JP7420321B1 - Machine structural parts and their manufacturing method - Google Patents

Abstract

The present invention provides a mechanical structural component that eliminates the problem of eccentricity, especially in components with a large aspect ratio having a shaft-shaped portion, by reliably reducing distortion after high-frequency heat treatment. C: 0.45-0.51%, Si: 0.15-0.35%, Mn: 0.60-0.90%, P: 0.030% or less, S: 0.025% or less, Al: 0.040-0.059%, Cr: 0.10-0.50% and N: 0.0060 ~0.0100%, and the remainder has a composition of Fe and impurities, and in a part that has a hardened layer formed by induction hardening and tempering, the area ratio of crystal grains in which the diameter of prior austenite grains is 80 μm or less in the hardened layer is 80% or more, and the number ratio of grains having a grain size that is twice or more the mode of grain size in the hardened layer is 5% or less.

Description

The present invention relates to a mechanical structural component having a hardened layer formed by induction hardening and tempering, which is used in the fields of construction industry machinery and automobiles, and particularly to a mechanical structural component having a shaft-shaped portion, and a method for manufacturing the same.

Machine structural alloy steels such as JIS standard SCr420 and SCM420 are used for power transmission parts such as drive shafts and axle shafts used in automobiles and construction machinery. The outline of the manufacturing method for this type of component is as follows. That is, a steel bar or wire rod made of alloy steel for machine structures is roughly formed into a part shape by hot forging and/or cold forging, and then finely formed by cutting. The product is then subjected to surface hardening treatments such as induction hardening and tempering (induction heat treatment) and carburizing and tempering (carburizing heat treatment). In induction heat treatment and carburizing heat treatment, the target member is hardened using transformation of the steel structure by heating and holding at 900°C or higher and then cooling. Therefore, during heating, heat treatment distortion occurs due to transformation of the steel structure. Particularly in parts with a large aspect ratio, such as shafts, etc., eccentricity of the part due to distortion cannot be ignored, and a correction process is required depending on the degree of eccentricity, leading to increased costs.

To address these issues, techniques described in Patent Documents 1 to 5, for example, have been proposed. That is, Patent Document 1 proposes a method for manufacturing steel in which coarsening of austenite grains is suppressed by precipitating AlN and heat treatment strain during carburizing heat treatment is reduced.

Patent Document 2 proposes controlling the size and precipitation density of AlN by controlling the cooling rate in a specified temperature range after hot working, and reducing the heat treatment strain during carburizing heat treatment.

Patent Document 3 proposes a steel that reduces decarburization by controlling the heating and hot rolling temperature after casting, and reduces the generation of coarse grains and heat treatment strain during carburizing heat treatment, and a method for manufacturing the same. .

Patent Document 4 proposes a steel and a method for producing the same in which generation of coarse grains during carburizing heat treatment and heat treatment strain are reduced by controlling the precipitation of TiN by adding Ti and the finishing temperature of hot rolling.

Patent Document 5 discloses that by controlling variations in the martensitic transformation point within a longitudinal vertical cross section of a steel bar, the occurrence of uneven martensitic transformation is suppressed, and the heat treatment strain that occurs during carburizing and quenching or carbonitriding and quenching is reduced. is proposed.

Japanese Unexamined Patent Publication No. 61-261427 Japanese Unexamined Patent Publication No. 8-199316 Japanese Patent Application Publication No. 2004-204263 Japanese Patent Application Publication No. 2006-265703 Japanese Patent Application Publication No. 2013-151719

Here, when carburizing heat treatment and high frequency heat treatment are compared, carburizing heat treatment causes greater strain after heat treatment. Therefore, conventionally, emphasis has been placed on reducing the strain after carburizing heat treatment. Therefore, although the techniques shown in Patent Documents 1 to 5 are effective in reducing the strain caused by carburizing heat treatment, they are insufficient to reduce the smaller strain caused by high frequency heat treatment. Ta.

