JP6263874B2 - Carburizing method for high Si carburizing steel - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a carburizing method capable of obtaining a higher surface carbon concentration more efficiently than in the past with respect to steel for high Si carburizing, which is a steel material that hardly penetrates carbon among case-hardened steel.

  The carburizing process is widely applied to parts that require excellent wear resistance on the surface, such as gears and CVT parts, which are parts that transmit rotational torque obtained by an engine in an automobile or the like to wheels. . And carburizing treatment is usually carried out for low carbon alloy steel having a carbon content of 0.30% or less, and by increasing the carbon content of the surface layer and obtaining a hard surface layer by quenching, Is hard and excellent in wear resistance, while the inside can easily obtain steel having high toughness, and is widely applied to the above parts.

  By the way, it is known that the carbon concentration of the surface layer after the carburizing process varies depending on the steel component to be processed when the same condition is applied. In particular, steel with a high Si content is less likely to penetrate carbon into the surface layer than a steel with a low Si content, making it difficult to stably obtain the target carbon concentration, and the hardening depth is shallow. There is a problem.

  On the other hand, steel with a high Si content is characterized by excellent softening resistance when the temperature rises. Therefore, especially for carburized parts such as gears and CVT parts that are used while being contacted at a high surface pressure, where the temperature of the contact surface is likely to rise during use, the application of case-hardened steel with a high Si content is particularly applicable. In many cases, it is necessary to develop a carburizing method capable of ensuring the target surface carbon concentration and hardening depth more stably for high Si case-hardened steel.

  Conventionally, for high-Si case-hardened steel, it is difficult for carbon to penetrate into the surface. As a carburizing method for increasing the surface carbon concentration or deepening the hardening depth, the carburizing time is increased or the carburizing treatment is performed. A method of increasing the temperature is used. In addition, as described in Patent Document 1, a method of setting a carbon potential in an atmospheric gas used at the time of carburizing higher than that of other steel that is easily carburized has been studied.

JP 2012-207247 A

  However, among the methods conventionally used for steel materials that are difficult to carburize, the method of increasing the carburizing time has a problem that productivity is lowered and cost is increased. In addition, increasing the processing temperature not only increases costs by requiring more energy, but also causes a problem that the crystal grains become coarse and cause a decrease in strength.

  Further, when the carbon potential is increased as described in Patent Document 1, not only the surface of the part is likely to be wrinkled, but also carbides such as cementite may be generated on the surface. Since carbides such as cementite are very brittle and easily cracked, there is a problem that the possibility of cracking increases in an environment where the above-described high surface pressure is applied.

  The cause of the poor carburizing property of the high Si case hardening steel is considered to be that an oxide film of Si or Cr is formed on the surface during carburizing, and this oxide film inhibits carbon intrusion. As a method for suppressing the formation of such an oxide film, vacuum carburizing is conceivable. However, since the carburizing equipment is very expensive, the carburizability of the high Si case-hardened steel can be improved without relying on the carburizing equipment. Development of carburizing methods that can be improved is desired.

  The present invention has been made in view of such a background, and despite having a great advantage of good softening resistance against heat, it is difficult to stably increase the hardening depth after carburizing treatment. An object of the present invention is to provide a new carburizing treatment method capable of easily ensuring a stable hardening depth and a high surface carbon concentration for high-hardening steel.

One embodiment of the present invention is mass%, C: 0.10 to 0.30%, Si: 0.50 to 2.00%, Mn: 0.30 to 1.50%, P: 0.035% Hereinafter, S: 0.035% or less, Cr: 0.80 to 3.00%, Mo: 0.80% or less (0 to impurity range included), Al: 0.020 to 0.060%, N: 0 In an oxygen-containing atmosphere having an oxygen partial pressure of 15000 Pa or more after carburizing treatment after processing into the final part shape using steel containing 0080-0.0250% and the balance being Fe and inevitable impurities This is a method for carburizing a steel for high Si carburizing, characterized in that after heating to form a Fe oxide film having a thickness of 1.5 μm or more on the surface , gas carburizing treatment is performed in an atmosphere composed of RX gas (claim) Item 1).

  The inventors have described in detail the cause of high Si case-hardened steel, which is often not the target value because the surface carbon concentration after carburization is not stable compared to other case-hardened steels with low Si content. Study was carried out. As a result, RX gas is used as an atmospheric gas for gas carburizing, but since Si and Cr are more easily oxidized than Fe, a small amount of oxygen contained in the atmospheric gas is combined with Si and Cr preferentially over Fe, An oxide film is formed. And it was found that this oxide film hinders the intrusion of carbon during the carburizing treatment and a stable surface carbon concentration cannot be obtained.

