JP7676892B2 - Manufacturing method of materials for processing - Google Patents

Description

Applicable under Article 30, Paragraph 2 of the Patent Act Publisher: Japan Society for Technology of Plasticity, Publication name: Proceedings of the 71st Joint Conference for Technology of Plasticity, Publication date: October 23, 2020 Meeting name: Japan Society for Technology of Plasticity 71st Joint Conference for Technology of Plasticity WEB Conference, Date held: November 14, 2020 Website posting date: March 2, 2021, Posting address: https://onsite.gakkai-web.net/isij/181/index.html Meeting name: The Iron and Steel Institute of Japan 181st Spring Conference, Date held: March 18, 2021

This invention relates to a method for manufacturing materials for processing, and more specifically, to a method characterized by technical means for suppressing deterioration of mechanical properties due to internal voids.

Ingots obtained by casting have internal voids (hereinafter sometimes referred to as "pores") that arise due to solidification and shrinkage during ingot making. Even if these internal voids are small, with a maximum diameter of 15 μm, if many of them occur, they clearly indicate poor quality, so they must be completely compressed by subsequent hot forging or hot rolling processes.
Conventionally, the reduction ratio at which the Zaku can be pressed was estimated using evaluation indexes such as the Q value and Gm+ value shown in the following Patent Document, and the ingot was pressed to that reduction ratio.

However, although these evaluation indices such as the Q value and Gm+ value are indicators of the occlusion of the cracks, there is no guarantee that the occluded cracks are completely compressed by the reduction and that the mechanical properties are equivalent to those of the parent phase (parent phase without the occurrence of cracks). Also, depending on the reduction method (described later), there are cases where the occlusion of the cracks is not sufficiently performed and the cracks remain.

JP 2010-89097 A

The present invention was made against the background described above, with the aim of providing a manufacturing method for materials for processing that can suppress the deterioration of mechanical properties caused by Zaku.

As a result of extensive investigations aimed at solving the above problems, the inventors of the present invention have arrived at the present invention as described below.
That is, the method for producing a material for processing of the present invention is as follows:
a pressing process in which the workpiece obtained from the ingot is pressed down in one direction in a hot state;
a heat treatment step of heating and holding the workpiece at a temperature at which internal pores in the workpiece that have been closed during the reduction step can be compressed by diffusion after the reduction step;
The present invention is characterized in that it is provided with:

According to the present invention, the bonding of the internal voids (pores) that have been closed by pressing from one direction is promoted by the subsequent heat treatment, so that the mechanical properties of the original pores inside the workpiece can be obtained that are equivalent to those of the parent phase (parent phase without pores). Note that the reason for limiting the pressing direction in the present invention to one direction is to prevent the pores that have been closed from reopening due to pressure from a different direction.

In the present invention, when the size of an internal void present in the workpiece before the rolling step in the planned rolling direction is defined as the internal void diameter, the value of the internal void maximum diameter/workpiece diameter is 0.18 or less for the workpiece,
The processing can be performed at a reduction ratio such that the minimum value of the zigzag closure parameter Q shown in the following formula (1) is 0.40 or more.

In addition, in the present invention, the shape of the internal void can be estimated prior to the rolling process, and the rolling direction in the rolling process can be set to the minor axis direction of the internal void.

FIG. 1 is a flow diagram showing the steps of a method for producing a material for processing according to one embodiment of the present invention. FIG. 1 is a diagram showing an example of a shape change of a workpiece during a rolling process. FIG. FIG. 1 shows an ingot prepared for evaluation. 1 is a radiograph of an ingot relating to evaluation material No. 2. 1 is a photomicrograph showing a cross section of an evaluation material. 1A is a stress-strain curve for evaluation material No. 1, and FIG. 1B is a stress-strain curve for evaluation material No. 2. 1A is a stress-strain curve for evaluation material No. 3, and FIG. 1B is a stress-strain curve for evaluation material No. 4.

Next, a method for producing a material for processing according to an embodiment of the present invention will be described in detail with reference to the drawings. Fig. 1 is a flow diagram showing the steps of the method for producing a material for processing according to the present embodiment.
In the figure, W is a workpiece (in this example, a cylindrical shape with a diameter _D0 ) obtained from an ingot produced by a known method, 2 is a heating furnace which heats the workpiece W before forging, 4 is a forging device which applies reduction to the workpiece W, and 6 is a heating furnace which performs heat treatment on the workpiece W after reduction.

