JP2006342368A - Heat treatment method for steel member - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a heat treatment method for a steel member including a quenching stage where, even in the case of a steel member such as a large-sized die, its toughening can be securely performed, and strain and cracks are hardly caused therein. <P>SOLUTION: The heat treatment method for a steel member includes a quenching stage where a steel member which contains at least 0.20 to 1.5 wt.% carbon and 0.5 to 25 wt.% carbide forming element(s) and is composed of a tool steel having a weight of ≥50 kg is subjected to the following cooling steps: the first cooling step where cooling is performed in a high temperature zone ht from a quenching temperature (1,030°C) to 600°C at the average cooling velocity Cl of >3°C/min in which the precipitation of a pearlite phase and a ferrite phase can be evaded; and the second cooling step where cooling is performed in a low temperature zone 1t from 500 to 130°C at the average cooling velocity of ≥1°C/min. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a heat treatment method for a steel member such as a die-casting die, a hot forging die, or a die made of tool steel and having a predetermined weight or more.

Conventionally, the quenching of hot die steel has required the following conditions in order to achieve martensite single phase.
(1) From the quenching temperature to around 600 ° C., in order to avoid precipitation of grain boundary carbides and pearlite transformation, rapid cooling is performed at a cooling rate above a certain level.
(2) Furthermore, in order to avoid bainite transformation, it is rapidly cooled to 250 to 150 ° C.
For example, to prevent distortion and cracking due to quenching and to obtain a tough steel mold that is less likely to cause heat cracking, from a quenching temperature to a temperature range of 650 to 300 ° C., troostite or grain boundary carbide precipitates. A die quenching method that suppresses precipitation of bainite by cooling at a rate faster than the cooling rate to be performed and then cooling in a polymer solution has been proposed (for example, see Patent Document 1).

JP-A-9-182948 (pages 1 to 12)

However, in the mold which is enlarged, since the cooling rate is difficult to make fast and uniform, (1) rapid cooling from the quenching temperature to around 600 ° C. to avoid precipitation of grain boundary carbides and pearlite transformation, and ( 2) It was difficult to satisfy both of the rapid cooling to 250 to 150 ° C. in order to avoid the bainite transformation. For this reason, it has been extremely difficult to toughen large dies by quenching.
In addition, quenching that satisfies both the above conditions (1) and (2) tends to increase the amount of deformation (distortion) of the mold, so that the number of steps for correction by finishing increases thereafter. There was also a problem.

  The present invention solves the problems described in the background art, and even a steel member such as a large metal mold can be reliably toughened and includes a quenching step in which distortion and cracking hardly occur. It is an issue to provide.

Means for Solving the Problems and Effects of the Invention

That is, the heat treatment method for steel members according to the present invention (Claim 1) includes at least 0.20 to 1.5 wt% carbon and 0.5 to 25 wt% carbide forming elements, and has a weight of 50 kg or more. It includes a quenching process for subjecting a steel member to the following cooling step.
First cooling step: In a high temperature range from the quenching temperature to a temperature at which at least one of the pearlite transformation and the ferrite transformation can start, cooling is performed at an average cooling rate that can avoid precipitation of the pearlite phase and the ferrite phase.
Second cooling step: Cooling is performed at an average cooling rate of 1 ° C./min or more in a low temperature zone from the bainite transformation start temperature to the bainite transformation finish temperature and the martensite transformation finish temperature.

Moreover, the heat treatment method (Claim 2) of the steel member (JIS: SKD61) according to the present invention includes at least 0.20 to 1.5 wt% carbon and 0.5 to 25 wt% carbide generating element. In addition, the steel member having a weight of 50 kg or more includes a quenching process in which the following cooling step is performed.
First cooling step: In a high temperature zone from the quenching temperature to 600 ° C., cooling is performed at an average cooling rate of more than 3 ° C./min that can prevent precipitation of a pearlite phase and a ferrite phase.
Second cooling step: Cooling is performed at an average cooling rate of 1 ° C./min or higher in a low temperature range of 500 ° C. or lower and up to 130 ° C.
Each temperature is preferably a temperature at a portion where the cooling rate is minimized in the cross section of the steel member to be processed, and such a portion is obtained from an estimation by numerical simulation, a real rule by inserting a thermocouple, an empirical rule, or the like.

