JP6408290B2 - Method for rapid softening annealing of carbon steel - Google Patents

Description

  The invention according to the present application relates to a method of annealing a machine structural part used for automobile parts and the like.

Conventionally, mechanical structural parts such as hollow rack bars used for automobile parts, etc. are processed by giving a predetermined thermal history to the steel material to give a predetermined hardness, and further by spheroidizing annealing and softening The improvement of the nature is aimed at. In the spheroidizing annealing process that has been conventionally performed, in order to spheroidize cementite, it was necessary to heat the steel material over a long time of several tens of hours or more at a temperature around Ae ₁ point. Therefore, various rapid continuous spheroidizing annealing methods have been developed to improve the spheroidizing annealing method having poor production efficiency.

For example, in the rapid continuous spheroidizing annealing method shown in Patent Document 1, as a basic process (1) for steel, the temperature is increased at a temperature rising rate of 1 K / second or more in a temperature range of Ae ₁ point or more and Ae ₁ point +150 K or less. temperature heating, after maintaining the temperature for less than 600 seconds 0 seconds in the region, the temperature region of Ae ₁ point + 50K~Ae ₁ point -150K 5K / sec is cooled below the cooling rate or of the temperature range And (2), heating is performed at a temperature rising rate of 1 K / second or more in a temperature range of Ae ₁ point + 80 K or more and Ae ₁ point + 300 K or less, and 0 second or more 120 in the temperature range. after holding the fractional seconds, or in combination to maintain the temperature within the region Ae ₁ point ～Ae ₁ point -150K or in the temperature range of the temperature is cooled below the cooling rate of 5K / sec, by itself 1 1 or more times It is characterized in Ukoto.

In the said patent document 1, the steel spheroidizing annealing process is shortened to 1 hour or less by adding a plastic working to the steel material in the temperature range below Ac ₁ point and 623K or more in the course of temperature rising, and passing through a predetermined heat treatment cycle. ing.

Moreover, the spheroidizing annealing method of the wire shown in Patent Document 2 is a steel in which the base material contains C: 0.15 to 0.6% and Mn—Cr: 0.8 to 3.0% by weight%. The wire in a strand state in which the structure after the hot rolling satisfies the following formulas (1) and (2) is heated to a temperature T1 at a heating rate of 100 ° C./second or more, and immediately until the temperature T2 C1 ° C. / It is characterized in that it is cooled at a cooling rate of 2 seconds and then charged in a furnace at temperature T3 and held for 3 to 30 minutes. In addition, Ae ₁ point + 20 ° C. ≦ T1 ≦ Ae ₁ point + 150 ° C., Ae ₁ point−50 ° C. ≦ T2 ≦ Ae ₁ point + 20 ° C., Ae ₁ point−80 ° C. ≦ T3 ≦ Ae ₁ point−20 ° C., {(T1 -Ae ₁ point) / 30} ≦ C1 ≦ {(T1-Ae ₁ point) /1.5}.
Formula (1) {(Bainite fraction) / (1-Ferrite fraction)} ≧ 0.2
Formula (2) Ferrite fraction ≧ 0.1

  In the said patent document 2, the spheroidizing annealing process of a wire is shortened to 30 minutes or less by controlling heating temperature, a heating rate, and a cooling rate.

Japanese Patent Laid-Open No. 08-246040 Japanese Patent Laid-Open No. 10-25521

  However, the spheroidizing annealing technique for softening as in each of the above-mentioned patent documents is a method of uniformly softening the entire material by mainly heating the material such as a steel plate or wire, There is a problem that a device for processing becomes a large scale. Further, as described in the above-described patent document, the spheroidizing annealing process requires about 30 minutes.

  Therefore, from the market, it has become possible to process the spheroidizing annealing of steel materials, which previously required a processing time of 30 minutes to 1 hour, in a shorter time, and realized a partial softening annealing process in the production line. Development of a spheroidizing annealing method that can be performed has been desired.

  As a result of diligent research, the inventors of the present invention can realize the annealing process of carbon steel in a very short time by adopting the following rapid softening annealing method of carbon steel, It was conceived that partial softening annealing could be performed.

That is, in the rapid softening annealing method of carbon steel according to the present invention, carbon in the chemical composition of the carbon steel is 0.27 mass% to 0.58 mass%, and silicon is 0.15 mass% to 0.35 mass. %, Manganese is 0.30% by mass to 1.50% by mass, phosphorus is 0.03% by mass or less, sulfur is 0.03% by mass or less, nickel is 0.25% by mass or less, and chromium is 0.35% by mass. In the following, the balance is iron and inevitable impurities, and the following steps are provided.
First heat treatment step: the carbon steel is heated, the step of increasing the surface of the carbon steel at _{Ac 1 + 20 ℃ ~Ac 1 +} 50 ℃ of the austenitizing temperature T1 10 ° C. / s or faster.
Second heat treatment step: Immediately after the completion of the first heat treatment step, the carbon steel is lowered at a rate of 1 ° C./s or more to a low temperature holding temperature T2 of Ms point to 600 ° C.
Third heat treatment step: A step of raising the carbon steel that has finished the second heat treatment step at a rate of 10 ° C./s or higher to a high temperature holding temperature T3 of Ae ₁ −130 ° C. to Ae ₁ −20 ° C.
Cooling step: A step of cooling the carbon steel that has finished the third heat treatment step to obtain a softened carbon steel.

  Further, the rapid softening annealing method for carbon steel according to the present invention includes the low temperature holding temperature T2 for 30 seconds or less in the second heat treatment step and / or the high temperature holding in the third heat treatment step. It is preferable to hold the temperature T3 for a time of 60 seconds or less.

  Moreover, the rapid softening annealing method of the carbon steel which concerns on this invention shall reduce Vickers hardness 8% or more by the said softening annealing process on the basis of the hardness of the carbon steel before the said process. Is preferred.

Furthermore, the rapid softening annealing method for carbon steel according to the present invention preferably uses a high frequency induction heating method in at least one of the first heat treatment step and the third heat treatment step.

Moreover, it is preferable that the rapid softening annealing method of the carbon steel which concerns on this invention uses a high frequency induction heating method in a said 2nd heat treatment process .

  Furthermore, in the rapid softening annealing method for carbon steel according to the present invention, the austenitizing temperature T1 in the first heat treatment step is 760 ° C. to 785 ° C., and the rate of temperature rise to the austenitizing temperature T1 is 90 ° C. It is preferable that it is degrees C / s-150 degrees C / s.

  In the method for rapid softening annealing of carbon steel according to the present invention, the cooling rate to the low temperature holding temperature T2 in the second heat treatment step is preferably 3 ° C./s to 170 ° C./s.

  Furthermore, in the rapid softening annealing method of carbon steel according to the present invention, the high temperature holding temperature T3 in the third heat treatment step is 600 ° C. to 700 ° C., and the temperature rising rate to the high temperature holding temperature T3 is 20 ° C. It is preferable that it is degrees C / s-350 degrees C / s.

  According to the rapid softening annealing method for carbon steel according to the present application, it is possible to realize the spheroidizing annealing treatment of carbon steel that has conventionally required a processing time of 30 minutes to 1 hour or more in an extremely short time. Become. Further, the rapid softening annealing method of carbon steel according to the present application can perform spheroidizing annealing of carbon steel in an extremely short time, and therefore, in the production line, partial softening annealing of carbon steel. Processing can be performed continuously. Therefore, the present invention can locally soften and anneal a portion of carbon steel that requires machining or plastic working, avoiding a decrease in hardness in a portion that requires hardness, and processing It becomes possible to remarkably improve the workability of a necessary part. Therefore, by adopting the rapid softening annealing method for carbon steel according to the present application, it becomes possible to provide a high quality product with excellent processing accuracy.

