WO2018061396A1 - Forged heat-treated product of case hardening steel - Google Patents

Abstract

In order to provide a forged heat-treated product of case hardening steel that has good machinability and that does not cause austenite grain coarsening during carburizing treatment in a practical temperature range, a forged heat-treated product of case hardening steel according to the present invention comprises a ferrite-pearlite structure steel that has 20-10,000 particles per µm² of an AlN precipitate having a particle size of 1-100 nm and that is the result of fine dispersion precipitation corresponding to the correlation f ≥ x when the number of particles per µm² is denoted by f and the average particle size is denoted by x (nm), and said forged heat-treated product of case hardening steel contains 0.10-0.35 wt% of C, 0.01-0.80 wt% of Si, 0.30-1.80 wt% of Mn, 0.020 wt% or less of P, 0.020 wt% or less of S, 0.15 wt% or less of Cu, 0.00-2.50 wt% of Ni, 0.30-2.50 wt% of Cr, 0.00-1.00 wt% of Mo, and 0.0020 wt% or less of O.

Description

Forged heat-treated products of bare steel

The present invention relates to a forged heat-treated product using bare-fired steel as a raw material.

Conventionally, in the manufacture of mechanical parts such as continuously variable transmission (Continuously Variable Transmission (hereinafter referred to as “CVT”), transmission gears, and differential gears, hot forging (cold forging is added using bare hardened steel. The same shall apply hereinafter), and after carrying out a normalization treatment to facilitate the subsequent machining, machining is performed to the shape of the final part, followed by carburizing and quenching. Further finishing processing was performed to obtain the final part.

By the way, in recent years, from the viewpoint of environmental friendliness, these components are becoming smaller and lighter and the shape of the components for realizing them is becoming more complex.
That is, (1) In order to reduce the size of these parts, high strength is required, the hardness of the forging material is increased, and the tool life during machining is shortened. (2) Complex shapes Since carburizing and quenching of parts, carburizing and quenching distortion is also increasing.
Therefore, (1) attempts to improve the tool life by providing a normalizing treatment after forging to soften the forged parts, and (2) in order to reduce carburizing quenching distortion, Semi-treatment and normalization + tempering have been performed.

On the other hand, with the widespread use of automobiles worldwide, such as BRICs, in addition to developed countries, production of extremely large amounts of parts has been forced. (1) On the molding surface, a method that combines hot forging and cold working, (2) In the carburizing process, in addition to high-temperature gas carburizing, a method of performing low-pressure high-temperature carburizing, a method of combining carburizing and nitriding (carbonitriding), or the like is employed. The former aims to simplify machining, and the latter aims to shorten the carburizing time and increase the functionality, but depending on the carburizing temperature, it may lead to a reduction in the strength of the parts due to coarsening of austenite grains, Optimization of these processes and cost reduction have been desired.

Takeshi Fujimatsu, 3 others, “Iron and Steel”, Vol. 93 (2007) No. 10, p. 35-40 Shuji Kinoshita, two others, "Iron and Steel", 59 (1973) No. 3, p. 88-95 Toshiyuki Tsuge, 3 others, “Iron and Steel”, '86 -S509, p. 103 Taiji Nishizawa, "Iron and Steel", 70th (1984) No.15, p.194-202

In view of the problems related to the production of the above-mentioned conventional forged parts, the present invention examines various simplifications of the process, and (1) molding by a method combining hot forging and cold working, or by low-pressure high-temperature carburizing. Even if the heat treatment time is shortened, (2) if the steel for forging, the forging heating temperature and the normalizing conditions are optimized, the machinability is good and the austenite grains are coarse by carburizing in the practical temperature range. It has been found that a forged heat-treated product of case-hardened steel with no deformation or carburizing quenching strain can be provided.

In order to achieve the above object, the forged heat-treated product of the case-hardened steel of the present invention uses the case-hardened steel as a raw material and is subjected to hot forging and heat treatment in the following steps (1) to (3). Manufactured.
(1) A step of heating the material at 1100 to 1280 ° C. and forging at a temperature of 950 to 1200 ° C.
(2) A process of cooling in the range of 0.10 to 2 ° C./s even if the cooling after forging is natural air cooling or controlled cooling.
(3) In the step of carrying out the normalization process, the temperature is increased at a temperature increase rate in the range of 0.10 to 0.40 ° C./s during the normalization process, and the temperature reaches a predetermined normalization temperature. The time from 860 ° C. in the temperature process to 860 ° C. in the cooling process after reaching the normalization temperature is set to 1800 s or less, and subsequently cooled to 550 ° C. at a cooling rate of 0.10 to 0.30 ° C./s. And a step of removing from the normalizing furnace (it can also be performed after the normalizing furnace is cooled to near room temperature). Here, when it can be determined that the ferrite pearlite transformation is completed at a temperature higher than 550 ° C., the furnace may be removed at that time.
(4) By passing through these steps, the forged parts can disperse fine precipitates such as AlN (Aluminum Nitride (hereinafter referred to as “AlN”) precipitates, and abnormalities of austenite grains during carburization. Grain growth is suppressed, and as a result, the carburizing and quenching distortion of the carburized parts is improved.
(5) Depending on the application, a step of cold forging under a conventionally known production condition can be added.