According to each of the above methods, in the high temperature zone of the first cooling step, the cooling rate (3 ° C./noble time) that can avoid the start temperature and start time (nose time) of the pearlite transformation and the ferrite transformation where the pearlite phase and the ferrite phase start to precipitate. Therefore, it is possible to pass through the high temperature zone using the same austenite phase as the quenching temperature as a base layer. As a result, precipitation of the pearlite phase and the ferrite phase can be prevented, and the toughening can be achieved by the transformation in the low temperature zone described later. In the first cooling step, carbides may precipitate in the austenite phase.
Furthermore, in the low temperature zone of the second cooling step, cooling is performed at an average cooling rate of 1 ° C./min or more, so that martensitic transformation or bainitic transformation is easily started between 400 ° C. and martensitic transformation start temperature. . Moreover, since it cools at the said speed | rate until it becomes 130 degreeC, a retained austenite phase can be reduced. As a result, transformation in a temperature range of 500 ° C. to over 400 ° C. can be prevented. That is, since a situation in which a martensite phase or a bainite phase of coarse crystal grains is precipitated can be prevented, the steel member can be toughened by performing a tempering step described later.
Therefore, for example, a relatively large steel member such as a die casting mold having a weight of several hundred kg to 1 ton can be reliably toughened and heat-treated without causing deformation or cracking. Can be prevented.

The steel member contains 0.20 to 1.5 wt% of carbon and 0.5 to 25 wt% of a carbide generating element in order to generate an appropriate amount of carbide and toughen. If the carbon content is less than 0.20 wt% and the carbide generating element is less than 0.5 wt%, the amount of carbide generated is insufficient and cannot contribute to high strength, while the carbon content exceeds 1.5 wt% and carbide is generated. If the element content exceeds 25 wt%, the amount of carbide generated is excessive, so that it cannot contribute to high toughness and the cost is high. In order to prevent these, the range of the addition amount of carbon and carbide forming elements is restricted to the above range.
The carbide generating elements include V, W, Ti, Nb, Cr and the like, and carbides such as VC, WC, TiC, NbC, Cr ₇ C ₃ are generated by these and carbon. In addition to these, Si, Mn, S and the like may be further added according to the characteristics required for the steel member to be heat-treated.
Further, the reason why the weight of the steel member that is the subject of the present invention is 50 kg or more is that, in the case of a relatively small steel member, it is easy to quench the conditions (1) and (2) at the same time. It is intended for steel members such as large dies that are difficult to increase in speed and uniformity. The steel member subjected to the heat treatment has a complicated shape like a die-casting mold. However, if the steel member having a weight of 50 kg or more is expressed in a cube, the volume is 6330 cm ³ and the length of each side is 18 .5 cm.

Further, in the present invention, in the low temperature zone in the second cooling step, in the temperature zone from 500 ° C. to any one of the bainite transformation start temperature and the martensite transformation start temperature, at least the bainite transformation and the martensite transformation. One is a heat treatment method for a steel member that is cooled at a cooling rate that starts in a range of 400 ° C. to a martensite transformation start temperature, and that is cooled at an average cooling rate of 1 ° C./min or more after the transformation starts. 3) is also included.
According to this, in the above temperature zone in the low temperature zone, cooling is performed at an average cooling rate of 1 ° C./min or more, and between 400 ° C. and the martensitic transformation start temperature, martensitic transformation, bainitic transformation, or these Both are started. As a result, it is possible to prevent a situation in which a martensite phase or a bainite phase of coarse crystal grains is precipitated when the transformation is started in a temperature range exceeding 400 ° C. Therefore, it is possible to surely strengthen the steel member. .

Furthermore, in the present invention, in the intermediate temperature zone between the high temperature zone of the first cooling step and the low temperature zone of the second cooling step, or between 600 ° C. and 500 ° C., the cross section of the steel member to be quenched. Also included is a steel member heat treatment method (Claim 4) that controls the difference between the maximum temperature region and the minimum temperature region to be 200 ° C. or less.
According to this, since the difference in the cooling temperature inside the steel member is suppressed to 200 ° C. or less, the deformation amount (strain) due to the cooling between the respective parts can be reduced to 0.2% or less. As a result, for example, even if the steel member is a large metal mold having a side length of 500 to 1000 mm, a correction process with a large number of man-hours is not required, and thus the cost can be reduced. In addition, when the temperature difference inside the cross section is set to 150 ° C. or less, the deformation amount of the steel member can be suppressed to 0.1% or less.