It is a schematic diagram of the heat processing cycle as embodiment which concerns on this invention. It is a microscope picture of the metal structure of the sample before the softening annealing process used in each Example. 2 is a schematic diagram of a heat treatment cycle of Example 1. FIG. 6 is a schematic diagram of a heat treatment cycle of Example 4. FIG. 6 is a schematic diagram of a heat treatment cycle of Example 7. FIG. 6 is a schematic diagram of a heat treatment cycle of Example 10. FIG. 10 is a schematic diagram of a heat treatment cycle of Example 11. FIG. 14 is a schematic diagram of a heat treatment cycle of Example 14. FIG. 18 is a schematic diagram of a heat treatment cycle of Example 15. FIG. 10 is a schematic diagram of a heat treatment cycle of Example 17. FIG. 18 is a schematic diagram of a heat treatment cycle of Example 18. FIG. 10 is a schematic diagram of a heat treatment cycle of Comparative Example 7. FIG. 10 is a schematic diagram of a heat treatment cycle of Comparative Example 8. FIG. It is a figure which shows the hardness with respect to austenitizing temperature T1. It is a figure which shows the hardness with respect to the low temperature holding temperature T2. It is a figure which shows the hardness with respect to the high temperature holding temperature T3. It is a cross-sectional schematic diagram of the test steel pipe used in Examples 17-19. It is a figure which shows the hardness in each measurement part of the test steel pipe before a process, and the test steel pipe of Example 17-19.

  Below, the embodiment of "the rapid softening annealing method of carbon steel" concerning this invention is explained in full detail.

  The rapid softening annealing method of carbon steel according to the present invention is a method of softening annealing a carbon steel having a predetermined hardness in an extremely short time, and includes a “first heat treatment step” and a “second heat treatment step”. The heat treatment step, the third heat treatment step, and the cooling step are provided as essential steps. First, after explaining the carbon steel to be treated, the first heat treatment step to the third heat treatment step and the cooling step will be described in detail.

1. Carbon steel The carbon steel used in the rapid softening annealing method according to the present invention includes a predetermined amount of components necessary for producing a carbon steel product such as a hollow rack bar that requires high strength. That is, the carbon steel used in the present invention is carbon 0.27 mass% to 0.58 mass%, silicon 0.15 mass% to 0.35 mass%, manganese 0.30 mass% to 1.50 mass%. %, Phosphorus is 0.03% by mass or less, sulfur is 0.03% by mass or less, nickel is 0.25% by mass or less, chromium is 0.35% by mass or less, and the balance is iron and inevitable impurities. It is preferable to have a composition. As an example of the carbon steel having the chemical composition, SMn433 (JIS G 4053) can be cited.

Carbon: The carbon content in steel materials for mechanical structures that require high strength such as hollow rack bars is preferably 0.27% by mass to 0.58% by mass, and 0.30% by mass to 0.36% by mass. % Is more preferable. Carbon as an alloy component is an element necessary for ensuring the core strength after the quenching treatment. If it is less than 0.27% by mass, its hardness is insufficient and exceeds 0.58% by mass. And in order to reduce the toughness of a core part, content was made into 0.27 mass%-0.58 mass%.

Silicon: Silicon is an element that greatly contributes to the improvement of the softening resistance of tempering, and is required to be 0.15% by mass or more in order to obtain a sufficient effect. On the other hand, when silicon is excessively contained, the machinability is lowered, and therefore, when used for a hollow rack bar or the like that performs machining such as cutting partially after the softening annealing treatment, The upper limit is preferably 0.35% by mass.

Manganese: The manganese content in steel for mechanical structures such as hollow rack bars is preferably 0.30% by mass to 1.50% by mass. Manganese is necessary as a deoxidizer during refining, and is an element that reduces oxide inclusions and increases the cleanliness of steel. Moreover, it is an element which improves hardenability and raises the core part hardness and hardened layer depth of steel. In order to use it for steel materials for mechanical structures such as a hollow rack bar that requires a predetermined hardness as in the present embodiment, it is necessary to contain 0.30% by mass or more of manganese in carbon steel. . On the other hand, if manganese is added excessively, the hardenability becomes excessive, the toughness deteriorates and the workability also decreases. Therefore, the upper limit of manganese content is set to 1.50% by mass.

Phosphorus: Phosphorus is an impurity that reduces the hot workability and toughness of steel. Therefore, it is preferable that the content of phosphorus in the carbon steel is small. The upper limit of the phosphorus content is 0.03% by mass or less.

Sulfur: The sulfur content in steel for mechanical structures such as hollow rack bars is preferably 0.030% by mass or less.

Nickel: The nickel content in steel for mechanical structures such as hollow rack bars is preferably 0.25% by mass or less.

Chromium: The chromium content in the steel for mechanical structures such as hollow rack bars is preferably 0.35% by mass or less.

  The carbon steel used in the present invention includes other components, such as molybdenum, vanadium, niobium, boron, titanium, tellurium, calcium, magnesium, zirconium, etc. depending on the use as a steel for machine structural use. May be used.

  The above-mentioned carbon steel, which is a processing target of the rapid softening annealing method of the present invention, has high hardness as it is, and has low machinability and plastic workability for processing teeth and the like. Therefore, in this invention, the rapid softening annealing process is performed to the part used as the process target of the said carbon steel, and the said process target part is made into the metal structure which is easy to machine.

  In this invention, it is preferable that Vickers hardness is reduced by 8% or more on the basis of the hardness of the carbon steel before a process by a softening annealing process. Based on the hardness of the carbon steel before the treatment, the hardness is reduced by 8% or more by the softening annealing treatment, so that the machinability, grindability and plastic working are compared with the carbon steel before the softening annealing treatment. Property is improved. Therefore, it is possible to provide a high quality product with excellent processing accuracy. Moreover, in this invention, it is more preferable that the Vickers hardness is reduced by 8% to 20% based on the hardness of the carbon steel before the treatment by the softening annealing treatment. For example, when the Vickers hardness of the carbon steel having the above-described chemical composition is 155 HV (1) to 190 HV (1), the Vickers hardness after the softening annealing treatment is 124 HV (1) to 174 HV (1). It is preferable. By reducing the Vickers hardness in the above-described range based on the hardness of the carbon steel before the treatment by the softening annealing treatment, it is possible to improve the workability while maintaining the hardness of the carbon steel to some extent. it can.

  Next, a specific rapid softening annealing method for realizing the above-described metal structure and hardness will be described for each step with reference to the schematic diagram of the heat treatment cycle of FIG.

2. First Heat Treatment Step In this first heat treatment step, carbon steel is heated, and the surface of the carbon steel is raised to an austenitizing temperature T1 of Ac ₁ + 20 ° C. to Ac ₁ + 50 ° C. at a rate of 10 ° C./s or more. Therefore, when the carbon steel with Ac ₁ of about 740 ° C. is used, the austenitizing temperature T 1 is preferably 760 ° C. to 785 ° C.

In the first heat treatment step, by rapidly heating the carbon steel to a temperature of Ac ₁ + 20 ° C. or higher, only a part of the carbide (cementite structure in the pearlite structure) of the carbon steel is austenitized. Can do. On the other hand, when the carbon steel is rapidly heated to a temperature exceeding Ac ₁ + 50 ° C. in the first heat treatment step, the amount of carbide austenitizing in the metal structure becomes excessive, and the amount of pearlite deposited in the metal structure after the treatment is large. Therefore, it is not preferable.

  Moreover, it is preferable that the temperature increase rate to the austenitizing temperature T1 in this 1st heat treatment process shall be 90 degreeC / s-150 degreeC / s. This is because by heating at the temperature increase rate, only a part of the carbide can be efficiently austenitized, and the time required for the softening annealing process can be shortened.

  In order to perform heating at the above-described temperature increase rate, it is preferable to use a high-frequency induction heating method as a heating method for carbon steel. This is because by using the high frequency induction heating method, the carbon steel can be easily heated to the austenitizing temperature T1 at a high temperature rising rate.

3. Second Heat Treatment Step In the second heat treatment step, immediately after the above-described first heat treatment step is completed, the carbon steel is lowered at a rate of 1 ° C./s or more to the low temperature holding temperature T2 of Ms point to 600 ° C. At this time, it is preferable to hold for 30 seconds or less at the low temperature holding temperature T2.

  In the second heat treatment step, the temperature of the carbon steel immediately after being heated to the austenitizing temperature T1 in the first heat treatment step is rapidly lowered to a low temperature holding temperature T2 that is equal to or higher than the Ms point. Of these, the austenite structure undergoes diffusion transformation from the austenitized structure to ferrite and carbide (cementite structure) without martensitic transformation, and at that time, the carbon is distributed to the remaining carbides, so that the growth of the carbides is achieved. Can be promoted. On the other hand, if the low temperature holding temperature T2 in the second heat treatment step is higher than 600 ° C., the driving force for diffusion transformation from the structure austenitized in the first heat treatment step to ferrite and carbide becomes low, and the diffusion transformation Is not preferable because it takes time. Therefore, when carbon steel having an Ms point of about 400 ° C. is used, the low temperature holding temperature T2 is preferably 450 ° C. to 600 ° C.