”Here,“ hardened steel ”means“ hardened steel ”defined in JIS G 0203. However, depending on the size of the part, it may deviate from the standard of the hardenability improving element, and these are included as targets. The “temperature increase rate” or “cooling rate” refers to an average temperature increase rate or average cooling rate between 800 ° C. and 500 ° C.

The forged heat-treated product of the case-hardened steel of the present invention thus produced has 20 to 10,000 AlN precipitates having a particle size of 1 to 100 nm per 1 μm ² , and 1 μm ² When the number of per particle is f and the average particle diameter is x nm, it is ferrite + pearlite structure steel finely dispersed and precipitated with a correlation of f ≧ x, and C: 0.10 to 0.35% by weight, Si: 0.01 to 0.80 wt%, Mn: 0.30 to 1.80 wt%, P: 0.020 wt% or less, S: 0.020 wt% or less, Cu: 0.15 wt% or less, Ni : 0.00-2.50 wt%, Cr: 0.30-2.50 wt%, Mo: 0.00-1.00 wt%, and O: 0.0020 wt% or less To do.
Here, among the AlN precipitates, it is preferable that the number of AlN precipitates having a particle diameter of 5 to 40 nm deposited alone is 20 or more and 300 or less per 1 μm ² .

In this case, Sol. Al: 0.010 to 0.060 wt%, Nb: 0.015 to 0.060 wt%, Ti: 0.0015 to 0.050 wt% and N: 0.010 to 0.025 wt% be able to.

The forged heat-treated products of the case-hardened steel of the present invention are carburized parts used for drive system parts of automobiles and construction machinery, among others, mechanical parts used as CVT, mission gear, differential gear, etc. (1) environmental considerations (energy saving, CO ₂ reduction), (2) reduce the carburized distortion, (3) the austenite grain abnormal grain growth inhibition during carburization, provided as (4) shortening the construction period, which contributes forged heat-treated product to reduce costs It can be done.

It is explanatory drawing of the manufacturing process of the forge heat processing components of this invention. It is a graph showing the correlation of the number of unit area (1 [mu] m ²⁾ per AlN precipitates and (f-number) and the average particle diameter (x nm). 6 is a structure photograph showing an example of FE-SEM observation of a forged heat-treated part.

Hereinafter, embodiments of the forging heat-treated product of the hardened steel according to the present invention will be described based on examples.

Forged heat-treated products of the case-hardened steel of the present invention utilize trace elements such as Al and N contained in the steel, optimize the heating temperature and forging temperature of hot forging, and the subsequent heat treatment conditions. Precipitates having a particle size of 1 to 100 nm, 20 to 10000 per unit area (1 μm ² ), f per unit area, and average particle size xnm, f ≧ x By making the ferrite + pearlite structure steel finely dispersed and precipitated by correlation, it is possible to soften before machining, suppress coarsening during carburizing, reduce carburizing quenching strain, and so on.
The AlN precipitate shown here was obtained by collecting a 10 mm square test piece from the surface layer of the forged heat-treated product, embedding and polishing, corroding with a 1.5% nital solution for about 30 seconds, vapor depositing gold, —SEM (Field Emission-Scanning Electron Microscope) (hereinafter referred to as “FE-SEM”). In the measurement, four fields of view were observed at a magnification of 40,000 times, and the size and number of AlN precipitates were measured from a total test area of 30 μm ² .
Since the AlN precipitates were mainly strip-shaped, and some of them were agglomerated, the measured area of the AlN precipitates was replaced with a circular area, and the diameter of the circle was taken as the particle diameter.
Identification as an AlN precipitate was performed by EDX analysis (Energy dispersive X-ray spectrometry), but it was difficult to discriminate when the size was 100 nm or less. Therefore, a 500 nm level precipitate estimated to be insoluble is identified, and only those having an Al peak are judged as AlN, and those below 100 nm existing in the vicinity thereof are assumed to be AlN precipitates. The particle size and number were measured. In order to calculate the pinning force, it is necessary to measure the number per unit volume, but measure the number per unit area for FE-SEM observation, and use the number per 1 μm ² as the notation. It was.
Here, the particle diameter is obtained by taking the electronic information of the image observed with the FE-SEM into a digital microscope (manufactured by Keyence Corporation), measuring the area of each precipitated particle with the attached analysis software, and obtaining the equivalent circle diameter. The particle diameter was converted.
Although there are AlN precipitates having a size exceeding 100 nm, the number thereof is small, and the pinning effect thereof is small as compared with precipitates having a size of 1 to 100 nm. Was excluded from the subject.
As for the measured precipitates, a region containing precipitates with a size of more than 100 nm and precipitates with a size of 100 nm or less from the FE-SEM observation sample was used using a FIB (Focused Ion Beam). Each sample is collected and identified as a representative one by electron beam diffraction, and confirmed as AlN precipitate.