In addition, the present invention includes a tempering step of heating and holding the quenched steel member at 50 to 750 ° C. after the quenching step (Claim 5). ) Is also included.
According to this, since the residual austenite phase remaining after the heat treatment step is decomposed and a stable structure of the martensite phase or bainite phase from which fine carbides are discharged is obtained, the original characteristics of the steel type are exhibited. be able to.
If the upper heating temperature is less than 50 ° C., the residual austenite phase cannot be decomposed. On the other hand, if the upper heating temperature exceeds 750 ° C., there is a high risk of reverse transformation to the austenite phase. For example, when the steel member is JIS: SKD61, the preferable tempering condition is 550 to 650 ° C. × 1 to 2 hours, but the tempering condition may be repeated about 2 to 5 times.

  In other words, a method for heat-treating a steel member, in which the steel member is a mold made of tool steel, may be included in the present invention. In this case, a relatively large steel member such as a die-casting die having a weight of several hundred kg can be reliably toughened and fired without causing deformation or cracking, and heat cracks can be prevented during use. Is possible.

In the following, the best mode for carrying out the present invention will be described.
FIG. 1 is a temperature-time graph showing the outline of the quenching process in the present invention and the comparative example, and FIG. 2 is a graph showing the temperature history of the quenching process and the tempering process of the present invention. Below, the steel member made into the object of heat processing consists of JIS: SKD61, and is a test piece of size 10.5 * 10.5 * 56mm.
As shown in FIGS. 1 and 2, the steel member of the example of the present invention is heated to a quenching temperature of 1030 ° C. and is soaked by holding for about 2 hours, and an austenite phase having an average crystal grain size of dγ ( γ phase).
For example, the steel member is inserted into oil, for example, and as shown by a curved solid line in FIG. 1, from 1030 ° C. (quenching temperature) to at least one transformation end temperature (line) Pf of pearlite transformation and ferrite transformation, Pf, In the high temperature zone ht up to Ff (specifically, 600 ° C.), a first cooling step is performed in which cooling (rapid cooling) is performed at an average cooling rate C1 that can avoid precipitation of pearlite phase and ferrite phase. The average cooling rate C1 is specifically more than 3 ° C./min. For example, methods such as standing cooling, blast cooling, and cooling in a high-temperature refrigerant are used.

The graph of FIG. 3 shows the result of conducting an impact test at room temperature on a 10 × 10 × 55 mm JIS3 test piece cut out from a plurality of the test pieces subjected to the heat treatment method by changing the average cooling rate C1. . According to this, when the average cooling rate C1 exceeds 3 ° C./min, the impact value (A) is at a high level. This indicates that, for example, VC (carbide) shown in FIG. 1 precipitates at the grain boundary of the austenite phase during cooling in the high temperature zone ht, but the adverse effect on toughness is small. On the other hand, at 3 ° C./min or less, the impact value (B) rapidly decreased. This is because the pearlite phase was precipitated because the average cooling rate C1 was too low.
From the above results, the average cooling rate C1 is more than 3 ° C./min. The most important thing in the high temperature zone ht is to avoid precipitation of ferrite phase and pearlite phase, and it has been found that some precipitation of carbide is acceptable.
In order to increase the impact value, as shown in the graphs of FIGS. 4 and 5, the hardness (HRC) after the tempering process described later is decreased, and the average crystal grain size dγ of the austenite phase at the quenching temperature is decreased. It was also confirmed that it was good.

