JP6551657B2 - Material for high temperature carburized parts excellent in anti-grain coarsening characteristics and method for producing the same - Google Patents

Description

  The present invention relates to a material for a high temperature carburized part having excellent anti-grain coarsening properties and a method for producing the same.

The high-temperature carburizing treatment is a carburizing method in which the carburizing temperature is set high (1000 ° C. or higher) to promote the reaction. According to this carburizing method, more carbon can penetrate and diffuse in a short time, so that the processing time can be shortened and carburized parts can be manufactured efficiently. On the other hand, according to this carburizing method, specific crystal grains may grow abnormally and become mixed grains. When such a structure is generated, fatigue strength and impact strength decrease and heat treatment distortion increases, so it is necessary to prevent abnormal grain growth.
Conventionally, as a technique for preventing abnormal grain growth, for example, as described in Patent Documents 1 to 3 below, a method of dispersing precipitates and pinning grain boundaries, and controlling a structure state before carburizing. There is known a method of suppressing the occurrence of abnormal grain growth. A method of pinning of grain boundaries by dispersion of precipitates is mainly used, and as precipitates, carbonitrides such as AlN and NbC are often used. Specifically, in the following patent documents 1 to 3, not only AlN but also Nb and Ti carbonitrides are efficiently dispersed as precipitates to be pinned, thereby preventing grain coarsening during high-temperature carburizing treatment. Like to do. Moreover, in the following Patent Document 1, the grain coarsening characteristic or the machinability is improved by defining the bainite fraction, the pearlite fraction, and the ferrite crystal grain size as the structural state of the steel before carburizing.

JP 2001-303174 A JP 2003-027135 A JP 2008-189989 A

  However, positively adding a constituent element of a precipitate such as Nb or Ti may lead to deterioration in manufacturing cost and workability. Further, when controlling the structure of steel before carburizing, simply focusing on the structure is not sufficient to prevent grain coarsening during high-temperature carburizing.

  The present invention has been made against the background described above, and the object thereof is a material for high temperature carburized parts capable of favorably preventing coarsening even when high temperature carburizing treatment is performed at 1000 ° C. or higher, and its material It is to provide a manufacturing method.

Means for Solving the Problems and Effects of the Invention

In order to achieve the above object, the material for high-temperature carburized parts having excellent crystal grain coarsening prevention characteristics in the present invention and a carburizing temperature of 940 to 1060 ° C. is in mass%, C: 0.10 to 0.30%, Si: 0.05 to 1.00%, Mn: 0.30 to 2.00%, P: 0.030% or less, S: 0.030% or less, Cr: 0.30 to 1.50%, Mo : 0.50% or less, Al: 0.016 to 0.060%, N: 0.0085 to 0.030%, the balance being composed of Fe and unavoidable impurities,
It is a ferrite and pearlite structure, and satisfies the following formula (1) when the N content is X _N and the Al content is X _Al in mass%, and the area per pearlite is Sp μm ² / piece, 1000 When the equilibrium precipitation amount of AlN in ° C. is Wp mass%, the following formula (2) is satisfied.
(X _N −8.5 × 10 ⁻³ ) (X _Al −1.6 × 10 ⁻² ) ≧ 5.2 × 10 ⁻⁵ (1)
Sp × Wp ≧ 4 (2)
Moreover, in order to achieve the above object, the method for producing a material for high-temperature carburized parts according to the present invention is a method for producing a material for high-temperature carburized parts described above, in mass%, and C: 0.10 to 0.30%. , Si: 0.05 to 1.00%, Mn: 0.30 to 2.00%, P: 0.030% or less, S: 0.030% or less, Cr: 0.30 to 1.50%, Mo: 0.50% or less, Al: 0.016 to 0.060%, N: 0.0085 to 0.030%, the balance being composed of Fe and unavoidable impurities,
After hot forging a steel material satisfying the above formula (1) when the N content is X _N and the Al content is X _{Al in} mass%, the steel material is heated to a predetermined reheating temperature in the range of 900 to 1000 ° C. Then, after heating and holding at the reheating temperature for 20 minutes or more, cooling to a predetermined cooling temperature within a range of 600 to 700 ° C. at a predetermined cooling rate of 0.5 ° C./sec or less, and at the cooling temperature for 30 minutes or more It is characterized by being heated and held.

