JP2013132659A - Method of manufacturing die component - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing a die component which is excellent in high strength and low temperature toughness, and for suppressing an alloy cost and reducing dispersion of a structure in a thickness direction after quenching and tempering by controlling the condition of a forging step.SOLUTION: In the method of manufacturing the die component by hot-forging an ingot, hot forging includes: a first term forging step of heating the ingot to 1,100°C or higher, then forging it above 950°C and granulating a metal structure; and a latter term forging step of successively forging the ingot below 950°C and miniaturizing the metal structure. A hammering ratio of the hot forging is 3.5 or higher.

Description

  The present invention relates to a mold part manufacturing method.

Since a rotary machine such as a centrifugal compressor is used at high speed or in a low temperature range, an impeller material which is a mold part constituting the rotary machine is required to have high strength and low temperature toughness characteristics. As such an impeller material, in recent years, a high-strength material of nickel / chromium / molybdenum steel, which is a low alloy steel, has been used.
The final characteristic value of such a high-strength material is determined by quenching and tempering after forging. However, in the case where the quenching process, which is the pre-tempering stage, is insufficient, the ratio of pearlite or ferrite increases in the structure, and even if proper tempering is performed, the strength and low temperature toughness may be deteriorated. Therefore, paying attention to the hardenability of such a high-strength material, a method for securing the strength and improving the low-temperature toughness has been proposed.

In order to ensure sufficient hardenability, it is conceivable to reduce the critical cooling rate as much as possible by adding alloy elements. However, excessive addition of alloy elements may increase costs or decrease low-temperature toughness. is there. On the other hand, if the alloy element is added too little, the hardenability is impaired and the strength may be deteriorated.
In order to solve such a problem, Patent Document 1 discloses a chemical component range in which hardenability is good and the critical cooling rate is small, and 80% or more of the metal structure after quenching is a bainite phase, or a bainite phase and a martensite phase. Thus, a method for achieving both strength and low temperature toughness is disclosed.

JP 2011-127203 A

  However, Patent Document 1 only adjusts the hardenability by defining the chemical component range, and the forging process, which is the previous process, has not been studied. Therefore, it was unstable to achieve both high strength and excellent low temperature toughness.

As described above, since the final characteristic value of the high strength material is determined by quenching and tempering after forging, it is effective to sufficiently perform a quenching treatment in order to ensure strength and low temperature toughness. However, when the forging process, which is the pre-quenching process, is insufficient, the austenite grain size becomes coarse, so that it is easy to quench and the strength can be secured, while the final structure after tempering is also coarse. As a result, there is a problem that the low temperature toughness deteriorates.
In the conventional forging process, as shown in FIG. 2, the forging conditions are evaluated and determined only by the required product dimensions, that is, the shape of the final mold part (material), and the metal structure is controlled. It did not stipulate the index of building the material. Furthermore, the forging temperature was not specified from the viewpoint of structure control.

  Furthermore, such a problem becomes more prominent as the material thickness increases. If the thickness of the material is thick, forging to the center of the material becomes insufficient, and the final metal structure after tempering becomes a mixed grain structure with a large difference in austenite grain size between the material center and the surface. Therefore, a difference in characteristics occurs between the center of the material and the surface, and it becomes unstable to secure strength and low temperature toughness, and it is difficult to stably manufacture a high quality material.

  In the present invention, in view of such circumstances, the forging step is a step of sizing the crystal grains at a temperature higher than the temperature at the temperature at which the crystal grains are difficult to grow after recrystallization of the metal structure. The process is divided into two processes, the process of refining on the low temperature side, and by further controlling the forging ratio in the forging process, the forging is fully introduced to the center of the material, and the metal structure varies in the thickness direction. It is what suppresses. In particular, the present invention suppresses the alloy cost of the raw material as a mold part and controls the conditions of the forging process, thereby reducing the variation in the structure in the thickness direction after quenching and tempering, and providing high strength and low temperature toughness. It is an object of the present invention to provide a mold part manufacturing method excellent in the above.

