JP2012140675A - Case-hardening steel excellent in cold-workability, and high fatigue-resistant strength carburized material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a case-hardening steel which is excellent in cold-workability, which restrains excessive carburizing without depending on the heat-treatment condition of the carburizing and which has the high fatigue-resistant characteristic after treatment of carburizing.SOLUTION: This steel has the component-composion including, by mass%, 0.10-0.35% C, 0.01-0.50% Si, 0.40-1.50% Mn, ≤0.02% P, ≤0.03% S, 0.04-0.10% Al, 0.5-2.5% Cr, 0.002-0.035% Sb, 0.0005-0.0050% B, ≤0.003% Ti and <0.0080% N and the balance Fe with inevitable impurities.

Description

  The present invention is mainly used for machine structural materials in the fields of industrial equipment and automobiles. Case-hardened steel that has excellent cold workability and can impart high fatigue resistance by carburizing, and carburization having excellent fatigue resistance. It relates to materials.

  For example, parts materials such as automobiles are often manufactured by cold forming steel bars, and therefore high cold formability is required. Therefore, spheroidizing heat treatment is performed to spheroidize carbides to improve cold workability, particularly cold forgeability.

In addition, from the viewpoint of the composition of steel, proposals have been made to reduce Si, which greatly affects deformation resistance. Furthermore, there is a proposal of steel that effectively uses the hardenability of B.
For example, in Patent Document 1, by reducing the amount of other alloy elements by the effect of improving the hardenability of B, the hardness is reduced from the normalizing step, and the cutting performance is dramatically improved compared to conventional steel. Improved carburized gear steels have been proposed.

  Furthermore, Patent Document 2 proposes a case hardening steel that secures cold workability by combining a component system that secures hardenability with B by reducing Si and Mn, which are solid solution strengthening elements, and manufacturing conditions. Has been.

  On the other hand, in recent years, gears used in automobiles and the like have been required to be reduced in size as the vehicle body has become lighter due to energy savings, and the load on the gears has increased as the engine output has increased. ing. The durability of the gear is determined mainly by the root bending fatigue failure and the tooth surface fatigue failure. Regarding the root bending fatigue fracture strength, it is considered effective to reduce the incompletely hardened layer generated on the surface layer during carburizing and to refine the prior austenite grain size. In addition, regarding the improvement of surface fatigue strength, the relationship with temper softening resistance has been pointed out, and components with increased Si content, components with Mo added, or steel with fine carbides dispersed in the carburized surface layer, Each has been proposed.

  For example, Patent Document 3 proposes a steel for carburizing in which fatigue strength and toughness are improved by setting the prior austenite grain size to 7 μm or less. Patent Document 4 proposes steels in which carbides are dispersed in a carburized layer on the surface.

  Further, in the carburizing heat treatment, the carbon concentration on the steel material surface is greatly influenced by the shape of the non-treated material. That is, even if the desired hardness and structure are obtained in the flat portion of the treated material, carburization is excessive in the corner portion, and there is a concern that the fatigue resistance characteristics are reduced due to the formation of coarse carbides. In particular, this tendency is remarkable in vacuum carburizing that has come to be used in recent years. In view of this, component compositions have been proposed in consideration of the effects on the intrusion diffusion of carbon and the formation behavior of carbides.

  For example, Patent Document 5 suppresses deterioration of fatigue resistance due to excessive carburization by increasing the amount of Si, Cu, and Ni, which are effective in suppressing the formation of carbides, and reducing the amount of Cr that tends to increase carbides. It has been proposed.

Japanese Patent No. 3551573 Japanese Patent No. 3764586 Japanese Patent No. 3063399 Japanese Patent No. 4056709 Japanese Patent No. 4254816

However, in the techniques described in Patent Documents 1 and 2 described above, improvements in cold workability and impact characteristics are recognized, but fatigue characteristics are equivalent to those of conventional steel.
In the techniques described in Patent Documents 3 and 4, there is a problem that, when a large amount of carbide forming elements such as Nb, Ti and V are used and finely precipitated, deformation resistance during processing is remarkably increased.
In the technique described in Patent Document 5, since a large amount of Si is added, there is a concern about a decrease in cold workability, and in the case of gas carburization, a problem of grain boundary oxidation occurs. Furthermore, Cu and Ni for securing the strength must be added along with the reduction of Cr, which raises a problem that the alloy cost increases.

  The present invention was developed in view of the above circumstances, and not only has excellent cold workability, but also suppresses excessive carburization without depending on the heat treatment conditions of carburization, and has high fatigue resistance after carburization. It is to provide a hardened steel and a carburized material using the same.