The present invention has been developed in view of the above-mentioned circumstances, and provides mechanical structural parts and machine structural parts that solve the problem of eccentricity, especially in parts with a large aspect ratio having shaft-shaped parts, by reducing distortion after high-frequency heat treatment. The purpose is to provide information on manufacturing methods.

The present inventors investigated the effects of steel components and steel manufacturing conditions on strain after high-frequency heat treatment in order to reduce strain in members after high-frequency heat treatment. As a result, we have found that the following (a) and (b) are important for reducing strain after high-frequency heat treatment.
(a) Reduce the variation in prior austenite grain size in the hardened layer after high-frequency heat treatment. (b) In the hardened layer after high-frequency heat treatment, prior austenite grains with a small grain size occupy a large area.

The present invention is based on the above findings, and the gist and structure thereof are as follows.
1. In mass% C: 0.45-0.51%,
Si: 0.15-0.35%,
Mn: 0.60-0.90%,
P: 0.030% or less,
S: 0.025% or less,
Al: 0.040-0.059%,
Cr: 0.10~0.50% and N: 0.0060~0.0100%
, the remainder has a composition of Fe and impurities, and has a hardened layer formed by induction hardening and tempering, and the area ratio of crystal grains in which the diameter of prior austenite grains in the hardened layer is 80 μm or less is 80 μm. % or more, and the number ratio of grains having a grain size that is twice or more the mode grain size in the hardened layer is 5% or less.

2. 2. The mechanical structural component according to item 1, wherein the component has a shaft-shaped portion.

3. In mass% C: 0.45-0.51%,
Si: 0.15-0.35%,
Mn: 0.60-0.90%,
P: 0.030% or less,
S: 0.025% or less,
Al: 0.040-0.059%,
Cr: 0.10~0.50% and N: 0.0060~0.0100%
, the remainder being Fe and impurities, hot-rolled at a rolling speed VSL that satisfies the following formula (1) to form a steel bar or wire rod, and after forging the steel bar or wire rod. A manufacturing method for mechanical structural parts that involves induction hardening and tempering at 900 to 1150°C.
Note VSL≦100/DL (m/s)...(1)
However, VSL is the rolling speed (m/s) just before passing through the final rolling stage, and DL is the diameter of the rolled material (mm) after rolling is completed.

According to the present invention, a mechanical structural component with low residual strain can be provided. In particular, since strain reduction is realized in mechanical structural parts having shaft-shaped parts, it is effective in suppressing eccentricity of this type of mechanical structural parts.

It is a graph showing the correlation between the area ratio of prior austenite grains with a grain size of 80 μm or less in a hardened layer and core runout (eccentricity).

Hereinafter, one embodiment of the present invention will be explained in detail in order from the composition of steel applied to the mechanical structural parts of the present invention. Note that "%" indicating the content of each element indicates "mass%" unless otherwise specified.
C: 0.45-0.51%
C is an essential element in order to ensure the strength of the hardened layer of the component when subjected to high frequency heat treatment. If the C content is less than 0.45%, the strength of the part will be insufficient. On the other hand, when the C content exceeds 0.51%, the amount of strain after high-frequency heat treatment increases. For the above reasons, the C content is determined to be in the range of 0.45 to 0.51%. From the viewpoint of balancing strength and strain, it is desirable that the C content be 0.47% or more. Similarly, it is desirable that the content be 0.49% or less.

Si: 0.15-0.35%
Si has the effect of reducing oxygen-based inclusions by deoxidizing the steel and suppressing the decrease in hardness during tempering heat treatment. That is, it has the effect of improving the mechanical properties of the product. On the other hand, if it is added in excess, the material will harden and cold workability will decrease. For the above reasons, the Si content is determined to be in the range of 0.15 to 0.35%. A more desirable range is 0.20% or more. A more desirable range is 0.30% or less.

Mn: 0.60-0.90%
Mn has the effect of greatly improving hardenability, and for this purpose it is necessary to add 0.60% or more. On the other hand, as the amount added increases, the material hardness increases and cold workability decreases, but up to 0.90% is acceptable. For the above reasons, the Mn content is set in the range of 0.60 to 0.90%. Note that a more desirable range is 0.70% or more. A more desirable range is 0.80% or less.