  In the present invention, the holding time of the low temperature holding temperature T2 in the second heat treatment step is preferably 30 seconds or less. By setting the holding time of the low temperature holding temperature T2 to 30 seconds or less, it is possible to significantly shorten the processing time as compared with the conventional spheroidizing annealing treatment.

  Further, the cooling rate to the low temperature holding temperature T2 in the second heat treatment step is more preferably 3 ° C./s to 170 ° C./s. By setting the cooling rate from the austenitizing temperature T1 to the low temperature holding temperature T2 to 3 ° C./s to 170 ° C./s, the carbon in the austenitic structure in the metal structure of the carbon steel is efficiently distributed to the cementite structure. This is because the carbide can be grown, and the time required for the softening annealing process can be greatly shortened.

  In addition, as a heating method of the carbon steel in the second heat treatment step, it is preferable to use a high frequency induction heating method as in the first heat treatment step. This is because the temperature can be easily controlled by using the high frequency induction heating method.

4). Third Heat Treatment Step In the third heat treatment step, the carbon steel that has finished the second heat treatment step is fed at a rate of 10 ° C./s or higher up to a high temperature holding temperature T3 of Ae ₁ −130 ° C. to Ae ₁ −20 ° C. Raise. For example, the high temperature holding temperature T3 is preferably 600 ° C to 700 ° C.

In the third heat treatment step, by rapidly heating the temperature of the carbon steel held at the low temperature holding temperature T2 in the second heat treatment step to a temperature of Ae ₁ -130 ° C. or higher, In the second heat treatment step, a carbide (cementite structure) in which carbon is distributed from the austenite structure can be efficiently grown, coalesced, aggregated, and spheroidized. When the high temperature holding temperature T3 in the third heat treatment step is lower than Ae ₁ -130 ° C, it is not preferable because the carbide growth rate is delayed and efficient spheroidizing treatment of the carbide becomes difficult. On the other hand, when the high temperature holding temperature T3 in the third heat treatment step is set to a temperature exceeding Ae ₁ -20 ° C, a part of the metal structure of the carbon steel is austenitized again, and martensite is transformed during rapid cooling. This is not preferable because the steel cannot be softened.

  Moreover, it is preferable that the rate of temperature increase from the low temperature holding temperature T2 to the high temperature holding temperature T3 in the third heat treatment step is 20 ° C./s to 350 ° C./s. This is because by heating at the temperature increase rate, the carbide can be efficiently spheroidized and the time required for the softening annealing process can be shortened.

  In the present invention, the holding time of the high temperature holding temperature T3 in the third heat treatment step is preferably 60 seconds or less. By setting the holding time of the high temperature holding temperature T3 to 60 seconds or less, it is possible to significantly shorten the processing time as compared with the conventional spheroidizing annealing process.

  In addition, as a heating method of the carbon steel in the third heat treatment step, it is preferable to use a high frequency induction heating method as in the first heat treatment step and the second heat treatment step. This is because by using the high frequency induction heating method, the carbon steel can be easily heated to the high temperature holding temperature T3 at a high temperature rising rate.

5. Cooling Step In this cooling step, the carbon steel that has been subjected to the third heat treatment step is cooled to obtain a softened carbon steel.

  In the cooling step, by cooling the carbon steel that has finished the third heat treatment step, the solid solution carbon in the ferrite structure is distributed among the metal structure of the carbon steel. The cooling rate in the cooling step is not particularly limited. That is, the cooling rate in the cooling step has almost no influence on the hardness of the obtained softened carbon steel. Therefore, the time required for the softening process can be shortened by adopting the rapid cooling method. As the rapid cooling method, forced air cooling, water cooling, or a cooling method using other coolants can be employed.

  By performing the series of the first heat treatment step to the cooling step, the carbon steel having a predetermined strength is softened by the spheroidizing annealing treatment, and the workability can be remarkably improved.

  In particular, according to the present invention, it is possible to realize the spheroidizing annealing of carbon steel, which has conventionally required a treatment time of 30 minutes to 1 hour or more, in a very short time from the first heat treatment step to the cooling step. Become. Therefore, since the rapid softening annealing method of carbon steel according to the present application can perform spheroidizing annealing of carbon steel in an extremely short time, the partial softening annealing of carbon steel in the production line Processing can be realized. Therefore, the rapid softening annealing method of carbon steel according to the present application can locally soften and anneal only the part of the carbon steel that requires machining or plastic working, so the part that requires hardness. In this case, it is possible to avoid a decrease in hardness and remarkably improve the workability of the part that needs to be processed. Therefore, by adopting the rapid softening annealing method for carbon steel according to the present application, it becomes possible to provide a high quality product with excellent processing accuracy.

  In the rapid softening annealing method according to the present invention, the cooling process is performed after sequentially performing the first heat treatment process, the second heat treatment process, and the third heat treatment process, but the present invention is not limited thereto. Is not to be done. For example, after performing the first heat treatment step and the second heat treatment step for a plurality of cycles, the third heat treatment step is performed, and then the cooling step is performed, or the first heat treatment step to the third heat treatment step is performed for a plurality of cycles. It is also preferable to perform a cooling process later. Furthermore, it is also preferable to perform a plurality of cycles from the first heat treatment step to the cooling step. In this way, by performing a plurality of cycles from the first heat treatment step and the second heat treatment step, or from the first heat treatment step to the third heat treatment step, or from the first heat treatment step to the cooling step, the carbon steel becomes even softer. A chemical annealing treatment can be performed. In particular, the degree of softening of the carbon steel can be adjusted with high accuracy by adjusting the number of repetitions of the cycle. Therefore, it becomes possible to adjust the hardness of carbon steel according to the grade and kind of processing, and it becomes possible to obtain carbon steel of desired hardness. In any case, since each heat treatment step can be executed in a very short time, even if a series of heat treatment cycles from the first heat treatment step to the cooling step are repeated several times, It is possible to execute the softening annealing process in an extremely short time compared to the conventional case.

  Next, the softened carbon steel obtained by using the above-described rapid softening annealing method for carbon steel will be described. The softened carbon steel is obtained by reducing the Vickers hardness by 8% or more on the basis of the hardness of the carbon steel before treatment. The softened carbon steel obtained by the rapid softening annealing method of the present invention is not limited to one obtained by uniformly softening the entire carbon steel, but also those obtained by softening annealing only the parts that require processing. included. Therefore, in the case of mechanical structural parts such as a hollow rack bar, only the molded portion that needs to be processed may be partially softened and annealed by using a high-frequency heating method. Therefore, the softening annealing treatment can be performed only on the portion that does not require the softening annealing treatment while maintaining the hardness of the material itself, and only the portion that needs to be processed, so that the workability is very good. Therefore, processing accuracy can be improved and high quality can be realized.

  Next, examples and comparative examples using the rapid softening annealing method for carbon steel according to the present invention will be described.

Examples 1 to 19 shown below were annealed by the rapid softening annealing method for carbon steel according to the present invention. In each example, a hollow rack bar blank (SMn433: JIS G 4053) was employed as a test material. It is a hollow tube made of a steel material having the component composition shown in Table 1 of the raw tube. In Table 1, the unit of numerical values is mass%, and the balance is iron and inevitable impurities. The raw tube has a diameter of 32 mm and a wall thickness of 5 mm. In each of Examples 1 to 16, a cylindrical test piece having a diameter of 3 mm and a length of 10 mm is taken from the raw tube in the length direction of the test piece. Was cut out by a wire cutter electric discharge machine so as to coincide with the length direction of the raw tube. Examples 16 to 18 used test steel pipes that were subjected to flattening after drawing with a cross-section reduction rate of about 2%. The Ac ₁ of SMn433 used as the test piece is about 740 ° C., the Ae ₁ is 713 ° C., and the Ms point is 386 ° C.

  Moreover, the microscope picture of the metal structure of the said raw tube, ie, the metal structure of the test piece before the softening annealing process used in each Example is shown in FIG. The magnifications of the micrographs were 400 times, 1000 times, and 2000 times. The microphotographs of 400 times and 1000 times were taken with an optical microscope, and the 2000 times micrograph was taken with a scanning electron microscope. The metal structure of the test piece before the softening annealing is in a state in which part of the cementite structure has been spheroidized in the pearlite structure, and the lumped pearlite structure remains. In FIG. 2, the metal structure of the element tube appears to be striped because the drawing process was performed as part of the pretreatment of the element tube.

