JP6197761B2 - Manufacturing method of cold processed products - Google Patents

Description

  The present invention is a cold working suitable for machine parts used as components of transport machines such as automobiles, construction machines and other industrial machines, particularly machine structural parts used by carburizing or carbonitriding. The present invention relates to a method for manufacturing a product.

  Automotive crankshafts and gears are required to have excellent fatigue characteristics, wear resistance and pitting resistance. Therefore, after carving chromium steel, chromium-molybdenum steel, nickel-chromium-molybdenum steel, etc. into a predetermined shape by plastic working and cutting represented by hot forging and cold forging, carburizing treatment called case hardening Ordinarily, it is manufactured by performing carbonitriding.

  Here, cold forging has a lower manufacturing cost than hot forging, a good product yield, and excellent surface properties and dimensional accuracy of the product. Therefore, it has been directed to switch parts that were conventionally manufactured by hot forging to manufacture by cold forging. As a result, in recent years, the proportion of parts manufactured by performing surface hardening treatment such as carburizing and carbonitriding after cold forging has increased rapidly.

In the above carburizing and carbonitriding processes, if heat treatment distortion occurs during quenching, for example, in the case of gears, not only causes noise and vibration, but also the shape is out of order, so a correction process such as polishing is required. Further, in the case of a shaft-shaped part, a bend occurs, and it is necessary to correct this, which causes the manufacturing cost to increase.
The occurrence of the heat treatment strain is particularly remarkable in carburized parts processed at high temperatures. This heat treatment strain is said to be caused by stress nonuniformity due to expansion during martensitic transformation because γ grains are locally coarsened during heat treatment and the hardenability becomes unstable.

Conventionally, in order to prevent coarsening of γ grains in carburizing treatment, the precipitation state of AlN in the steel material after hot rolling, the precipitation amount and distribution when fine precipitation such as Nb and Ti carbonitrides, etc. Attempts have been made to take advantage of the pinning effect of these precipitates by controlling.
For example, in Patent Document 1 and Patent Document 2, the coarsening of γ grains is suppressed by the pinning effect of Al and Nb nitride, which is expressed by adjusting the thermal history of steel and the amounts of Al, Nb, and N. Has been proposed. However, Al and Nb nitrides are likely to become coarse when passing through a thermal history such as hot rolling and annealing, and with the coarsening of precipitates, γ grains during heating in carburizing and carbonitriding treatments. There was a problem that the ability to suppress coarsening was significantly reduced.

  In Patent Document 3 and Patent Document 4, the content of nitride, carbide, carbonitride-forming elements such as Al, Nb, and Ti is adjusted, and the size and distribution of each precipitate are defined as rolling conditions. A method for controlling is disclosed. However, in an actual line where various sizes of rolling are performed, it was difficult to manufacture stably, and it was still difficult to say that the effect was stably exhibited.

  In Patent Document 5, the content of C, Ti, and Mo in steel is controlled within a predetermined range, and nanoprecipitates having a particle size of less than 10 nm are dispersed in the ferrite phase, so that γ grains at the time of carburizing treatment are obtained. There has been proposed a technique for preventing the coarsening of the steel and reducing the heat treatment strain. However, this technique has a problem that in addition to low toughness, the strength other than the carburized layer is not sufficient and it is difficult to apply to a shaft or gear.

JP 58-45354 A JP 61-261427 JP Japanese Unexamined Patent Publication No. 11-50191 Japanese Patent Laid-Open No. 11-335777 Japanese Patent Laid-Open No. 2003-321731

  The present invention has been developed in view of the above-mentioned problems, and the coarseness of crystal grains that cause heat treatment distortion while ensuring the strength and toughness inside the component even when subjected to carburizing treatment or carbonitriding treatment. It is an object of the present invention to propose an advantageous method for producing a cold-worked product that can effectively suppress the formation of the cold-processed product.