The forging device 4 is a forging device equipped with a pair of anvils 10A, 10B for pressing the workpiece W. The upper anvil 10A is movable in the vertical direction, while the lower anvil 10B is fixed in position. When the upper anvil 10A descends, it cooperates with the lower anvil 10B to press (forge and stretch) the workpiece W.
If necessary, the forging device 4 may be provided with a heat-retaining chamber having a storage space for storing the workpiece W, a heating means for heating the storage space, and the like.

In the manufacturing method of this example, these devices are used to carry out the rolling process and the subsequent heat treatment process. In the rolling process, the workpiece W is rolled down in one direction in a hot state to close the internal pits (e.g., pits 8 shown in FIG. 1). Then, in the subsequent heat treatment process, the workpiece W is reheated to compress the closed pits by diffusion. In this way, the closed pits can have mechanical properties equivalent to those of the parent phase (parent phase without pits). The material of the workpiece W can be carbon steel for mechanical construction such as JIS S45C, alloy steel for mechanical construction, steel for molds, Ni-based alloy, etc.

(Regarding the reduction process)
In the rolling down process, the workpiece W is first heated in the heating furnace 2. Then, as shown in FIG. 2, one end of the workpiece W is held by the manipulator 12, and the part of the workpiece W to be processed is placed between a pair of anvils 10A and 10B. Then, the part to be processed is forged and extended by the pair of anvils 10A and 10B so as to achieve a predetermined rolling down ratio. Thereafter, the workpiece W is repeatedly forged and extended while being moved sequentially in the longitudinal direction, and the entire part to be processed of the workpiece W is pressed to achieve a predetermined rolling down ratio. As a result, the workpiece W, which had a height D ₀ , becomes a flat shape with a height D ₁ as shown in FIG. 3, and the burrs that existed inside the workpiece W at this time are closed. In the rolling down process of this example, in addition to the case where a predetermined rolling down ratio is obtained by one pressing operation, it is also possible to obtain a predetermined rolling down ratio by accumulating two or more pressing operations. Even when two or more rolling down operations are performed, the rolling down is performed from one direction.

The processing conditions in the reduction step can be exemplified as follows.
Heating temperature before forging: 1000-1200℃
In order to ensure a suitable hot working temperature in a series of forging and stretching operations, in this example, the pre-forging heating temperature of the workpiece W is set to 1000° C. or higher. However, since excessive heating leads to excessive energy consumption and melting of the workpiece due to heat generated during forging, the upper limit is preferably set to 1200° C.

Forging temperature: 900° C. or higher If the forging temperature is less than 900° C., the deformation resistance increases and the load on the forging device increases. For this reason, in this example, the forging temperature is set to 900° C. or higher.

The rolling reduction ratio is a ratio where the ratio of the maximum diameter of internal voids in the workpiece W to the diameter of the workpiece W is 0.18 or less, and the minimum value of the Zak closure parameter Q is 0.40 or more. Here, the internal void diameter (Zak diameter) is the size of the internal voids (Zak) present inside the workpiece W before the rolling process in the planned rolling direction, and corresponds to, for example, the dimension L of Zak 8 shown in Fig. 1. The internal void maximum diameter is the largest internal void diameter among the multiple Zaks present inside the workpiece W.
According to the research of the present inventors, the value of the maximum diameter of the internal void/the diameter of the workpiece material is 0.18 or less in most of the workpiece materials W. It has been confirmed that for the workpiece materials W in which the maximum diameter of the internal void/the diameter of the workpiece material is 0.18 or less, it is possible to close the internal voids by forging the workpiece materials W at a reduction rate at which the minimum value of the void closure parameter Q shown in formula (1) is 0.40 or more.

The value of the Zak closure parameter Q is distributed in the center of the workpiece (when rolling a rod-shaped workpiece from the short direction, the center is the longitudinal axis). Therefore, it is preferable to roll so that Q in the Zak portion is maximized, and for example, it is desirable to measure or estimate the Zak portion before rolling and determine the forging conditions. Alternatively, it is preferable to determine the forging conditions so that the minimum value of Q is sufficient for Zak crimping.