  Further examination revealed that when heating was performed in an atmosphere having a relatively high oxygen partial pressure, such as in the atmosphere, a large amount of oxygen was present in the atmosphere with a high oxygen partial pressure, and when an oxide was formed during heating, an alloy Oxygen in the atmosphere gas is overwhelmingly excessive with respect to Si and Cr which are present only in a few percent, so this excess oxygen combines with Fe, which occupies most of the alloy, and consists of Fe oxide. As long as the oxide film is formed and the Fe oxide film is pre-formed in the pretreatment, the target surface carbon concentration and hardening depth can be secured even if carburizing is performed in an atmosphere gas consisting of RX gas. I found out that I can do it. The present invention has been made based on the above findings.

  Even within the scope of the present invention, the thicker the Fe oxide film, the higher the effect of improving the surface carbon concentration. However, when a thick Fe oxide film is formed, the prior heat treatment time becomes longer and thicker. Since the effect is saturated even if it becomes too much, there is a possibility that the total required energy cost including the carburizing treatment will increase. Therefore, it is preferable to determine the optimum value of the thickness of the Fe oxide film as a result of determining the optimum heat treatment conditions in consideration of the total energy cost.

  In the present invention, heating is performed before carburizing in an atmosphere having a higher oxygen partial pressure compared to RX gas, such as in the air, and an Fe oxide film having a thickness of 1.5 μm or more is previously formed on the surface, followed by gas carburizing. Process. Even when carburizing a steel material with poor carburizing properties, such as high Si case-hardened steel, by carrying out gas carburizing treatment after setting the state of Fe oxide film formed on the surface, it is aimed stably after carburizing treatment. The surface carbon concentration and the curing depth can be ensured.

  In addition, by applying the above manufacturing method, a carburized component having a stable and excellent strength can be easily manufactured.

  In the present invention, the heating temperature during the Fe oxide film formation treatment is preferably within a range of 550 to 650 ° C. (Claim 2).

  When the heating temperature is less than 550 ° C., a sufficient Fe oxide film is not formed, and thus the effect of the present invention cannot be obtained. Also, when heated at a temperature exceeding 650 ° C., an Fe oxide film can be formed, but it is not formed as a strong film on the surface, and the possibility of peeling off as a scale increases. This is because the effects of the present invention may not be sufficiently obtained. The heating time may be appropriately adjusted so that the thickness of the oxide film is 1.5 μm or more. Considering productivity, the higher the temperature, the faster the oxide film is formed. Therefore, it is preferable to set the temperature higher in a range where a brittle and easy-to-peel scale is not formed.

  Further, in the present invention, during the heat treatment performed before the gas carburizing treatment, the oxygen partial pressure is somewhat high, and a large amount of oxygen in the atmosphere compared to the amount of Si and Cr existing near the surface of the target steel. It is necessary to heat in the atmosphere where exists. This is because if heating is performed in an atmosphere having a low oxygen partial pressure, an oxide film of Si or Cr may be formed. In this regard, the atmosphere contains 21% oxygen and is suitable as an atmosphere for forming the Fe oxide film of the present invention. Therefore, when forming the Fe oxide film of the present invention, heating is performed in the atmosphere. (Claim 3). Even when heating is performed in an atmosphere other than the air, the oxygen partial pressure is preferably set to 15000 Pa or more. Thus, by heating in an atmosphere having a certain oxygen partial pressure or higher, an Fe oxide film can be reliably formed on the surface, and the effects of the present invention can be obtained.

Next, the reason for limiting the chemical composition of the steel targeted in the present invention will be described.
The present invention is an invention characterized by a carburizing method. In order to clarify the range of steel types to which the carburizing method of the present invention can be applied, the component ranges and reasons for limitation will be described below.

C: 0.10% to 0.30%,
C is an indispensable element for securing the internal hardness of the carburized component to be manufactured and for ensuring the required component strength after the carburizing process, and it is necessary to contain 0.10% or more. On the other hand, when C is added excessively, machinability and cold forgeability are lowered. Carburized parts are often subjected to cold forging and machining to obtain the desired product shape, and if these characteristics deteriorate, it becomes difficult to process into the desired part shape, so the upper limit of the C content is 0.30. %.

