JP2015172234A - Slow cooling method for steel material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a slow cooling method for steel materials which allows carrying out a spheroidization treatment of steel in a short time.SOLUTION: A slow cooling method for steel materials includes a first cooling step of cooling from a uniformizing temperature to a transformation initiation temperature at a first cooling rate (20-60°C/h), a second cooling step of cooling from the transformation initiation temperature to a transformation completion temperature at a second cooling rate (10-30°C/h) and a third cooling step of cooling from the transformation completion temperature at a pre-set radiational cooling initiation temperature at a third cooling rate, and the slow cooling period is taken as a heat pattern comprising the three cooling steps. When the previous step of a heat treatment of the spheroidization of a steel material is a wire drawing step, the third cooling rate, e.g. 25°C/h, is adopted, and, when the previous step is a rolling treatment step, the third cooling rate, e.g. 5-20°C/h, is adopted. For the cooling rates, the second cooling rate is equal to or lower than the first cooling rate, and the third cooling rate is equal to or higher than the second cooling rate for wire-drawing steel materials and equal to or lower than the second cooling rate for rolling-treatment steel materials.

Description

  The present invention relates to a spheroidizing annealing method capable of performing spheroidizing annealing of steel materials such as steel wires and steel bars in a short time.

  Conventionally, as part of the heat treatment of carbon steel or chrome steel, spheroidizing annealing treatment has been performed in order to spheroidize carbides present in carbon steel or chrome steel. If the spheroidizing annealing treatment is performed, the toughness of the steel is increased and the workability in the subsequent process is also improved.

The spheroidizing annealing is a heat treatment in which the austenite structure is transformed into a ferrite + cementite structure in the process of gradually cooling the steel in the austenite structure heated to the transformation point or higher.
In the prior art spheroidizing annealing treatment, generally slow cooling takes about 6 hours for carbon steel, about 8 hours for chrome steel (SCR-4), and about 11 hours for bearing steel (SUJ-2). It took a very long time.

FIG. 8 is a diagram showing a heat pattern of a slow cooling process of a spheroidizing annealing process of carbon steel in the prior art.
In carbon steel, the transformation point which is the transformation start temperature is 720 ° C. The transformation completion temperature is believed to be between 700 ° C. and 710 ° C. with some variation. Generally, after soaking at 740 ° C higher than the transformation point, slow cooling is started, and the slow cooling is progressed at a rate of 10 ° C per hour toward a uniform 680 ° C that is sufficiently lower than the transformation completion temperature. The way to go is taken. In the case of carbon steel, since the temperature difference is 60 ° C., it is gradually cooled over 6 hours. In this slow cooling over 6 hours, the transformation complete temperature is passed from the transformation point of the transformation start temperature, and spheroidization is performed.

  FIG. 9 is a diagram showing a heat pattern of a slow cooling process of a spheroidizing annealing process of chrome steel (SCR-4) in the prior art. In chrome steel (SCR-4), the transformation point of the transformation start temperature is 740 ° C. The transformation completion temperature is considered to be around 720 ° C. with some variation. In general, after heat treatment to 760 ° C., which is higher than the transformation point, and soaking, slow cooling is started. The slow cooling is uniformly 10 ° C. per hour toward 680 ° C. as in FIG. Continue slow cooling at the rate of. In the case of SCR-4 steel, since the temperature difference is 80 ° C., it is gradually cooled over 8 hours. In this slow cooling over 8 hours, the transformation completion temperature is passed from the transformation start point of the transformation start temperature, and spheroidization is performed.

  FIG. 10 is a diagram showing a heat pattern of a slow cooling process in a spheroidizing annealing process of chrome steel (SUJ-2) in the prior art. In chrome steel (SUJ-2), the transformation point of the transformation start temperature is 750 ° C. The transformation completion temperature is considered to be around 720 ° C. with some variation. In general, after heat treatment to 795 ° C., which is higher than the transformation point, and soaking, slow cooling is started. The slow cooling is uniformly 10 ° C. per hour toward 680 ° C. as in FIG. Continue slow cooling at the rate of. In the case of SUJ-2 steel, since the temperature difference is 115 ° C., it is gradually cooled over 11.5 hours. In this slow cooling over 11.5 hours, the transformation complete temperature is passed from the transformation point of the transformation start temperature, and spheroidization is performed.

Regarding heat treatment of steel, there are known literatures describing various heat patterns, but for spheroidizing annealing treatment, the temperature is uniformly directed from 680 ° C to 680 ° C and uniformly at a rate of 10 ° C per hour. This has been done by gradually cooling.
The rate of slow cooling is generally at a rate of 10 ° C. per hour, but as shown in FIG. 11, some have disclosed that it may be performed uniformly at a rate of 15 ° C. per hour. is there.