On the other hand, as shown by the alternate long and short dashed line on the right side in FIG. 1, Comparative Example A that conflicts with the start lines Ps and Fs of the pearlite transformation or the ferrite transformation is out of the present invention because the pearlite phase or the ferrite phase is precipitated. .
Further, Comparative Example B targeting a relatively small steel member having a weight of less than 50 kg is cooled at a higher cooling rate than the example of the present invention, as indicated by a two-dot chain line curved to the left in FIG. Even so, since the entire steel member is cooled substantially uniformly, it does not easily cause cracking or deformation. In addition, the small steel member of Comparative Example B is relatively easy to toughen because it is easy to avoid the precipitation of carbides in the high temperature zone ht. For these reasons, the present invention is intended for steel members weighing 50 kg or more.

As shown in FIG. 1, the steel member of the present invention example that is subsequently cooled has an intermediate temperature zone mt of 600 to 500 ° C., and the difference between the highest temperature portion and the lowest temperature portion in the cross section is 200 ° C. or less. It is controlled to cool at an average cooling rate such as 3 ° C./min. As a result, in the intermediate temperature zone mt, the deformation amount of the steel member accompanying cooling can be suppressed to 0.2% or less, and the next second cooling step can be performed well. When the temperature difference was 150 ° C. or less, the deformation amount could be suppressed to 0.1% or less.
Next, from the bainite transformation start temperature Bs to the control cooling finish temperature Tf lower than the bainite transformation finish temperature Bf and the martensite transformation finish temperature Mf, that is, in the steel member, up to 500 ° C. and the control finish temperature of 130 ° C. In the low temperature zone lt, as shown in FIG. 2, cooling is performed at an average cooling rate CT of 1 ° C./min or more.

As shown in FIG. 2, the average cooling rate CT is, strictly speaking, the average cooling rate C2U from 500 ° C. to the martensite transformation start temperature Ms (or the bainitic transformation start temperature Bs), and the martensite transformation end temperature from now on. It is divided into two stages, that is, an average cooling rate C2L up to a control cooling end temperature Tf (for example, 130 ° C.) which is Mf (or bainite transformation end temperature Bf).
Hereinafter, the martensitic transformation start temperature Ms is simply referred to as Ms, and the bainite transformation start temperature Bs is simply referred to as Bs.
The average cooling rate C2U is such a value that at least one of martensitic transformation and bainite transformation starts in the range of 400 ° C. to Ms, for example, 2 ° C./min or more. As a result, it is possible to prevent the martensite phase or the bainite phase from being mixed in the coarse crystal grains that are precipitated when the transformation is started in a temperature range exceeding 400 ° C.

Here, the influence of the transformation start temperature (Ms or Bs) in the target steel member (JIS: SKD61) was examined.
First, the graph of FIG. 6 shows the relationship between the average cooling rate C2U and the transformation start temperature (Ms or Bs) with the average crystal grain size dγ of the austenite phase and the average cooling rate C1 being constant. According to this, it was found that as the average cooling rate C2U is smaller, the transformation start temperature becomes higher, and particularly when the average cooling rate C2U is less than 5 ° C./min, the transformation start temperature becomes remarkably higher. Therefore, it is recommended that the speed C2U is 5 ° C./min or more.
Further, the relationship between the transformation start temperature (Ms or Bs) and the hardness after tempering process (HRC) with the average crystal grain size dγ, the average cooling rate C1, the average cooling rate C2L, and the control end temperature Tf being constant. Is shown in the graph of FIG. According to this, the higher the transformation start temperature (Ms or Bs) and the higher the hardness, the lower the impact value. Therefore, to increase the impact value, the transformation start temperature (Ms or Bs) is set to 400 ° C. or lower, It was also found that the hardness needs to be lowered.

Further, assuming that the average cooling rate C1, the average cooling rate C2L, the control end temperature Tf, and the hardness (HRC) after the tempering step are constant, the average crystal grain size dγ of the austenite phase and the transformation start temperature (Ms or Bs). ) Is shown in the graph of FIG. According to this, it was found that the impact value can be increased as the average crystal grain size dγ is smaller and the transformation start temperature (Ms or Bs) is lower.
From the results of the graphs of FIGS. 6 to 8 described above, it was found that the transformation start temperature (Ms or Bs) needs to be between 400 ° C. and Ms. Note that cooling in a refrigerant at a relatively low temperature (20 to 200 ° C.) is effective for a cooling method in which the transformation start temperature (Ms or Bs) is started between 400 ° C. and Ms. Such refrigerants include water, oil, water-soluble quenching agents and the like. Further, the steel member of the present invention exhibits either martensitic transformation or bainitic transformation in the low temperature zone lt, as shown by the curved solid line in FIG. 1, depending on the composition, cooling conditions, and the like.

