JPH0570685B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for manufacturing rods and wires to be subjected to cold forging. [Conventional technology] Cold forging is often used as a method for forming mechanical structural parts from steel materials, but due to problems such as short tool life and easy processing cracks, spheroidizing annealing is used before forging. Processing is taking place. This heat treatment usually requires 10 hours or more, and shortening this time has been a long-standing challenge from the standpoint of productivity and energy conservation. There are two ways to shorten the spheroidizing annealing time: improving the steel material and improving the annealing method. Examples of improved steel materials include JP-A-60-152627 and JP-A-60-255922.
There are publications etc. After rolling the steel material, these materials are rapidly cooled to form a mixed structure of fine ferrite-pearlite and bainite or martensite, thereby increasing the rate of spheroidization of cementite. However, in this method, the cementite is excessively fine and the hardness is considerably higher than that in the conventional method, so that the intended purpose of spheroidizing annealing cannot be achieved. On the other hand, as an improvement of the annealing method,
Publication No. 15930 and Special Publication No. 61-15930. These are made of rods and wires in the form of strands and heated to a temperature 30°C or more higher than the Ac ₁ transformation point using a special heating device.
After rapid heating at 3000℃/h or higher and holding for a short time,
By performing slow cooling at a rate of 30°C/h or higher, the annealing time is shortened to less than 60 minutes. However, this method not only requires modification of equipment such as special heating equipment, but also tends to result in insufficient spheroidization, and also has the problem that variations in material properties are carried over after annealing, increasing variations in material properties of annealed materials. Ta. As described above, all of the conventional methods for shortening the spheroidizing annealing time have problems and have not been put to practical use. [Problems to be solved by the invention] When cold forging steel rods and wire rods for machine structural use, spheroidizing annealing is performed for 10 hours or more in order to spheroidize and soften the cementite, but this does not improve productivity or save energy. Therefore, shortening the annealing time has been a long-standing issue. The present invention is for cold forging, which combines increasing the ferrite fraction through controlled rolling, which had been done unrelatedly in the past, improving the material by making it finer, shortening the holding time, and improving the annealing method by slow cooling only in a specific temperature range. The present invention provides a method for manufacturing rods and wires. [Means for Solving the Problems] In order to solve the problems of the prior art, the present inventors conducted research on shortening the spheroidizing annealing time and obtained the following findings. (1) In the spheroidizing annealing process, the slow cooling process is the most important among the heating and holding to a temperature just above the Ac ₁ transformation point and slow cooling process, and especially the cooling rate in a specific range centered on the temperature at which cementite reprecipitates. If the process is accelerated, the spheroidization of cementite will be significantly worsened and the hardness will also increase. (2) However, if the ferrite-pearlite crystal grains of the material, which were conventionally considered to be almost unrelated, are refined, the degree of spheroidization can be improved even if the slow cooling rate during annealing is increased. (3) When such fine-grained steel is annealed, the heating rate to just above the Ac ₁ transformation point should be moderately fast, and
By appropriately shortening the holding time at the temperature just above the Ac ₁ transformation point, the degree of spheroidization is further improved. (4) After maintaining the Ac ₁ transformation point temperature or higher, by increasing the cooling rate to just above the A ₁ transformation point, cementite precipitates quickly in the subsequent slow cooling process that crosses the Ar ₁ point, and forms a spherical shape. It is possible to improve the degree of oxidation. (5) Furthermore, by applying the above-mentioned appropriate short-time annealing to a steel material that not only refines the crystal grains of the steel material through controlled rolling but also increases the ferrite microstructure ratio, it is possible to improve not only the degree of spheroidization but also the degree of spheroidization. A good annealed material with hardness comparable to that of the conventional spheroidizing annealing method using long-term holding and slow cooling can be obtained. Based on these findings, we came up with the present invention. That is, in the present invention, when hot rolling steel containing 0.1 to 0.6% C into a steel bar or wire rod, the final finish rolling is performed at a temperature range of 700 to 800°C, and ferrite and pearlite with a grain size of No. 8 or higher are mixed. After reheating the textured steel material to Ac ₁ +15±10℃ and holding it for 5 to 60 minutes, it is cooled at a rate of 10℃/h to less than 30℃/h in the range from 720℃ to 680℃ or less. This is a method for producing rods and wire rods for cold forging, which is characterized by a heating rate of 300°C/h or more when reheating to Ac ₁ +15±10°C, and a heating rate of 300°C/h or more from this temperature to 720°C. The gist of this is that the cooling rate is 30°C/h or more. [Function] The steel material components, rolling conditions, and rolling conditions limited in the present invention are as follows:
The reason for the annealing conditions will be explained. If C is less than 0.1%, sufficient strength as a mechanical structural component cannot be obtained, and the softening effect of spheroidizing annealing is also reduced. On the other hand, if it exceeds 0.6%, the hardness will be high even after spheroidizing annealing, making it unsuitable for cold working, so it was limited to a range of 0.1 to 0.6%. Elements other than C do not need to be particularly limited when applying the present invention, but are preferably within the following ranges. Although Si is effective for increasing product strength and hardenability, it has a large effect on increasing the hardness of annealed materials, and the upper limit should be 0.35%. The lower limit of Mn needs to be 0.15% for hardenability and prevention of hot embrittlement due to S, but the upper limit is 1.5% because it increases the hardness of the annealed material. Although S is effective in improving machinability, it increases inclusions that cause hot embrittlement and processing cracks, so the upper limit is set at 0.10%. Cr, Mo, and Ni are effective in increasing the hardenability, but since they increase the hardness of the annealed material, the upper limits are set to 1.2%, 0.5%, and 0.5%, respectively. Although a small amount of B is effective in increasing hardenability, it hardly increases the hardness of the annealed material. Therefore
It is an element useful for reducing hardness after spheroidizing annealing by replacing it with hardenability increasing elements such as Mn, Cr, and Mo. However, if it is added excessively, the product toughness will deteriorate, so to prevent this, the upper limit should be set at 0.003%. Next, we will discuss the rolling conditions and structure of the steel material. The reason why the upper limit of final finishing rolling was set at 800℃ is that by performing final rolling below this temperature, the austenite crystal grains are refined, the ferrite and pearlite that are transformed from these grains are refined, and the transformation temperature is lowered. By increasing the temperature,
This is because it is possible to increase the amount of ferrite and make it soft. Therefore, if the temperature exceeds 800°C, ferrite and pearlite will become coarse and such an effect cannot be expected. On the other hand, if the temperature is lower than 700℃, the rolling load will be significantly large, making it difficult to carry out finish rolling, and in order to keep the steel material temperature below 700℃ during finish rolling, cooling treatment such as water cooling must be performed during rolling. First, a lower transformed structure such as hard bainite occurs in the surface layer, increasing the hardness of the annealed material, so 700°C was set as the lower limit. It is desirable to add slow cooling to this controlled rolling at a cooling rate of 0.5° C./sec or less after rolling, since this increases the ferrite fraction of the material and further reduces the hardness of the annealed material. Note that in low-alloy steel containing large amounts of Cr, Mo, Ni, etc., the generation of bainite etc. can be greatly suppressed by lowering the rolling temperature, but slow cooling after rolling is required to create a structure consisting only of ferrite and pearlite. Required. The reason for limiting the grain size to No. 8 or higher is that when the spheroidizing annealing time is shortened as described below, the spheroidization of cementite is insufficient in conventional steel materials, but by setting the grain size to No. 8 or higher, Therefore, a good spheroidal structure can be obtained. Next, the annealing conditions will be described. The heating rate from room temperature to the holding temperature above the Ac ₁ transformation point has a relatively small effect on the annealing material, but for fine-grained steel with a grain size of No. 8 or higher, the degree of spheroidization can be reduced by heating at 300°C/h or higher. Becomes good. However, if the heating rate is too high, spheroidization will be insufficient.
It is best to keep it within 2500℃/h. Next, if the holding temperature is lower than Ac ₁ +15-10℃, undissolved cementite remains and the degree of spheroidization deteriorates, causing Ac ₁ +
On the other hand, if the temperature exceeds 15 + 10°C, most of the cementite will dissolve into solid solution, producing recycled pearlite and significantly deteriorating the degree of spheroidization, so the temperature was limited to the range of Ac ₁ +15 +/-10°C. The time to maintain this temperature was limited to 5 to 60 minutes because if it was less than 5 minutes, the solid solution of cementite would not be sufficient, and if it exceeded 60 minutes, it would be excessively dissolved, but in order to shorten the annealing time. , preferably within 5 to 30 minutes. Slow cooling in the 720 to 680℃ range is most important for cementite spheroidization.If the slow cooling starts below 720℃ or slow cooling ends above 680℃, the precipitation of spheroidal cementite will be hindered and the spheroidization will deteriorate significantly. , the slow cooling section was limited to the section from 720°C to a temperature below 680°C. The slow cooling rate during this time was set to be less than 30°C/h because if it was 30°C/h or more, recycled pearlite would occur, the degree of spheroidization would deteriorate, and the hardness would increase. Further, the slower the annealing rate, the better; however, if it is less than 10°C/h, even if it is slower, the effect will be small and the annealing time will be unnecessarily extended, so the lower limit was set at 10°C/h. When the hot-rolled material of the present invention is used, by setting the cooling rate from the holding temperature to the slow cooling start temperature to 30°C/h or more, it is possible to improve the degree of spheroidization without increasing hardness. . [Example] Examples according to the present invention will be specifically described below in comparison with comparative examples according to the conventional method. Table 1 shows the chemical composition of the sample materials. All of the test materials met the mechanical structural molten carbon steel and low alloy steel composition standards specified by JIS. All of these were cast by continuous casting after melting in a converter furnace. After blooming into a 120 mm square steel billet, it was rolled into a 32 mm round steel bar under the rolling conditions shown in Table 2. After rolling, one was left to cool on a cooling bed (steel cooling rate 0.8℃/sec), and the other was covered with a heat insulation cover immediately after rolling and slowly cooled (steel cooling rate 0.3℃/sec).