JP 2008-121032 A

  However, the slow cooling process of the conventional spheroidizing annealing process requires a very long time. Originally, in steel mills, etc., heat treatment is performed on large-capacity steel, and its thermal inertia is extremely large. In addition, it takes a very long time to heat up and cool down in order to provide a heat history of heating and cooling. . For this reason, productivity is very low and the heat processing cost becomes high.

For example, in the case of carbon steel, it takes 8 hours for temperature rise, 8 hours for soaking, 6 hours for slow cooling, and a total of 22 hours. In the case of bearing steel (SUJ-2), in general, it takes 8 hours for temperature increase, 8 hours for soaking, and 11.5 hours for slow cooling, which totals 26.5 hours.
Such slow cooling over 6 to 11.5 hours also prolongs the manufacturing time. Therefore, if the slow cooling time can be shortened, productivity is improved and heat treatment costs are reduced.

Inventor Takaya Nagaya has come up with a method capable of shortening the slow cooling process in the spheroidizing annealing process of steel while engaging in steel-related work for many years and conducting research on heat treatment.
In view of the above problems, the present invention provides an innovative steel annealing method that can process a spheroidizing annealing process of steel, which has conventionally required a processing time of about 10 hours, in a short time. With the goal.

  In order to achieve the above object, the steel material annealing method of the present invention is a method of gradually cooling a steel material while maintaining a soaking temperature for a predetermined time after raising the temperature to a predetermined heating temperature equal to or higher than the transformation start temperature. A first cooling step of cooling within the first cooling rate range from the soaking temperature to the transformation start temperature, and a second from the transformation start temperature to the transformation completion temperature. The wire is drawn from the transformation completion temperature to the cooling start temperature on the condition that the second cooling step of cooling within the cooling rate range of the steel material and the pre-process of the heat treatment of the spheroidizing annealing in the steel material is a wire drawing treatment step. A third cooling step for cooling within the third cooling rate range for the treated steel material, and starting to cool from the transformation completion temperature on the condition that the pre-treatment of the heat treatment of the spheroidizing annealing in the steel material is a rolling treatment step Rolled steel up to temperature A third third cooling step slow cooling treatment method of a steel material, characterized that it has a heat pattern having the cooling in the cooling rate range of use.

  With the above configuration, three cooling steps are set according to important changes in the slow cooling treatment of the steel material, and the first cooling step from the soaking temperature to the transformation start temperature, from the transformation start temperature to the transformation completion temperature. The second cooling step, the third cooling step from the transformation completion temperature to the cooling start temperature, and by gradually cooling at each cooling rate in each cooling step, while maintaining the quality of the steel material to be produced at a high quality Shorten the production time of steel. Further, the third cooling process is classified by condition, and if it is a wire-drawn steel material, the spheroidization rate is relatively high at the stage where the second cooling step is finished, so the third cooling for the wire-drawn steel material. The process may be at a relatively high speed, and if it is a rolled steel, the third cooling process for the rolled steel is relatively slow because the spheroidization rate is relatively low when the second cooling process is completed. It controls to do.

Inventor Takaya Nagaya paid attention to the fact that two important changes occur in steel during the slow cooling of steel. One is spheroidizing annealing that occurs from the transformation start temperature to the transformation completion temperature. The other is a change in hardness determined from the transformation completion temperature to the cooling start temperature.
In general, in order to perform high-quality spheroidizing annealing and hardness change control, it is necessary to slowly cool slowly. Therefore, in a conventional heat pattern, all steel materials descend at a low speed, for example, soaking at 10 ° C / h. Cooling was performed at a constant low speed from the crystallization temperature.

  However, the inventor Takaya Nagaya does not need to rely on slow cooling at a low speed in order to perform high-quality spheroidizing annealing treatment and hardness change control, and the process preceding the spheroidizing annealing treatment. It was found that the spheroidizing annealing process and the hardness change control with good quality can be realized even if the process is classified according to the above conditions, and the process is gradually cooled at a speed higher than the conventional slow cooling rate. The process before the heat treatment of the spheroidizing annealing of the steel material is classified according to whether it is a steel material that has been drawn or a steel material that has been rolled.

First, a case where the preprocessing is a wire drawing process will be described.
In the case of wire-drawn steel, the first cooling rate in the first cooling step from the soaking temperature to the transformation start temperature does not affect the quality regardless of whether the first cooling rate is high or low. The speed range is selected from the range of 20 ° C./h or more to the forced cooling temperature by the radiant tube, and the cooling speed may be the fastest in the three cooling steps in order to shorten the entire processing time. Preferably, it is between 20 ° C./h and 60 ° C./h, and as an example, the first cooling rate is 50 ° C./h.

