JP7006052B2 - Steel material for soaking treatment - Google Patents

Description

The present invention relates to a steel material and a steel material for poking treatment.

Nitriding using carburizing treatment using the formation of high carbon martensite and precipitation strengthening by alloy nitride as a surface hardening treatment method for thin parts that are one part of planetary gears in automobile transmissions such as ring gears. Processing is widely used. Here, the carburizing treatment has a problem of the magnitude of the treatment strain, and the nitriding treatment has a problem that the treatment time is long and the hardened layer is shallow, and improvement is required. Therefore, a distillation-quenching treatment method has been developed as a new surface hardening treatment method in which the treatment strain is smaller than that of the carburizing treatment and a relatively thick hardened layer can be obtained in a shorter time than the nitriding treatment, and it has begun to be used industrially.

The immersion quenching treatment method is a surface hardening method in which nitrogen is diffused and permeated from the steel surface in a temperature range of A1 point (863K) or higher to obtain high nitrogen austenite on the surface layer, and then quenched to generate hard high nitrogen martensite. Is. Since the carburizing and quenching treatment method is performed at a lower temperature than the carburizing treatment, the thermal strain is small, and the transformation strain is also small in the temperature range where the base metal does not become austenite. In addition, it is expected that the processing time will be shorter because the processing is performed at a higher temperature than that of nitriding.
As a conventional method for such an immersion treatment, for example, the methods described in Patent Documents 1 to 6 have been proposed.

Japanese Unexamined Patent Publication No. 2009-102733 Japanese Unexamined Patent Publication No. 2004-285474 Japanese Patent Application Laid-Open No. 2003-027211 Japanese Unexamined Patent Publication No. 2000-204464 Japanese Unexamined Patent Publication No. 11-22627 Japanese Unexamined Patent Publication No. 10-147814

However, in the conventional carburizing treatment, the temperature is lower than that in carburizing, so that it is difficult to deepen the effective hardened layer.

An object of the present invention is to solve the above problems.
That is, an object of the present invention is to have a low strain and a deep effective cured layer, and by considering the control of the nitride generated during the carburizing treatment, it is possible to have a bending fatigue strength equivalent to that of the carburized material. It is to provide a steel material suitable for nitrogen treatment.

The present inventor has made diligent studies to solve the above problems and completed the steel material of the present invention.
The steel material of the present invention is
C: 0.05 to 0.25% by mass,
Si: 0.30% by mass or less,
Mn: 0.61 to 2.0% by mass,
P: 0.1% by mass or less,
S: 0.1% by mass or less,
Cr: 0.7% by mass or less,
Al: 10-800ppm,
N: 10-300ppm,
Contained in, the balance consists of Fe and unavoidable impurities,
Equation 1 = -0.225 x Si + 0.083 x Mn + 0.091 x Cr + 0.035 x Cu + 0.047 x Ni + 0.092 x Mo
When it is defined as, the equation 1 ≧ 0.04 is satisfied, and
Equation 2 = -20 + 74 x C + 7 x Si + 15 x Mn + 26 x Cr + 10 x Cu + 12Ni + 30 x Mo
When it is defined as, the equation 2 ≦ (60 × C + 31) / 2 is satisfied, and
Equation 3 = 25.4 × Si + 20.3 × Cr + 32.8 × Si × Mn
It is a steel material for poking treatment that satisfies the formula 3 ≦ 22 when defined as.

The steel material of the present invention is
C: 0.05 to 0.25% by mass,
Si: 0.30% by mass or less,
Mn: 0.61 to 2.0% by mass,
P: 0.1% by mass or less,
S: 0.1% by mass or less,
Cr: 0.7% by mass or less,
Al: 10-800ppm,
N: 10-300ppm,
Contained in
Moreover,
Cu: 0.3% by mass or less and / or
Ni: 0.5% by mass or less and / or
Mo: 0.6% by mass or less and / or
V: 0.3% by mass or less and / or
Nb: Contains 0.1% by mass or less, and the balance consists of Fe and unavoidable impurities.
Equation 1 ≧ 0.04 is satisfied,
Equation 2 ≦ (60 × C + 31) / 2 is satisfied,
It is preferable that the steel material for poking treatment satisfies the formula 3 ≦ 22.