As a result of intensive studies to solve the above problems, the present inventors have obtained the following knowledge.
First, in order to improve fatigue resistance, the inventors have intensively studied ways to finely disperse carbides by suppressing the formation of coarse carbides in the carburized surface layer of case-hardened steel.
Here, in FIG. 1, the relationship of the amount of Al, B, and Ti which affects the maximum particle size of the carbide in the carburized surface layer of the case hardening steel is shown. As can be seen from the figure, in order to suppress the formation of coarse carbides and finely disperse the carbides, it is important to control the amounts of Al and B and to suppress the amount of Ti added. FIG. 1 also shows the results of measuring the surface fatigue strength of some steels, but it can also be seen that high surface fatigue strength can be obtained by suppressing the formation of coarse carbides.

In addition, the experiment which shows a result in FIG. 1 is based on 0.2 mass% C-0.1 mass% Si-0.6 mass% Mn-1.5 mass% Cr-0.02 mass% Nb steel. The steel materials to which B and B are added are prepared, and the maximum particle diameter (μm) and the surface fatigue strength (MPa) of the carbides after these steel materials are treated under the following conditions are evaluated.
That is, from a steel material, a 25 mm diameter round bar is processed, carburized at a high carbon concentration of 2% at 950 ° C for 5 hours, once cooled to 600 ° C, held again at 850 ° C for 30 minutes, and then After oil cooling at 60 ° C., a tempering treatment was performed at 170 ° C. for 2 hours. After cutting the sample subjected to this treatment, the cut surface was corroded with Piracool liquid, the region from the surface to a depth of 30 μm was observed over a scanning electron microscope over 6000 μm ^2, and the maximum particle size of the carbide was determined by image analysis. . In addition, a roller pitching test piece was collected from the round bar, and a roller pitching test was performed on the samples subjected to the above-described high-concentration carburization to tempering treatment at a sliding rate of 40% and an oil temperature of 80 ° C. Then, the strength 10 ⁷ times (the limit strength at which pitting occurs on the surface of the test piece) was evaluated as the surface fatigue strength.

  Furthermore, we focused on Sb as an element that has the effect of suppressing excessive carbon intrusion and diffusion from the steel surface. That is, by adding Sb to steel, it was found that excessive carburization at the corners can be prevented and the difference in carbon content between the flat and corners can be reduced. Further, it was confirmed that the same result as that shown in FIG. 1 was obtained even when Sb was added to 0.035 mass%.

The present invention has been made on the basis of the above knowledge, and the gist of the present invention is as follows.
(1) C: 0.10 to 0.35 mass%,
Si: 0.01 to 0.50 mass%,
Mn: 0.40-1.50 mass%,
P: 0.02 mass% or less,
S: 0.03 mass% or less,
Al: 0.04 to 0.10% by mass,
Cr: 0.5 to 2.5% by mass,
Sb: 0.002 to 0.035 mass%,
B: 0.0005-0.0050 mass%,
Case-hardened steel excellent in cold workability containing Ti: 0.003 mass% or less and N: less than 0.0080 mass%, with the balance being a component composition of Fe and inevitable impurities.

(2) In addition to the above component composition,
Nb: 0.06% by mass or less,
Cu: 1.0 mass% or less,
Ni: 0.5% by mass or less,
The case-hardened steel having excellent cold workability according to (1), containing one or more selected from Mo: 0.5% by mass or less and V: 0.5% by mass or less.

(3) In addition to the above component composition,
Ca: 0.0005 to 0.0050 mass% and
Mg: 0.0002 to 0.0020 mass%
The case hardening steel excellent in cold workability as described in said (1) or (2) containing 1 type or 2 types chosen from these.

(4) A carburized material obtained by carburizing the case-hardened steel according to any one of (1) to (3), wherein the maximum diameter of carbide in the surface layer region from the surface to 0.4 mm is A high fatigue strength carburized material having an average particle size of 10 μm or less and an average particle size of 4 μm or less.

  According to the present invention, not only excellent cold workability but also excessive carburization is suppressed regardless of the heat treatment conditions of carburizing, and a case-hardened steel having excellent fatigue resistance after carburizing treatment, and the same are used. Since a carburized material can be provided, it is very useful industrially.

It is a graph which shows the influence of the amount of Al, B, and Ti which acts on the precipitation state of carbide. It is a figure which shows the test piece shape used for the investigation of the presence or absence of excessive carburization of a corner | angular part. It is a graph which shows carburizing heat treatment conditions.