  Next, in the case of a wire-drawn steel material, the influence on the spheroidization rate of the second cooling rate in the second cooling step from the transformation start temperature to the transformation completion temperature is small, whether it is high or low, When the pretreatment is a wire drawing treatment, the spheroidizing annealing rate is relatively high. Therefore, the second cooling rate range is 10 ° C./h or more to the forced cooling temperature or less by the radiant tube, and the cooling rate may be relatively high in order to shorten the entire processing time. However, the second cooling rate is controlled so as to be slower than the first cooling rate. The temperature is preferably between 10 ° C. and 30 ° C., and as an example, the second cooling rate is 20 ° C./h.

  Next, in the case of the wire-drawn steel material, since the spheroidizing rate in the second cooling period is high and the quality is high, the hardness change of the third cooling rate in the third cooling period from the transformation completion temperature to the cooling start temperature. The influence on is not significant and may be a relatively large speed. The third cooling rate is controlled to be larger than the second cooling rate. Preferably, the third cooling rate selected is in the range of 10 ° C./h or more and 30 ° C./h or less, and the selected third cooling rate is faster than the selected second cooling rate. As an example, the third cooling rate is 25 ° C./h.

Next, a case where the pretreatment is a rolling treatment will be described.
In the case of rolled steel, the first cooling rate in the first cooling step from the soaking temperature to the transformation start temperature affects the quality whether it is high or low, as in the case of the wire-drawn steel. Therefore, the first cooling rate range is selected from the range of 20 ° C / h or more to the forced cooling temperature by the radiant tube, and the cooling rate is the highest in the three cooling processes in order to shorten the overall processing time. It may be fast. Preferably, it is between 20 ° C./h and 60 ° C./h, and as an example, the first cooling rate is 50 ° C./h.

  Next, in the case of rolled steel, although the influence on the spheroidization rate of the second cooling rate in the second cooling step from the transformation start temperature to the transformation completion temperature is small, even at high speed or low speed, When the pretreatment is rolled steel, the spheroidization rate is relatively low. Therefore, the second cooling rate range is 10 ° C./h or more to the forced cooling temperature or less by the radiant tube, and the cooling rate may be relatively high in order to shorten the entire processing time. However, the second cooling rate is controlled so as to be slower than the first cooling rate. The temperature is preferably between 10 ° C. and 30 ° C., and as an example, the second cooling rate is 20 ° C./h.

  Next, in the case of rolled steel, since the spheroidization rate in the second cooling step is low and the quality is insufficient, the third cooling rate in the third cooling step from the transformation completion temperature to the cooling start temperature is The effect on hardness change is large. Therefore, in order to increase the spheroidization rate and control the hardness change to be favorable, the third cooling rate is set to be low as in the prior art. As an example, the third cooling rate is 10 ° C./h in the range of 5 ° C. to 20 ° C., preferably between 5 ° C./h and 15 ° C./h.

In this way, the pre-process of the heat treatment of the spheroidizing annealing of the steel material is classified according to whether it is a steel material that has been drawn or rolled, and each of the treatments is cooled by a separate heat pattern, thereby satisfying the standard. It has been discovered that excellent steel materials can be produced that are spheroidized with a spheroidizing rate and are sticky so that the hardness of the steel material is within the specified range.
As described above, it is possible to achieve the quality of spheroidizing annealing, achieve the quality of good hardness, and shorten the overall slow cooling time.

  In the state of the metal structure of steel, the transformation start temperature is specified relatively accurately, and the transformation start temperature is rapidly cooled to the transformation start temperature at the first cooling rate of the first cooling step having a large cooling rate. On the other hand, the transformation completion temperature has a relatively wide range compared to the transformation start temperature, and it is better to treat with a certain range of temperature depending on the steel part and physical environmental fluctuation. Therefore, the transformation completion temperature is an estimated temperature that is estimated to complete the transformation.

  Since the temperature range of the first cooling rate and the temperature range of the second cooling rate have a certain range, it is possible to make the first cooling rate and the second cooling rate the same. is there. In this case, temperature control is simplified.

  In addition, as steel, it can apply widely, without being specifically limited. For example, the present invention can also be applied to coiled carbon steel, SCR-4 chrome steel, SUJ-2 chrome steel, etc. for cold forging wire.

  According to the steel material slow cooling method according to the present invention, it is possible to achieve the quality of spheroidizing annealing, achieve the quality of good hardness, and shorten the overall slow cooling time.