According to the present invention, it is possible to provide a steel material suitable for a carburizing treatment, which can have a low strain and a deep effective hardened layer and can have a bending fatigue strength equivalent to that of a carburized material.

It is a schematic side view of the test piece used in the measurement of fatigue strength. It is a figure explaining the soaking treatment applied in an Example and a comparative example (excluding Comparative Examples 14 and 15). It is a figure explaining the nitriding treatment applied in the comparative examples 14 and 15. It is a schematic perspective view of the test piece used in the measurement of strain. It is a graph which shows the relationship between the effective curing depth and the calculation result of the formula 1. It is a graph which shows the relationship between the Rockwell hardness (J5) and the amount of deformation (strain) from a perfect circle. It is a graph which shows the relationship between the nitride precipitation ratio and fatigue strength. It is a tissue photograph (SEM, 2000 times) about the comparative example 2. It is a tissue photograph (FE-EPMA, 2000 times) about the comparative example 2.

The inventor of the present invention can make the effective hardened layer deep with low strain in terms of (1) effective hardened layer depth, (2) hardenability, and (3) nitride formation amount, and has bending fatigue strength equivalent to that of carburized material. We investigated the composition of steel materials suitable for quenching treatment that is possible.

(1) Effective hardened layer When assuming the load stress on practical parts, the depth of the hardened layer is required to the extent that it is not damaged even inside. Therefore, the increase in the effective curing depth for the portion damaged at the internal origin directly leads to the increase in strength.
As a means for deepening the effective cured layer, I. Raise the processing temperature, II. Prolong the processing time, III. There are three types to improve the hardenability of steel materials.
Here I. Since the strain increases as the temperature rises, we would like to carry out the treatment at as low a temperature as possible.
II. From the viewpoint of productivity, we want to carry out the processing in the shortest possible time.
III. So far, almost no studies have been conducted on the effect of steel components on the increase in effective hardening depth.

(2) Hardenability Hardenability needs to be increased as necessary in order to increase the depth of the effective hardened layer. Generally, in order to improve hardenability, elements such as Si, Cr, Mn, Cu, Ni and Mo are added. Further, it is said that the hardenability is improved as the amount of added elements is increased. On the other hand, if the hardenability is too high, the amount of martensitic transformation increases and the strain increases. Therefore, from the viewpoint of strain reduction, it is better that the hardenability is low. Here, the distance from each water-cooled end measured in the hardenability jominy test has a different cooling rate, and the farther from the water-cooled end, the slower the cooling rate.

(3) Nitride ratio When nitrides are formed during the nitrification treatment, the elements added to ensure hardenability are deprived of the matrix, resulting in the formation of an incompletely hardened structure or solid solution of N. It is expected that the fatigue strength will decrease because the effect of improving the hardenability is not sufficiently obtained. Therefore, it is better that the amount of nitride is small. However, the relationship between the amount of added elements and the amount of nitride produced and its effect on fatigue strength have not been clarified so far.

<About Equation 1>
The inventor of the present invention investigated a component system for obtaining an effective cured layer 1.4 times as much as the central component of JIS S15C. The diffusion depth of N and C increases only by the square root with respect to the processing time. That is, if the treatment time is doubled, the effective curing depth becomes 1.4 times. Controlling the steel composition and increasing the effective hardening depth by 1.4 times corresponds to halving the processing time compared to S15C.
As a result of examination by the inventor of the present invention, I. It was clarified that the penetration depth of N changes depending on the amount of alloying element added. Specifically, since Si and Cr generate nitrides, the penetration N depth is reduced. Mn reduces the activity of N and increases the stability of N in the steel material, so that the intrusion N depth increases. II. Further, even when the concentration is the same, the hardness changes due to the change in the hardenability of the steel material. I. , II. The effect of the steel component on the effective hardening depth was examined in consideration of the effect of. Then, when equation 1 = −0.225 × Si + 0.083 × Mn + 0.091 × Cr + 0.035 × Cu + 0.047 × Ni + 0.092 × Mo is defined and equation 1 ≧ 0.04 is satisfied, it is compared with JIS S15C. It was found that 1.4 times as much steel material could be obtained.
In addition, Si, Mn, Cr, Cu, Ni, Mo have Si content (mass%), Mn content (mass%), Cr content (mass%), Cu content (mass%), Ni content. (Mass%), Mo content (mass%). Further, as will be described later, the content of Si, Cr, Cu, Ni, and Mo may be zero. In this case, zero may be substituted for the corresponding term in Equation 1. The same applies to Equations 2 and 3 described later.