Now, in order to achieve the above-mentioned object, the inventors have conducted intensive research on a cold forging material and a cold forging method.
As a result, by optimizing the component composition of the material for cold forging, even if annealing before cold forging is omitted, cold forging with excellent dimensional accuracy is realized, and further annealing is omitted by omitting annealing It has been found that avoiding coarsening of fine carbonitrides in the steel can prevent the coarsening of γ grains during carburizing heating, and obtain machine parts with much superior dimensional accuracy after carburizing and quenching. . Furthermore, the knowledge that advantageous improvement was also achieved in terms of fatigue strength by preventing the coarsening of the prior γ grains after carburizing treatment was also obtained.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. In manufacturing a cold-worked product by performing carburizing treatment after hot rolling and then cold forging on the steel material,
The number of annealing performed before or in the middle of the cold forging satisfies the following formula (1):
In the cold-worked product after performing the carburizing treatment, the area ratio of crystal grains having an austenite grain size of 50 μm or less is 80% or more, and the area ratio of crystal grains having an austenite grain size of more than 300 μm is 10% or less. The manufacturing method of the cold work goods characterized by the above-mentioned.
Record
nA ≦ 3-εc (1)
(If nA <0, nA = 0)
εc: Maximum value of equivalent plastic strain (sum in the case of multiple cold forgings) introduced by cold forging

2. The steel material is mass%,
C: 0.10 to 0.35%,
Si: 0.01 to 0.13%
Mn: 0.30 to 0.80%
P: 0.03% or less,
S: 0.03% or less,
Al: 0.01-0.045%,
Cr: 0.5-3.0%
B: 0.0005-0.0040%,
2. Nb: 0.003-0.080% and N: 0.0080% or less, Ti mixed as impurities is suppressed to 0.005% or less, and the balance has a component composition of Fe and inevitable impurities A manufacturing method for cold processed products.

3. The steel material is further mass%,
Sb: 0.0003 to 0.50% and
Sn: 0.0003 to 0.50%
The method for producing a cold-worked product as described in 2 above, which comprises one or more selected from among the above.

  According to the present invention, a combination of an optimal cold forging material and a cold forging method makes it possible to obtain an optimal cold-worked product for a machine structural component having excellent dimensional accuracy. Therefore, by using this cold-worked product for machine structural parts, it is possible to reduce the noise and further increase the fatigue strength of transportation machines, construction machines, and other industrial machines.

It is a figure which shows annealing conditions. It is a figure which shows the shape and dimension of an intermediate material. It is a figure which shows the shape and dimension of a test piece. It is a figure which shows the conditions of carburizing quenching and tempering.

The present invention will be described in detail below.
In the method for producing a cold-worked product of the present invention, it is essential to perform surface hardening by carburizing treatment. For example, in order to obtain high dimensional accuracy and fatigue strength as a machine structural part, crystal grains after carburizing treatment It is important to make the diameter fine.
Specifically, in the cold-worked product after carburizing treatment, the area ratio of crystal grains with an austenite grain size of 50 μm or less is 80% or more, and the area ratio of crystal grains with an austenite grain size of more than 300 μm is 10% or less. There is a need to. Preferably, the area ratio of crystal grains having an austenite grain size of 50 μm or less is 90% or more, and the area ratio of crystal grains having an austenite grain size exceeding 300 μm is 5% or less.

  That is, the reason why the area ratio of crystal grains having an austenite grain size of 50 μm or less is set to 80% or more is to sufficiently improve fatigue strength. Furthermore, the reason why the area ratio of crystal grains having an austenite grain size exceeding 300 μm is 10% or less is to ensure the dimensional accuracy after the carburizing treatment and to sufficiently improve the fatigue strength.

In order to obtain the above-described structure, it is important to limit the number of times of annealing, which is essential when performing cold forging. This point will be described in detail below.
Now, the steel material is subjected to a carburizing process after the cold forging process to obtain a cold-worked product. However, there are some cases where the fatigue strength deteriorates after the carburizing process.
Thus, as a result of repeated studies on this point, the inventors have found that when fatigue strength deteriorates, the crystal grains are coarsened after the carburizing treatment. Furthermore, when the cause was investigated, it became clear that the coarsening of this crystal grain had a strong correlation with the frequency | count of annealing at the time of cold forging.