The reason why the precipitation form of the forged heat-treated product constituting the invention is specified will be described below.

The particle diameter was 1 to 100 nm. In the carburizing and quenching treatment, for example, the treatment is performed at 930 ° C. for 6 hours and in the austenite region for a long time at a high temperature. In the grain growth of austenite grains during carburizing and quenching treatment, it is said that coarsening should not occur because it affects the fatigue strength and wear resistance of automobile parts after carburizing and quenching. In order to suppress grain growth, fine precipitates such as AlN are utilized. However, as described in Non-Patent Document 4, a large number of such fine precipitates are precipitated in steel, and grain boundaries during carburizing treatment are used. It is known as an effective method to pin the movement (granular growth). In this study, it is difficult to control the form of the precipitate smaller than 1 nm. In addition, the size of 100 nm or more has a smaller number of precipitates and the pinning is larger than that of 1-100 nm precipitates with a large number of precipitates. Since the influence on the effect is small, the range is set to 1 to 100 nm.
The number per unit area (1 μm ² ) was 20 or more and 10,000 or less. In the case of 20 or less per unit area, there is no sufficient pinning effect, and in order to deposit 10000 or more, it is necessary to make AlN solid solution once at the forging heating temperature and reprecipitate. There are also restrictions on the forging heating temperature, and there are upper limits on the amounts of Al and N added, so the amount of precipitation depends on the amount of solid solution, and the upper and lower limits are naturally determined. In this patent, the number per unit area (1 μm ² ) is set to 20 or more and 10,000 or less from a practical aspect.
The number f per unit area (1 μm ² ) of fine precipitates and the average particle diameter x were controlled by the correlation of f ≧ x. Here, f is the number of fine precipitates such as AlN per unit area (1 μm ² ), x is the average particle diameter of the fine precipitates, and the range of the particle diameter is defined as 1 to 100 nm. The particle diameter was also set to 1 to 100 nm. The number of fine precipitates per unit area and the average particle size cannot be controlled independently, but can be controlled within a certain addition amount. On the other hand, the magnitude of the pinning effect depends on the contact area with the inhibition precipitate during grain boundary growth. That is, the pinning effect per precipitate increases as the particle size of the precipitate increases, and the effect decreases as the particle size decreases. Therefore, as the size of the precipitate approaches 100 nm, the pinning force per precipitate increases. Increase, but reduce the total number of precipitates. That is, when the total volume of precipitates is the same, the larger the total number of precipitates, the larger the total cross-sectional area of the precipitates.Therefore, if the number of fine precipitates is increased, the pinning effect is increased. growing. Therefore, the average particle diameter (x) and the number per unit area (f) are controlled by the correlation of f ≧ x.
The precipitation of AlN is generally performed by a normalizing process after forging. The heating rate, processing time, cooling rate, etc. of the normalizing process will be described separately, but here, the precipitation state of AlN will be described. As described in Non-Patent Document 4, as described above, the smaller the number of AlN precipitates, the greater the overall pinning force, which is effective in suppressing the growth of austenite grains during carburizing. However, in addition to the size of the precipitate, in addition to the single precipitate, a plurality of composite precipitates may be formed, and it is necessary to define from the viewpoint of the pinning effect. In the present study, it was found that the pinning effect of the composite precipitate in the case of two or more composite precipitates is small even if it is the same size as the single precipitate. Photo 3 shows an example of simple precipitation, Photo 4 shows an example of two composite precipitations, and Photo 5 shows an example of three composite precipitations as FE-SEM observation photographs. In the present invention, the precipitation form of the AlN precipitate is “a single precipitate having a particle size of 5 to 40 nm is deposited in a total of 20 to 10,000, preferably 20 to 300, per 1 μm ² . “Forged heat-treated product” was defined as a state in which the effects of the present invention can be fully utilized.

A specific method for exhibiting the effects of the present invention will be described below.
1. Material steel Generally, carburized parts used in automobiles and construction machinery are manufactured by carburizing and quenching using the case-hardened steel specified in JIS G 0203.
Specifically, case hardening steel is defined as low carbon steel and low carbon alloy steel, and is mainly defined as the name of steel used for carburized parts. ing.

Here, the case hardening steel used as a raw material includes C: 0.10 to 0.35 wt%, Si: 0.01 to 0.80 wt%, Mn: 0.30 to 1.80 wt%, P: 0.020% by weight or less, S: 0.020% by weight or less, Cu: 0.15% by weight or less, Ni: 0.00-2.50% by weight, Cr: 0.30-2.50% by weight , Mo: 0.00-1.00% by weight and O: 0.0020% by weight or less of case hardening steel is used.
Furthermore, this hardened steel includes Sol. Al: 0.010 to 0.060 wt%, Nb: 0.015 to 0.060 wt%, Ti: 0.0015 to 0.050 wt% and N: 0.010 to 0.025 wt% A case-hardened steel can be used, which can suppress coarsening of austenite grains during carburizing.