  And in each Example 1- Example 19, about each test piece, the rapid softening annealing process which concerns on this invention using a Formaster test machine (The Fuji Radio Industry Co., Ltd. make, transformation point measuring device Formaster-EDP). Went. Below, Table 2 which put together the rapid softening annealing process conditions of each Example 1- Example 19 is shown. The rice marks in Table 2 indicate those obtained by performing the first heat treatment step to the third heat treatment step or the first heat treatment step to the second heat treatment step for 3 cycles.

  First, the rapid softening annealing process of Example 1 is demonstrated with reference to the schematic diagram of the heat processing cycle of FIG. FIG. 3 shows the heat treatment cycles of Example 2 and Example 3 in addition to Example 1. In Example 1, the test piece was heated to the austenitizing temperature T1 of 760 ° C. in the first heat treatment step. In Example 1, since heating was performed from room temperature to 760 ° C. in 8 seconds, the rate of temperature increase in the first heat treatment step was about 91.3 ° C./s. In the first heat treatment step, immediately after the test piece was heated to 760 ° C., the temperature of the test piece was lowered to the low temperature holding temperature T2 of 550 ° C. in the second heat treatment step, and then the low temperature holding temperature T2 was held for 10 seconds. In Example 1, since the temperature was decreased from 760 ° C. to 550 ° C. in 2 seconds, the cooling rate of the second heat treatment step was 105 ° C./s. After finishing the second heat treatment step, in the third heat treatment step, the test piece was heated to a high temperature holding temperature T3 of 670 ° C., and the high temperature holding temperature T3 was held for 10 seconds. In Example 1, since the temperature was increased from 550 ° C. to 670 ° C. in 1 second, the rate of temperature increase in the third heat treatment step was 120 ° C./s. After finishing the third heat treatment step, the test piece was rapidly cooled to room temperature in the cooling step to obtain the softened carbon steel of Example 1. The time required for cooling to room temperature in the cooling step in Example 1 was 10 seconds. From the above, in Example 1, the time required to execute the series of heat treatment cycles was 41 seconds (0.68 minutes).

  Example 2 was different from Example 1 only in the austenitizing temperature T1 in the first heat treatment step, and the other process conditions were the same. That is, in Example 2, the test piece was heated to the austenitizing temperature T1 of 770 ° C. in the first heat treatment step. In Example 2, since heating was performed from room temperature to the austenitizing temperature T1 in 8 seconds as in Example 1, the rate of temperature increase in the first heat treatment step was about 92.5 ° C./s. Then, also about Example 2, the softened carbon steel of Example 2 was obtained through the 2nd heat treatment process, the 3rd heat treatment process, and the cooling process like Example 1. In Example 2, since the temperature was lowered from 770 ° C. to 550 ° C. in 2 seconds, the cooling rate of the second heat treatment step was 110 ° C./s.

  Example 3 was different from Example 1 only in the austenitizing temperature T1 in the first heat treatment step, and the other process conditions were the same. That is, in Example 3, the test piece was heated to the austenitizing temperature T1 of 785 ° C. in the first heat treatment step. In Example 3, since heating was performed from room temperature to the austenitizing temperature T1 in 8 seconds as in Example 1, the temperature increase rate in the first heat treatment step was about 94.4 ° C./s. Then, also about Example 3, similarly to Example 1, the softened carbon steel of Example 3 was obtained through the second heat treatment step, the third heat treatment step, and the cooling step. In Example 3, since the temperature was lowered from 785 ° C. to 550 ° C. in 2 seconds, the cooling rate of the second heat treatment step was 118 ° C./s.

  Next, the rapid softening annealing process of Example 4 will be described with reference to the schematic diagram of the heat treatment cycle of FIG. In FIG. 4, in addition to the fourth embodiment, the heat treatment cycles of the above-described first and fifth embodiments are also shown. Example 4 differs from Example 2 described above only in the low-temperature holding temperature T2 in the second heat treatment step, and the other process conditions are the same. That is, in Example 4, as in Example 2, immediately after heating the test piece to the austenitizing temperature T1 of 770 ° C. in the first heat treatment step, the test piece was brought to the low temperature holding temperature T2 of 450 ° C. in the second heat treatment step. After the temperature was lowered, the low temperature holding temperature T2 was held for 10 seconds. In Example 4, since the temperature was lowered from 770 ° C. to 450 ° C. in 2 seconds, the cooling rate of the second heat treatment step was 160 ° C./s. After finishing the second heat treatment step, the softened carbon steel of Example 4 was obtained in Example 4 through the third heat treatment step and the cooling step as in Example 2.

  Example 5 differs from Example 2 described above only in the low-temperature holding temperature T2 in the second heat treatment step, and the other process conditions are the same. That is, in Example 5, as in Example 2, immediately after the test piece was heated to the austenitizing temperature T1 of 770 ° C. in the first heat treatment step, the test piece was heated to the low temperature holding temperature T2 of 500 ° C. in the second heat treatment step. After the temperature was lowered, the low temperature holding temperature T2 was held for 10 seconds. In Example 5, since the temperature was lowered from 770 ° C. to 500 ° C. in 2 seconds, the cooling rate of the second heat treatment step was 135 ° C./s. After finishing the second heat treatment step, the softened carbon steel of Example 5 was obtained in Example 5 through the third heat treatment step and the cooling step as in Example 2.

  Example 6 differs from Example 2 described above only in the low-temperature holding temperature T2 in the second heat treatment step, and the other process conditions are the same. That is, in Example 6, as in Example 2, immediately after heating the test piece to the austenitizing temperature T1 of 770 ° C. in the first heat treatment step, the test piece was brought to the low temperature holding temperature T2 of 600 ° C. in the second heat treatment step. After the temperature was lowered, the low temperature holding temperature T2 was held for 10 seconds. In Example 6, since the temperature was decreased from 770 ° C. to 600 ° C. in 2 seconds, the cooling rate of the second heat treatment step was 85 ° C./s. After finishing the second heat treatment step, the softened carbon steel of Example 6 was obtained in Example 6 through the third heat treatment step and the cooling step as in Example 2.

  Next, the rapid softening annealing process of Example 7 will be described with reference to the schematic diagram of the heat treatment cycle of FIG. FIG. 5 shows the heat treatment cycles of Example 8 and Example 9 in addition to Example 7. Example 7 was different from Example 2 described above only in the high temperature holding temperature T3 in the third heat treatment step, and the other process conditions were the same. That is, in Example 7, after performing the first heat treatment step and the second heat treatment step under the same conditions as in Example 2, in the third heat treatment step, the test piece was heated from a low temperature holding temperature T2 of 550 ° C. to a high temperature of 600 ° C. The temperature was heated to the holding temperature T3, and the high temperature holding temperature T3 was held for 10 seconds. In Example 7, since the temperature was raised from 550 ° C. to 600 ° C. in one second, the rate of temperature increase in the third heat treatment step was 50 ° C./s. After finishing the third heat treatment step, the test piece was rapidly cooled to room temperature in the cooling step to obtain a softened carbon steel of Example 7.

  Example 8 is different from Example 2 described above only in the high temperature holding temperature T3 in the third heat treatment step, and the other process conditions are the same. That is, in Example 8, after the first heat treatment step and the second heat treatment step were performed under the same conditions as in Example 2, in the third heat treatment step, the test piece was moved from a low temperature holding temperature T2 of 550 ° C. to a high temperature of 650 ° C. The temperature was heated to the holding temperature T3, and the high temperature holding temperature T3 was held for 10 seconds. In Example 8, since the temperature was raised from 550 ° C. to 650 ° C. in 1 second, the rate of temperature increase in the third heat treatment step was 100 ° C./s. After finishing the third heat treatment step, the test piece was rapidly cooled to room temperature in the cooling step to obtain a softened carbon steel of Example 8.

  Example 9 was different from Example 2 described above only in the high temperature holding temperature T3 in the third heat treatment step, and the other process conditions were the same. That is, in Example 9, after performing the first heat treatment step and the second heat treatment step under the same conditions as in Example 2, in the third heat treatment step, the test piece was heated from a low temperature holding temperature T2 of 550 ° C. to a high temperature of 700 ° C. The temperature was heated to the holding temperature T3, and the high temperature holding temperature T3 was held for 10 seconds. In Example 9, since the temperature was raised from 550 ° C. to 700 ° C. in 1 second, the rate of temperature increase in the third heat treatment step was 150 ° C./s. After finishing the third heat treatment step, in the cooling step, the test piece was rapidly cooled to room temperature to obtain a softened carbon steel of Example 9.