Cold forging is usually performed a plurality of times, and annealing is appropriately performed between the plurality of cold forgings for the purpose of softening the steel material. When the number of times of annealing nA does not satisfy the following formula (1), it has been found that the crystal grains become coarse and the fatigue strength deteriorates.
In other words, coarsening of crystal grains during carburization can be suppressed by fine dispersion of Al nitride and Nb carbonitride, but if annealing is performed multiple times, Al nitride and Nb carbonitride are coarsened. Consequently, as a result, the ability to suppress coarsening of crystal grains during carburization is lost, and thus annealing needs to be limited to the number of times that satisfies the following expression (1). More preferably, the number of times satisfies the following expression (2).
Record
nA ≦ 3-εc (1)
nA ≦ 2.5−εc (2)
(If nA ≦ 0, nA = 0)

  Here, the reason why the number of annealing is specified according to the equivalent plastic strain introduced by cold forging is as follows. That is, as the equivalent plastic strain is larger, the ferrite in the structure after annealing is more easily refined. By this refinement, the number of nucleation sites of reverse transformed austenite during carburizing heating increases, and austenite at the initial stage of carburizing becomes refined. Extremely refined austenite tends to cause abnormal grain growth. Therefore, abnormal grain growth during the carburizing process can be prevented by decreasing the number of annealing and suppressing the refinement of austenite at the initial stage of carburizing as the equivalent plastic strain increases.

  The cold-worked product obtained by performing cold forging according to this rule and carburizing treatment has an area ratio of crystal grains with an austenite grain size of 50 μm or less of 80% or more and an austenite grain size of 300 μm or more. The area ratio is 10% or less.

  The εc is the maximum value of the equivalent plastic strain introduced by cold forging (the sum in the case of multiple cold forgings), and is a simulation when setting forging conditions using the usual FEM analysis. Can be sought.

  In addition, there is no restriction | limiting in particular about the annealing conditions at the time of cold forging, What is necessary is just to carry out on conventionally well-known conditions. A preferable annealing condition is about 10 to 300 min in a temperature range of about 760 to 780 ° C. In the present invention, the cold forging conditions are not particularly limited, and may be performed under conventionally known conditions.

Moreover, there is no restriction | limiting in particular about carburizing process conditions, What is necessary is just to carry out on conventionally well-known conditions. As a general carburizing process, after carburizing in a carburizing gas atmosphere, for example, in an RX gas (CO / CO ₂ ) atmosphere at 900 to 960 ° C. and 1 to 20 hours, from this carburizing temperature, or from 840 to It is recommended to hold for 10 to 120 minutes at 880 ° C. and then quench and then temper at 120 to 250 ° C. for 30 to 300 minutes.

Next, with regard to the above-described component composition of the steel material, the preferred range and reason for limitation of each component will be described in detail. In addition, unless otherwise indicated, the "%" display regarding a component shall mean "mass%".
First, as basic components, C: 0.10 to 0.35%, Si: 0.01 to 0.13%, Mn: 0.30 to 0.80%, P: 0.03% or less, S: 0.03% or less, Al: 0.01 to 0.045%, Cr: 0.5 -3.0%, B: 0.0005-0.0040%, Nb: 0.003-0.080% and N: 0.0080% or less, Ti mixed as impurities is suppressed to 0.005% or less, and the balance may be Fe and inevitable impurities preferable.

C: 0.10 to 0.35%
C is preferably set to 0.10% or more in order to obtain sufficient hardness at the center of the forged product by quenching after carburizing treatment applied to the cold forged product. On the other hand, if the C content exceeds 0.35%, cold forging under the conditions satisfying the above-mentioned formula (1) becomes difficult as the hardness of the cold forging material increases, and further, the center after carburizing and quenching is difficult. Since the toughness of the part deteriorates, the C content is preferably in the range of 0.10 to 0.35%. In view of toughness and cold forgeability, it is more preferably 0.25% or less, and still more preferably 0.20% or less.

Si: 0.01-0.13%
Si is useful as a deoxidizer and is preferably added in an amount of at least 0.01%. However, Si not only preferentially oxidizes at the carburized surface layer and promotes grain boundary oxidation, but also enhances solid solution strengthening of ferrite to increase deformation resistance and deteriorate cold forgeability, so the upper limit is made 0.13%. preferable. More preferably, it is 0.02 to 0.10%, and still more preferably 0.02 to 0.09%.

Mn: 0.30 to 0.80%
Mn is an element effective for improving hardenability, and the effect is exhibited by addition of 0.30% or more. However, excessive addition of Mn causes an increase in deformation resistance due to solid solution strengthening and deteriorates cold forgeability, so the upper limit is preferably made 0.80%. More preferably, it is 0.60% or less, More preferably, it is 0.55% or less.