The reason why the component range of the hardened steel used as a raw material is specified is shown below.
[C: 0.10 to 0.35% by weight]
C is an element effective for improving the strength of the carburized component by improving the hardness after carburizing and quenching. This effect is poor when the content is less than 0.10% by weight. On the other hand, when the content exceeds 0.35% by weight, toughness and impact strength are lowered.
[Si: 0.01 to 0.80% by weight]
Si is an element effective for improving the hardenability and static strength of steel. This effect is poor when the content is less than 0.01% by weight, and the desired static strength cannot be secured. On the other hand, when the content exceeds 0.80% by weight, the toughness is deteriorated. Was from 0.01 to 0.80% by weight.
[Mn: 0.30 to 1.80% by weight]
Mn has a deoxidizing action and a desulfurizing action of molten steel. Although it is indispensable for improving the toughness of the steel material, if the content is less than 0.30% by weight, the desired effect cannot be obtained in the above-mentioned work. In view of this, the content was made 0.30 to 1.80% by weight.
[P: 0.020% by weight or less]
P is an impurity element and causes a decrease in strength. For this reason, it restrict | limits to 0.020 weight% or less.
[S: 0.020% by weight or less]
S is an impurity element and causes a decrease in strength. For this reason, it restrict | limits to 0.020 weight% or less.
[Cu: 0.15 wt% or less]
Since Cu is an impurity element and has an effect of improving hardenability, it is necessary to regulate this to stabilize the hardenability of steel. For that purpose, it is necessary to regulate to 0.15 weight% or less.
[Ni: 0.00-2.50 wt%]
Ni is an element effective for improving the hardenability of steel and the toughness after quenching / tempering, but is not necessarily an element to be added. However, even if added in excess of 2.50% by weight, the effect is saturated, and conversely the workability is impaired, so 0.00 to 2.50% by weight.
[Cr: 0.30 to 2.50% by weight]
Cr is an element effective for improving the hardenability of steel and the toughness after quenching and tempering, and is contained in an amount of 0.30 or more. However, if added over 2.50% by weight, the toughness and workability are lowered, so 0.30 to 2.50% by weight.
[Mo: 0.00 to 1.00% by weight]
Mo is an element effective for improving the hardenability of steel and the toughness after quenching and tempering, but it is not necessarily an element to be added. However, even if added in excess of 1.00% by weight, the effect tends to be saturated, and the addition cost is increased, so 0.00 to 1.00% by weight was set.
[O: 0.0020% by weight or less]
O exists in steel as a hard oxide such as Al ₂ O ₃ , reduces the rolling fatigue strength of the sliding surface of the carburized gear or carburized and quenched pulley, and reduces the strength as an automobile part. Become. Therefore, O excludes oxides such as Al ₂ O _{3 in} the manufacturing process of the steel refining process and casting process, and is made 0.0020% by weight or less. Therefore, the content of O in the steel is set to 0.0020% by weight or less.
[Sol. Al: 0.010 to 0.060 wt%]
Sol. Al (acid-soluble Al) reacts with N in steel to form AlN, and has an effect of preventing austenite grains from coarsening during carburizing. If it is less than 0.010% by weight, the effect of addition is poor, while Sol. When the Al content exceeds 0.060% by weight, the effect of preventing the coarsening of crystal grains is saturated. Therefore, it is defined within the above range.
[Nb: 0.015 to 0.060% by weight]
Nb is an element required to produce Nb carbide and Nb carbonitride by combining with carbon C and nitrogen N in steel, and has an effect of preventing austenite grains from coarsening during carburization. . If it is less than 0.015% by weight, the effect of preventing grain coarsening is poor. As the amount of Nb added increases, the temperature at which the crystal grains begin to coarsen also becomes higher, but the effect is saturated even if added over 0.060% by weight, and the hardness increases due to precipitation hardening. In order to deteriorate the machinability, the upper limit value is defined as 0.060% by weight.
[Ti: 0.002 to 0.050% by weight]
Ti is an element required to form Ti carbide and Ti carbonitride, or NbTi carbide and NbTi carbonitride by combining with carbon C and nitrogen N in steel, as with Nb. This has the effect of preventing coarsening of the austenite grains. If it is less than 0.002% by weight, the effect of preventing grain coarsening is poor. As the amount of Ti is increased, the temperature at which the crystal grains begin to coarsen becomes higher, but the effect is saturated even if added over 0.050% by weight, and the hardness increases due to precipitation hardening. In order to deteriorate the machinability, the upper limit value was specified to 0.050% by weight.
[N: 0.010 to 0.025% by weight]
N represents Sol. It is an element necessary for precipitating AlN by reacting with Al, and has the effect of improving toughness. If it is less than 0.010% by weight, the effect of addition is poor. On the other hand, if the N content exceeds 0.025% by weight, the effect of preventing crystal grain coarsening will be saturated. Excessive N addition causes blowholes and reduces the strength. Therefore, it is defined within the above range.