  Next, the rapid softening annealing process of Example 10 will be described with reference to the schematic diagram of the heat treatment cycle of FIG. Example 10 was different from Example 2 described above only in the cooling rate in the cooling process, and the other process conditions were the same. That is, in Example 10, after performing the first heat treatment process to the third heat treatment process under the same conditions as in Example 2, in the cooling process, the test piece was moved from a high temperature holding temperature T3 of 670 ° C. to 450 ° C. at 5 ° C. / After cooling at the rate of s, it was cooled to room temperature at 20 ° C./s to obtain the softened carbon steel of Example 10. The time required for cooling to room temperature in the cooling step in Example 10 was 65 seconds. As described above, in Example 10, the time required for executing the series of heat treatment cycles was 96 seconds (1.60 minutes).

  Next, the rapid softening annealing process of Example 11 will be described with reference to the schematic diagram of the heat treatment cycle of FIG. FIG. 7 shows the heat treatment cycles of Example 12 and Example 13 in addition to Example 11. Example 11 was different from Example 2 described above only in the holding time of the low temperature holding temperature T2 in the second heat treatment step and the cooling rate in the cooling step, and the conditions of the other steps were the same. That is, in Example 11, in the first heat treatment step, the test piece was heated from room temperature to an austenitizing temperature T1 of 770 ° C. at a rate of 92.5 ° C./s. Immediately after heating the test piece to 770 ° C. in the first heat treatment step, the test piece is lowered to the low temperature holding temperature T2 of 550 ° C. at a cooling rate of 110 ° C./s in the second heat treatment step, and then kept at the low temperature. The temperature T2 was held for 5 seconds. After finishing the second heat treatment step, in the third heat treatment step, the test piece was heated to a high temperature holding temperature T3 of 670 ° C. at a temperature rising rate of 120 ° C./s, and the high temperature holding temperature T3 was held for 10 seconds. After finishing the third heat treatment step, in the cooling step, the test piece was cooled to room temperature at a cooling rate of 1 ° C./s to obtain a softened carbon steel of Example 11. The time required for cooling to room temperature in the cooling step in Example 11 was 640 seconds. As described above, in Example 11, the time required for executing the series of heat treatment cycles was 666 seconds (11.10 minutes).

  Example 12 differs from Example 11 described above only in the holding time of the high temperature holding temperature T3 in the third heat treatment step, and the conditions of the other steps are the same. That is, in Example 12, after performing the first heat treatment step and the second heat treatment step under the same conditions as in Example 11, in the third heat treatment step, the test piece was moved from a low temperature holding temperature T2 of 550 ° C. to a high temperature of 670 ° C. The temperature was heated to the holding temperature T3, and the high temperature holding temperature T3 was held for 60 seconds. After finishing the third heat treatment step, in the cooling step, the test piece was cooled to room temperature at a cooling rate of 1 ° C./s to obtain a softened carbon steel of Example 12. As described above, in Example 12, the time required for executing the series of heat treatment cycles was 716 seconds (11.93 minutes).

  Example 13 differs from Example 11 described above only in the holding time of the high temperature holding temperature T3 in the third heat treatment step, and the conditions of the other steps are the same. That is, in Example 13, after performing the first heat treatment step and the second heat treatment step under the same conditions as in Example 11, in the third heat treatment step, the test piece was moved from a low temperature holding temperature T2 of 550 ° C. to a high temperature of 670 ° C. The temperature was heated to the holding temperature T3, and the high temperature holding temperature T3 was held for 900 seconds. After finishing the third heat treatment step, in the cooling step, the test piece was cooled to room temperature at a cooling rate of 1 ° C./s to obtain a softened carbon steel of Example 13. As described above, in Example 13, the time required for executing the series of heat treatment cycles was 1556 seconds (25.90 minutes).

  Next, the rapid softening annealing process of Example 14 is demonstrated with reference to the schematic diagram of the heat processing cycle of FIG. In Example 14, after performing the first heat treatment step to the third heat treatment step under the same conditions as in Example 11, in the cooling step, the test piece was changed from a high temperature holding temperature T3 of 670 ° C. to 450 ° C. at 5 ° C./s. After cooling at a speed, it was cooled to room temperature at 20 ° C./s to obtain a softened carbon steel of Example 14. The time required for cooling to room temperature in the cooling step in Example 14 was 65 seconds. As described above, in Example 14, the time required for executing the series of heat treatment cycles was 91 seconds (1.52 minutes).

  Next, the rapid softening annealing method of Example 15 will be described with reference to FIG. In Example 15, after performing three cycles from the first heat treatment step to the third heat treatment step under the same heat treatment conditions from the first heat treatment step to the third heat treatment step in Example 13 described above, Example 13 The softening carbon steel of Example 15 was obtained by performing the cooling step under the same conditions as in Example 1. As described above, in Example 15, the time required for executing the series of heat treatment cycles was 1604 seconds (26.70 minutes).

  Next, the rapid softening annealing method of Example 16 will be described. In Example 16, after performing three cycles from the first heat treatment step to the second heat treatment step under the same heat treatment conditions from the first heat treatment step to the third heat treatment step in Example 11 described above, Example 11 was performed. The softening carbon steel of Example 16 was obtained by performing the third heat treatment step and the cooling step under the same conditions. As described above, in Example 16, the time required for executing the series of heat treatment cycles was 696 seconds (11.60 minutes).

  Next, the rapid softening annealing process of Example 17 will be described with reference to the schematic diagram of the heat treatment cycle of FIG. In Example 17, a test steel pipe subjected to drawing and flattening was used. The test steel pipe subjected to the drawing process and the flattening process is used for manufacturing a hollow rack bar. On the flattened surface subjected to the drawing and flattening, teeth are formed by plastic working after the softening annealing treatment.

  In Example 17, the test steel pipe that had been subjected to drawing and flattening in the first heat treatment step was heated to an austenitizing temperature T1 of 752 ° C. In Example 17, since heating was performed from room temperature to 752 ° C. in 5 seconds, the rate of temperature increase in the first heat treatment step was 144.4 ° C./s. In the first heat treatment step, immediately after the test steel pipe was heated to 752 ° C., the temperature was lowered to the low temperature holding temperature T2 of 525 ° C. in the second heat treatment step. In Example 17, since the temperature was lowered from 752 ° C. to 525 ° C. in 72 seconds, the cooling rate of the second heat treatment step was about 3 ° C./s. After finishing the second heat treatment step, the test steel pipe was heated to a high temperature holding temperature T3 of 625 ° C. in the third heat treatment step. In Example 17, since the temperature was increased from 525 ° C. to 625 ° C. in 5 seconds, the rate of temperature increase in the third heat treatment step was 20 ° C./s. After finishing the third heat treatment step, in the cooling step, the test steel pipe was cooled to 543 ° C. at a rate of about 1.4 ° C./s and then cooled to room temperature at 20 ° C./s. Carbonized steel was obtained. The time required for cooling to room temperature in the cooling step in Example 17 was 80 seconds. As described above, in Example 17, the time required for executing the series of heat treatment cycles was 162 seconds (2.70 minutes).

  Next, the rapid softening annealing process of Example 18 will be described with reference to the schematic diagram of the heat treatment cycle of FIG. In Example 18, the test steel pipe subjected to drawing and flattening in the first heat treatment step was heated to an austenitizing temperature T1 of 767 ° C. In Example 18, since heating was performed from room temperature to 767 ° C. in 5 seconds, the rate of temperature increase in the first heat treatment step was 147.4 ° C./s. Immediately after the test steel pipe was heated to 767 ° C. in the first heat treatment step, the temperature was lowered to the low temperature holding temperature T2 of 539 ° C. in the second heat treatment step. In Example 18, since the temperature was lowered from 767 ° C. to 539 ° C. in 72 seconds, the cooling rate of the second heat treatment step was about 3 ° C./s. After finishing the second heat treatment step, the test steel pipe was heated to a high temperature holding temperature T3 of 665 ° C. in the third heat treatment step. In Example 18, since the temperature was increased from 539 ° C. to 665 ° C. in 5 seconds, the rate of temperature increase in the third heat treatment step was 25.2 ° C./s. After finishing the third heat treatment step, in the cooling step, the test steel pipe was cooled to 565 ° C. at a rate of about 1.4 ° C./s, and then cooled to room temperature at 20 ° C./s. Carbonized steel was obtained. The time required for cooling to room temperature in the cooling step in Example 18 was 80 seconds. As described above, in Example 18, the time required for executing the series of heat treatment cycles was 162 seconds (2.70 minutes).