P: 0.030% or less (including 0%)
P segregates at prior austenite grain boundaries after induction hardening and has the effect of reducing the fatigue properties of the hardened layer. Therefore, it is preferable to keep the P content as low as possible. For the above reasons, the P content is set at 0.030% or less. Note that it is more preferably suppressed to 0.012% or less. Of course, it may be 0%.

S: 0.025% or less (including 0%)
S exists as a sulfur-based inclusion and is an effective element for improving machinability, but addition of more than 0.025% has a negative effect on manufacturability during casting, so the upper limit is set to 0.025%. Note that if it is necessary to improve machinability, it may be added in an amount of 0.010% or more, and a range of 0.010 to 0.015% is preferable. Note that if there is no need to take machinability into consideration, it may be 0%.

Al: 0.040-0.059%
Since Al combines with N to form AlN, it has the effect of suppressing coarsening of austenite grains during rolling of steel bars and wire rods and during induction hardening. Since austenite grain size control during rolling and induction hardening of steel bars and wire rods is effective in suppressing strain, it is an important element in the present invention. If the Al content is low, the above effects cannot be expected. On the other hand, if the Al content is excessive, inclusions will increase and the number of starting points for fatigue fracture will increase, causing a decrease in fatigue strength. For the above reasons, the Al content was determined to be in the range of 0.040 to 0.059%. Preferably it is 0.045% or more. Preferably it is 0.055% or less.

Cr: 0.10~0.50%
Cr effectively acts to improve hardenability and strength of steel. On the other hand, when the Cr content increases, a decrease in workability due to increased hardness is inevitable. For the above reasons, the Cr content was determined to be in the range of 0.10 to 0.50%. Preferably it is 0.10% or more. Preferably it is 0.20% or less.

N: 0.0060-0.0100%
Like Al, N is an important element in the present invention because it combines with Al to form AlN. Containing N at 0.0060% or more is necessary to control the austenite grain size during rod/wire rod rolling and induction hardening. On the other hand, if the N content increases, cracks will occur during solidification and will remain as flaws in subsequent steps. If these flaws remain, the flaws will open and cracks will occur significantly, making it impossible to use the product as a product. For the above reasons, the N content was determined to be in the range of 0.0060 to 0.0100%. Preferably, it is 0.0060% or more. Preferably it is 0.0080% or less.

In the mechanical structural component of the present invention, the balance of the component composition includes Fe and impurities. Impurities are those that are mixed in from ores used as raw materials, scrap, or the manufacturing environment when steel materials are manufactured industrially, and are allowed within a range that does not adversely affect the characteristics of the present embodiment.

Next, the regulations regarding the prior austenite grain size of the hardened layer by high-frequency heat treatment in the present invention will be explained.
The mechanical structural component of the present invention is formed into a component shape, such as a shape having a shaft, using steel having the above-mentioned composition, and then subjected to high-frequency heat treatment such as induction hardening and tempering. In the hardened layer formed by this high-frequency heat treatment, the area ratio of crystal grains in which the diameter of prior austenite grains is 80 μm or less is 80% or more, and the number of grains having a grain size that is at least twice the mode grain size. The ratio must be 5% or less.

[Hardened layer]
In the mechanical structural component of the present invention, a molded body shaped into a component is subjected to high-frequency heat treatment such as induction hardening and tempering to form a hardened layer on the surface layer. This hardened layer is a portion hardened by high frequency heat treatment. Specifically, we measure the hardness distribution (e.g. Vickers hardness) from the surface of the part toward the center after high-frequency heat treatment, and determine the depth at which a predetermined hardness (e.g. HV450) is maintained in the obtained hardness distribution. The effective hardened layer depth (ECD) is defined as the effective hardened layer depth (ECD), and the area of this effective hardened layer depth is defined as the hardened layer.