  The inventors of the present invention focused on the relationship between AlN as the pinning precipitate and the structural state of the steel before carburizing as the material for high temperature carburizing parts. It has been found that there is an appropriate equilibrium precipitation amount and an appropriate structure state in order to improve the crystal grain coarsening characteristics. That is, by defining the equilibrium precipitation amount of AlN and the structural state of the steel before carburizing so as to satisfy the above formulas (1) and (2), even when high-temperature carburizing treatment at 1000 ° C. or higher is performed, the grain size is large Can be satisfactorily prevented. And it is possible to obtain the carburized component adjusted to the appropriate structure | tissue state by giving a predetermined carburizing process to such a raw material for high temperature carburized components.

(A) is a process diagram of temperature-time in hot forging before carburizing. (B) is a flow chart of temperature-time in isothermal annealing applied after (A). (C) is a flow chart of temperature-time in carburizing applied after (B). The graph which shows the solubility product of Al and N used as the basis for calculating AlN equilibrium precipitation amount Wp. The graph which shows the relation of AlN equilibrium precipitation amount Wp-abnormal grain growth temperature. The graph which shows the area | region which satisfy | fills the conditions of Formula (1). The graph which shows the relationship of Sp * Wp- abnormal grain growth generation temperature. The graph which shows the relationship of the cooling rate-per pearlite area in heat treatment before carburizing. (A)-(C) are explanatory drawings which show the method of calculating area Sp per pearlite.

  Hereinafter, the composition limitation reason and limitation condition of each element in the high-temperature carburized part material of the present invention will be described.

(1) C: 0.10 to 0.30%
C is an essential element for ensuring the core strength of steel parts that have been quenched after carburization. However, excessive addition causes deterioration of processability. Preferably it is 0.15 to 0.25%.

(2) Si: 0.05 to 1.00%
Si improves the hardenability of steel and is an effective element as a deoxidizing element of steel. However, excessive addition causes a decrease in the processability of the material. Preferably, it is 0.10 to 0.50%.

(3) Mn: 0.30 to 2.00%
Mn is an element effective for enhancing the hardenability of steel, but excessive addition causes deterioration of workability. Preferably it is 0.50 to 1.50%.

(4) P: 0.030% or less P is required to minimize the content of P because it embrittles the grain boundaries. If the content is 0.030% or less, the effect of lowering the grain boundary strength is slight. On the other hand, extremely reducing the content causes an extension of the refining process and increases costs, which is not industrially preferable.

(5) S: 0.030% or less S is inevitably present in the steel, and combines with Mn to form MnS inclusions which become a starting point of stress concentration. Excessive content increases the amount of MnS inclusions, which in turn leads to a decrease in fatigue strength. However, if the content is 0.030% or less, the decrease in fatigue strength is extremely slight.

(6) Cr: 0.30 to 1.50%
Cr is an element that enhances the hardenability of the steel, and addition of 0.30% or more is necessary to secure the hardenability of the steel. On the other hand, excessive addition will reduce the machinability, so the upper limit is 1.50%. Preferably it is 0.80 to 1.20%.

(7) Mo: 0.50% or less Mo is an element that enhances the hardenability of steel and is effective in improving wear resistance, but is expensive and therefore requires industrial minimization of its content. ing. Preferably it is 0.50% or less.

(8) Al: 0.016 to 0.060%
Al has a deoxidizing action. Further, it is an element which is easily combined with N to form AlN. AlN has an effect of preventing coarsening of crystal grains, and in order to obtain this effect, the content of 0.016% or more is required. On the other hand, when the content exceeds 0.060%, inclusions increase and the bending fatigue strength decreases, so the upper limit is 0.060%.

(9) N: 0.0085 to 0.030%
N combines with Al to form nitrides and has the effect of preventing the coarsening of crystal grains. To obtain this effect, N must be contained in an amount of 0.0085% or more. On the other hand, if added over 0.030%, the hardness of the material is increased, so 0.030% is made the upper limit.