The present inventors perform forging treatment under suitable conditions, and by preliminarily crystallizing and refining the crystal grains before quenching and tempering treatment, forging can be put to the center in the thickness direction, and the surface of the material It is possible to suppress the difference in structure between the central part and the central part, and as a result, the present invention has been reached by a technical idea based on the knowledge that low-temperature toughness after quenching can be secured stably and sufficiently. is there.
The gist of the present invention aimed at solving the above problems is as follows.

  The mold part manufacturing method of the present invention is a method of manufacturing a mold part by hot forging an ingot, and after the hot forging heats the ingot to 1100 ° C. or higher, 950 The hot forging comprising: the forging step forging the metal structure by forging at a temperature exceeding ℃; and the subsequent forging step for forging the ingot at 950 ° C. or less to refine the metal structure. The training ratio is 3.5 or more.

  In the method for manufacturing a mold part of the present invention, it is preferable that the forging ratio in the first forging step is 1.5 or more and the forging ratio in the latter forging step is 2.0 or more.

  Moreover, in the manufacturing method of the metal mold components of this invention, the said ingot is the mass%, C: 0.29-0.35%, Si: 0.15-0.35%, Mn: 0.60 0.90%, Ni: 1.60 to 2.00%, Cr: 0.77 to 1.00%, Mo: 0.15 to 0.30%, and the balance from iron and inevitable impurities It is preferable to become.

  According to the present invention, the forging step includes a step of adjusting the crystal grains on the high temperature side from the temperature at which the crystal grains are difficult to grow after recrystallization of the metal structure, and the low temperature side. It is divided into two processes, the process of miniaturization, and by further controlling the forging ratio of the forging process, it is possible to sufficiently forge up to the center of the material and suppress the variation in the metal structure in the thickness direction. . In particular, by controlling the forging process conditions while reducing the alloy costs of the raw materials as mold parts, the variation in the structure in the thickness direction after quenching and tempering is reduced, and high strength and sufficient low temperature toughness are obtained. be able to.

It is a schematic diagram which shows the temperature change of the ingot in the process of the hot forging of this embodiment. It is a schematic diagram which shows the temperature change of the ingot in the process of the conventional hot forging. It is a graph which shows the dispersion | variation in the austenite particle size of each of the raw material surface part and center part after the forge of a present Example.

  Hereinafter, although the manufacturing method of the mold component of this embodiment is demonstrated, the knowledge of the present inventors who led to this invention is demonstrated first.

The method of manufacturing a mold part according to the present embodiment is a method of manufacturing a mold part (hereinafter also simply referred to as a material) by hot forging the ingot, and the hot forging is performed at 1100 ° C. After heating up to above, forge process for forging above 950 ° C. to refine the metal structure, and subsequently forging process for forging the ingot at 950 ° C. or less to refine the metal structure The forging ratio of hot forging is 3.5 or more. In addition, the forging ratio of hot forging can be calculated | required by (the raw material cross-sectional area after a late forging process) / (ingot cross-sectional area before a pre-forging process).
Hereafter, each process of the hot forging which concerns on this embodiment is demonstrated in detail using FIG.

FIG. 1 is a schematic diagram showing a temperature change of the ingot in the hot forging process of the present embodiment.
In the hot forging process according to this embodiment, first, as described above, the ingot is heated to 1100 ° C. or higher. This is because the forging to be performed later is performed at a temperature at which the metal structure of the ingot becomes an austenite single phase, that is, an austenite region, and the crystal grain size of the forging is made fine. In order to suppress the coarsening of the crystal grain size, the heating temperature before forging is preferably 1150 ° C. or higher.

Next, an early forging process in which the ingot heated to 1100 ° C. or higher is forged at a temperature exceeding 950 ° C. to refine the metal structure, and a late forging process in which the metal structure is forged at 950 ° C. or lower to refine the metal structure And then forging in order. In the present invention, the forging ratio of hot forging needs to be 3.5 or more.
Thus, by setting the forging ratio to 3.5 or more, sufficient strain can be introduced also in the center in the thickness direction. That is, after forging, not only the surface portion of the material but also the metal structure in the central portion in the thickness direction can be sufficiently refined. As a result, stable and sufficient low temperature toughness can be secured by the subsequent quenching and tempering treatment.
Hereinafter, each process in hot forging will be described.