<About Equation 2>
Assuming thin-walled parts, the cooling rate of the core of the thin part is about J3 to J5, which is a jominy value, and the steel material of the present invention can greatly reduce strain by reducing the amount of martensite at these cooling rates. ..
The hardness of fulmartensite can be arranged by the amount of carbon, and can be expressed as (60C + 31) HRC in the range of 0.05 to 0.3%. When the J5 value is less than half (60C + 31) / 2 of the hardness at which full martensite is obtained, the martensite structure that greatly affects strain is not generated, and ferrite + pearlite + a small amount of bainite structure is formed, and the strain is sufficiently small. can do.
Based on such findings, the influence of the alloying element on the jominy value J5 of the steel material was examined, and it was found that the strain can be reduced when the formula 2 described later satisfies the formula 2 ≦ (60 × C + 31) / 2.

<About Equation 3>
If the amount of N that becomes a nitride in the invaded surface layer N exceeds a certain ratio (22% or more) with respect to the bending fatigue strength, an incompletely hardened structure is formed, and the effect of improving the hardenability by solid melting of N is obtained. It was clarified that the fatigue strength varied greatly because it could not be obtained sufficiently, and the amount of alloy component added was specified. It was also clarified that when Si and Mn are added at the same time, Mn is also produced as a nitride.
Then, it was found that it is necessary to satisfy the formula 3 ≦ 22 when the formula 3 = 25.4 × Si + 20.3 × Cr + 32.8 × Si × Mn.

The composition of the steel material of the present invention will be described.

The content of the C component is 0.05 to 0.25% by mass, preferably 0.07 to 0.20% by mass.
If the content of the C component is too low, the hardness of the core after quenching and quenching tends to decrease.
On the contrary, if the content of the C component is too high, the cold forging property, the toughness of the core portion, and the processability tend to decrease. Further, when the content of the C component is high, the amount of expansion during quenching tends to increase, and the strain tends to increase.

The content of the Si component is 0.30% by mass or less, preferably 0.01 to 0.15% by mass.
This is because nitrides are produced even in a small amount of Si, and if the content of the Si component is too high, nitrides are formed and the fatigue strength tends to decrease.

The content of the Mn component is 0.61 to 2.0% by mass, preferably 0.65 to 1.5% by mass.
Mn is effective for improving the effective curing depth during the immersion treatment. If the content of the Mn component is too low, the hardenability of the core portion at the time of immersion quenching tends to decrease.
On the contrary, if the content of the Mn component is too high, the machinability and workability tend to decrease, and the hardenability tends to increase too much.

The content of the P component is 0.1% by mass or less, preferably 0.05% by mass or less.
This is because if the content of the P component is too high, grain boundary cracks tend to occur easily.

The content of the S component is 0.1% by mass or less, preferably 0.05% by mass or less.
This is because if the content of the S component is too high, MnS-based inclusions are generated and the fatigue strength tends to decrease.

The content of the Cr component is 0.7% by mass or less, preferably 0.01 to 0.5% by mass.
Cr is an element effective for increasing the effective curing depth after the immersion treatment. If the content of the Cr component is too high, nitrides are likely to be formed, and the fatigue strength varies widely.

The content of the Al component is 10 to 800 ppm, preferably 50 to 500 ppm.
This is because if the content of the Al component is too low, a sufficient amount of AlN that prevents the grain coarsening is not deposited, and the crystal grain coarsening is likely to occur.
On the contrary, if the content of the Al component is too high, coarse Al ₂ O ₃ inclusions tend to occur and the strength tends to decrease.

The content of the N component is 10 to 300 ppm, preferably 10 to 250 ppm.
This is because if the content of the N component is too low, a sufficient amount of AlN that prevents grain coarsening does not precipitate, and crystal grain coarsening is likely to occur.
On the contrary, if the content of the N component is too high, voids (parts that are not densely filled) tend to be generated during casting.