Hereinafter, the case-hardened steel of the present invention will be specifically described.
First, the reason why the component composition of steel is limited to the above range in the present invention will be described in detail for each component element. In addition, unless otherwise indicated, the "%" display regarding content of each element shall mean "mass%".

C: 0.10 to 0.35%
C requires a content of 0.10% or more in order to increase the hardness of the core part (center part) to be treated by quenching after the carburizing heat treatment, but if the content exceeds 0.35%, the toughness of the core part is increased. Since it falls, C amount was limited to 0.10 to 0.35% of range. Preferably, it is 0.3% or less of range.

Si: 0.01-0.50%
Si is necessary as a deoxidizing agent, and it is necessary to add at least 0.01%. However, Si is an element that preferentially oxidizes in the carburized surface layer and promotes grain boundary oxidation. In addition, the upper limit is set to 0.50% in order to enhance the solid solution strengthening of the ferrite to increase the deformation resistance and deteriorate the cold workability. Preferably, it is 0.03 to 0.35%.

Mn: 0.40 to 1.50%
Mn is an element effective for hardenability and needs to be added at least 0.40%. However, Mn tends to cause grain boundary oxidation, and excessive addition increases residual austenite and causes a decrease in surface hardness. Therefore, the upper limit is made 1.50%. Preferably, it is 0.45 to 1.40% of range.

P: 0.02% or less P is segregated at the grain boundaries and lowers the toughness. Therefore, the lower the content, the better, but 0.02% is acceptable. Preferably, it is 0.018% or less.

S: 0.03% or less S is an element that exists as sulfide inclusions and is effective in improving machinability. However, excessive addition causes a decrease in fatigue resistance, so the upper limit was made 0.03%.

Al: 0.04-0.10%
Al is an important element for obtaining the quenching effect of B by fixing N in steel as AlN. In order to obtain this effect, it is necessary to add at least 0.04%. However, when the content exceeds 0.10%, the formation of Al ₂ O ₃ inclusions harmful to fatigue resistance is promoted, so the content is limited to 0.04 to 0.10%.

Cr: 0.5-2.5%
Cr is an element that contributes not only to hardenability but also improves resistance to temper softening and is also useful for promoting the spheroidization of carbides. However, if its content is less than 0.5%, its addition effect is poor. If it exceeds 2.5%, the formation of retained austenite in the carburized part is promoted, and the fatigue resistance may be adversely affected. Therefore, the Cr content is limited to a range of 0.5 to 2.5%. Preferably, it is 0.6 to 2.0% of range.

Sb: 0.002 to 0.035%
Sb is a particularly important element in the present invention. Sb can suppress the intrusion and diffusion of excessive carbon from the steel material surface, and can reduce the difference in carbon content between the flat portion and the corner portion. In order to exhibit this effect, addition of 0.002% or more is necessary. On the other hand, excessive addition causes a decrease in forgeability and the like, so the upper limit was made 0.035%. Further, it is preferably 0.003 to 0.025%.

B: 0.0005-0.0050%
B is the most important element in the present invention. B segregates at the austenite grain boundaries during the quenching heat treatment, thereby improving the hardenability and contributing to an increase in the hardness of the material. By this effect, other strengthening elements can be reduced, and as a result, an improvement in cold workability due to a decrease in deformation resistance can be obtained. In order to exert this effect, it is necessary to add at least 0.0005% or more. On the other hand, excessive addition causes a decrease in toughness and workability (forgeability), so the upper limit was made 0.0050%. Furthermore, Preferably, it is 0.0007 to 0.0040%.

Ti: 0.003% or less
Ti is a component that preferably avoids mixing into steel as much as possible. Ti is easy to combine with N to form coarse TiN. In this way, the upper limit is made 0.003% in order to cause coarsening of the carbide on the carburized surface layer and decrease in fatigue resistance.

N: Less than 0.008% N is a component that preferably avoids mixing into steel as much as possible. Therefore, N is made less than 0.008% in order to ensure the hardenability of B and to suppress the formation of TiN.

In the present invention, in order to improve hardenability, the above basic components are further selected from Nb: 0.06% or less, Cu: 1.0% or less, Ni: 0.5% or less, Mo: 0.50% or less, and V: 0.5% or less. It can contain seeds or two or more.
Nb: 0.06% or less
In addition to improving hardenability, Nb forms NbC in steel and suppresses the coarsening of the austenite grain size during the carburizing heat treatment due to the pinning effect. For that purpose, it is preferable to add at 0.010% or more. On the other hand, if added over 0.06%, there is a risk of reducing the coarsening suppression ability due to coarse NbC precipitation and deterioration of fatigue resistance, so 0.06% or less. More preferably, it is 0.045% or less.