Even if the transformation is started between 400 ° C. and Ms, a tough steel member cannot be obtained after the tempering process unless the subsequent average cooling rate C2L is set to 1 ° C./min or more. The reason for this is that, particularly when the average cooling rate C2L during the bainite transformation is low, the newly transformed transformation phase (martensite phase, bainite layer, or a mixed layer thereof, hereinafter the same) becomes coarse, and the steel after tempering. This is to prevent toughening of the member.
Here, the influence of the average cooling rate C2L after the start of the transformation was examined.
First, assuming that the average crystal grain size dγ, the average cooling rate C1, C2U, and the control end temperature Tf are constant, the average cooling rate C2L after the start of the transformation, the impact value and the hardness (HRC) after tempering The relationship is shown in the graph of FIG.

According to this, it has been found that as the average cooling rate C2L increases and the hardness (HRC) decreases, the impact value increases, particularly at 1 ° C./min or higher. This is because the newly precipitated transformation phase is refined, and is particularly remarkable in the bainite phase during the bainite transformation.
The average cooling rate C1, C2U, the control end temperature Tf, and the hardness after tempering (HRC) are constant, and the average cooling rate C2L after the start of transformation, the average crystal grain size dγ, and the impact value are The relationship is shown in the graph of FIG. It has been found that the impact value increases as the average cooling rate C2L increases and the average crystal grain size dγ decreases.
From the above results, in order to increase the impact value of the steel member, the average crystal grain size dγ is decreased, the average cooling rate C2L is set to 1 ° C./min or more, and the hardness after tempering (HRC) is decreased. It turns out that there is a need.
The method for obtaining the average cooling rate C2L is effective in cooling in a relatively low-temperature refrigerant (water, oil, water-soluble quenching agent).

  Subsequently, as shown in FIGS. 1 and 2, the steel member of the present invention cooled at the average cooling rate C2L is subjected to the controlled cooling at the rate C2L by the martensite transformation end temperature Mf or the bainite transformation end temperature Bf. (Hereinafter, simply described as Mf or Bf) or continued to 130 ° C. or lower, which is lower than this. The reason for this is that when a large amount of austenite phase remains at the time when control cooling at an average cooling rate C2L of 1 ° C./min or more is completed, the remaining austenite phase becomes a coarse bainite phase in the subsequent steps. This is to inhibit the toughening of the steel member. Although it depends on the steel composition, the representative value of Mf is about 130 ° C., and the representative value of Bf is in the range of 250 to 130 ° C.

Here, the influence of the control cooling end temperature Tf in the target steel member (JIS: SKD61) was examined. First, assuming that the average crystal grain size dγ and the average cooling rate C1, CT (C2U, C2L) are constant, the relationship between the impact value of the steel member, the controlled cooling end temperature Tf, and the hardness after tempering (HRC) is shown in FIG. 11 graphs. According to this, the lower the control cooling end temperature Tf, the higher the impact value, and the higher the value, particularly at 130 ° C. or less. Further, the impact value tended to be higher as the hardness after tempering (HRC) was lower.
Further, the average cooling rate C1, CT (C2U, C2L) and the hardness after tempering (HRC) are constant, and the relationship between the impact value of the steel member, the controlled cooling end temperature Tf, and the average crystal grain size dγ. Is shown in the graph of FIG. Even in this case, the lower the control cooling end temperature Tf, the higher the impact value, and it was particularly remarkable at 130 ° C. or lower. Further, the impact value tended to increase as the average crystal grain size dγ decreased.
From the above results, it was found that after the start of martensitic transformation (or bainite transformation), it is necessary to maintain the controlled cooling at the average cooling rate C2L up to the controlled cooling end temperature Tf of 130 ° C. or lower.