The content of the Cu component is 0.3% by mass or less, preferably 0.01 to 0.25% by mass.
This is because if the content of the Cu component is too high, the hot forging property tends to decrease.

The content of the Ni component is 0.5% by mass or less, preferably 0.4% by mass or less.
This is because if the content of the Ni component is too high, the processability tends to decrease.

The content of the Mo component is 0.6% by mass or less, preferably 0.5% by mass or less.
This is because if the content of the Mo component is too high, the workability and machinability tend to decrease, and the hardenability tends to increase too much.

The content of the V component is 0.3% by mass or less, preferably 0.25% by mass or less.
This is because if the content of the V component is too high, the machinability tends to decrease.

The content of the Nb component is 0.1% by mass or less, preferably 0.05% by mass or less.
This is because if the content of the Nb component is too high, the processability tends to decrease.

The steel material of the present invention is a steel material containing C, Si, Mn, P, S, Cr, Al, N, Cu, Ni, Mo, V, and Nb in a specific ratio as described above (however, Si, P, S, Cr, Cu, Ni, Mo, V, Nb may not be included), the balance is Fe and unavoidable impurities.

The steel material of the present invention can be preferably used as a thin-walled part by subjecting it to a distillation treatment.
Here, the thin wall means a part having a thickness of 1 to 15 mm at the thinnest part. Further, examples of the thin-walled component include a ring gear in a planetary gear incorporated in an automatic transmission.

<Manufacturing of test pieces>
Hereinafter, examples of the present invention will be described.
For each of Examples 1 to 38 and Comparative Examples 1 to 15 shown in Table 1, raw materials were mixed so as to have the composition shown in Table 1 (the balance is Fe and unavoidable impurities), and a 150 kg high frequency induction furnace was used. It was melted and cast to obtain a steel ingot A.

<Measurement of fatigue strength>
The ingot A was hot-rolled or hot-forged to obtain a round bar having a cross-sectional diameter of 105 mm, and then hot-forged to obtain a round bar having a cross-sectional diameter of 22 mm. Then, after the baking treatment (925 ° C. × 1 HrAC), a round bar (length 210 mm) having a cross-sectional diameter of 15 mm is cut out from this round bar and further processed to obtain a diameter of a parallel portion as shown in FIG. A steel piece having a diameter of 12 mm, a notch diameter of 8 mm, and a notch bottom of 0.5 R (radius 0.5 mm) was obtained.
Next, the steel piece was subjected to the distillation treatment X to obtain a test piece B.

In the following, the immersion treatment X means the following treatment.
Steel pieces were placed in a gas carburizing nitriding furnace, ammonia was supplied as a carburizing gas, propane gas was supplied as a carburizing gas, and the carburizing treatment was performed at 800 ° C. for 180 minutes. At this time, the CP was controlled to 0.3 by adjusting the partial pressures of carbon monoxide and carbon dioxide. By performing such an immersion treatment, the C concentration on the outermost surface was set to 0.3% by mass so that the change in the amount of C contained in the base metal did not affect the effective curing depth.
After such soaking treatment, the test piece is quenched by immersing it in hot oil at 150 ° C., and then tempered at 180 ° C. for 120 minutes using a heating furnace to obtain the test piece. obtain.
However, in the case of Comparative Example 14 and Comparative Example 15, the CP was set to 0.7, and the gas carburizing treatment was performed at 880 ° C. for 60 minutes.
FIG. 2 shows an outline of the infiltration treatment in Examples and Comparative Examples (however, Comparative Examples 14 and 15 are excluded). The outline of the carburizing treatment in Comparative Examples 14 and 15 is shown in FIG.

Using the test piece B thus obtained, an Ono-type rotary bending fatigue test was conducted by a method conforming to JIS Z 2274, and the fatigue strength was investigated. The test conditions are a rotation speed of 3500 rpm and the test temperature is room temperature. Further, the value of fatigue strength means the fatigue limit, which is the maximum stress that does not break after the number of repetitions of ¹⁰⁷ times.
The results are shown in Table 2.