  In Example 19, after performing three cycles from the first heat treatment step to the third heat treatment step under the same heat treatment conditions from the first heat treatment step to the third heat treatment step in Example 18 described above, Example 18 The softening carbon steel of Example 19 was obtained by performing the cooling step under the same conditions as in Example 1. The time required for cooling to room temperature in the cooling step in Example 19 was 80 seconds. As described above, in Example 19, the time required to execute the series of heat treatment cycles was 486 seconds (8.10 minutes).

Comparative example

  In Comparative Examples 1 to 8 shown below, softened carbon steel was produced using the same test pieces as those of Examples 1 to 16 described above. Table 3 below summarizes the softening annealing conditions of Comparative Examples 1 to 8.

[Comparative Example 1]
Comparative Example 1 was different from Example 1 only in the austenitizing temperature T1 in the first heat treatment step, and the other process conditions were the same. That is, in Comparative Example 1, the test piece was heated to the austenitizing temperature T1 of 740 ° C. in the first heat treatment step. In Comparative Example 1, since heating was performed from room temperature to austenitizing temperature T1 in 8 seconds as in Example 1, the rate of temperature increase in the first heat treatment step was 89 ° C./s. Then, also about the comparative example 1, the softened carbon steel of the comparative example 1 was obtained through the 2nd heat processing process, the 3rd heat processing process, and the cooling process similarly to Example 1. FIG. In Comparative Example 1, since the temperature was lowered from 740 ° C. to 550 ° C. in 2 seconds, the cooling rate of the second heat treatment step was 95 ° C./s.

[Comparative Example 2]
Comparative Example 2 was different from Example 1 only in the austenitizing temperature T1 in the first heat treatment step, and the other process conditions were the same. That is, in Comparative Example 2, the test piece was heated to the austenitizing temperature T1 of 800 ° C. in the first heat treatment step. In Comparative Example 2, since heating was performed from room temperature to austenitizing temperature T1 in 8 seconds as in Example 1, the rate of temperature increase in the first heat treatment step was 96 ° C./s. Then, also about the comparative example 2, the softened carbon steel of the comparative example 2 was obtained through the 2nd heat processing process, the 3rd heat processing process, and the cooling process similarly to Example 1. FIG. In Comparative Example 2, since the temperature was lowered from 800 ° C. to 550 ° C. in 2 seconds, the cooling rate of the second heat treatment step was 125 ° C./s.

[Comparative Example 3]
Comparative Example 3 was different from Example 2 described above only in the low temperature holding temperature T2 in the second heat treatment step, and the other process conditions were the same. That is, in Comparative Example 3, as in Example 2, immediately after the test piece was heated to the austenitizing temperature T1 of 770 ° C. in the first heat treatment step, the test piece was brought to the low temperature holding temperature T2 of 100 ° C. in the second heat treatment step. After the temperature was lowered, the low temperature holding temperature T2 was held for 10 seconds. In Comparative Example 3, since the temperature was lowered from 770 ° C. to 100 ° C. in 2 seconds, the cooling rate of the second heat treatment step was 335 ° C./s. After finishing the second heat treatment step, the softened carbon steel of Comparative Example 3 was obtained in Comparative Example 3 through the third heat treatment step and the cooling step as in Example 2.

[Comparative Example 4]
Comparative Example 4 differs from Example 2 described above only in the low-temperature holding temperature T2 in the second heat treatment step, and the other process conditions are the same. That is, in Comparative Example 4, as in Example 2, immediately after the test piece was heated to the austenitizing temperature T1 of 770 ° C. in the first heat treatment step, the test piece was brought to the low temperature holding temperature T2 of 650 ° C. in the second heat treatment step. After the temperature was lowered, the low temperature holding temperature T2 was held for 10 seconds. In Comparative Example 4, since the temperature was lowered from 770 ° C. to 650 ° C. in 2 seconds, the cooling rate of the second heat treatment step was 60 ° C./s. After finishing the second heat treatment step, the softened carbon steel of Comparative Example 4 was obtained in Comparative Example 4 through the third heat treatment step and the cooling step as in Example 2.

[Comparative Example 5]
Comparative Example 5 was different from Example 2 described above only in the high temperature holding temperature T3 in the third heat treatment step, and the other process conditions were the same. That is, in Comparative Example 5, after performing the first heat treatment step and the second heat treatment step under the same conditions as in Example 2, in the third heat treatment step, the high temperature holding temperature T3, which is the same as the low temperature holding temperature T2, is 550 ° C. Maintained for 10 seconds. In this case, since there is no temperature change from the low temperature holding temperature T2 to the high temperature holding temperature T3, the rate of temperature increase in the third heat treatment step is 0 ° C./s. After finishing the third heat treatment step, the test piece was rapidly cooled to room temperature in the cooling step to obtain a softened carbon steel of Comparative Example 5.

[Comparative Example 6]
Comparative Example 6 was different from Example 2 described above only in the high temperature holding temperature T3 in the third heat treatment step, and the other process conditions were the same. That is, in Comparative Example 6, after performing the first heat treatment step and the second heat treatment step under the same conditions as in Example 2, in the third heat treatment step, the test piece was moved from a low temperature holding temperature T2 of 550 ° C. to a high temperature of 720 ° C. The temperature was heated to the holding temperature T3, and the high temperature holding temperature T3 was held for 10 seconds. In Comparative Example 6, since the temperature was raised from 550 ° C. to 720 ° C. in 1 second, the rate of temperature increase in the third heat treatment step was 170 ° C./s. After finishing the third heat treatment step, the test piece was rapidly cooled to room temperature in the cooling step to obtain a softened carbon steel of Comparative Example 6.

[Comparative Example 7]
Next, with reference to the schematic diagram of the heat treatment cycle of FIG. 12, Comparative Example 7 describes the softening annealing process. In Comparative Example 7, the test piece was heated to 760 ° C. at a temperature increase rate of 100 ° C./s, and then the temperature was held for 10 seconds. Thereafter, the test piece was cooled to 650 ° C. at a rate of 5 ° C./s and then cooled to room temperature at 20 ° C./s to obtain a softened carbon steel of Comparative Example 7. The time required for cooling to room temperature in the cooling step in Comparative Example 7 was 53 seconds. As described above, in Comparative Example 7, the time required for executing the series of heat treatment cycles was 71 seconds (1.18 minutes).

[Comparative Example 8]
Next, with reference to the schematic diagram of the heat treatment cycle of FIG. 13, Comparative Example 8 describes the softening annealing process. In Comparative Example 8, the test piece was heated to 760 ° C. at a temperature increase rate of 100 ° C./s, and then the temperature was held for 3600 seconds (1 hour). Then, the test piece was cooled to room temperature in 15 seconds, and the softened carbon steel of the comparative example 8 was obtained. The time required for cooling to room temperature in the cooling step in Comparative Example 8 was 15 seconds. As described above, in Comparative Example 8, the time required for executing the series of heat treatment cycles was 3623 seconds (60.38 minutes).

[Evaluation]
Vickers hardness HV (1) is measured for the softened carbon steel of Examples 1 to 16 obtained as described above, the softened carbon steel of Comparative Examples 1 to 8 and the material before the softening annealing treatment. And evaluated. Moreover, Vickers hardness HV (5) was measured and evaluated about the softened carbon steel of Examples 17-19 obtained by the above-mentioned, and the raw material before a softening annealing process. Table 4 below summarizes the measurement results of Examples 1 to 16, Comparative Examples 1 to 8, and Vickers hardness of the raw material before processing. In Table 4, for Examples 1 to 10 and Comparative Examples 1 to 6, eight Vickers hardness measurements were performed and the average values thereof were shown. Examples 11 to 16 and About the comparative example 7 and the comparative example 8, 3 points | pieces of Vickers hardness measurement are implemented and those average values are shown.