[The area ratio of crystal grains with a prior austenite grain diameter of 80 μm or less is 80% or more]
This specification of the prior austenite grain size is an index indicating the properties of the prior austenite grains that can suppress strain after high-frequency heat treatment. Here, in the examples described later, the relationship between the area ratio of prior austenite grains with a grain size of 80 μm or less and core runout (if it is 0.25% or less, distortion in the part can be suppressed) is shown in Figure 1. When the area ratio of crystal grains with prior austenite grains having a diameter of 80 μm or less satisfies 80% or more, core runout can be effectively suppressed, that is, strain in the part can be suppressed. Furthermore, prior austenite grains with a diameter of 80 μm or less are subject to regulation because prior austenite grains with a grain size larger than 80 μm have a large effect on suppressing core runout, and crystal grains with a diameter exceeding 80 μm are regulated. This is because the desired center runout suppressing ability can be obtained. Therefore, the area ratio of crystal grains with a diameter of prior austenite grains exceeding 80 μm is set to be less than 20%, in other words, the area ratio of crystal grains with a diameter of prior austenite grains of 80 μm or less is set to 80% or more.

Here, the prior austenite grain size can be obtained by appropriately corroding and observing the parts after high-frequency heat treatment. For example, after corroding the hardened layer formed on the surface layer of the part with a picric acid solution to reveal the prior austenite grain boundaries, the prior austenite grain structure is photographed, processed by image processing software, and the circle equivalent of each prior austenite grain is In addition to obtaining the diameter, the area ratio of crystal grains of 80 μm or less can be determined.

[The percentage of particles with a particle size that is twice or more the mode particle size is 5% or less]
In the hardened layer, by controlling the percentage of grains having a grain size twice or more the mode grain size to 5% or less, distortion and eccentricity of the part after induction hardening can be suppressed. Even when the area ratio of prior austenite grains described in the previous section is satisfied, if a certain number of prior austenite grains become significantly coarser than other grains (specifically, more than twice the grain size mode), eccentricity may be suppressed. does not reach the desired degree. Thus, it was found that the above conditions are appropriate as standards for suppressing eccentricity.

Here, the particle size mode can be obtained by appropriately corroding and observing the parts after high-frequency heat treatment. For example, after corroding the hardened layer formed on the surface layer of a part with a picric acid solution to reveal the prior austenite grain boundaries, the prior austenite grain structure is photographed and processed using image processing software to obtain a grain size histogram. It can be found at Furthermore, the number ratio of grains having the particle size can be determined from the histogram.

Next, a method of manufacturing a mechanical structural component according to the present invention will be explained.
That is, a steel material having the above-mentioned composition is hot-rolled at a rolling speed that satisfies the following formula (1) to form a steel bar or wire rod, and after the steel bar or wire rod is forged into a part shape, Machine structural parts are manufactured by induction hardening at ℃. Note that the steel material is not particularly limited, and may include, for example, billet as a typical example, or a slab.

Here, in order to satisfy the above-mentioned regulation regarding the prior austenite grain size in the hardened layer, in addition to adjusting the above-mentioned composition, the steel material, for example, the slab, must be rolled at a rolling speed that satisfies the following formula (1). It is necessary to hot-roll the steel and make it into steel bars or wire rods.
Note VSL≦100/DL (m/s)...(1)
However, VSL is the rolling speed (m/s) just before passing through the final rolling stage, and DL is the diameter of the rolled material (mm) after rolling is completed.

The above formula (1) is an index showing the rolling speed when hot rolling a cast slab into a steel bar or wire rod that becomes a forging material for making a machine structural part. By setting an appropriate rolling speed according to the diameter of the steel bar or wire rod, the temperature gradient inside the rolled material can be reduced and the structure of the rolled material can be controlled. By setting a rolling speed that satisfies this formula, it is possible to secure the time necessary for cooling the inside of the rolled material, and as a result, the temperature difference between the surface layer and the inside of the rolled material is suppressed, resulting in a homogeneous structure after rolling. I can do it. Therefore, if the rolling speed and diameter of the rolled material do not satisfy the above formula (1), the prior austenite grain size in the final part may not satisfy the above conditions even if the heat treatment conditions for the induction heat treatment described below are satisfied. Can not.