Cu: 1.0% or less
Cu is an element effective for improving hardenability and is preferably added at 0.1% or more. However, the addition of a large amount causes deterioration of the surface properties of the steel material and increase of the alloy cost, so the upper limit was set to 1.0%. .

Ni: 0.5% or less
Mo: 0.5% or less V: 0.5% or less
Ni, Mo, and V are effective elements for improving hardenability and toughness, and are preferably added in amounts of 0.1% or more, 0.05% or more, and 0.02% or more, respectively. It was.

  In addition, the present invention controls the form of the sulfide, and in order to improve the machinability and cold workability, the above component is further added to one of the elements selected from Ca: 0.0005 to 0.0050% and Mg: 0.0002 to 0.0020%. Two types can be added. In order to obtain the above effects by Ca and Mg, it is preferable to add at least 0.0005% each. On the other hand, when excessively added, coarse inclusions are formed and the fatigue resistance is adversely affected. Therefore, the upper limits of Ca and Mg are set to 0.0050% and 0.0020%, respectively.

  A carburized material can be obtained by subjecting the case-hardened steel according to the component composition described above to a carburizing treatment after cold working to form a part shape. The carburized material that has undergone this carburizing treatment has a carbon content of 0.80% by mass or more in the surface layer region up to 0.4 mm below the surface, and the maximum diameter of the carbide formed here is 10 μm or less and the average particle size is 4 μm or less. It is particularly preferred that there is. Within this range, the surface fatigue strength is particularly improved. Conversely, if the maximum diameter or average particle diameter of the carbide exceeds this range, improvement in surface fatigue strength cannot be expected.

That is, when the amount of carbon in the surface layer region is less than 0.80%, a sufficient amount of carbide cannot be obtained and the surface fatigue strength cannot be improved. On the other hand, if the maximum diameter of the carbide exceeds 10 μm, the fatigue life is reduced due to the coarse carbide becoming the starting point of the fatigue crack. Similarly, when the average particle diameter exceeds 4 μm, the fatigue life is reduced.
Incidentally, the upper limit of the amount of carbon in the surface layer region is preferably 1.8% or less from the viewpoint of suppressing the formation of excessive retained austenite.

In addition, in order to obtain the carbide | carbonized_material according to above-mentioned prescription | regulation, it is preferable to perform carburizing heat processing on the following conditions.
That is, hold at carbon potential 1.5% or more and 900 to 1050 ° C. for about 2 to 10 hours, once cool to 700 ° C. or less, and then hold again at 800 to 900 ° C. for 30 minutes or more and then oil cooling. Thereafter, tempering is preferably performed, and the tempering temperature is preferably in the range of 170 to 200 ° C.

Next, examples of the present invention will be described.
First, steels with the composition shown in Table 1 were melted as test steels, heated once to 1150 ° C or higher, then used as an intermediate material with a 170 mm x 170 mm square cross section, and further heated to (Ar ₃ + 100 ° C) or higher Then, it was formed into a round bar having a diameter of 60 mm by hot rolling. The obtained round bar was evaluated for cold workability and fatigue resistance.

Here, the cold workability was evaluated by two items of the limit upsetting rate and the deformation resistance.
In other words, the deformation resistance was measured by taking a test piece with a diameter of 10 mm and a height of 15 mm from a 1/4 depth position of the diameter of the rolled steel bar, and using a 300-ton press to apply a compression load at 60% upsetting. It measured and calculated | required using the deformation resistance measuring method by the end surface restraint compression which the Japan Society for Technology of Plasticity advocated. Further, the limit upsetting rate was defined as the upsetting rate when the end portion was cracked by compressing by a method of measuring deformation resistance.
If the deformation resistance value is 899 MPa or less and the critical crack rate is 74% or more, it can be said that the cold workability (cold forgeability) is good.

Further, a test piece having a corner (angle: 120 °) shown in FIG. 2 is taken from the test steel shown in Table 1, and the effective hardening depth (Hv ≧ 550) in the flat portion is obtained according to the thermal history shown in FIG. Carburizing heat treatment was performed with a target depth of 1.2 mm from the surface, and the carbon concentration and carbides of the corner portion 1 and the flat portion 2 of the test piece after the carburizing heat treatment were measured.
The carbide was measured by etching with a Picral solution and then observing a region from the surface to a depth of 30 μm over a scanning electron microscope over 6000 μm ² and obtaining the maximum and average diameters of the carbide by image analysis. That is, the maximum value of the equivalent circle diameter was defined as the maximum diameter, and the average value of the equivalent circle diameter was defined as the average diameter. Carbides were also observed in other depth regions from the surface to 0.4 mm, and it was confirmed that the maximum diameter and the average diameter were the largest from the surface to a depth of 30 μm. Here, in the observation of carbides, those having an equivalent circle diameter of 0.5 μm or more can be identified as carbides.
The carbon concentration was measured by EPMA line analysis from the surface to a depth of 0.4 μm.