<Jominy test>
The ingot A was hot-rolled or hot-forged to obtain a round bar having a cross-sectional diameter of 105 mm, and then hot-forged to obtain a round bar having a cross-sectional diameter of 30 mm. Then, a normalizing treatment (925 ° C. × 1 HrAC) was carried out. A test piece C having a cross-sectional diameter of 25 mm and a length of 100 mm was obtained from this round bar.
Then, such a test piece C was subjected to a Jominy type one-sided quenching test (925 ° C., 30 minutes) specified in JIS G0561. Then, the Rockwell hardness (J5) at 5 mm from the hardened end was measured.
The results are shown in Table 2.

<Measurement of strain (deformation from a perfect circle)>
The steel ingot A is hot-rolled or hot-forged to obtain a round bar having a cross-sectional diameter of 105 mm, and after normalizing (925 ° C. × 1 HrAC), the round bar is processed and as shown in FIG. A ring-shaped test piece material having an outer diameter of 100 mm, an inner diameter of 90 mm, and a thickness of 20 mm was obtained.
Next, the ring-shaped test piece material was subjected to a distillation treatment X to obtain a test piece D. Then, the length of the inner diameter portion of the test piece D is measured at three points before and after immersion, the average value is calculated, the inner circumference length is obtained from the average value, and the inner diameter length before and after immersion is obtained. The amount of deformation (strain) from a perfect circle was measured by calculating the difference between the average values of.
The results are shown in Table 2.

<Surface C concentration>
After hot rolling the steel ingot A to obtain a round bar having a cross-sectional diameter of 105 mm, the steel ingot A has a cross-sectional diameter of 30 mm by hot forging, and further subjected to normalizing treatment (925 ° C. × 1 HrAC). A test piece material obtained by cutting out a round bar (length 100 mm) having a cross-sectional diameter of 25 mm was subjected to a normalizing treatment X to obtain a test piece E.
Then, the test piece E was scraped to a position of 0.05 mm from the surface thereof, and the C concentration in the obtained chips (Dalai powder) was measured. The combustion-infrared absorption method was used for the measurement.
The results are shown in Table 2.

<Surface N concentration>
The N concentration in the chips (Dalai powder) obtained by scraping from the surface of the test piece E to a position of 0.05 mm was measured. Melt-thermal conductivity measurement was used for the measurement.
The results are shown in Table 2.

<Nitride precipitation amount>
Chips (Dalai powder) obtained by scraping from the surface of the test piece E to a position of 0.05 mm are dissolved with methanol bromide, and the precipitate is extracted using a 0.2 μm filter to determine the amount of nitride precipitate. It was measured. The distillation separation-nitrogen analysis method was used for the measurement.
The results are shown in Table 2.

<Measurement of effective cured layer depth>
The steel ingot A is hot-rolled or hot-forged to obtain a round bar having a cross-sectional diameter of 105 mm, and then hot-forged to obtain a round bar having a cross-sectional diameter of 30 mm, and after normalizing (925). (° C. × 1 HrAC), a steel piece obtained by cutting out a round bar (length 10 mm) having a cross-sectional diameter of 25 mm from this round bar was subjected to a normalizing treatment X to obtain a test piece F.
Then, the Vickers hardness of the test piece F was measured from the surface layer in the thickness direction from one end face thereof with a test force of 2.94 N. Here, the hardness was determined from the surface layer to 0.5 mm at a pitch of 0.025 mm, and thereafter at a pitch of 0.1 mm to a position of 1 mm. Then, the hardness at each obtained position was continuously combined to determine the effective hardened layer depth. The surface layer hardness is defined as a hardness at a position of 0.05 mm, and the effective cured layer depth is defined as a depth from the surface of 513 HV. Specifically, it was calculated from the hardness of two points before and after 513HV, which is the closest to 513HV, and the effective cured layer depth at 513HV was calculated.
The results are shown in Table 2.

<Microstructure observation>
Microstructure observation was performed on the test piece F. The test piece was divided into two equal parts in a semicircular shape, the cut surface was filled with resin so as to be the surface to be inspected, and the surface was mirror-polished. The polished surface was corroded with nital, and the structure was observed using an optical microscope at a magnification of 100 to 400 times and an SEM at a magnification of 2000 times. In addition, using FE-EPMA using the sample after mirror polishing, the precipitation state of the nitride on the surface layer was confirmed at a magnification of 2,000 times.