Even if the average cooling rate CT is 1 ° C./min or more, the impact value of the steel member is lowered unless both of the average cooling rates C2U and C2L constituting the average cooling rate CT are 1 ° C./min or more. That is, when one of the average cooling rates C2U and C2L is less than 1 ° C./min, the transformation starts at a high temperature, and the transformation phase becomes coarse, or the newly precipitated transformation phase becomes coarse. Decrease the value. For this reason, both the average cooling rates C2U and C2L need to be 1 ° C./min or more.
Further, immediately after the quenching step, the austenite phase remaining in the steel member is decomposed into a coarse bainite phase by subsequent slow cooling (between 130 ° C. and room temperature or during cooling after the tempering step). It was also found that if the amount is less than 40 vol%, the adverse effect on the impact value is extremely small. From such a result, it is desirable that 60 vol% or more of the austenite phase existing at the time of quenching is transformed into a martensite phase or a bainite phase when a control cooling end temperature Tf of 130 ° C. or less is reached.

And the steel member cooled to room temperature through the quenching process is heated to a temperature Tt of 50 to 750 ° C. and held for 1 to 2 hours and then cooled to room temperature as shown in FIG. Is given. When the steel member is JIS: SKD61, it is tempered under the conditions of 550 to 650 ° C. × 1 to 2 hours. As a result, the retained austenite phase in the steel member is decomposed, the entire structure becomes a stable martensite phase or bainite phase, and the original characteristics of each steel type can be made obvious, and the hardness (HRC) should be 40-50. Can do. The tempering process may be repeated a plurality of times.
According to the heat treatment method of the present invention that undergoes the quenching process and the tempering process as described above, for example, even a large steel member such as a die-casting mold can be reliably toughened, and deformation and tempering cracks are generated. Therefore, heat cracks can be prevented during use after the heat treatment.

In the following, specific examples of the present invention will be described together with comparative examples.
The actual machine verification consists of JIS: SKD61, a die casting mold with a weight of 750 kg, 900 x 450 x 210 mm, with a recess in the center and four legs in four corners. Prepared. In advance, twelve thermocouples were inserted into the cross section of the mold, and temperature histories in various quenching processes were sampled.
The above molds were soaked at 1030 ° C. (quenching temperature) for 30 minutes, and subjected to quenching and tempering processes with various cooling patterns shown in Table 1 (average cooling rates C1, C2U, C2L, etc.). . The soaking time for quenching and tempering is all the same, and in the quenching process, each mold was inserted into a furnace at 500 ° C., and the method of reducing the maximum temperature difference in the cross section was used. .
Further, the hardness (HRC) after the tempering step was adjusted so that all the molds became HRC45 by controlling the tempering conditions. Furthermore, each mold was applied to actual die casting under the same conditions, and their lifetimes are shown in Table 1 by relative ratios.

As shown in Table 1, in Examples 1 to 12 according to the present invention, the deformation after tempering is 0.2% or less, the impact value is 26 J / cm ² or more, and the life ratio is the longest of Examples 1 and 7. The ratio was 0.92 or more when 1 was 1. Moreover, there was no burning crack.
From these results, according to Examples 1 to 12, since the amount of deformation during heat treatment is small, there is no need for subsequent processes such as correction and re-creation, and the impact value is relatively high, so even though it is a large mold. It was confirmed that it had a long life and excellent durability.

On the other hand, as shown in Table 1, in Comparative Examples 1 to 5, although the deformation amount is as small as 0.17% or less, the impact value is as low as 14 J / cm ² or less, and the mold life ratio is also 0.78 or less. And shortened. The cause is that in Comparative Example 1, the average cooling rate C1 is as low as 2.1 ° C./min, in Comparative Example 2, the control cooling end temperature Tf is as high as 200 ° C., and in Comparative Example 3, the transformation start temperature Ms (Bs ) Is 408 ° C. higher, and in Comparative Example 4, the average cooling rate C2L is as low as 0.9 ° C./min. Further, in Comparative Example 5, the average cooling rate C1, the average cooling rate C2L, the state start temperature Ms (Bs), and the average cooling rate C2L were out of the specified range of the present invention. And are the most declined.
From the results of Examples 1 to 12 as described above, the effects of the present invention were confirmed.