sec) was manufactured. Table 2 also shows the grain size, structure, and ferrite structure fraction of this rolled material based on JIS G0552. Furthermore, the Ac ₁ transformation temperature of each steel material, spheroidizing annealing conditions, and material evaluation results of the spheroidizing annealing material are also shown. Evaluation of spheroidized annealed materials is based on hardness and JIS G3539
Two points of the degree of spheroidization defined by . The target material quality achieved with spheroidizing annealing is hardness (Hv of 105
×(%C+%Si/3+%Mn/6+%Cr/19)+
72.6 (points) or less, degree of spheroidization is No. 2 or less 2
Both conditions must be satisfied. No. 2, 4, 6, with a circle in the level number in Table 2
Nos. 11, 15, 17, 19, and 21 are examples of the present invention, and compared with examples using conventional release (long-term retention + long-time slow cooling) shown in level Nos. 1, 3, 16, 18, and 20. Even if the total annealing time (t) was reduced to 1/2 to 1/3 of the conventional one, the hardness and degree of spheroidization were equal to or higher than those of conventional annealing, and both of the softening targets mentioned earlier were achieved. I can see that you are satisfied. The nine examples shown in level No. 5, 7-11, 12-14, and 22 are:
All of these are examples in which the softening target could not be achieved. No. 5 is an example in which the material was annealed for a short time without performing controlled rolling during rolling, and the cementite spheroidization was poor and the hardness was somewhat high. No. 7 is an example in which the holding temperature (T _H ) was set below the lower limit of the range of the present invention, and the degree of spheroidization was poor due to insufficient solid solution of cementite, and the hardness was also extremely high. On the contrary, No. 8 is an example in which the value was set above the upper limit, and cementite was dissolved excessively to form recycled pearlite, resulting in poor spheroidization and slightly high hardness. No. 9 has a retention time (t _H ) shorter than the range of the present invention.
Similar to No. 7 above, the degree of spheroidization is poor due to insufficient solid solution of cementite, and the hardness is also somewhat high. On the other hand, No. 10 is an example where the retention time (t _H ) was too long.
Similarly to No. 8, cementite is dissolved in solid solution excessively, and the degree of spheroidization is poor. In No. 12, the slow cooling start temperature (T ₁ ) is lower than the lower limit of the range of the present invention, pearlite is precipitated without spheroidal cementite precipitated, and both hardness and degree of spheroidization are extremely poor. No. 13 is an example in which the cooling rate (V ₂ ) during slow cooling was faster than the upper limit of the range of the present invention, and in this case, most of the cementite did not become spheroidized but precipitated as pearlite, resulting in hardness and spheroidization. It is always defective. No. 14 is an example in which the slow cooling end temperature (T ₂ ) was set above the upper limit of the range of the present invention, and because the slow cooling was finished before the precipitation of spherical cementite was completely completed, some of the parts became pearlite and hardened. Both the shape and the degree of spheroidization are high. In addition, No. 22 is an example in which low alloy steel was not slowly cooled after rolling, and because bainite was generated in the material structure, the degree of spheroidization was good after short-time annealing, but the hardness was extremely high. In addition, No. 6 is an example in which the heating rate (V _H ) is slow as in the conventional method, and although it satisfies the targets for both hardness and spheroidization, the total annealing time (t) is longer and the hardness is lower than No. 4. - Spheroidization degree is also somewhat poor. No. 11 is an example where the cooling speed (V ₁ ) until the slow cooling starts is slow.
Although this also satisfies the target, compared to No. 4, the hardness and degree of spheroidization are not improved even though the total annealing time (t) is long. No.15 is slowly cooled immediately after rolling (cooling rate 0.3
℃/sec), and the hardness was further reduced after annealing compared to No. 4, which was not slowly cooled.

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Claims (1)

[Claims] 1. When hot rolling steel containing 0.1 to 0.6% C into steel bars or wire rods, the final finishing rolling is carried out at 700 to 800°C.
Ac ₁ +
Reheat to 15±10℃ and hold for 5-60 minutes, then 720℃
From 10℃/h to a temperature of 680℃ or less
A method for producing a rod and wire rod for cold forging, characterized by cooling at a rate of less than 30°C/h. 2 Ac ₁ Heating rate up to +15±10℃ at 300℃/h
The method for manufacturing a rod and wire rod for cold forging according to claim 1, wherein the cooling rate from this temperature to 720°C is 30°C/h or more.