  The Vickers hardness of the carbon steel before the treatment used in each Example and Comparative Example shown in Table 4 is 186HV (1), and the Vickers hardness of the softened carbon steel of each of the Examples 1 to 16 after the treatment. All were 170 HV (1) or less. In Examples 1 to 14 in which only one cycle from the first heat treatment step to the third heat treatment step was performed, the time required for these heat treatments was 41 seconds to 25 minutes 56 including the cooling step as described above. Second. In Example 15 in which the first heat treatment step to the third heat treatment step were performed for 3 cycles, the time required for the heat treatment was 26 minutes 44 seconds. In Example 16 in which the third heat treatment step was performed after three cycles of the first heat treatment step to the second heat treatment step, the time required for the heat treatment was 11 minutes and 36 seconds. On the other hand, in Comparative Example 7 in which the softening annealing process can be performed in a relatively short time (1.11 seconds), it is 183HV (1), and in Comparative Example 8 in which the softening annealing process is performed over 1 hour. 164HV (1).

  From the above measurement results, the conventional rapid softening annealing method as shown in Comparative Example 7 is based on the hardness of the carbon steel before processing, but the Vickers hardness can be softened only by 1.6%. On the other hand, it can be seen that the softened carbon steel subjected to the rapid softening annealing process according to the present invention can reduce the Vickers hardness by 8% or more in a short time of less than 30 minutes. According to the rapid softening annealing process of the present invention, it is possible to reduce the hardness of the carbon steel as in the softening annealing process that requires one hour for the processing time as shown in Comparative Example 8. Recognize.

  In particular, by shortening the cooling time in the cooling process, spheroidizing annealing of carbon steel can be performed in a very short time, and therefore, partial softening annealing of carbon steel is continuously performed in the production line. It becomes possible to do. Therefore, the present invention can locally soften and anneal a portion of carbon steel that requires machining or plastic working, avoiding a decrease in hardness in a portion that requires hardness, and processing It becomes possible to remarkably improve the workability of a necessary part. Therefore, by adopting the rapid softening annealing method for carbon steel according to the present application, it becomes possible to provide a high quality product with excellent processing accuracy.

  Next, the optimum conditions for the temperatures T1 to T3 in the present invention, the high temperature holding time in the third heat treatment step, the cooling rate in the cooling step, and the number of heat treatment cycles will be described with reference to the above-described examples and comparative examples.

(1) Austenitizing temperature T1
FIG. 14 is a diagram showing the relationship between the austenitizing temperature T1 and hardness of Examples 1 to 3, Comparative Example 1, and Comparative Example 2. As shown in FIG. 3, Example 1 to Example 3, Comparative Example 1 and Comparative Example 2 differ only in the conditions for the austenitizing temperature T1, and all other conditions are subjected to the same heat treatment cycle. The austenitizing temperature T1 was 740 ° C. in Comparative Example 1, 760 ° C. in Example 1, 770 ° C. in Example 2, 785 ° C. in Example 3, and 800 ° C. in Comparative Example 2.

  From FIG. 14, when the austenitizing temperature T1 is in the range of 760 ° C. to 785 ° C., the carbon steel whose pre-treatment Vickers hardness was 186 HV (1) is less than 170 HV (1). It can be seen that the softening annealing process could be performed. On the other hand, when the austenitizing temperature T1 was lower than 760 ° C. and exceeded 785 ° C., the softening annealing treatment was only possible up to Vickers hardness of 172 HV (1) or 174 HV (1). I understand.

  From the above, by adopting austenitizing temperature T1 in the first heat treatment step of 760 ° C. to 785 ° C., the carbon steel before treatment which was 186 HV (1) is softened to 170 HV (1) or less. It turns out that it becomes possible. Therefore, it can be confirmed that by employing the austenitizing temperature T1, it is possible to reduce the Vickers hardness by 8% or more based on the hardness of the carbon steel before the treatment.

(2) Low temperature holding temperature T2
FIG. 15 is a graph showing the relationship between the low temperature holding temperature T2 and the hardness of Example 1, Examples 4 to 6, Comparative Example 3, and Comparative Example 4. As shown in FIG. 4, Example 1, Example 4 to Example 6, and Comparative Example 3 and Comparative Example 4 differ only in the condition of the low temperature holding temperature T2, and the same heat treatment cycle was performed for all other conditions. Is. The low temperature holding temperature T2 was 100 ° C in Comparative Example 3, 450 ° C in Example 4, 500 ° C in Example 5, 550 ° C in Example 1, 600 ° C in Example 6, and 650 ° C in Comparative Example 4. .

  From FIG. 15, when the low temperature holding temperature T2 is in the range of the Ms point of the carbon steel (in this case, 386 ° C.) to 600 ° C., the carbon steel whose pre-treatment Vickers hardness was 186 HV (1) It can also be seen that the softening annealing treatment was able to be performed to a level below 170 HV (1). On the other hand, when the low temperature holding temperature T2 is lower than 386 ° C. which is the Ms point of the carbon steel, it can be understood that the softening annealing treatment can be performed only to Vickers hardness of up to 178HV (1). Moreover, when the low temperature holding temperature T2 exceeds 600 degreeC, it turns out that Vickers hardness became 194HV (1) and the hardness became higher than before a process.

  From the above, the low temperature holding temperature T2 in the second heat treatment step adopts the Ms point of 386 ° C. to 650 ° C. which is the Ms point of the carbon steel, and the carbon steel before the treatment, which was 186 HV (1), is 170 HV. (1) It can be seen that softening is possible below. Therefore, it can be confirmed that by adopting the low temperature holding temperature T2, it is possible to reduce the Vickers hardness by 8% or more on the basis of the hardness of the carbon steel before the treatment.

(3) High temperature holding temperature T3
FIG. 16 is a diagram showing the relationship between the high temperature holding temperature T3 and the hardness of Example 1, Examples 7 to 9, Comparative Example 5, and Comparative Example 6. As shown in FIG. 5, Example 1, Example 7 to Example 9, Comparative Example 5 and Comparative Example 6 differed only in the condition of the high temperature holding temperature T3, and all other conditions were subjected to the same heat treatment cycle. It is. The high temperature holding temperature T3 was 550 ° C in Comparative Example 5, 600 ° C in Example 7, 650 ° C in Example 8, 670 ° C in Example 1, 700 ° C in Example 9, and 720 ° C in Comparative Example 6. .

  From FIG. 16, when the high temperature holding temperature T3 is in the range of 600 ° C. to 700 ° C., the carbon steel whose pre-treatment Vickers hardness was 186 HV (1) is softened to 170 HV (1) or less. It can be seen that the annealing treatment was possible. On the other hand, when the high temperature holding temperature T3 is lower than 600 ° C., it can be seen that the softening annealing process can be performed only to Vickers hardness of up to 177HV (1). Moreover, when high temperature holding temperature T3 exceeds 700 degreeC, it turns out that Vickers hardness became 227HV (1) and the hardness became higher than before a process.

  From the above, by adopting 600 ° C. to 700 ° C. as the high temperature holding temperature T3 in the third heat treatment step, the carbon steel before treatment, which was 186 HV (1), is softened to 170 HV (1) or less. It turns out that it becomes possible. Therefore, by adopting the high temperature holding temperature T3, it can be confirmed that the Vickers hardness can be reduced by 8% or more based on the hardness of the carbon steel before the treatment.

(4) High temperature holding time in the third heat treatment step As shown in FIG. 7, Example 11 to Example 13 differ only in the time for holding the high temperature holding temperature T3 in the third heat treatment step, and all other conditions are the same. A heat treatment cycle was performed. The holding time of the high temperature holding temperature T3 was 10 seconds in Example 11, 60 seconds in Example 12, and 900 seconds in Example 13.

  As can be seen from Table 4 above, the carbon steel having a Vickers hardness of 186 HV (1) before the treatment was obtained regardless of whether the holding time of the high temperature holding temperature T3 was 10 seconds to 900 seconds. It can be seen that the softening annealing treatment can be performed to a level below 160 HV (1). That is, it can be seen that there is almost no difference between the case where the holding time of the high temperature holding temperature T3 is 900 seconds and the case where the holding time is 10 seconds so that the softening can be performed. Therefore, in consideration of shortening of the processing time, it can be said that it is appropriate to perform the processing in a shorter time, for example, 60 seconds or less.

(5) Cooling rate in the cooling step The above-described Example 2 and Example 10 differ only in the cooling rate in the cooling step, and all other conditions are the same heat treatment cycle. In Example 2, rapid cooling was performed in the cooling step. In Example 10, after cooling to 450 ° C. at a rate of 5 ° C./s, cooling was performed to room temperature at 20 ° C./s. Moreover, as shown in FIG. 8, Example 11 and Example 14 differ only in the cooling rate in a cooling process, All other conditions performed the same heat processing cycle. Example 11 is cooled at a rate of 1 ° C./s in the cooling step, and Example 14 is cooled at a rate of 5 ° C./s to 450 ° C. in the cooling step, and then cooled to 20 ° C./s to room temperature. went.