The present invention is not limited to the embodiments and examples described above.
For example, the present invention can be applied to any steel member that contains the carbon and carbide generating elements and has a weight of 50 kg or more, not limited to a steel type other than tool steel or a die such as a die. It is.
In addition, the cooling method can be water cooling, oil cooling, blast cooling, or into a water-soluble medium as long as the average cooling rates C1, CT, C2U, and C2L in the quenching step and the tempering cooling rate can be observed. Various methods such as dipping are appropriately selected.
The present invention can be variously modified without departing from the spirit of the present invention.

The temperature-time graph which shows the hardening process in the heat processing method of this invention. The graph which shows the temperature history of the quenching process and tempering process of this invention. The graph which shows the relationship between the average cooling rate C1 in a hardening process, and an impact value. FIG. 4 is a graph similar to FIG. 3 according to the classification of hardness after tempering. FIG. 4 is a graph similar to FIG. 3 according to the classification of the average grain size dγ during quenching. The graph which shows the relationship between transformation start temperature and average cooling rate C2U. The graph which shows the relationship between the transformation start temperature in a hardening process, and an impact value. FIG. 8 is a graph similar to FIG. 7 according to the classification of the average grain size dγ during quenching. FIG. 8 is a graph similar to FIG. 7 in terms of hardness classification after tempering. The graph which shows the relationship between average cooling rate C2L and an impact value. The graph which shows the relationship between control cooling end temperature Tf and an impact value. FIG. 12 is a graph similar to FIG. 11 according to the classification of the average grain size dγ during quenching.

Explanation of symbols

ht ……………………………… High temperature zone mt ……………………………… Intermediate temperature zone lt ……………………………… Low temperature zone Pf… …………………………… Perlite transformation end temperature Ff ……………………………… Ferrite transformation end temperature Ms ……………………………… Martensite transformation start temperature Bs ……………………………… Banite transformation start temperature C1, CT, C2U, C2L… Average cooling rate Tf ……………………………… Control cooling end temperature Tt ………… …………………… Tempering temperature

Claims (5)

Including a quenching step in which at least 0.20 to 1.5 wt% of carbon and 0.5 to 25 wt% of a carbide generating element are included, and a steel member having a weight of 50 kg or more is subjected to the following cooling step. A method for heat treating a steel member.
First cooling step: In a high temperature range from the quenching temperature to a temperature at which at least one of the pearlite transformation and the ferrite transformation can start, cooling is performed at an average cooling rate that can avoid precipitation of the pearlite phase and the ferrite phase.
Second cooling step: Cooling is performed at an average cooling rate of 1 ° C./min or more in a low temperature zone from the bainite transformation start temperature to the bainite transformation finish temperature and the martensite transformation finish temperature. A quenching process in which at least a 0.20 to 1.5 wt% carbon and a 0.5 to 25 wt% carbide-forming element and a steel member having a weight of 50 kg or more are subjected to the following cooling step. A heat treatment method for a steel member, comprising:
First cooling step: In a high temperature zone from the quenching temperature to 600 ° C., cooling is performed at an average cooling rate of more than 3 ° C./min that can prevent precipitation of a pearlite phase and a ferrite phase.
Second cooling step: Cooling is performed at an average cooling rate of 1 ° C./min or higher in a low temperature range of 500 ° C. or lower and up to 130 ° C. Among the low temperature zones in the second cooling step, in the temperature zone from 500 ° C. to any one of the bainite transformation start temperature and the martensite transformation start temperature, at least one of the bainite transformation and the martensite transformation is 400 ° C. to martensite. Cool at a cooling rate that starts at the site transformation start temperature range, and after the transformation starts, cool at an average cooling rate of 1 ° C./min or more.
The method for heat-treating a steel member according to claim 1 or 2. In the intermediate temperature zone between the high temperature zone of the first cooling step and the low temperature zone of the second cooling step or between 600 ° C. and 500 ° C., the highest temperature portion and the lowest temperature in the cross section of the steel member to be quenched. Control so that the difference with the part is 200 ° C. or less.
The method for heat treating a steel member according to any one of claims 1 to 3. After the quenching step, the steel member subjected to the quenching has a tempering step of heating and holding at 50 to 750 ° C.
The method for heat treatment of a steel member according to any one of claims 1 to 4, wherein the heat treatment method is performed.

Images