JP2016199784A - Raw material for high temperature carburization component excellent in crystal grain coarsening prevention characteristic and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a raw material for high temperature carburization component excellent in crystal grain coarsening prevention characteristics.SOLUTION: A raw material for high temperature carburization component consists of, by 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% and the balance Fe with inevitable impurities. The raw material has a ferrite and perlite structure before carburization and satisfies following formula (1), when N content is defined as Xand Al content is defined as X, and satisfies following formula (2) when an area per one perlite is defined as Sp and equilibrium precipitation amount of AlN at 1,000°C is defined as Wp. (X-8.5×10)(X-1.6×10)≥5.2×10(mass%)...(1), Sp(μm/unit)×Wp(mass%)≥4...(2).SELECTED DRAWING: Figure 4

Description

  The present invention relates to a high-temperature carburized component material excellent in crystal grain coarsening prevention characteristics 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 are reduced and heat treatment strain is increased, so that 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 are known methods for suppressing the occurrence of abnormal grain growth. A grain boundary pinning method by dispersion of precipitates is mainly used, and carbonitrides such as AlN and NbC are often used as the precipitates. 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 in the background as described above, and the purpose thereof is a material for high-temperature carburized parts that can satisfactorily prevent coarsening of crystal grains even when a high-temperature carburization treatment of 1000 ° C. or higher is performed, and its 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 of the present invention is in mass%, C: 0.10 to 0.30%, Si: 0.05 to 1.00%, Mn: 0.30 to 2 0.000%, 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 is made of Fe and inevitable 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 It is characterized in that the following formula (2) is satisfied when the equilibrium precipitation amount of AlN at ° C. is Wp mass%.
(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 said objective, the manufacturing method of the raw material for high-temperature carburized components of this invention is the mass%, C: 0.10-0.30%, 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%, the balance is made of Fe and inevitable 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 a precipitate to be pinned and the structural state of steel before carburization as a material for high-temperature carburized 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) Process drawing of temperature-time in hot forging before carburizing. (B) Process drawing of the temperature-time in the isothermal annealing performed after (A). (C) Process drawing of temperature-time in the carburizing performed 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 relationship of AlN equilibrium precipitation amount Wp- abnormal grain growth generation 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 in the heat processing before carburizing-the area per pearlite. (A)-(C) is explanatory drawing which shows the method of calculating the area Sp per pearlite.

  Hereinafter, the reasons for limiting the composition of each element and the limiting conditions in the raw material for high-temperature carburized parts 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 workability. Preferably it is 0.15-0.25%.

(2) Si: 0.05 to 1.00%
Si enhances the hardenability of steel and is an effective element as a deoxidizing element for steel. However, excessive addition causes a decrease in workability 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 unavoidably present in the steel, and combines with Mn to generate MnS inclusions that serve as starting points for 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 very slight.

(6) Cr: 0.30 to 1.50%
Cr is an element that enhances the hardenability of steel, and 0.30% or more of addition is necessary to ensure the hardenability of steel. On the other hand, since excessive addition reduces machinability, 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 that is easily bonded to N to form AlN. AlN has an effect of preventing coarsening of crystal grains, and in order to obtain this effect, it is necessary to contain 0.016% or more. On the other hand, if the content exceeds 0.060%, inclusions increase and, on the other hand, the bending fatigue strength decreases, so 0.060% is made the upper limit.