<First forging process>
After the ingot is heated to 1100 ° C. or higher, as shown in FIG. 1, forging is performed in a temperature range exceeding 950 ° C. As described above, since the austenite grains can be sized by performing the forging process in a high temperature region exceeding 950 ° C., the metal structure of the material obtained by quenching and tempering after forging is made uniform. As a result, the low-temperature toughness of the material can be stably secured.
The reason why the forging temperature in the first forging step is specified to be higher than 950 ° C. is that 950 ° C. corresponds to a temperature at which crystal grains hardly grow after recrystallization of the metal structure of the ingot. Moreover, in this embodiment, it is preferable that the upper limit temperature of a previous forging process shall be 1050 degrees C or less. This is because if forging in a high temperature region exceeding 1050 ° C., the influence of grain growth becomes large, and it may be difficult to refine the austenite grains in a subsequent process.

Moreover, in this embodiment, it is preferable that the forge ratio of the said forge process is 1.5 or more from a viewpoint of the granulation of austenite grain. Thus, by setting the forging ratio in the first forging step to 1.5 or more, the structure of the central portion in the thickness direction can be further sized. The forging ratio in the first forging process can be obtained by (ingot cross-sectional area after the first forging process) / (ingot cross-sectional area before the first forging process).
Although the upper limit of the forging ratio in the first forging process is not particularly defined, it is preferably set to 2.0 or less from the viewpoint of equipment cost and the like.

<Late forging process>
Subsequently, forging is performed at a temperature range of 950 ° C. or lower after performing the above-described forging step. Thus, by performing the late forging process in a low temperature region of 950 ° C. or lower, the metal structure that has been grain-sized in the early forging process can be refined. Therefore, low temperature toughness can be sufficiently obtained by quenching and tempering performed later.
The reason why the forging temperature in the late forging process is defined as 950 ° C. or lower is the same as the temperature in the early forging process, because the crystal grains are difficult to grow after recrystallization of the metal structure, and strain is accumulated inside the structure. This is because it corresponds to a temperature at which it is easy to perform. In consideration of the fact that the temperature dependence of austenite grain refinement is small and that the deformation resistance of the ingot increases as the temperature becomes lower, the forging temperature in the late forging process aimed at refinement of the structure is 950. It is preferable that the temperature is in the vicinity of ℃.

In the present embodiment, the forging ratio in the late forging step is preferably 2.0 or more from the viewpoint of refining austenite grains. Thus, by setting the forging ratio in the late forging process to 2.0 or more, not only the surface portion of the material but also the austenite grains in the center portion in the thickness direction can be further refined. As a result, it is possible to suppress variations in low temperature toughness characteristics in the thickness direction of the material. The forging ratio in the early forging process can be determined by (material cross-sectional area after late forging process) / (ingot cross-sectional area after early forging process).
In addition, the upper limit of the forging ratio in the late forging process is not particularly specified, but is preferably set to 2.5 or less from the viewpoint of the equipment cost and the saturation of the material structure refinement effect.

  Here, in the hot forging process according to the present invention, it is divided into an early forging process for the purpose of sizing the metal structure and a late forging process for the purpose of miniaturization, and these early and late forging processes are sequentially performed. As described above, the forging ratios of the early forging process and the late forging process are preferably 1.5 or more and 2.0 or more, respectively. It may be the same, and further, the training ratio in the late forging process may be larger than the training ratio in the early forging process. That is, in the present invention, hot forging is performed in which two stages of forging are sequentially performed, a first forging step for sizing the metal structure and a second forging step for refining the metal structure thus sized, It is very important that the forging ratio of this hot forging is 3.5 or more, and the forging ratios of the first and second forging processes are the same, or the forging ratio of the latter forging process is higher than the forging ratio of the first forging process. Even if hot forging is performed under larger conditions, the effects of the present invention can be fully enjoyed.

  Moreover, in this embodiment, it is preferable to slow down the cooling rate after a late forging process and before quenching process. The austenite grain size before quenching affects the low temperature toughness. That is, if the austenite grain size before quenching is small, the grain interfacial area of austenite grains serving as the starting point of martensite increases. Therefore, in order to obtain more excellent low temperature toughness, it is preferable that the cooling after the late forging process and before the quenching process is furnace cooling or air cooling.