  As can be seen from Table 4 shown earlier, even if the cooling rate in the cooling process is different, the carbon steel whose Vickers hardness before processing was 186 HV (1) is 164 HV (1). It can be seen that the softening annealing process was able to be performed to an extent below. Therefore, it can be seen that the cooling rate in the cooling step hardly affects the degree of softening. Therefore, it can be said that it is appropriate to cool in a shorter time in consideration of shortening of the processing time.

(6) Number of heat treatment cycles The above-described Example 13 and Example 15 differ only in the number of heat treatment cycles performed from the first heat treatment step to the third heat treatment step, and all other conditions were performed in the same manner. is there. In Example 13, the heat treatment cycle from the first heat treatment step to the third heat treatment step was performed once, and then the cooling step was performed. In Example 15, the above-described heat treatment cycle was repeated three times. The cooling process is performed.

  As can be seen from Table 4 shown above, when the number of heat treatment cycles is one, the carbon steel having a Vickers hardness of 186 HV (1) before treatment should be softened to 157 HV (1). It can be seen that when the number of heat treatment cycles was 3, the softening treatment could be further performed up to 156 HV (1). Therefore, it can be seen that the hardness of the carbon steel can be slightly reduced by repeating the number of executions of the heat treatment cycle. Therefore, when it is required to perform the softening annealing process with higher accuracy, it can be said that it is effective to adjust the number of executions of the heat treatment cycle.

  Further, Example 11 and Example 16 described above differ only in the number of executions of the heat treatment cycle from the first heat treatment step to the second heat treatment step, and all other conditions were performed in the same manner. In Example 11, the heat treatment cycle from the first heat treatment process to the third heat treatment process was performed once, and then the cooling process was performed. In Example 16, the process from the first heat treatment process to the second heat treatment process was performed. The third heat treatment step and the cooling step are performed after the heat treatment cycle is repeated three times.

  As can be seen from Table 4 shown above, when the number of heat treatment cycles to the first heat treatment step and the second heat treatment step is one, the carbon steel whose Vickers hardness before treatment was 186 HV (1) Can be softened to 159 HV (1), and when the number of heat treatment cycles to the first heat treatment step and the second heat treatment step is three, softening treatment can be further performed to 157 HV (1). I understand that. Therefore, it can be seen that the hardness of the carbon steel can be slightly reduced by repeating the number of executions of the heat treatment cycle. Therefore, when it is required to perform the softening annealing process with higher accuracy, it can be said that it is effective to adjust the number of executions of the heat treatment cycle.

  Next, the hardness of each part of Examples 17 to 19 in which the flattened test steel pipe is softened and annealed by the rapid softening annealing method of the present invention will be described. In Examples 17 to 19, Vickers hardness HV (5) was measured for each part and evaluated. FIG. 17 shows a schematic cross-sectional view of the test steel pipe used in Examples 17 to 19 and the hardness measurement position. FIG. 18 shows the test steel pipe before treatment and the test steel pipes of Examples 17 to 19. The hardness at each measurement position is shown. Table 5 shows the hardness reduction rate and the evaluation results of whether or not the softening treatment is properly performed, together with the hardness of each measurement position of the test steel pipes of Examples 17 to 19. The measurement position of the hardness is as follows: the surface of the flattened surface as the measurement unit 1 is 0.5 mm, the surface of the flattened surface as the measurement unit 2 is 0.5 mm, the surface of the flattened surface as the measurement unit 3 The position is 0.5 mm.

  The test steel pipe before the treatment had a Vickers hardness of about 210 HV (5) in the measuring unit 1 and the measuring unit 3 having a low degree of processing, but the Vickers hardness was 259 HV (5) in the measuring unit 3 having a high degree of processing. It was so high.

  As shown in FIG. 18 and Table 5, the test steel pipes of each Example 17 and Example 18 subjected to the rapid softening annealing treatment of the present invention were such that the Vickers hardness of the entire surface of the test steel pipe was substantially uniform. It can be seen that the softening annealing is performed. Moreover, the test steel pipe of each Example 17 and Example 18 is softening annealing process even if it is the measurement part 1 and the measurement part 3 with a small workability which were conventionally difficult to perform the softening annealing process in a short time. You can see that

  Moreover, as can be seen from FIG. 18 and Table 5, the hardness of the measuring unit 3 is greatly reduced with reference to the test steel pipe before processing. This is thought to be because the recovery and recrystallization of the processed metal structure at the corners having a high degree of processing by flattening were also caused by the rapid softening annealing according to the present invention.

  From the above, according to the rapid softening annealing method of the present invention, softening annealing is performed so that the entire surface of the flattened surface is uniform in the test steel pipe subjected to flattening. It can be said that it can be processed. Therefore, it can be seen from the above that the hardness of the entire flattened surface is uniformly softened, so that machining or plastic working on the flattened surface can be performed with high accuracy.

  18 and Table 5 also show the test steel pipe of Example 19 in which the heat treatment cycle under the same conditions as in Example 18 was repeated three times. 18 and Table 5, as described above, when the heat treatment cycle from the first heat treatment step to the third heat treatment step is repeated three times, the Vickers hardness of the entire surface of the flattened surface is further increased. , You can see that it is falling. From this, as described above, it can be said that the hardness of the carbon steel can be reduced by repeating the number of heat treatment cycles.

  The rapid softening annealing method according to the present invention can realize softening annealing of carbon steel in a very short time. Moreover, a partial softening annealing process is also possible in the production line. Therefore, it is possible to soften only the portion that needs to be processed without reducing the hardness of the portion that requires strength, and to perform machining with high accuracy, and to provide high-quality mechanical structural components.

Claims (8)

A method of rapid softening annealing of carbon steel,
The chemical composition of the carbon steel is 0.27 mass% to 0.58 mass% for carbon, 0.15 mass% to 0.35 mass% for silicon, 0.30 mass% to 1.50 mass% for manganese, Phosphorus is 0.03% by mass or less, sulfur is 0.03% by mass or less, nickel is 0.25% by mass or less, chromium is 0.35% by mass or less, and the balance is iron and inevitable impurities,
A method for rapid softening annealing of carbon steel, comprising the following steps.
First heat treatment step: the carbon steel is heated, the step of increasing the surface of the carbon steel at _{Ac 1 + 20 ℃ ~Ac 1 +} 50 ℃ of the austenitizing temperature T1 10 ° C. / s or faster.
Second heat treatment step: Immediately after the completion of the first heat treatment step, the carbon steel is lowered at a rate of 1 ° C./s or more to a low temperature holding temperature T2 of Ms point to 600 ° C.
Third heat treatment step: A step of raising the carbon steel that has finished the second heat treatment step at a rate of 10 ° C./s or higher to a high temperature holding temperature T3 of Ae ₁ −130 ° C. to Ae ₁ −20 ° C.
Cooling step: A step of cooling the carbon steel that has finished the third heat treatment step to obtain a softened carbon steel.   The low temperature holding temperature T2 is maintained for 30 seconds or less in the second heat treatment step and / or the high temperature holding temperature T3 is maintained for 60 seconds or less in the third heat treatment step. Rapid softening annealing method for carbon steel.   The rapid softening annealing method of carbon steel according to claim 1 or 2, wherein the softening treatment reduces Vickers hardness by 8% or more on the basis of the hardness of the carbon steel before the treatment. . 4. The rapid carbon steel according to claim 1, wherein a high-frequency induction heating method is used in at least one of the first heat treatment step and the third heat treatment step. 5. Softening annealing method. The rapid softening annealing method for carbon steel according to claim 2, wherein a high-frequency induction heating method is used in the second heat treatment step.   The austenitizing temperature T1 in the first heat treatment step is 760 ° C to 785 ° C, and the rate of temperature rise to the austenitizing temperature T1 is 90 ° C / s to 150 ° C / s. The method for rapid softening annealing of carbon steel according to any one of the above.   The rapid softening annealing method of carbon steel according to any one of claims 1 to 6, wherein a cooling rate to the low temperature holding temperature T2 in the second heat treatment step is 3 ° C / s to 170 ° C / s.   The high temperature holding temperature T3 in the third heat treatment step is 600 ° C to 700 ° C, and the rate of temperature rise to the high temperature holding temperature T3 is 20 ° C / s to 350 ° C / s. The method for rapid softening annealing of carbon steel according to any one of the above.