(10) Remainder: Fe and inevitable impurities In Table 1, descriptions of Fe and inevitable impurities are omitted.

Next, it is possible to satisfactorily prevent grain coarsening at a carburizing temperature of 1000 ° C. or higher. In other words, in order for the abnormal grain growth occurrence temperature to be 1000 ° C. or higher, the N content is X _N and Al content by mass% When the amount is X _Al , the following formula (1) is satisfied, the area per pearlite is Sp μm ² / piece, and the equilibrium precipitation amount of AlN at 1000 ° C. is Wp mass%, the following formula (2) Explain that it is necessary to satisfy
(X _N −8.5 × 10 ⁻³ ) (X _Al −1.6 × 10 ⁻² ) ≧ 5.2 × 10 ⁻⁵ (1)
Sp × Wp ≧ 4 (2)

  The following tests were conducted to derive the equations (1) and (2). As a test material for the test, a steel consisting of steel types A to F (the balance being Fe and unavoidable impurities) having the chemical components shown in Table 1 was used. Each test material was melted in an electric furnace and rolled to make a φ80 bar, and a φ8 × 12 mm test piece was collected. The test piece was subjected to hot forging under the conditions shown in FIG. 1 (A) by a processing formaster tester, and then isothermal annealing shown in FIG. 1 (B) and Table 2 was performed. From the test pieces subjected to such heat treatment, measurement of the area per pearlite and carburizing treatment were performed under the conditions shown in FIG. 1 (C), and the abnormal grain growth occurrence temperature was evaluated.

Next, it is expected that there is some correspondence between the AlN equilibrium precipitation amount (mass%) and the grain coarsening at a high temperature of 1000 ° C. or higher, and the AlN equilibrium precipitation amount at 1000 ° C. (1000 ° C.) in each test piece. The amount of precipitation of AlN that would be obtained if held for a long time was calculated based on FIG. In FIG. 2, the X-axis shows the Al content (mass%), the Y-axis shows the N content (mass%), and the line 1 shows the stoichiometric ratio of Al (atomic weight 27) and N (atomic weight 14). A line of 1: 1 is shown. Also, line 2 indicates the solubility product of Al and N (WCLeslie; Trans. ASM, Vol 46 (1954), p. 1470),
log [Al] [N] = 1.03-6770 / T (K).

Consider a case in which the AlN equilibrium precipitation amount is obtained when the Al content and the N content have the composition indicated by A (x1, y1) in FIG. In this case, the intersection B of the line 1 ([N] −y1 = 14/27 × ([Al] −x1)) obtained by translating the line 1 through A (x1, y1) and the line 2 ([Al ] And [N] are solid solution components, and the remaining (difference) is precipitation components. Then, precipitated Al (mass%) and N (mass%) are as follows.
Precipitation Al = containing Al-solid solution Al = x1- [Al] formula (3)
Precipitation N = containing N-solid solution N = y1- [N] formula (4)
Also, since AlN molecular weight = 41, Al atomic weight = 27, N atomic weight = 14,
AlN equilibrium precipitation amount = (41/27) × precipitation Al
= (41/27) x (x 1-[Al]) formula (5)

  Therefore, by substituting the given A (x1, y1) and the line 1 ′ into the line 2 to obtain [Al] and substituting this [Al] into the equation (5), the AlN equilibrium precipitation amount can be obtained. it can. Specifically, since the steel type A in Table 1 is (x1, y1) = (0.030, 0.014), if this and T = 1273 (K: 1000 ° C.) are substituted into the line 2, [ Al] = 0.1116 is obtained. Therefore, a value of 0.0280 is obtained as the AlN equilibrium precipitation amount at 1000 ° C. from the equation (5) (see Table 2). Similarly, in each steel type B to F, values of 0.0332, 0.0326, 0.0428, 0.0197, and 0.0107 are obtained as the AlN equilibrium precipitation amount (mass%) at 1000 ° C., respectively.