Further, from the viewpoint of strain suppression, it is preferable to set the rolling speed to a constant on the right side of equation (1) between 100 and 90. That is, hot rolling at a rolling speed that satisfies the following formula (2) is suitable for suppressing strain.
Note VSL≦90/DL (m/s)...(2)
However, VSL is the rolling speed (m/s) just before passing through the final rolling stage, and DL is the diameter of the rolled material (mm) after rolling is completed.

Incidentally, originally it would be appropriate to use the diameter of the rolled material immediately before the final stage of rolling as an indicator of rolling speed, but the final stage of rolling steel bars or wire rods usually involves a slight reduction to adjust the dimensions. Since the change in diameter is minute, the diameter after rolling can be used.

The steel bar or wire rod produced as described above is processed into a part shape by hot forging and/or cold forging, cutting, etc., and then subjected to high frequency heat treatment to become a part. In order for the prior austenite grains in the hardened layer after induction heat treatment to satisfy the above-mentioned grain size conditions, the induction hardening temperature needs to be 900 to 1150°C.

The above temperature range is defined based on the fact that complete austenite transformation occurs during heating within the range of the component composition of the present invention, and that significant austenite grain growth does not occur during heating. Note that the tempering heat treatment after induction hardening may be performed under known conditions.

Hereinafter, the configuration and effects of the present invention will be specifically explained according to Examples. However, the present invention is not limited by the following examples, and can be modified as appropriate within the scope that fits the gist of the present invention, and all of these are within the technical scope of the present invention. It will be done.

Slabs produced by continuously casting steel having the composition shown in Table 1 were processed into billets, and then hot-rolled into round bars of various diameters. The hot rolling conditions were as follows: diameter of the rolled material DL = 30 mm after completion of rolling, and rolling speed VSL = 3.0 m/s immediately before passing through the final rolling stage. The properties required for mechanical structural steel were investigated for the obtained round bar. The hardness of the round bar after hot rolling is measured as a property required of steel for machine structural use, that is, the property of the steel itself, and the hardness distribution is measured after the round bar is subjected to induction heat treatment as described below to evaluate hardenability. I did it.

The hardness was measured by measuring the Vickers hardness at 300 gf at a depth of 1/4 of the diameter of the round bar from the circumferential surface of the round bar. It was measured at 10 arbitrary points, and the average value was calculated and evaluated. The Vickers hardness here is desirably HV195 or less from the viewpoint of cold workability.

The hardness distribution measurement after high frequency heat treatment was carried out by high frequency heat treating a round bar. The induction heat treatment was performed at a frequency of 8.5 kHz, a maximum heating temperature of 1000°C, and moving hardening. Tempering was carried out using a heating furnace at 180°C for 30 minutes. Thereafter, the Vickers hardness was measured at 300gf from the surface to the center on a cross section perpendicular to the axis of the round bar. That is, the first point was a depth position of 1 mm radially inward from the surface of the round bar, and thereafter measurements were made radially inward at 1 mm pitches to evaluate the radial hardness distribution. Based on the results, the layer from the surface to the position where HV450 or higher was evaluated as the effective hardened layer depth (ECD). This effective hardened layer depth region is the hardened layer of the present invention. In order to ensure the strength of the parts, it is desirable that the ECD is 10% or more of the diameter of the round bar.
The results of these measurements are shown in Table 2.

Furthermore, using the same slabs as those used for the hardness measurement and hardness distribution measurement described above, billets made from each slab were hot rolled and induction hardened under the component manufacturing conditions shown in Table 3. A shaft part was fabricated using the following methods. The shape of the part is a round bar that has been extruded from a hot-rolled steel bar with an area reduction rate of 20%, and the ECD (i.e., the thickness of the hardened layer) is approximately 10% of the shaft diameter due to induction heat treatment. Each experiment was carried out after adjusting the conditions as follows.