  Hereinafter, a processing example of the steel material slow cooling method according to the present invention will be described with reference to the drawings. The present invention is not limited to these examples.

[For carbon steel]
As Example 1, a cooling process in the case of carbon steel will be described.
The cooling process in the case of SRC-4 steel will be described in Example 2, and the cooling process in the case of SUJ-2 steel will be described in Example 3.

FIG. 1 is a flowchart showing a step of slow cooling treatment of a steel material of the present invention.
FIG. 2 is a view showing a heat pattern when applying the steel material annealing method of the present invention to carbon steel.
The flowchart of FIG. 1 is drawn including the steps before and after the slow cooling process. Steps S103 to S106 are the slow cooling process of the present invention.
As shown in FIG. 1, prior to the slow cooling treatment process of the steel of the present invention, a pretreatment process (step S101) and a soaking process (step S102) are performed on the steel material.
The steel slow cooling process of the present invention includes three cooling processes, a first cooling process (step S103), a second cooling process (step S104), and a third cooling process (step S106).

(1) Pretreatment process for steel (step S101)
The pretreatment process for the steel material is a machining process.
Steel is formed from steel ingots into steel materials such as steel wires and steel bars, and there are rolling processing and wire drawing processing.
The rolling process is a process of extending the steel ingot into a rod shape, and is stretched by applying a large pressure by being pushed between two adjacent rolls.
The wire drawing process is a process in which the steel ingot is stretched into a rod shape, and a rod or wire is formed in such a way that the crystal is oriented in the wire drawing direction in order to push out the member with an inner diameter called a die that is gradually reduced in diameter with a large pressure. It is processed into a shape. The steel material is formed into a desired shape by this pretreatment process.
In addition, as will be described later, in the present invention, in step S103, the method of proceeding the slow cooling process differs depending on whether the pretreatment process is a wire drawing process or a rolling process.
Here, the pretreatment process is a wire drawing process.

(2) Soaking process (step S102)
In order to equalize the heat-treated product processed in the pretreatment process to the spheroidizing treatment, eliminating the temperature difference between the inside and outside from the surface to the inside of the heat-treated product, put the heat-treated product in the furnace as needed Hold for a certain time to reach a predetermined temperature.
The temperature of the soaking process depends on the type of steel material and the manufacturing process, but an example is given.
In the case of carbon steel, the transformation start temperature is known and is 720 ° C. The soaking temperature is set to + 20 ° C. from the transformation start temperature, and the soaking temperature in the soaking process is 740 ° C.

(3) First cooling step (step S103)
The first cooling step is a cooling process in the first cooling step in which cooling is performed from the soaking temperature to the transformation start temperature at the first cooling rate.
In the first cooling step, as shown in the heat pattern of FIG. 2, cooling is performed at a first cooling rate from a soaking temperature of 740 ° C. to a transformation start temperature of 720 ° C.
Here, the range of the first cooling rate is selected from the range of 20 ° C./h or more to the forced cooling temperature by the radiant tube. Preferably, the first cooling rate is selected from the range of 30 ° C./h to 60 ° C./h. Here, as an example, the cooling rate is lowered at a rate of 50 ° C./h. Thus, since it cools at the cooling rate of 50 degreeC per hour, it has contributed to the time shortening of the whole slow cooling process. This may be the same whether it is a drawn steel or a rolled steel, or may be different.
In this example, it takes 24 minutes for the first cooling step.
Note that since the spheroidizing annealing itself has not started from the soaking temperature of 740 ° C. to the transformation start temperature of 720 ° C., the quality is not particularly affected.

(4) Second cooling step (step S104)
The second cooling step is a cooling process in the second cooling period in which cooling is performed at the second cooling rate from the transformation start temperature to the transformation completion temperature.
In the case of carbon steel, the transformation completion temperature is relatively wide compared to the transformation start temperature, and it is better to treat with a certain range of temperature depending on the steel part and physical environmental fluctuations. It is considered that there is a transformation completion temperature at a temperature slightly higher than the starting temperature of −20 ° C. of 720 ° C., for example, around 705 ° C.
In this way, in the second cooling step, as shown in FIG. 2, the cooling is performed from the transformation start temperature of 720 ° C. to the transformation completion temperature of 705 ° C. Spheroidizing annealing is performed during the second cooling step.

  Here, the range of the second cooling rate in the second cooling step is a range from 10 ° C./h or more to the forced cooling temperature by the radiant tube and the first cooling rate or less. Preferably, it selects from the range of 10 degreeC / h to 30 degreeC / h. Here, it is set as the cooling rate which descend | falls at a speed | rate of 20 degrees C / h as an example. This may be the same whether it is a drawn steel or a rolled steel, or may be different.