2. Hot forging heat treatment Manufacturing processes such as CVT, mission gear, differential gear, etc., produced using hardened steel are (1) hot forging, (2) normalizing treatment, (3) shot peening, (4) It consists of a plurality of processes such as machining, (5) carburizing treatment, and (6) finishing.

The present invention targets CVT, transmission gear, differential gear, etc., and is a hot forging heat treatment composed of the steps (1) and (2) among the steps (1) to (6). For products. FIG. 1 briefly shows the outline.
The restrictions of the manufacturing conditions defined for each process will be described below.

(1) Hot forging (a) Temperature rise time In the case of hot forging, for example, the material steel is forged by heating from room temperature to a predetermined temperature by high-frequency heating, but the temperature rise time is 10 to 120 seconds. Is common. Since it varies depending on the capacity of the apparatus and the size of the material, only a guide is described here.
(B) Heating temperature (1100-1280 ° C)
In consideration of the forging load, it is necessary to heat to 1100 ° C. or higher in order to keep the load low, but in order to utilize the precipitated particles that have a pinning effect of austenite grain growth such as AlN, it is necessary to dissolve the solution once. It is necessary to reprecipitate in a wide temperature range. For this purpose, it is preferable to heat in a temperature range exceeding 1150 ° C. On the other hand, if the temperature exceeds 1280 ° C., the effect is saturated, and the possibility of decarburization and burning cannot be excluded, and the life of the high-frequency heating coil is severely reduced.
(C) Forging temperature (950-1200 ° C)
Hot forging is often performed for the purpose of forming a metal hot and adjusting the size of the metal structure (sizing). In order to make the metal structure sized, it is generally performed by utilizing recrystallization by hot working, and is preferably performed at 950 ° C. or higher. In addition, when hot forging is performed at a temperature of 950 ° C. or lower, depending on the additive element, (i) recrystallization may be greatly suppressed, and grain size may not be achieved. Further, (ii) processing in a low temperature γ region such as 950 ° C. or lower causes precipitates such as AlN, Nb (CN), NbTi (CN), etc., to be induced by processing as relatively large precipitates. This reduces the amount of fine nitride and carbonitride precipitates effective for the stopping effect. On the other hand, when forging above 1200 ° C., precipitation of precipitates is not accompanied, but grain growth after forging may be extremely fast and abnormal grain growth may occur. Even if recrystallized by hot working, As a result, mixed grains may be formed, so the forging temperature was set to 950 ° C. to 1200 ° C.
(D) Cooling rate after hot forging The structure after hot forging of the hardened steel is generally a mixed structure of ferrite + pearlite + partially bainite. Automotive parts such as CVT, transmission gear, and differential gear manufactured using the hot forging process are formed by machining after hot forging, and are subjected to surface hardening treatment such as carburizing and quenching. Used as a part. Of these processes, a hard metal structure such as a bainite structure is often mixed in the hot forging state, which degrades the machinability (increased tool wear and reduced chip cutting performance). It is common to improve the machinability by performing a normalization treatment before.
In Non-Patent Document 1, if there is a difference in the metal structure before carburizing treatment, the austenite initial grains are greatly affected, and if the initial grains are small, the driving force for grain growth of austenite grains increases. In order to suppress the growth, it is important to reduce the grain growth driving force by increasing the ferrite + pearlite structure before carburizing and increasing the austenite initial grains during carburizing treatment. Here, in order to obtain a uniform and large ferrite + pearlite structure not only by simply enlarging the structure before carburizing but also by a normalizing process, it is necessary to align the structure after forging with sized particles. That is, if the quenched structure after carburizing takes over the mixed grains of the previous structure, it is said that the fatigue strength reduction and wear of the carburized and quenched parts are promoted and carburizing and quenching distortion occurs, and the cooling rate after forging is the metal structure. For the purpose of control, it is necessary to perform natural cooling or control within a range of 0.10 to 2 ° C./s. That is, when cooling at a rate higher than 2 ° C./s, a bainite or martensite structure is introduced, and the structure after normalization becomes a mixed grain or only a partly coarse structure, and abnormal grain growth of austenite grains during carburizing It is easy to cause. On the other hand, when the cooling is slower than 0.10 ° C./s, a coarse ferrite + pearlite structure can be obtained more easily. However, since the effect of the coarsening is limited and only costs increase, the lower limit is set to 0.10 ° C./s. Accordingly, the cooling rate after hot forging was set to natural cooling or 0.10 to 2 ° C./s.

(2) Normalizing treatment After performing the hot forging treatment, as described in the above (d), it is necessary to have a ferrite + pearlite structure in order to improve the machinability, and the normalizing treatment is performed. Is common.