Si: 0.50% to 2.00%,
Si is an element having an effect of suppressing a decrease in hardness due to an increase in temperature during use as a part by improving tempering softening resistance and suppressing a decrease in pitching strength and wear resistance due to a decrease in hardness. Therefore, when it is necessary to obtain the above effect sufficiently, it is necessary to contain at least 0.50% or more. In addition, if the content is less than 0.50%, the deterioration of carburizing property is not a big problem, and the carburizing treatment can be performed without performing the pretreatment as proposed in the present invention. On the other hand, if Si is added excessively, machinability such as machinability and cold forgeability deteriorates, and it becomes difficult to machine into the desired part shape, so the upper limit is made 2.00%.

Mn: 0.30% to 1.50%,
Mn is an indispensable element for securing the internal hardness of a carburized part to be produced in the same manner as C, ensuring the necessary hardenability, and ensuring the required part strength after carburizing treatment. It is necessary to contain 30% or more. However, if Mn is added excessively, the machinability is lowered and the retained austenite is increased, so that the internal hardness may be lowered. Therefore, the upper limit is made 1.50%.

P, S: 0.035% or less,
P and S are impurity elements that are unavoidably contained in production. However, when the content is increased, P may segregate at the grain boundaries to reduce toughness, and S may increase sulfide inclusions. There is a possibility that it becomes a starting point of fatigue fracture and causes a decrease in fatigue strength. Therefore, both P and S need to be 0.035% or less at most.

Cr: 0.80 to 3.00%,
Cr is an important element that ensures the performance of case-hardened steel, and is an element effective for ensuring internal hardness, suppressing troostite, pitching strength, and wear resistance, so 0.80% or more is contained is necessary. However, even if it is added too much, the cold workability deteriorates, so the upper limit was made 3.00%.

Al: 0.020 to 0.060%,
Al combines with N to become AlN, and is an element that has the effect of preventing crystal grain coarsening that may occur due to heating associated with the carburizing treatment due to the pinning effect. In order to obtain the effect, 0.020% or more It is necessary to contain. However, if the content is excessive, the above effect is saturated, and Al-containing oxide inclusions increase, which becomes a starting point and deteriorates fatigue characteristics. Therefore, the upper limit was made 0.060%.

N: 0.0080 to 0.0200%,
N is an element contained as an inevitable impurity in production, but in the present invention, as described above, it combines with Al to become AlN, which is effective in preventing grain coarsening after carburizing due to the pinning effect. It is an important element that works as an element. Therefore, in order to acquire the said effect, it is necessary to adjust so that it may contain at least 0.0080% or more. However, even if contained excessively, not only the above effects are saturated, but also nitride inclusions increase, which increases the possibility that the fatigue characteristics will decrease, so the upper limit was made 0.0200%.

Mo: 0.80% or less (0 to impurity range included),
Mo is an effective element for improving the strength and toughness, but is expensive, so the upper limit is limited to 0.80% as an element that can be optionally added as necessary.

Next, configurations other than the components of the production method of the present invention will be described.
In the present invention, a steel material having the above-described components is hot-rolled to obtain a steel material having an appropriate cross-sectional dimension, and then hot forging, cold forging, machining, etc. are appropriately combined to obtain a desired final product shape. . Here, normally, the carburizing process is performed as it is. However, in the present invention, heating is performed in an oxygen-containing atmosphere in advance before the carburizing process to form an Fe oxide film of 1.5 μm or more on the surface.

  During this heat treatment, it is necessary to heat in an atmosphere containing oxygen that is overwhelmingly excessive as compared to elements that are easily oxidized such as Si and Cr present in the steel as described above. . By this treatment, an Fe oxide film can be formed on the component surface instead of the Si or Cr oxide film that is more easily oxidized than Fe.

As the oxygen-containing atmosphere in this heat treatment, it is desirable that the oxygen partial pressure is 15000 Pa or more, and the atmosphere that is the most easily prepared atmosphere is most preferable because it is suitable as this oxygen-containing atmosphere.

  Here, the Fe oxide film needs to be formed with a thickness of 1.5 μm or more. Although it is not clear how the Fe oxide film changes during the carburizing process, it was confirmed by experiments that the carburized parts in which the Fe oxide film was secured to 1.5 μm or more were carburized. In this case, it was confirmed that the target surface carbon concentration can be obtained even when a steel material having a high Si content is used.