In the present embodiment, as shown in FIG. 1, a quenching process is performed after the late forging step, but the conditions are not particularly defined. In the present embodiment, as described above, by regulating the metal structure by defining the conditions of the hot forging process, the structure can be sized and refined in advance to the center in the thickness direction. By performing the quenching process to be performed, the low temperature toughness of the material can be secured stably without variation in the thickness direction.
Tempering is performed by heating the material to 550 to 650 ° C.

Here, each cooling rate in the present embodiment is an average speed at the center of the thickness of the ingot or the obtained material, and each temperature such as a heating temperature is an average temperature of the ingot or the material.
As a method for measuring the temperature, for example, there is a method of measuring several ingot or material surface temperatures with a radiation thermometer.

  Next, the alloy composition of the ingot according to the present embodiment will be described in detail. Hereinafter,% means mass%.

The ingot according to the present embodiment is preferably nickel / chromium / molybdenum steel, and specifically, by mass%, C: 0.29 to 0.35%, Si: 0.15 to 0.35. %, Mn: 0.60 to 0.90%, Ni: 1.60 to 2.00%, Cr: 0.77 to 1.00%, Mo: 0.15 to 0.30%, The balance is preferably an alloy steel made of iron and inevitable impurities.
Hereinafter, the reason which limited the component of the ingot of this embodiment is demonstrated.

(C: 0.29 to 0.35%)
C is an element effective for improving the strength required for the impeller. Moreover, in order to ensure sufficient baking after forging, it is preferable to make C content into 0.29% or more. On the other hand, if the C content is too large, weldability deteriorates and the amount of bainite structure in the structure after quenching decreases, so the C content is preferably 0.35% or less. In addition, more preferable C content is 0.30 to 0.35%, and still more preferable content is 0.30 to 0.34%.

(Si: 0.15-0.35%)
Si is an element useful for deoxidation and strength improvement. In order to obtain these effects, the Si content is preferably 0.15% or more. However, if Si is contained more than necessary, the toughness deterioration due to segregation may occur, so the upper limit is preferably made 0.35%.

(Mn: 0.60 to 0.90%)
Mn is an element that is used as a deoxidizing agent and improves the hardenability like Si. Therefore, in the present invention, the Mn content is preferably 0.60% or more. However, if it is contained more than necessary, a grain boundary embrittlement phenomenon occurs, and there is a danger that delayed fracture will be accelerated and the grain boundary strength may be lowered. Therefore, the upper limit is preferably made 0.90%.

(Ni: 1.60 to 2.00%)
Ni forms a solid solution in the base and strengthens the solid solution, and imparts toughness. Moreover, in order to ensure sufficient baking after forging, the Ni content is preferably 1.60% or more. On the other hand, even if the Ni content increases, the weldability deteriorates and the effect of solid solution strengthening is saturated to increase the cost. Therefore, the Ni content is preferably set to 2.00% or less. A more preferable Ni content is 1.65 to 1.95%, and a more preferable content is 1.70 to 1.90%.

(Cr: 0.77 to 1.00%)
Cr, like Mn, enhances hardenability and forms carbides with C to impart strength and wear resistance. Moreover, in order to ensure sufficient baking after forging, the Cr content is preferably 0.77% or more. However, even if it is contained in a large amount, the weldability deteriorates and the effect of hardenability is saturated and the cost is increased, so the Cr content is preferably made 1.00% or less. A more preferable Cr content is 0.77 to 0.95%, and a still more preferable content is 0.77 to 0.90%.

(Mo: 0.15-0.30%)
Mo relieves the transformation stress of steel and enhances hardenability, toughness and temper softening resistance. Moreover, it has the effect of producing fine carbides in the base and improving wear resistance. In order to obtain these effects, the Mo content is preferably 0.15% or more. However, when the Mo content increases, the occurrence of cracking is promoted and the effect of hardenability is saturated and the cost is increased, so the Mo content is preferably set to 0.30% or less. More preferable Mo content is 0.17 to 0.27%, and still more preferable content is 0.20 to 0.27%.