(A) Temperature rising rate during normalizing treatment The temperature raising rate of the normalizing treatment needs to be carried out at a temperature rising rate of 0.10 to 0.40 ° C./s up to a predetermined normalizing temperature. That is, as shown in Non-Patent Document 2, AlN precipitates are likely to precipitate during the temperature raising process of the normalization treatment, and in particular, when AlN precipitation treatment is performed at 600 ° C. to 700 ° C., the austenite coarsening start temperature increases. It is known that it is difficult to coarsen (Non-patent Document 3). In order to finely deposit the AlN precipitate having the effect of preventing the coarsening, precipitation in the temperature rising process is more effective than the temperature lowering process as described in Non-Patent Document 2. For these precipitations, it is not possible to secure the precipitation amount of AlN necessary for pinning unless the temperature is raised at a rate higher than 0.40 ° C./s. On the other hand, if the temperature is increased at a rate slower than 0.10 ° C./s, the effect of precipitation can be obtained, but the size of the precipitate may be increased, and the effect of improving the pinning effect is saturated while the cost increases. Therefore, the lower limit was set to 0.10 ° C./s.
(B) Normalization temperature during normalization treatment The normalization temperature is not clearly defined in textbooks. According to a textbook of corona steel material (written by Shozo Okamoto), it is a description of "a steel with hypoeutectoid structure is heated to a temperature of Ac3 point or higher to become austenite". In general, the normalization temperature is usually determined in a practical range between the part-using side (for example, car manufacturer) and the part material-delivering side (for example, forging manufacturer), and is in the range of 900-950 ° C. It is decided each time. In the case of bare steel, 910 ° C or 920 ° C is often used. In this patent, a temperature range of 900 to 950 ° C. is assumed as the normalizing temperature, but the normalizing temperature is not particularly defined.
(C) Processing time from 860 ° C. in the temperature raising process to reach the normalization temperature to 860 ° C. in the temperature lowering process Defines a substantial austenite processing time. In the austenite region, it has been reported that AlN precipitates partially dissolve or agglomerate and coarsen (Non-patent Documents 1 and 2). That is, increasing the treatment time during this period promotes the aggregation and coarsening of AlN precipitates, and reduces the pinning force of austenite grain growth as shown in Non-Patent Document 4. That is, from the viewpoint of suppressing the growth of the austenite grains at the carburizing temperature, it is preferable to shorten the time between them as much as possible. In order to ensure that the heat-treated parts are austenitized in a normalizing furnace, it is necessary to secure 860 ° C. or higher in case-hardened steel. On the other hand, when held in the austenitized region, oxide (scale) is generated on the surface unless completely cut off in a vacuum atmosphere, and in some cases, it remains after machining, and CVT, mission gear, Since the roughness of the sliding surface of the differential gear becomes rough, shot peening after hot forging and pickling and descaling are necessary. The longer the treatment at 860 ° C. or higher, the more the scale is generated, so it is necessary to make it as short as possible. That is, in order to shorten the shot peening treatment time and the pickling time, it is necessary to shorten the treatment time, and for that purpose, it is preferably 1800 s or less, and is set to 1800 s or less.
(D) Cooling rate during normalizing treatment The cooling rate from the normalizing temperature to 550 ° C. was 0.10 to 0.30 ° C./s. This means a cooling rate for obtaining a ferrite + pearlite structure. To explain further, the slower the cooling rate, the higher the transformation from high temperature, so that a coarser ferrite + pearlite structure can be obtained. According to Non-Patent Document 1, it is said that the ferrite + pearlite structure with a coarser metal structure before carburizing has a larger initial austenite grain size during carburizing heat treatment, lowers the driving force for grain growth, and can increase the coarsening start temperature. . The case-hardened steel used here needs to be slower than 0.30 ° C./s in order to obtain a ferrite + pearlite structure. However, if the cooling rate is slower than 0.10 ° C./s, the heat treatment takes a long time and the cost increases too much. Therefore, the cooling rate is set to 0.10 to 0.30 ° C./s. However, it is not always necessary to gradually cool to 550 ° C. at a prescribed cooling rate in the furnace, and if there is a means for judging that the ferrite + pearlite transformation has been completed, air cooling may be performed at that time.

Hereinafter, the effects of the present invention will be described while showing more specific examples and comparative examples.

Table 1 shows chemical components of test materials (steel materials) used in Examples and Comparative Examples. Steel A to steel C and steel E are steel materials included in the component range of this patent, while steel D is a steel material in which the addition amounts of Al and N deviate significantly from the component range of this patent.

The raw steel was melted using a vacuum melting furnace, cast into a mold, die cut, and processed into a round bar having a diameter of 80 mm by hot forging. Then, it peeled off to a 70-mm-diameter round bar by peeling, and it used for the trial manufacture for demonstrating the effect of this invention.

Table 2 shows specific examples of forging heat treatment prototypes.