FIG. 5 shows the relationship between the effective curing depth shown in Table 2 and the calculation result of Equation 1.
From FIG. 5, it is understood that when the value of Equation 1 is 0.04 or more, the effective curing depth is 1.4 times that of S15C. If the treatment time is set to 1.4 times, it corresponds to the JIS 15C comparison treatment time being halved in order to obtain the same effective curing depth.
Therefore, it can be said that the steel material of the present invention is a steel material capable of obtaining the required effective hardening depth in a short time (about 3 to 4 hours).

The relationship between the Rockwell hardness (J5) shown in Table 1 and the amount of deformation (strain) from a perfect circle is shown in FIG.
From FIG. 6, it can be seen that the amount of deformation is small when the Rockwell hardness (J5) is about 20 HRC or less. The hardness of fulmartensite can be arranged by the amount of carbon, and can be expressed as (60C + 31) HRC in the range of 0.05 to 0.3%. Generally, if the Rockwell hardness (J5) is half ((60C + 31) / 2) or less (20HRC in the case of 0.15%), which is the full martensite, the strain (deformation amount) is greatly affected. No structure is formed, and ferrite + pearlite + a small amount of bainite structure is formed. That is, if the equation 2 ≦ ((60C + 31) / 2) is satisfied, the amount of martensite can be reduced and the strain can be significantly reduced. Further, Comparative Examples 14 and 15 are SCR415 and SCR420, which are general-purpose hardened steels, respectively, and it can be seen that the strain is significantly reduced with respect to them.
Assuming thin-walled parts of an actual machine, the cooling rate of the core of the thin parts is about J3 to J5, which is a jominy value, and the steel material of the present invention has large distortion by reducing the amount of martensite at these cooling rates. Can be reduced.

From the surface N concentration and the nitride precipitation amount shown in Table 2, the nitride precipitation ratio (= nitride precipitation amount / surface N concentration × 100 (%)) was determined. The relationship between the obtained nitride precipitation ratio and the fatigue strength is shown in FIG.
From FIG. 7, it can be seen that when the ratio of N that has become a nitride in the surface layer N is 22% or more, the variation in fatigue strength becomes large. From the microstructure photograph (SEM 2000 times, Fig. 8), the element added to ensure hardenability was deprived from the matrix due to the formation of nitride, and the incompletely hardened structure formed by the reduction of solid solution N was formed. It is confirmed. Furthermore, the formation of nitrides can be seen from FE-EPMA (2000 times, FIG. 9). In all the test pieces with reduced fatigue strength, such incompletely hardened structure and a large amount of nitride formation were confirmed, and a causal relationship with fatigue strength was clarified.
It can be said that the steel material of the present invention is a steel material in which the fatigue strength does not vary or decrease because the amount of nitride precipitates is suppressed.

Claims (2)

C: 0.09 to 0.25% by mass,
Si: 0.30% by mass or less,
Mn: 0.61 to 2.0% by mass,
P: 0.1% by mass or less,
S: 0.1% by mass or less,
Cr: 0.02 to 0.7 % by mass,
Cu: 0.01-0.3% by mass,
Al: 10-800ppm,
N: 10-300ppm,
Contained in, the balance consists of Fe and unavoidable impurities,
Equation 1 = -0.225 x Si + 0.083 x Mn + 0.091 x Cr + 0.035 x Cu + 0.047 x Ni + 0.092 x Mo
When it is defined as, the equation 1 ≧ 0.04 is satisfied, and
Equation 2 = -20 + 74 x C + 7 x Si + 15 x Mn + 26 x Cr + 10 x Cu + 12Ni + 30 x Mo
When it is defined as, the equation 2 ≦ (60 × C + 31) / 2 is satisfied, and
Equation 3 = 25.4 × Si + 20.3 × Cr + 32.8 × Si × Mn
A steel material for poking treatment that satisfies the formula 3 ≦ 22 when defined as. Ni : 0.5% by mass or less,
Mo: 0.6% by mass or less,
V: 0.3% by mass or less,
Nb: 0.1% by mass or less,
The steel material for poking treatment according to claim 1, further comprising one or more kinds selected from the group consisting of.

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