In the hardened layer of the obtained parts, the number ratio of grains having a grain size twice or more than the mode grain size and the area ratio of crystal grains with a prior austenite grain size of 80 μm or less were investigated.

[Number ratio of grains having a grain size that is twice or more than the mode grain size]
Observation of prior austenite grains in the hardened layer of the component was carried out by cutting out a sample whose observation surface was a cross section perpendicular to the axis of the shaft component. The cut sample was corroded with a 3% picric acid aqueous solution, and the prior austenite grain structure was photographed in 10 fields at 200x magnification using an optical microscope at a position halfway across the ECD. Based on the photographed photograph, the prior austenite grain boundaries were traced, the trace image was processed by image processing software ImageJ, and the diameter of each prior austenite grain was rounded to an integer as a circle equivalent diameter. From the obtained particle size data, the particle size with the largest number of particles was defined as the mode, and the ratio of particles larger than twice that number to the total number of particles was calculated. Note that when there are multiple candidates for the mode, the smallest one among them was treated as the mode.

[Area ratio of crystal grains with prior austenite grain diameter of 80 μm or less]
By image analysis of the trace image obtained in the same manner as above, the area ratio of prior austenite grains with a grain size of 80 μm or less was calculated.

Furthermore, the obtained parts were evaluated for torsional fatigue life and part center runout.
[Torsional fatigue life]
Torsional fatigue life was measured using an electric servo type torsional fatigue testing machine. Loading was carried out at 2 Hz so that the maximum shear stress was 300 MPa, and the number of repetitions until failure was measured. If it shows a fracture life of 15,000 cycles or more in this test, it can be said that it has sufficient fatigue strength.

[Component runout]
The strain of the parts was measured by an eccentricity tester. In other words, by dividing the range of change in displacement (the difference between the maximum displacement value and the minimum displacement value) by the diameter of the measurement part when the part is made to go around the part by supporting the hole drilled in the center of both ends before high-frequency heat treatment, Center runout (%) was calculated. In this test, if the core runout is 0.25% or less, it can be said that the distortion of the part is sufficiently suppressed.

The obtained evaluation results are also shown in Table 3. Furthermore, FIG. 1 summarizes the relationship between the area ratio of prior austenite grains with a grain size of 80 μm or less and core runout. Note that the figure shows only examples whose fatigue life is within the preferred range.

Claims (3)

In mass% C: 0.45-0.51%,
Si: 0.15-0.35%,
Mn: 0.60-0.90%,
P: 0.030% or less,
S: 0.025% or less,
Al: 0.040-0.059%,
Cr: 0.10~0.50% and N: 0.0060~0.0100%
, the remainder has a composition of Fe and impurities, and has a hardened layer formed by induction hardening and tempering, and the area ratio of crystal grains in which the diameter of prior austenite grains in the hardened layer is 80 μm or less is 80 μm. % or more, and the number ratio of grains having a grain size that is twice or more the mode of grain size of prior austenite grains in the hardened layer is 5% or less. The mechanical structural component according to claim 1, wherein the component is a component having a shaft-shaped portion. In mass% C: 0.45-0.51%,
Si: 0.15-0.35%,
Mn: 0.60-0.90%,
P: 0.030% or less,
S: 0.025% or less,
Al: 0.040-0.059%,
Cr: 0.10~0.50% and N: 0.0060~0.0100%
, the remainder being Fe and impurities, hot-rolled at a rolling speed VSL that satisfies the following formula (1) to form a steel bar or wire rod, and after forging the steel bar or wire rod. The method for manufacturing a mechanical structural part according to claim 1 or 2 , wherein the tempering is performed by induction hardening at 900 to 1150°C.
Note VSL≦100/DL (m/s)...(1)
However, VSL is the rolling speed (m/s) just before passing through the final rolling stage, and DL is the diameter of the rolled material (mm) after rolling is completed.

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