In this prototype, a steel material with a diameter of 70 mm was used. After forging heat treatment to CVT parts under the conditions shown in Table 2, the metal structure and surface hardness were investigated. The metal structure was measured by measuring the internal structure near 3 mm from the surface part of the part, and the hardness was measured by measuring the surface hardness (HB) and converted to HRB hardness. The metal structure and hardness are used for judgment of machinability. When bainite is mixed in the metal structure and becomes harder than HRB87, the chips are connected to hinder the operation of the automatic processing line. Therefore, the ferrite + pearlite structure and HRB <87. Was used as a criterion. In addition, after the forging heat treatment, shot peening is performed, and the ratio of the processing time in the normalizing process is obtained in comparison with the processing time (time until the removal of the surface scale by visual observation) performed by the conventional method. The improvement effect was shown.
For shot peening, a steel ball having a diameter of 0.8 mm was used, and the effect was indicated by the reduction rate of the projection time. Carburizing and quenching was performed after machining, and the carburized and quenched particle size was measured.
The carburizing and quenching particle size was measured by carrying out the usual gas carburizing and quenching.
The quenching temperature is specified in the table. Here, the carburizing time was 2 hours. In the table, the carburized and quenched particle size measured by corroding with picral was described, but when coarse particles of 50 μm or more were observed, it was judged as “x” and judged as NG.

Table 2 specifically shows examples and comparative examples, which will be described below.
Examples of the present invention are shown in Prototype Examples 1 to 3 and 9 to 11 in Table 2, and Comparative Examples are shown in Prototype Examples 4 to 8 in Table 2.
As shown in Table 2, the prototype No. Examples 1 to 3 are examples of this patent that satisfy the components of the steel material of this patent and satisfy the forging conditions and heat treatment conditions. In any case, the obtained metal structure was a ferrite + pearlite structure, and the machinability was also in the range of 80 to 85 in HRB hardness, which was a satisfactory level. Further, the shot peening process was also 50 to 60%, which was simplified to about half the processing time as compared with the normal shot peening process. By shortening the normalization time shown in Table 2 and FIG. 1 to 1800 s or less, the carburized and quenched particle size was 25 μm or less, and no coarsening occurred.
Prototype No. No. 4 is a component system in which the contents of Al and N deviate significantly from the components of this patent, and NG was selected because coarsening could not be suppressed even when carried out under the production conditions of this patent. This is considered to be because the solid solution of AlN could not be sufficiently achieved even by heating at 1250 ° C.
Prototype No. No. 5 has a forging temperature as low as 940 ° C., and it is considered that AlN was processed and precipitated during forging, resulting in a weak pinning effect during carburizing and coarsening. In this prototype, the cooling rate after normalization is as high as 0.90 ° C./s, the surface hardness is HRB87, and the metal structure is ferrite + pearlite + bainite. If the cooling rate during normalization was not slower than 0.30 ° C./s, it was found that bainite was mixed into the metal structure to form a hard structure, and the machinability deteriorated, so that it was determined as NG.
Prototype No. No. 6 has a processing time of 860 ° C. or higher at the time of normalization of 3000 s, and the prototype is manufactured under conditions significantly exceeding 1800 s. Thus, when the treatment for 1800 s or more is performed in the temperature range of 860 ° C. or more, the AlN precipitate exhibiting the pinning effect does not satisfy the relationship of f = 10, x = 100 and f ≧ x, Agglomerated to lose the pinning effect, leading to coarsening of the carburized and quenched γ grains. In addition, the shot peening time is 300%, and it has been found that time is required for descaling due to an increase in the amount of scale generation, which is NG.
Prototype No. No. 7 is a component system in which Nb and Ti, both of the atomizing elements, are added together, and the forging heating temperature is lowered to 1000 ° C., and if the forging temperature is lowered to 940 ° C., even if other production conditions are satisfied, As a result, granulation could not be prevented and it was determined as NG.
Prototype No. In No. 8, the heating rate during normalization was increased to 1.00 ° C./s, and the cooling rate during normalization was increased to 0.85 ° C./s. The component of the steel material is a component system in which Al and N are deviated to a high level, and sufficient AlN solid solution cannot be achieved even by 1200 ° C. forging heating, and in addition, it is a prototype condition in which the heating rate and cooling rate during normalization are increased. . As a result of insufficient solid solution of AlN during forging heating and precipitation of AlN during normalization, the carburized and quenched γ grains are coarse.
That is, according to the present invention, if forging and normalizing treatment is carried out according to specific components and production conditions, appropriate AlN fine dispersion can be achieved, and coarsening of carburized and quenched γ grains can be eliminated, resulting in reduction of carburizing strain. It was found that a CVT part having high fatigue strength can be manufactured.
By the way, prototype No. The precipitation status of AlN precipitates 1 to 8 is also shown in Table 2, but no. 1: x = 20 nm, f = 500 / μm ² , No. 1 2: x = 15 nm, f = 130 / μm ² , No. ² 3: x = 25 nm, f = 100 / μm ² , No. 3 4: x = 200 nm, f = 4 / μm ² , No. 4 5: x = 100 nm, f = 10 / μm ² , No. 5 6: x = 180 nm, f = 5 / μm ² , No. 6 7: x = 500 nm, f = 3 / μm ² , No. 7 8: x = 300 nm, f = 3 / μm ² .
FIG. 2 shows the correlation between the number (f) of AlN precipitates per unit area (1 μm ² ) and the average particle diameter (xnm).
Here, x is used as the average particle size because the particle size of all the precipitates cannot be plotted in FIG. 2, and therefore the average particle size was used as the representative particle size of each sample.
Photo 1 shown in FIG. 3 shows an example of FE-SEM observation of Prototype Example 3 (Example) in Table 2, and Photo 2 shows an example of FE-SEM observation of Prototype Example 8 (Comparative Example) in Table 2. Indicates.
Moreover, the change of the precipitation form of the AlN precipitate by the processing time of the austenite area | region of 860 degreeC or more at the time of a normalization process was investigated using E steel. The processing time is three conditions of 300 s, 3000 s, and 6000 s. The FE-SEM observation photograph of the precipitate at that time corresponds to photograph 3 (No. 9), photograph 4 (No. 10), and photograph 5 (No. 11) shown in FIG. In order to evaluate the difference in the pinning effect due to the difference in the form of precipitation, the carburization temperature was higher than the practical temperature at 980 ° C. The carburizing time was set to 2 hours like the others. No. No. 9 is an AlN precipitate mainly composed of a single precipitate, the average particle diameter x is 16 nm, the number f per unit area (1 μm ² ) is 150, the austenite particle diameter is 15 μm, and is not coarsened. No. 10 was x = 33 nm and f = 70, and the austenite grain size was 24 μm with the same 980 ° C. carburization. Photo 4 shows a case where the treatment was performed for 3000 s for a long time, and two composite deposits were observed. Precipitates deposited by the 300 s treatment are partially dissolved, reprecipitated and agglomerated, and the average particle size is increased to 33 nm, but the number of precipitates has decreased to f = 70, so the pinning effect has been reduced. It was. No. 11 is a sample processed for a long time of 6000 s. Three or more composite precipitates were also observed, and agglomeration was further promoted. Due to the treatment for a long time, the average particle size of the precipitates is 51 nm, but the number of precipitates has increased only to f = 100, so the pinning effect is not improved and the austenite grain size grows to 29 μm and further grows. However, coarsening was avoided. That is, when it is kept for a long time in a temperature range of 860 ° C. or higher, the AlN precipitates undergo a solid solution, reprecipitation / aggregation process, and precipitate as single or two or more composite precipitates. It can be seen that the pinning effect is small even when the size of the precipitate is small, and among the AlN precipitates, AlN precipitates having a particle diameter of 5 to 40 nm as a simple substance are 20 or more and 300 or less per 1 μm ^2. It can be said that it is preferable.
As previously discussed, the case-hardened steel must basically have a component system that takes into account coarsening during carburization. Needless to say, it is possible to produce a forged heat-treated product.