Inventor Takaya Nagaya, if the pretreatment process of spheroidizing annealing is a wire drawing process, the quality of spheroidizing annealing even when cooled at a fast rate from the transformation start temperature to the transformation completion temperature where spheroidizing annealing is performed I found that did not fall. In other words, it is not only the cooling rate that affects the quality of spheroidizing annealing, but also whether the pretreatment process is a wire drawing process or a rolling process. Even found that the quality of spheroidization is high. In addition, if the pretreatment step of spheroidizing annealing is a rolling treatment step, the spheroidizing is carried out from the transformation start temperature where the spheroidizing annealing is performed to the transformation completion temperature, even if it is cooled at a fast rate or carefully at a slow rate. It was discovered that the quality of annealing did not improve and reached its peak, and it did not become as high as the wire-drawn steel. In other words, it is not only the cooling rate that affects the quality of spheroidizing annealing, but also whether the pretreatment process is a wire drawing process or a rolling process. Even found that the quality of spheronization is not so high.
Here, since the pretreatment step of spheroidizing annealing is a wire drawing treatment step, the cooling rate is set at a rate of 20 ° C./h.
In this example, the time required for the third cooling step is 45 minutes.

(5) Condition satisfaction check (step S105)
The treatment process is divided depending on whether the pretreatment process for the spheroidizing annealing in the steel material is a wire drawing process or a compression treatment process. The condition is, for example, whether or not the pretreatment process is a wire drawing process. If the pretreatment process is a wire drawing process (step 105: Yes), the process proceeds to step S106-1, and if the pretreatment process is a rolling process (step 105: No), the process proceeds to step S106-2.

(6) Third cooling step (step S106)
The third cooling step is a process of cooling from the transformation completion temperature of 705 ° C. to the cooling start temperature of 680 ° C. at the third cooling rate.
If the pretreatment process of the heat treatment for spheroidizing annealing is a wire drawing process, the quality of the spheroidizing annealing is high, and therefore the hardness tends to be low even if the third cooling process is cooled at a relatively fast rate (hardness is low). If it is low, it becomes sticky and good quality steel). The third cooling rate is assumed to be faster than the second cooling rate, and is selected from the range of 10 ° C./h to 30 ° C./h, for example. Here, as an example, the cooling rate is lowered at a rate of 25 ° C./h (solid line in FIG. 2).
In this example, the time required for the third cooling step for the wire-drawn steel material is 60 minutes.

On the other hand, if the pretreatment step of the heat treatment for spheroidizing annealing is a rolling treatment step, the quality of spheroidizing annealing is not high, so consider that the hardness is not increased by cooling at a relatively slow rate in the third cooling step. To do. The third cooling rate is assumed to be slower than the second cooling rate, and is selected from the range of 5 ° C./h to 20 ° C./h, for example. Here, as an example, it is assumed that the cooling rate drops at a rate of 10 ° C./h (dashed line in FIG. 2).
In this example, the time required for the third cooling step for the rolled steel material is 150 minutes.

With the heat pattern shown in FIG. 2, in the steel material slow cooling method of the present invention, the total processing time is calculated as 2 hours 9 minutes for the wire-drawn steel material and 3 hours 39 minutes for the rolled steel material.
On the other hand, since it takes 6 hours for the conventional spheroidizing annealing method, it can be seen that the annealing time of the steel material of the present invention can effectively shorten the processing time.
In addition, after a 3rd cooling process, a natural cooling process is performed (step S107).
In addition, the comparative example of the trial manufacture example of steel materials is described in the latter part of Example 3 by making SUJ-2 steel into an example.

[In the case of SCR-4 chrome steel]
FIG. 3 is a diagram showing a heat pattern when applying the steel material slow cooling method of the present invention to SCR-4 chrome steel.

(1) Pretreatment process for steel (step S101)
Regarding the pretreatment process for the SCR-4 chrome steel material, as with the carbon steel of Example 1, there are a rolling process and a wire drawing process.

(2) Soaking process (step S102)
In the case of SCR-4 chrome steel, the transformation start temperature is known and is 740 ° C. The soaking temperature is set to + 20 ° C. from the transformation start temperature, and the soaking temperature in the soaking process is 760 ° C. (heat pattern in FIG. 3).

(3) First cooling step (step S103)
In the first cooling step, as shown in FIG. 3, cooling is performed from a soaking temperature of 760 ° C. to a transformation start temperature of 740 ° C. In this example, the cooling rate is 50 ° C. per hour.
In this example, the time required for the first cooling step is 24 minutes.
Note that since the spheroidizing annealing itself has not started from the soaking temperature of 760 ° C. to the transformation start temperature of 740 ° C., the quality is not particularly affected.