(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) The point which needs to satisfy | fill is demonstrated.
(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 specimen, steel made of steel types A to F having the chemical components shown in Table 1 (the balance being Fe and inevitable impurities) was used. Each specimen was melted in an electric furnace and rolled to prepare a φ80 bar, and a test piece of φ8 × 12 mm was collected. This test piece was subjected to hot forging under the conditions shown in FIG. 1 (A) by a machining formaster test machine, and then subjected to isothermal annealing shown in FIG. 1 (B) and Table 2. 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. Line 2 shows 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 ], [N]) becomes a solid solution, and the remainder (difference) becomes a precipitate. Then, precipitated Al (mass%) and N (mass%) are as follows.
Precipitated Al = Contained Al-Solubility Al = x1- [Al] Formula (3)
Precipitation N = Contained N-Solubility N = y1- [N] Formula (4)
Moreover, since AlN molecular weight = 41, Al atomic weight = 27, and N atomic weight = 14,
AlN equilibrium precipitation amount = (41/27) × precipitated Al
= (41/27) × (x1- [Al]) (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] = 0.116 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, 10 visual field observations were randomly performed with an optical microscope (100 times), and it was determined that abnormal grain growth occurred on the condition that at least one coarse grain having a particle size of 6 or less was present in the visual field. The results are shown in FIG.

  As can be seen from FIG. 3, even when the AlN equilibrium precipitation amount is the same, if the cooling rate is different, the abnormal grain growth occurrence temperature varies and is distributed within the range of 20 to 40 ° C., and the cooling rate is slow. It turns out that the abnormal grain growth occurrence temperature shifts to the higher temperature side. For example, when the AlN equilibrium precipitation amount is set to 0.0280% by mass, the abnormal grain growth occurrence temperature exceeds 1000 ° C. at most cooling rates, but the cooling rate is too fast (2.5 ° C. / sec), the abnormal grain growth 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. Equation (1) is obtained by translating the curve at 1000 ° C.) by that amount.

  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. This is a necessary condition for preventing abnormal grain growth even at a high temperature of ℃ or higher.

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

As is apparent from FIG. 5, the abnormal grain growth temperature becomes 1000 ° C. or higher 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 pearlites in a measurement area of 0.4 mm ² was measured from the photographed tissue photograph. In this case, for example, as a representative pearlite P1 surrounded by a broken line in FIG. 7B, the number of pearlites in the connected state is regarded as one. Further, pearlite having a side of 5 μm or less was not included. Next, as shown in FIG. 7C, the taken tissue photograph was binarized (using predetermined image editing software), and the pearlite 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 AlN equilibrium precipitation amount used the calculation value in 1000 degreeC for every steel type.

  In FIG. 5, data 12 corresponds to a test piece having a steel type as B, the abnormal grain growth temperature is about 1060 ° C., the calculated value of the AlN equilibrium precipitation amount corresponding to that temperature is 0.0284, 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. For this reason, when the carburizing temperature is set 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 if it is used, it is considered that there is no problem in performing the evaluation based on the formula (2). In this case, in relation to using the calculated value of the AlN equilibrium precipitation amount Wp at 1000 ° C., it is desirable to set the area Sp per pearlite to at least 150 or more in order to satisfy the formula (2). 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 can be seen from FIG. 6, when the cooling rate is 0.5 ° C./sec or less, the area Sp per pearlite increases remarkably. 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%. However, when the cooling rate exceeds 0.5 ° C./sec, the abnormal grain growth temperature is increased. Is observed below 1000 ° C.

  As is clear from the above explanation, according to the material for high-temperature carburized parts of this example, even when a high-temperature carburizing process of 1000 ° C. or higher is performed by satisfying the above formulas (1) and (2), Crystal grain coarsening can be satisfactorily prevented. In addition, by setting the cooling temperature before carburizing to 0.5 ° C./sec or less, the area per pearlite can be secured to a predetermined size or more, thereby preventing grain coarsening even better. 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)

% By 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 inevitable 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 excellent in crystal grain coarsening prevention characteristics characterized by satisfying the following formula (2) when the equilibrium precipitation amount of AlN at ℃ is Wp mass%.
(X _N −8.5 × 10 ⁻³ ) (X _Al −1.6 × 10 ⁻² ) ≧ 5.2 × 10 ⁻⁵ (1)
Sp × Wp ≧ 4 (2) % By 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 inevitable 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 A method for producing a material for high-temperature carburized parts, comprising:
(X _N −8.5 × 10 ⁻³ ) (X _Al −1.6 × 10 ⁻² ) ≧ 5.2 × 10 ⁻⁵ (1)