The occlusion parameter Q shown in formula (1) is a conventionally known concept described in the above-mentioned Patent Document 1 and the like. For example, it can be determined by deformation analysis in a computer simulation in which the reduction rate of the workpiece W is changed after setting the shape, elastic coefficient, Poisson's ratio, friction coefficient, deformation resistance, rolling conditions (working speed, forging feed rate, working temperature), etc. of the workpiece W. In addition, the relationship between the Q value and the rolling rate can be determined.

Here, for flat-shaped ribs, it is easier to close the ribs by pressing down from the short diameter direction of the ribs. For this reason, in this embodiment, it is preferable to estimate the rib shape inside the workpiece W prior to the pressing process by computer simulation, X-ray projection observation, etc., and set the pressing direction in the pressing process to the short diameter direction of the ribs.

(About the heat treatment process)
In the heat treatment process, the workpiece W that has been forged to a predetermined reduction ratio in the reduction process is heated and held at a predetermined temperature, and the zigzags (the original zigzags) inside the workpiece W that were blocked in the reduction process are compressed by diffusion.

Here, it is desirable to set the heat treatment temperature so that the temperature inside the workpiece is within the range of 600 to 1450°C. If the internal temperature is less than 600°C, diffusion in the Zaku closure portion may not proceed sufficiently, which may lead to insufficient mechanical properties. On the other hand, if the internal temperature exceeds 1450°C, not only the Zaku but also the workpiece itself may melt, which may also lead to insufficient mechanical properties. Furthermore, the temperature holding time is not particularly limited as it is sufficient for the Zaku to be compressed by diffusion, and can be, for example, 4 hours or more. Moreover, since excessively long heating times saturate the diffusion effect and result in a lot of waste, it is desirable to keep the holding time to 40 hours or less. After heating and holding, the workpiece W is then cooled by air cooling.

The processing material of this embodiment obtained in this manner can be used as a material for hot processing such as hot forging and hot rolling, and in some cases as a material for cold processing.

Four types of evaluation materials shown in Table 1 below were prepared and evaluated for residual zigzag and tensile properties. These four types of evaluation materials differ in the reduction ratio during forging and the presence or absence of reheating after forging. The Q value in the table is the minimum value at the longitudinal axis center and was calculated using the plastic processing simulation software "FORGE (Transvalor)". The simulation was performed under the following conditions: hot forging (forging temperature 1200°C, processing speed 100 m/s), deformation resistance: 60 MPa (strain rate 1/s, 1200°C)/100 MPa (strain rate 1/s, 1000°C)/170 MPa (strain rate 1/s, 800°C), feed rate: conditions in Table 1, and reduction rate: conditions in Table 1. The deformation resistance takes into account the temperature drop during forging, and the feed rate and reduction rate are the same as those used to prepare the evaluation materials.

(Preparation of evaluation materials)
An ingot (casting lump) 30 having a cylindrical portion 31 of Φ100 mm×350 mm shown in FIG. 4 was produced. The material was S45C. A cooling ring with a faster cooling rate than other portions was arranged in a part of the mold used, and lumps were intentionally formed inside a specific portion of the obtained ingot 30 (specifically, the cylindrical portion 31). In FIG. 4, 31A is an area where lumps are generated, and 31B is an area where lumps are not generated.
The obtained ingot 30 was radiographed to examine the shape and dimensions of the pits. Figure 5 shows a radiograph of the ingot of evaluation material No. 2. In this figure, the presence of the pits is recognized at approximately the center of the cylindrical portion of the ingot in the longitudinal direction.
The maximum diameter of the slits measured for each ingot is shown in Table 1. The maximum diameter of the slits shown in Table 1 is the maximum value of the diameters measured for multiple slits.

Next, the ingot 30 was heated at 1200°C for 2.5 hours or more, and then forged and stretched in one direction (see Figure 3). As shown in Table 1, the reduction ratios due to the forging and stretching process were 30% for evaluation material No. 1, 45% for evaluation material No. 2, and 60% for evaluation materials Nos. 3 and 4. After the forging and stretching process, evaluation material No. 4 was moved to a heating furnace and reheated under conditions of 1200°C x 4 hours (note that evaluation materials Nos. 1, 2, and 3 were not reheated after the forging and stretching process). The ingots (evaluation materials) 30 that had completed the scheduled processing were cooled by air cooling.