P: 0.03% or less P is segregated at the grain boundaries to lower toughness. Therefore, the lower the mixing, the better, but 0.03% is acceptable. Preferably it is 0.025% or less. There is no problem even if the lower limit is not particularly limited, but unnecessary lowering of P leads to an increase in the refining time and an increase in the refining cost. Therefore, the lower limit is preferably 0.010% or more. More preferably, it is 0.013% or more.

S: 0.03% or less S exists as a sulfide inclusion and is an effective element for improving machinability, but excessive addition causes a decrease in cold forgeability, so the upper limit is made 0.03%. Is preferred. The lower limit is not particularly limited, but may be 0.010% or more in order to ensure particularly excellent machinability. From the viewpoint of machinability, it is more preferably 0.012% or more.

Al: 0.01-0.045%
When Al is added excessively, N in the steel is fixed as AlN, and the hardenability effect of B is expressed. In order to stabilize the strength of the parts after the carburizing treatment, it is important not to exhibit the hardenability effect of B. For this reason, the upper limit of the Al amount is 0.045%. On the other hand, since it is also an element effective for deoxidation, the lower limit is made 0.01%. Preferably it is 0.01 to 0.040%, More preferably, it is 0.015 to 0.035% of range.

Cr: 0.5-3.0%
Cr contributes not only to hardenability but also to improvement of temper softening resistance, and is also an element useful for promoting spheroidization of carbides. However, if the content is less than 0.5%, the effect of addition is poor. On the other hand, if it exceeds 3.0%, excessive carburization and residual austenite are promoted, and fatigue strength is adversely affected. Therefore, the Cr content is preferably in the range of 0.5 to 3.0%. More preferably, it is 0.7 to 2.5%, and still more preferably 1.0 to 1.8%.

B: 0.0005-0.0040%
B has the effect of reducing the solid solution N by combining with N in the steel. Therefore, it is possible to reduce the dynamic strain aging at the time of cold forging by the solid solution N. Contributes to lowering deformation resistance. For this purpose, B addition of 0.0005% or more is desirable. On the other hand, when the amount of B exceeds 0.0040%, the effect of reducing deformation resistance is saturated, but rather, the added B is in a solid solution state due to a decrease in toughness. Further, since the hardenability becomes extremely unstable, the B content is preferably limited to the range of 0.0005 to 0.0040%. More preferably, it is 0.0005 to 0.0030% of range.

Nb: 0.003-0.080%
Nb has the effect of forming NbC in steel and suppressing the coarsening of austenite grains during carburizing by pinning. In order to obtain this effect, addition of at least 0.003% is necessary. However, if added over 0.080%, there is a possibility that deterioration of coarsening suppression ability and deterioration of fatigue strength due to precipitation of coarse NbC may be caused. . For this reason, it is preferable to make Nb amount into the range of 0.003-0.080%. More preferably, it is 0.010 to 0.060%, and still more preferably 0.015 to 0.045%.

N: 0.0080% or less N is a preferred component to avoid mixing as much as possible because N dissolves in steel and causes dynamic strain aging during cold forging and increases deformation resistance. Therefore, the N content is preferably 0.0080% or less. More preferably, it is 0.0070% or less, More preferably, it is 0.0065% or less.

Ti: 0.005% or less
Ti is a component that preferably avoids mixing into steel as much as possible. That is, Ti is likely to bond with N to form coarse TiN, and simultaneous addition with Nb makes it easier to produce coarse precipitates, leading to a decrease in fatigue strength. However, 0.005% or less is acceptable. Preferably it is 0.003% or less.

The basic components have been described above. However, in the present invention, the following elements can be appropriately contained as needed.
Sb: 0.0003 to 0.50%
Sb is an effective element for suppressing the decarburization of the steel surface and preventing the decrease in surface hardness. However, excessive addition degrades the cold forgeability, so Sb is preferably added in the range of 0.0003 to 0.50%. More preferably, it is 0.0010-0.050%, More preferably, it is 0.0015-0.035% of range.

Sn: 0.0003 to 0.50%
Sn is an element effective in improving the corrosion resistance of the steel material surface. However, since excessive addition deteriorates cold forgeability, Sn is preferably contained in a range of 0.0003 to 0.50%. More preferably, it is 0.0010 to 0.050%, and still more preferably 0.0015 to 0.035%.

  Hereinafter, according to an Example, the structure and effect of this invention are demonstrated more concretely. Note that the present invention is not limited by the following examples, and can be appropriately modified from the following examples within a range that can be adapted to the gist of the present invention. Included in the scope.