  Further, in the present embodiment, the balance other than the above-described elements is substantially made of Fe, and it is possible to add a trace amount of elements that do not impair the effects of the present invention, including inevitable impurities.

  Note that the thickness of the mold part in the present embodiment is not limited, but 200 to 400 mm is particularly effective.

  According to the method of manufacturing a mold part according to the present invention, by controlling the hot forging conditions, the forging can be sufficiently inserted to the center part of the mold part (material), and uniformly in the thickness direction of the material. The metal structure can be sized and refined. In particular, by suppressing the alloy costs of the raw materials and controlling the conditions of the forging process, the variation in the structure in the thickness direction after quenching and tempering can be reduced, and high strength and sufficient low temperature toughness can be stably obtained. Can do.

  Hereinafter, the effects of the present invention will be described with reference to examples, but the present invention is not limited to the conditions used in the following examples.

In this example, first, steel having chemical components shown in Table 1 is melted and cast according to a conventional method, and then a hot forging process is performed, and a material having a thickness shown in Table 2 (mold part). Manufactured. Table 2 shows the forging temperature and forging ratio in the first forging process in the hot forging process, and the forging temperature and forging ratio in the latter forging process, and the forging ratio (total forging ratio) in the entire hot forging process. Condition. In addition, the heating temperature before performing hot forging was 1100-1150 degreeC.
Further, in this example, the cooling method from the late forging process to the quenching process was furnace cooling or air cooling, and the quenching process was performed according to a conventional method. Thereafter, the obtained material was heated again and tempered.

In this example, the austenite particle size (γ particle size) was measured before the tempering treatment in order to evaluate the sizing and refinement of the metal structure after the quenching treatment.
Specifically, after subjecting the material produced by the hot forging process to the quenching treatment, the microstructure of the material surface portion and the thickness direction center portion was observed with an optical microscope, and the average γ particle size was measured. .
Furthermore, in order to evaluate the size regulation of the austenite grains, (γ grain size of the material surface portion / γ grain size in the center in the thickness direction) was obtained, and the variation of the metal structure after quenching was evaluated.
The above results are shown in Table 2 and FIG.
In addition, about refinement | miniaturization of the austenite grain, from the viewpoint of ensuring a favorable low-temperature toughness value, the γ grain size was evaluated as 80 μm or less, and 60 μm or less was evaluated as particularly favorable. In addition, regarding the agglomeration of austenite grains, the above (gamma grain size of the raw material surface portion / gamma grain size of the central portion in the thickness direction) was evaluated as good when 0.8 or more.

Production No. Nos. 3 to 5 are examples of the present invention, and the austenite grain size was a good result of 80 μm or less even in the surface portion of the material and the central portion in the thickness direction. Further, in the hot forging process, the forging ratios of the early forging process and the late forging process were respectively set to 1.5 or more and 2.0 or more, so that a particularly fine metal structure could be obtained.
In addition, production No. All the variations of 3 to 5 austenite grains were good.

  On the other hand, production No. Nos. 1 and 2 both have a low total forging ratio in the hot forging step, and as shown in FIG. In the case of 2, the variation of austenite grains also became large. This is because the total training ratio is the production number. It is considered that the strain was larger than 2 and sufficient strain could be introduced into the surface portion and miniaturized, but the strain was not sufficiently introduced into the central portion in the thickness direction.

Claims (3)

A method of manufacturing a mold part by hot forging an ingot,
The hot forging is
After heating the ingot to 1100 ° C. or higher, the forging step forging the metal structure by forging above 950 ° C .;
Subsequently, a late forging step of forging the ingot at 950 ° C. or less to refine the metal structure,
With
A die part manufacturing method, wherein the hot forging forging ratio is 3.5 or more.   2. The method for producing a die part according to claim 1, wherein a forging ratio in the first forging step is 1.5 or more and a forging ratio in the second forging step is 2.0 or more. The ingot is mass%,
C: 0.29 to 0.35%,
Si: 0.15-0.35%,
Mn: 0.60 to 0.90%,
Ni: 1.60 to 2.00%,
Cr: 0.77 to 1.00%,
Mo: 0.15 to 0.30%,
The mold part manufacturing method according to claim 1, wherein the balance is made of iron and inevitable impurities.

Images