  Next, the temperature at which abnormal grain growth occurs, that is, the temperature at which crystal grains coarsen was measured for each test piece, and the relationship between the abnormal grain growth occurrence temperature and the AlN equilibrium precipitation amount was examined. In this case, it was determined that abnormal grain growth occurred under the condition that at least one coarse grain having a grain size of 6 or less was present in the visual field by performing 10 visual field observation randomly with an optical microscope (100 ×). The results are shown in FIG.

  As apparent from FIG. 3, even if the amount of AlN equilibration precipitation is the same, when the cooling rate is different, the abnormal grain growth occurrence temperature becomes dispersed in a range of 20 to 40 ° C., and the cooling rate is slow. It turns out that abnormal grain growth generation temperature shifts to the high temperature side so that it is. For example, when the AlN equilibrium precipitation amount is set to 0.0280 mass%, the abnormal grain growth generation temperature exceeds 1000 ° C. at most cooling rates, but the cooling rate is too fast (2.5 ° C. / and the abnormal grain growth onset temperature was slightly below 1000 ° C. Based on the test results of each test piece, it was determined that an AlN equilibrium precipitation amount of at least about 0.025% by mass was necessary for the abnormal grain growth occurrence temperature to be 1000 ° C. or higher.

Conditions under which the AlN equilibrium precipitation amount satisfies 0.025 mass% or more are as shown in FIG. Here, when AlN is 0.025 mass%, the contents of N and Al are 8.5 × 10 ⁻³ mass% and 1.6 × 10 ⁻² mass%, respectively. By translating the curve at 1000 ° C. by that amount, equation (1) is obtained.

  The N content and the Al content satisfying the expression (1) are represented by a region 11 indicated by hatching in FIG. 4, and the carburizing temperature is 1000 when both the N content and the Al content are in the region 11. It is a necessary condition to prevent abnormal grain growth even at high temperatures of over ° C.

  Next, in order to prevent coarsening at a high temperature of 1000 ° C. or more, it is expected that there is some correspondence with the structure before carburizing as well as from the viewpoint of the amount of AlN equilibrium precipitation, abnormal grain growth in each test piece The relationship between the generation temperature and the product of the area Sp per pearlite and the AlN equilibrium precipitation amount Wp was investigated. The results are shown in FIG.

As apparent from FIG. 5, it can be seen that the abnormal grain growth generation temperature is 1000 ° C. or more in the range of Sp × Wp ≧ 4. Here, the Sp, in each specimen, the pearlite area in the measurement area 0.4mm ^2, likewise by dividing the perlite number of measurement area 0.4mm ^2, and the area per perlite. Specifically, the area per pearlite was calculated by the procedure shown in FIGS. First, as shown in FIG. 7A, the structure of each test piece (ferrite + pearlite structure) was photographed with an optical microscope (picral was used as a corrosive solution).

The number of pearlite in a measurement area of 0.4 mm ² was measured from the photographed tissue photograph. In this case, for example, as shown as a representative of the pearlite P1 surrounded by a broken line in FIG. 7B, the pearlite in the connected state was regarded as one. In addition, it was decided not to include pearlite having a side of 5 μm or less. Next, as shown in FIG. 7C, the photographed tissue photograph was binarized (using a predetermined image editing software), and the perlite area was measured. Then, the pearlite area in the measurement area 0.4mm ^2, likewise by dividing the perlite number of measurement area 0.4mm ^2, was determined area per perlite. Moreover, the calculated value of the equilibrium precipitation amount of AlN at 1000 ° C. for each steel type was used.

  In FIG. 5, data 12 corresponds to the test specimen of steel type B, the abnormal grain growth generation temperature is about 1060 ° C., the calculated value of the AlN equilibrium precipitation amount corresponding to the temperature is 0.0284, and pearlite While the area Sp per piece is 326, the calculated value of the AlN equilibrium precipitation amount Wp at 1000 ° C. of the test piece of the steel type B is 0.0332 as shown in Table 2. Therefore, when setting the carburizing temperature to 1000 ° C. to 1060 ° C., the calculated value of the AlN equilibrium precipitation amount Wp at 1000 ° C. is used instead of the AlN equilibrium precipitation amount calculated based on the actual abnormal grain growth occurrence temperature. Even when used, it is considered that there is no problem in performing the evaluation based on the formula (2). In this case, it is desirable to set the area Sp per pearlite to at least 150 or more in order to satisfy the equation (2), also in connection with using the calculated value of the AlN equilibrium precipitation amount Wp at 1000 ° C. More preferably, it is desired to set it to 250 or more.