As described above, the forged heat-treated product of the hardened steel according to the present invention has been described based on the embodiment. However, the present invention is not limited to the configurations described in the above examples, and does not depart from the spirit of the present invention. The configuration can be changed as appropriate.

The forged heat-treated products of the case-hardened steel of the present invention are excellent in machinability by (1) optimizing hot forging and heat treatment, and (2) controlling the precipitation form of fine precipitates in the forged heat-treated products. As austenite grain coarsening can be suppressed at the same time, and carburizing and quenching distortion can be reduced, carburized parts used in driving system parts of automobiles and construction machinery, among others, CVT, mission gear, differential gear, etc. It can use suitably for the use which manufactures the machine parts used.

Claims (3)

AlN precipitate particle diameter of 1 ~ 100 nm is, 1 [mu] m ² per has 20 or more 10000 or less, and, f-number and the number per 1 [mu] m ^2, when the average particle diameter and xnm of f ≧ x Correlated and finely dispersed precipitated ferrite + pearlite structure steel, and C: 0.10 to 0.35 wt%, Si: 0.01 to 0.80 wt%, Mn: 0.30 to 1.80 wt% %, P: 0.020 wt% or less, S: 0.020 wt% or less, Cu: 0.15 wt% or less, Ni: 0.00 to 2.50 wt%, Cr: 0.30 to 2.50 A forged heat-treated product of case-hardened steel, comprising: wt%, Mo: 0.00 to 1.00 wt% and O: 0.0020 wt% or less. ^2. The bare according to claim 1, wherein among the AlN precipitates, there are 20 or more and 300 or less AlN precipitates having a particle size of 5 to 40 nm deposited as a single substance per 1 μm ^2. Forged heat-treated products of hardened steel. Sol. Al: 0.010 to 0.060 wt%, Nb: 0.015 to 0.060 wt%, Ti: 0.002 to 0.050 wt%, and N: 0.010 to 0.025 wt% The forged heat-treated product of case-hardened steel according to claim 1 or 2.

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