(4) Second cooling step (step S104)
Similarly, in the case of SCR-4 chrome steel, the transformation completion temperature is relatively wide compared to the transformation start temperature, and it is better to treat with a certain range of temperature depending on the steel part and physical environmental fluctuation. However, here, it is considered that the transformation completion temperature is around −20 ° C. of the transformation start temperature of 740 ° C., for example, around 720 ° C.

In the second cooling step, as shown in FIG. 3, cooling is performed from a transformation start temperature of 740 ° C. to a transformation completion temperature of 720 ° C.
In the second cooling step, the cooling rate drops at a rate of 20 ° C./h.
In this example, the time required for the second cooling step is 60 minutes.

(5) Condition satisfaction check (step S105)
The treatment process is divided depending on whether the pretreatment process for the spheroidizing annealing in the steel material is a wire drawing process or a compression treatment process. The condition is, for example, whether or not the pretreatment process is a wire drawing process. If the pretreatment process is a wire drawing process (step 105: Yes), the process proceeds to step S106-1, and if the pretreatment process is a rolling process (step 105: No), the process proceeds to step S106-2.

(6) Third cooling step (step S106)
The third cooling step is a process of cooling from the transformation completion temperature of 720 ° C. to the cooling start temperature of 680 ° C. at the third cooling rate.
If the pretreatment process of the heat treatment for spheroidizing annealing is a wire drawing process, the quality of the spheroidizing annealing is high, so the third cooling rate is faster than the second cooling rate, for example from 10 ° C./h Select from the range of 30 ° C / h. Here, as an example, the cooling rate is lowered at a rate of 25 ° C./h (solid line in FIG. 3).
In this example, the time required for the third cooling step for the wire-drawn steel material is 100 minutes.

If the pretreatment step of the heat treatment for spheroidizing annealing is a rolling treatment step, since the quality of spheroidizing annealing is not high, care is taken so that the third cooling step is cooled at a relatively slow rate and the hardness is not increased. The third cooling rate is assumed to be slower than the second cooling rate, and is selected from the range of 5 ° C./h to 20 ° C./h, for example. Here, as an example, it is assumed that the cooling rate descends at a rate of 10 ° C./h (dashed line in FIG. 3).
In this example, the time required for the third cooling step for the rolled steel material is 240 minutes.

With the heat pattern shown in FIG. 3, in the steel material slow cooling method of the present invention, the total processing time is calculated as 3 hours and 4 minutes for the wire-drawn steel material and 5 hours and 24 minutes for the rolled steel material.
On the other hand, since it takes 8 hours for the conventional spheroidizing annealing method, it can be seen that the annealing time of the steel material of the present invention can effectively shorten the processing time.
In addition, after a 3rd cooling process, a natural cooling process is performed (step S107).
In addition, the comparative example of the trial manufacture example of steel materials is described in the latter part of Example 3 by making SUJ-2 steel into an example.

[For SUJ-2 steel]
FIG. 4 is a diagram showing a heat pattern when applying the steel material annealing method of the present invention to SUJ-2 chrome steel.

(1) Pretreatment process for steel (step S101)
Regarding the pretreatment process for the SUJ-2 chrome steel material, there are a rolling treatment process and a wire drawing treatment process as in the carbon steel of Example 1.

(2) Soaking process (step S102)
In the case of SUJ-2 chrome steel, the transformation start temperature is known and is 750 ° C. The soaking temperature is set to + 45 ° C. from the transformation start temperature, and the soaking temperature in the soaking process is about 795 ° C. (heat pattern in FIG. 4).

(3) First cooling step (step S103)
In the first cooling step, as shown in FIG. 4, cooling is performed from a soaking temperature of 795 ° C. to a transformation start temperature of 750 ° C. In this example, the cooling rate is 50 ° C. per hour.
In this example, the time required for the first cooling step is 54 minutes.
Note that since the spheroidizing annealing itself has not started from the soaking temperature of 795 ° C. to the transformation start temperature of 750 ° C., the quality is not particularly affected.

(4) Second cooling step (step S104)
Similarly, in the case of SUJ-2 chrome steel, the transformation completion temperature is relatively wide compared to the transformation start temperature, and it is better to treat the steel with a certain range of temperature depending on the steel part and physical environmental fluctuation. However, here, it is considered that there is a transformation completion temperature at a temperature around −30 ° C. of 750 ° C. of the transformation start temperature, for example, around 720 ° C.
In the second cooling step, as shown in FIG. 4, cooling is performed from a transformation start temperature of 750 ° C. to a transformation completion temperature of 720 ° C.
The second cooling step is a cooling rate that descends at a rate of 20 ° C./h.
In this example, the time required for the second cooling step is 90 minutes.