  Each test piece was cooled at three cooling rates as shown in FIG. 1B, but between the cooling rate and the area Sp per pearlite, FIG. It was found that there is a relationship as shown in. As apparent from FIG. 6, it can be seen that the area Sp per pearlite remarkably increases when the cooling rate is 0.5 ° C./sec or less. As shown in Table 2, the abnormal grain growth occurrence temperature is increased even when the cooling rate exceeds 0.5 ° C./sec as the value of the AlN equilibrium precipitation amount Wp at 1000 ° C. is larger than 0.025 mass%. In many cases where the temperature exceeds 1000 ° C, the abnormal grain growth temperature when the cooling rate exceeds 0.5 ° C / sec if the AlN equilibrium precipitation amount Wp exceeds 0.025% by mass and the degree is small. Is observed below 1000 ° C.

  As apparent from the above description, according to the material for high temperature carburized parts of the present embodiment, even when high temperature carburizing treatment is performed at 1000 ° C. or more by satisfying the above formulas (1) and (2), Crystal grain coarsening can be satisfactorily prevented. Further, by setting the above-mentioned cooling temperature before carburizing to 0.5 ° C./sec or less, it is possible to prevent the coarsening of the crystal grain more satisfactorily because the area per pearlite can be secured to a predetermined size or more. become able to.

Point B Example 11 of solid solution ([Al], [N]) in Leslie's equation N region satisfying equation (1) and Al content region 12 Example of data satisfying equation (2)

Claims (2)

In mass%,
C: 0.10 to 0.30%,
Si: 0.05-1.00%,
Mn: 0.30 to 2.00%
P: 0.030% or less,
S: 0.030% or less,
Cr: 0.30 to 1.50%,
Mo: 0.50% or less,
Al: 0.016 to 0.060%,
N: 0.0085 to 0.030%,
And the balance consists of Fe and unavoidable impurities,
It is a ferrite and pearlite structure, and satisfies the following formula (1) when the N content is X _N and the Al content is X _Al in mass%, and the area per pearlite is Sp μm ² / piece, 1000 A material for high-temperature carburized parts having excellent grain coarsening prevention characteristics, characterized by satisfying the following formula (2) when the equilibrium precipitation amount of AlN at W ° C. is Wp mass%, and having a carburizing temperature of 940 to 1060 ° C. .
(X _N −8.5 × 10 ⁻³ ) (X _Al −1.6 × 10 ⁻² ) ≧ 5.2 × 10 ⁻⁵ (1)
Sp × Wp ≧ 4 (2) It is a manufacturing method of the raw material for high temperature carburized parts according to claim 1,
In mass%,
C: 0.10 to 0.30%,
Si: 0.05-1.00%,
Mn: 0.30 to 2.00%
P: 0.030% or less,
S: 0.030% or less,
Cr: 0.30 to 1.50%,
Mo: 0.50% or less,
Al: 0.016 to 0.060%,
N: 0.0085 to 0.030%,
And the balance consists of Fe and unavoidable impurities,
After hot forging a steel material satisfying the following formula (1) when the N content is X _N and the Al content is X _{Al in} mass%, the steel material is heated to a predetermined heating temperature in the range of 900 to 1000 ° C., After heating and holding at that heating temperature for 20 minutes or more, cooling to a predetermined cooling temperature in the range of 600 to 700 ° C. at a predetermined cooling rate of 0.5 ° C./sec or less, and holding at that cooling temperature for 30 minutes or more The manufacturing method of the raw material for high temperature carburized parts characterized by having.
(X _N −8.5 × 10 ⁻³ ) (X _Al −1.6 × 10 ⁻² ) ≧ 5.2 × 10 ⁻⁵ (1)