Cold forward extruding (cold reduction) with a cross-section reduction rate of 0 to 40% per round using round bar steel having the components shown in Table 1 and various diameters (material diameters shown in Table 2) 14mmφ intermediate material was obtained by repeating the forging process. Annealing was performed before or during repeated cold forging to obtain this intermediate material. This number is nA. The annealing was performed under the conditions shown in FIG. Next, the intermediate material was made into the dimensional shape shown in FIG. 2 by cold sealed forging, and further made into the dimensional shape shown in FIG. 3 by cutting.
At that time, the maximum load in the cold forging step to the shape of FIG. 2 was measured. The equivalent plastic strain εc introduced by cold forward extrusion and cold hermetic forging is the cross-sectional area A ₀ (= (material diameter / 2) ² × π) of the material and the cross-sectional area after cold hermetic forging. From A (= (8.5 / 2) ² × π), it was calculated by the following equation (3).
εc = −ln (A / A ₀ ) (3)

  The obtained cold-worked material was carburized, quenched and tempered under the conditions shown in FIG. For the test piece having the shape shown in FIG. 3 obtained in this way, the C cross section (cross section orthogonal to the extrusion forging direction) of the center of the parallel portion (8 mmφ portion) is sampled and 400 from the carburized portion in the cross section of the 8 mmφ portion. The microstructure of 10 fields of view was photographed at random, the prior austenite grain size distribution was measured for the photographed tissue, and the area ratio of crystal grains having a diameter of 50 μm or less and the area ratio of crystal grains exceeding 300 μm were respectively determined. Quantified. In addition, the diameter of the 8 mmφ portion was measured at 5 locations at intervals of 2 mm in the length direction of the parallel portion and 6 locations at intervals of 30 ° for each location, for a total of 30 locations, and the standard deviation of the values was determined. Furthermore, the said test piece was used for the Ono type | formula rotation bending fatigue test, and the fatigue characteristic was investigated. Table 2 shows the conditions of each example and the results of characteristic investigation.

When the implementation conditions including the number of annealing in repeated cold forging satisfy all the specified range of the present invention, a cold-worked product having good dimensional accuracy and fatigue strength is obtained. When the composition range of the steel material satisfies the preferred range of the present invention (No. 1-6, 8 and 10-12), No. 1 using steel G whose composition falls outside the preferred range. Compared with cases where the values of εc of 13 and 14 are equivalent, it can be seen that the deformation resistance at the time of cold forging into the shape of FIG. 2 is low, and the life of the forging die can be improved in actual forming. .
On the other hand, Nos. 7 and 9 in which the number of annealing times exceeded the specified number of times resulted in coarsening of crystal grains during carburizing and quenching, resulting in deterioration of dimensional accuracy and fatigue strength of the final shape.

Claims (2)

% By mass
C: 0.10 to 0.35%,
Si: 0.01 to 0.13%
Mn: 0.30 to 0.80%
P: 0.03% or less,
S: 0.03% or less,
Al: 0.01-0.045%,
Cr: 0.5-3.0%
B: 0.0005-0.0040%,
Nb: 0.003-0.080% and N: 0.0080% or less, Ti mixed as impurities is suppressed to 0.005% or less, the balance is composed of Fe and inevitable impurities, the steel material is hot rolled, then cooled After cold forging, carburizing is performed to manufacture cold-worked products.
The equivalent plastic strain εc introduced by the cold forging is 1.0 to 2.2 ,
The number of annealing performed before or in the middle of the cold forging satisfies the following formula (1):
The carburizing treatment is performed at a carburizing temperature of 950 ° C. or more and 960 ° C. or less ,
In the cold-worked product after performing the carburizing treatment, the area ratio of crystal grains having an austenite grain size of 50 μm or less is 80% or more, and the area ratio of crystal grains having an austenite grain size of more than 300 μm is 10% or less. The manufacturing method of the cold work goods characterized by the above-mentioned.
Record
nA ≦ 3-εc (1)
(If nA <0, nA = 0)
εc: Maximum value of equivalent plastic strain (sum in the case of multiple cold forgings) introduced by cold forging The steel material is further mass%,
Sb: 0.0003 to 0.50% and
Sn: 0.0003 to 0.50%
The method for producing a cold-worked product according to claim 1, comprising one or two selected from among them.

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