(5) Condition satisfaction check (step S105)
The treatment process is divided depending on whether the pretreatment process for the spheroidizing annealing in the steel material is a wire drawing process or a compression treatment process. The condition is, for example, whether or not the pretreatment process is a wire drawing process. If the pretreatment process is a wire drawing process (step 105: Yes), the process proceeds to step S106-1, and if the pretreatment process is a rolling process (step 105: No), the process proceeds to step S106-2.

(6) Third cooling step (step S106)
The third cooling step is a process of cooling from the transformation completion temperature of 720 ° C. to the cooling start temperature of 680 ° C. at the third cooling rate.
If the pretreatment process of the heat treatment for spheroidizing annealing is a wire drawing process, the quality of the spheroidizing annealing is high, so the third cooling rate is faster than the second cooling rate, for example, 10 ° C./h± Select from the range of 30 ° C / h. Here, as an example, the cooling rate is lowered at a rate of 25 ° C./h (solid line in FIG. 4).
In this example, the time required for the third cooling step for the wire-drawn steel material is 100 minutes.

If the pretreatment step of the heat treatment for spheroidizing annealing is a rolling treatment step, the quality of spheroidizing annealing is not high, so the third cooling step is cooled at a relatively slow rate so that the quality of hardness is increased. To do. The third cooling rate is slower than the second cooling rate, and is selected from the range of 5 ° C./h to 15 ° C./h, for example. Here, as an example, it is assumed that the cooling rate descends at a rate of 10 ° C./h (dashed line in FIG. 4).
In this example, the time required for the third cooling step for the rolled steel material is 240 minutes.

According to the heat pattern shown in FIG. 4, in the steel material slow cooling method of the present invention, the total processing time is calculated as 4 hours and 4 minutes for the wire-drawn steel material and 6 hours and 24 minutes for the rolled steel material.
On the other hand, since it takes 11.5 hours for the conventional spheroidizing annealing method, it can be seen that the annealing time of the steel material of the present invention can effectively shorten the processing time.
In addition, after a 3rd cooling process, a natural cooling process is performed (step S107).

Next, a comparative example of a steel material prototype is shown.
Next, a prototype example of a wire-drawn steel material prototyped by the slow cooling treatment method of the present invention, a prototype example of a conventional wire-drawn steel material slowly cooled over the time of the conventional method, and a rolled steel material while being a rolled steel material The trial manufacture example of the rolling process steel material made as a trial by the slow cooling method for the wire drawing steel materials of invention is compared and shown.

  FIG. 5 is an enlarged photograph of a cross-section of a steel material of a prototype of a wire-drawn steel material that was experimentally produced by the slow cooling treatment method of the present invention. FIG. 5 shows that the quality of spheroidization is sufficiently ensured in the cross section of the steel material. The hardness was 85.8 in HRB.

  FIG. 6 is an enlarged photograph of a cross section of a steel material of a prototype of a conventional wire-drawn steel material generated at a cooling rate of 10 ° C. per hour from 795 ° C. to 680 as a conventional heat pattern. FIG. 6 shows that the quality of spheroidization is sufficiently ensured in the cross section of the steel material. The hardness was 86.8 in HRB.

  From the comparison between FIG. 5 and FIG. 6, it can be seen that the wire-drawn steel produced by the slow cooling treatment method of the present invention is finely spheroidized. Furthermore, when the hardness of both is compared, the hardness of the wire-drawn steel produced by the slow cooling treatment method of the present invention is 85.8 in HRB, which is equivalent to the hardness 86.8 of the conventional wire-drawn steel. It can be seen that the hardness is low and the hardness quality is good.

  Next, FIG. 7 is an enlarged photograph of a cross-section of a steel material of a trial example of a rolled steel material that is a rolled steel material, which was experimentally manufactured by the annealing method for the wire-drawn steel material of the present invention. When FIG. 7 is seen, the part where spheroidization is sparse in the cross section of steel materials is seen, and it turns out that quality is not fully ensured. The hardness was 90.4 in HRB.

  As can be seen by comparing FIG. 5, FIG. 6 and FIG. 7, the quality of the spheroidizing annealing is not good if the third cooling step is made as fast as the wire-drawn steel material while being a steel material that has been rolled. It can also be seen that the hardness is HRB 90.4 and the quality deteriorates.