(Assessment of Zaku's survival)
In the evaluation materials obtained as described above, a test piece for micro observation was cut out from a portion where the presence of the roughness was recognized before forging and the cross section was observed to confirm the state of blockage of the roughness. The judgment of the remaining roughness was made as "present" when the remaining roughness was recognized in the above micro observation, and "absent" when the remaining roughness was not recognized, and the results are shown in Table 1. FIG. 6 also shows microscopic photographs of the cross sections of each evaluation material. According to these, for evaluation material No. 1 with a rolling reduction of 30% and evaluation material No. 2 with a rolling reduction of 45%, the remaining roughness was still recognized even after rolling (the portion surrounded by the dashed line in the photograph after etching in FIG. 6 is the remaining roughness). On the other hand, for evaluation materials No. 3 and 4 with a rolling reduction of 60%, the remaining roughness was not recognized, and it can be seen that the roughness was well blocked.

(Evaluation of Tensile Properties)
Tensile test specimens (size: total length 15.4 mm x width 3.4 mm x thickness 1.0 mm) were cut from the prepared evaluation material from the area where the occurrence of the roughness was observed (within region 31A in FIG. 4) and the area where the roughness was not observed (within region 31B in FIG. 4) so that the rolling direction was the longitudinal direction of the test specimen. Then, tensile tests were performed on each test specimen, and it was evaluated whether the test specimen cut from the area where the occurrence of the roughness was observed (original rough portion) had the same tensile properties as the test specimen cut from the area where the roughness was not observed. The obtained stress-strain curves are shown in FIG. 7 and FIG. 8.

FIG. 7(A) is a stress-strain curve for evaluation material No. 1 produced under conditions of a rolling reduction of 30% and no reheating, and FIG. 7(B) is a stress-strain curve for evaluation material No. 2 produced under conditions of a rolling reduction of 45% and no reheating.
In these materials, the cracks have not yet been fully closed, so the test pieces cut from the areas where the cracks were found (original cracked areas) did not have the same properties as the test pieces cut from areas where the cracks were not found, and the tensile properties shown in Table 1 were judged to be "X".

Next, Figure 8 (A) is a stress-strain diagram for evaluation material No. 3, which was produced with a rolling reduction of 60% and no reheating. A rolling reduction of 60% corresponds to the minimum value of the occlusion parameter Q, 0.42, and evaluation material No. 3 is understood to be a material in which the cracks are well occluded, but as shown in Figure 8 (A), the test piece cut from the original cracked area does not have the same properties as the test piece cut from a location where no cracks have occurred. For evaluation material No. 3, the tensile properties shown in Table 1 are also judged to be "X".

In contrast, as shown in Figure 8 (B), in evaluation material No. 4, which was produced under conditions of a rolling reduction rate of 60% and reheating, the test piece cut from the original rough part had a tensile strength equivalent to that of the test piece cut from the part without roughness, and a value of 60% or more of the nominal strain (total elongation) in the part without roughness was obtained. In other words, the tensile properties shown in Table 1 for evaluation material No. 4 were judged to be "○". In this way, it was confirmed that as a result of reheating in evaluation material No. 4, compression by diffusion was promoted in the part where the roughness was present (original rough part), and mechanical properties equivalent to those of the parent phase (parent phase without roughness) were obtained.

The manufacturing method of the material for processing of the present invention has been described in detail above, but the present invention is not limited to the above-mentioned embodiment and examples. For example, the shape of the workpiece may be a polygonal shape other than a cylindrical shape, and the means for applying a rolling pressure to the workpiece may be different from the above-mentioned embodiment. The present invention can be implemented in various modified forms without departing from the spirit of the invention.

Claims (1)

a pressing process in which the workpiece obtained from the ingot is heated and then subjected to a forging process in one direction in a hot state to press down the workpiece ;
a heat treatment step of heating and holding the workpiece at a temperature at which internal pores in the workpiece that have been closed during the reduction step can be compressed by diffusion after the reduction step;
A method for producing a material for processing comprising:
When the size of the internal pores present in the workpiece before the rolling step in the planned rolling direction is defined as the internal pore diameter, the value of the internal pore maximum diameter/workpiece diameter is 0.18 or less for the workpiece,
A method for manufacturing a material for processing, comprising the steps of: processing at a reduction ratio such that the minimum value of the Zak closure parameter Q, as shown in the following formula (1), is 0.40 or more.

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