Furthermore, the fatigue resistance was evaluated by two items, that is, rotational bending fatigue and surface fatigue.
That is, from the position of the 1/4 depth of the diameter of the steel bar, rotating bending specimens and roller pitching specimens are collected, and these specimens are usually subjected to carburization and high-concentration carburizing to generate a large amount of carbides. Two types of heat treatment were performed.
In normal carburizing, carburizing was performed at 930 ° C. and 7 hours under a carbon potential of 1.1% by mass, then held at 850 ° C. for 30 minutes, oil-cooled at 60 ° C., and tempered at 170 ° C. for 2 hours.
On the other hand, high-concentration carburization is held at 950 ° C for 5 hours under the condition of 2% by mass of carbon potential, once cooled to 600 ° C, then again held at 850 ° C for 30 minutes, oil-cooled at 60 ° C, and 170 ° C A tempering treatment for 2 hours was performed.
A rotating bending fatigue test and a roller pitching test were performed on each test piece after carburization. First, the rotating bending fatigue test was carried out at a rotational speed of 3500 rpm and evaluated with a fatigue strength of 10 ⁷ times. Further, the roller pitting test, the slip ratio of 40% was evaluated by 10 ⁷ times the intensity at oil temperature 80 ° C. conditions (limit strength pitching occurs in the test piece surface).

  As shown in Table 2, the above evaluation results are shown in Table 2. All the inventive examples according to the present invention are excellent in cold workability, stable carburizing treatment is performed regardless of the shape of the member, and excellent in fatigue resistance. You can see that

Further, a test piece having a corner (angle: 120 °) shown in FIG. 2 is taken from the test steel shown in Table 1, and the effective hardening depth (Hv ≧ 550) in the flat portion is obtained according to the thermal history shown in FIG. Carburizing heat treatment was performed with a target depth of 1.2 mm from the surface, and the carbon concentration and carbides of the corner portion 1 and the flat portion 2 of the test piece after the carburizing heat treatment were measured.
The carbide was measured by etching with a Picral solution and then observing a region from the surface to a depth of 30 μm over a scanning electron microscope over 6000 μm ² and obtaining the maximum and average diameters of the carbide by image analysis. That is, the maximum value of the equivalent circle diameter was defined as the maximum diameter, and the average value of the equivalent circle diameter was defined as the average diameter. Carbides were also observed in other depth regions from the surface to 0.4 mm, and it was confirmed that the maximum diameter and the average diameter were the largest from the surface to a depth of 30 μm. Here, in the observation of carbides, those having an equivalent circle diameter of 0.5 μm or more can be identified as carbides.
The carbon concentration was measured by EPMA line analysis from the surface to a depth of 0.4 mm .

Claims (4)

C: 0.10 to 0.35 mass%,
Si: 0.01 to 0.50 mass%,
Mn: 0.40-1.50 mass%,
P: 0.02 mass% or less,
S: 0.03 mass% or less,
Al: 0.04 to 0.10% by mass,
Cr: 0.5 to 2.5% by mass,
Sb: 0.002 to 0.035 mass%,
B: 0.0005-0.0050 mass%,
Case-hardened steel excellent in cold workability containing Ti: 0.003 mass% or less and N: less than 0.0080 mass%, with the balance being a component composition of Fe and inevitable impurities. In addition to the component composition,
Nb: 0.06% by mass or less,
Cu: 1.0 mass% or less,
Ni: 0.5% by mass or less,
The case-hardening steel excellent in cold workability according to claim 1, comprising one or more selected from Mo: 0.5% by mass or less and V: 0.5% by mass or less. In addition to the component composition,
Ca: 0.0005 to 0.0050 mass% and
Mg: 0.0002 to 0.0020 mass%
The case hardening steel excellent in cold workability of Claim 1 or 2 containing 1 type or 2 types chosen from these.   A carburized material obtained by carburizing the case-hardened steel according to any one of claims 1 to 3, wherein the maximum diameter of carbide in the surface layer region from the surface to 0.4 mm is 10 µm or less and the average particle diameter Is a high fatigue strength carburized material having a thickness of 4 μm or less.

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