  As mentioned above, according to the slow cooling treatment method of the steel material of the present invention, compared with the conventional spheroidizing annealing method, the processing time of the spheroidizing annealing treatment can be shortened without reducing the quality of the spheroidizing annealing. .

  As mentioned above, although preferred embodiment in the structural example of the slow cooling processing method of the steel material of this invention was illustrated and demonstrated, it is understood that various changes are possible without deviating from the technical scope of this invention. Will.

  The steel material slow cooling method of the present invention can be used for spheroidizing annealing of various steels, and can be widely applied to, for example, carbon steel, SCR-4 chrome steel, SUJ-2 chrome steel and the like.

It is a flowchart which shows the process of the rapid spheroidization annealing process of steel of this invention. It is a figure which shows the heat pattern at the time of applying the slow cooling processing method of the steel materials of this invention to carbon steel. It is a figure which shows the heat pattern at the time of applying the slow cooling processing method of the steel material of this invention to SCR-4 chrome steel. It is a figure which shows the heat pattern at the time of applying the slow cooling processing method of the steel material of this invention to SUJ-2 chrome steel. It is an enlarged photograph of the section of the steel material of the prototype which processed the steel material which satisfy | fills special slow cooling conditions by the slow cooling processing method of steps S101 to S107 of FIG. It is an enlarged photograph of the section of the steel material of the trial example processed with the cooling rate of the conventional heat pattern. FIG. 2 is an enlarged photograph of a cross section of a steel material in a prototype of a steel material obtained by processing a steel material that does not satisfy a special slow cooling condition by the slow cooling processing method of steps S101 to S107 in FIG. It is a figure which shows the heat pattern of the slow cooling process of the spheroidizing annealing process of the carbon steel in a prior art. It is a figure which shows the heat pattern of the slow cooling process of the spheroidizing annealing process of chrome steel (SCR-4) in a prior art. It is a figure which shows the heat pattern of the slow cooling process of the spheroidizing annealing process of chrome steel (SUJ-2) in a prior art. It is a figure which shows the example of the heat pattern disclosed by Unexamined-Japanese-Patent No. 2008-121032 in a prior art.

Claims (5)

It is a heat treatment method in which after heating up to a predetermined heating temperature equal to or higher than the transformation start temperature, maintaining the soaking temperature over a predetermined time, gradually cooling the steel material and spheroidizing annealing,
A first cooling step for cooling within the first cooling rate range from the soaking temperature to the transformation start temperature;
A second cooling step of cooling within the second cooling rate range from the transformation start temperature to the transformation completion temperature;
The cooling is performed within the third cooling rate range for the wire-drawn steel from the transformation completion temperature to the cooling start temperature on the condition that the pre-treatment of the spheroidizing annealing in the steel is a wire drawing treatment. Within the third cooling rate range for the rolled steel, from the transformation completion temperature to the cooling start temperature, provided that the cooling step 3 and the pretreatment of the spheroidizing annealing in the steel material are rolling treatment steps. A method of annealing a steel material, characterized in that the heat pattern includes a third cooling step for cooling. The first cooling rate range is a range from 20 ° C./h or more to a forced cooling temperature by a radiant tube,
The second cooling rate range is from 10 ° C./h or more to a forced cooling temperature by a radiant tube or less, and the selected second cooling rate is the selected first cooling rate or less,
The second cooling rate in which the third cooling rate range for the wire-drawn steel material is in a range from 10 ° C./h or more to a forced cooling temperature by a radiant tube and a selected third cooling rate is selected. More than,
The third cooling rate range for the rolled steel is in the range of 5 ° C./h to 20 ° C./h and the selected third cooling rate is the selected second cooling rate or less. The method of slow cooling a steel material according to claim 1, wherein: The first cooling rate is 20 ° C./h to 60 ° C./h;
The second cooling rate is 10 ° C./h to 30 ° C./h, and the selected second cooling rate is equal to or lower than the selected first cooling rate;
The third cooling rate range for the wire-drawn steel material is 10 ° C./h to 30 ° C./h, and the selected third cooling rate is equal to or higher than the selected second cooling rate. ,
The third cooling rate range for the rolled steel material is 5 ° C./h to 15 ° C./h, and the selected third cooling rate is equal to or less than the selected second cooling rate. The method for slow cooling a steel material according to claim 2.   The method for gradually cooling a steel material according to any one of claims 1 to 3, wherein the first cooling rate and the second cooling rate are the same.   The method for gradually cooling a steel material according to any one of claims 1 to 4, wherein the steel material is any one of coiled carbon steel, SCR-4 chrome steel, and SUJ-2 chrome steel.