire rod. Among the four types of steel, looking at the effect of finish rolling temperature on steel type I, the finish rolling was performed at 490°C.
In test No. 6 added in , the structure became bainite, and although some wire breakage occurred during wire drawing, it can be seen that raw processing is possible. However, the die wears out a lot during peeling, making it difficult to use. In addition, the finishing degree 9 shown as a comparative example
%, in the case of 16.5φ wire rod, bainite structure is mixed,
It can be seen that wire breakage is observed during wire drawing, and therefore, when the degree of processing is less than 10%, both the structure and properties are in a somewhat unstable state. Effects of the invention As explained above, according to the method of the invention,
It has high hardenability with an ideal critical diameter (DI) of 100 mm or more, and can easily process high-alloy steel wire rods, which previously could not be processed with hot-rolled steel. Since the rolled wire is wound, the variation in quality within the coil is extremely small. Furthermore, as is clear from the above examples, the Ar ₁ transformation can be completed in constant temperature holding for about 3 hours according to the method of this invention, even for steels with greatly different hardenability, so they can be continuously produced under the same manufacturing conditions. This makes it possible to achieve extremely high productivity.

[Brief explanation of drawings]

FIGS. 1, 2, 3, 4, and 5 are schematic diagrams showing an example of a direct softening heat treatment apparatus for carrying out the method of the present invention. 1...Rolling mill, 2, 22...Pot furnace, 3...
Laying head, 4, 14, 24... heating element,
5, 15, 25... Furnace lid, 6, 16, 26... Conveyor, 17... Entrance door, 18... Exit door, 27
... Motor, 28 ... Take-up reel, M ... Wire rod,
M′...Coil.

Claims (1)

[Claims] 1. The ideal critical diameter calculated by the following formula is 100 mm.
After heating the steel with hardenability above to Ac ₁ point or more, it is rolled into a wire rod with a working degree of 10% or more in the temperature range of Ac ₁ point or Ar ₁ point or more, and then rolled into a wire rod in a heat retention furnace or a slow cooling furnace. A method for producing a wire rod with excellent cold workability, characterized by winding the wire rod with a wire rod. DI (mm) = 7.95√% × {1 + 0.64 [Si%]} × {1 + 4.10 [Mn%]} × {1 + 2.87 [P%]} × {1-0.69 [S%]} × { 1+2.33[Cr%]} ×{1+0.52[NI%]} ×{1+3.14[Mo%]} ×{1+0.27[Cu%]} ×{1+1.5(0.9−[C%] } DI (mm): Ideal critical diameter However, {1+1.5(0.9-[C%]} has a C content of 0.9%)
Applicable only to the following steels to which B is added.