  Although the change of the Fe oxide film during the carburizing process is not clear, the iron oxide in the Fe oxide film is reduced during the carburizing process because it is processed in an atmosphere having a low oxygen partial pressure. If the Fe oxide film is thin, the Fe oxide film disappears at an early stage of the carburizing process, and oxygen present in a small amount in the atmosphere gas for carburizing process such as RX gas is an element that is more easily oxidized than Fe. It is considered that there is a possibility of binding to Si or Cr and inhibiting C intrusion as described in the problem section. Then, by starting the carburizing process in a state where the Fe oxide film is 1.5 μm or more, the cause of inhibition of the carburizing reaction due to the cause can be appropriately eliminated.

The effects obtained by the carburizing method for high Si carburizing steel according to the present invention described above will be clarified by showing examples below.
First, Table 1 shows chemical components of the test materials used as examples. Among the steel materials shown in Table 1, steel No. Steel materials Nos. 1 to 10 are steel materials targeted by the carburizing method of the present invention, and the Si content is as high as 0.50 to 1.80%. Nos. 11 and 12 are SCM420 and SCr420, which are conventional steels most frequently used as carburizing steel, and the Si contents are 0.25% and 0.24%, respectively, which are significantly lower than the steel materials. .

  And the steel material which consists of a component of Table 1 was melt | dissolved with the electric furnace, Then, the test piece of diameter 30mm and length 40mm was prepared by machining from the steel bar manufactured by hot rolling. Since the test piece is prepared by machining, there is no oxide film on the surface at this point. Next, the heat treatment, which is a feature of the present invention, was performed on the test piece in the atmosphere at a temperature of 600 ° C. and a heating time of 45 minutes. Then, after heat treatment, it is confirmed by EPMA that an oxide film of Fe is formed on the surface, and the thickness of the Fe oxide film formed on the surface is confirmed by using Auger electron spectroscopy and SEM in combination. As a result, it was in the range of 1.6 to 2.0 μm.

  About the test piece in which the Fe oxide film was formed on the surface by the heat treatment as described above, gas carburizing treatment was then performed under the conditions of 950 ° C. and a treatment time of 320 minutes (target surface carbon concentration 0.65%), and the surface The carbon concentration was measured by EPMA. As a comparison, an experiment of carburizing under the same conditions was performed simultaneously without performing the heat treatment before the carburizing treatment, and the effect of the heat treatment was evaluated. The results are shown in Table 2.

  As apparent from Table 2, steel No. which is a conventional case-hardened steel. Nos. 11 and 12 obtained steel No. 11 which is a test material having a high Si content, whereas a high surface carbon concentration of about 0.80% was obtained without prior heat treatment. Steel materials carburized without heat treatment for the test materials 1 to 10 have a surface carbon concentration of less than 0.65% and less than 0.60% for all the test materials. It was confirmed that the carburizability was greatly reduced due to the high content. On the other hand, the specimens that were further heat-treated in advance and formed a Fe oxide film having a thickness of 1.5 μm or more on the surface and then carburized were treated with a target surface carbon concentration of 0 for all specimens. .65% or more was secured, and some of the test materials were able to obtain a greatly improved surface carbon concentration of about 0.70%.

Next, another example when the heat treatment conditions are changed and an experiment is performed will be described.
Table 3 shows steel No. 1 having a Si content of 1.00%. 3 shows the measurement results of the thickness of the Fe oxide film and the surface carbon concentration when the same experiment as described above was performed while changing the heat treatment conditions.

  As is clear from the results in Table 3, the effect of improving the surface carbon concentration is greatly related to the thickness of the Fe oxide film. When the Fe oxide film is thin, a sufficient effect of increasing the surface carbon concentration can be obtained. Can not. It can be seen that a target surface carbon concentration can be obtained by forming an Fe oxide film having a thickness of approximately 1.5 μm or more.

Claims (3)

In mass%, C: 0.10 to 0.30%, Si: 0.50 to 2.00%, Mn: 0.30 to 1.50%, P: 0.035% or less, S: 0.035 % Or less, Cr: 0.80 to 3.00%, Mo: 0.80% or less (0 to impurity range included), Al: 0.020 to 0.060%, N: 0.0080 to 0.0250% Is used, and the balance is Fe and inevitable impurities, and after processing to the final part shape, before the carburizing treatment, the steel is heated in an oxygen-containing atmosphere with an oxygen partial pressure of 15000 Pa or more, A carburizing method for steel for high Si carburizing, characterized in that after forming an Fe oxide film having a thickness of 5 μm or more , gas carburizing treatment is performed in an atmosphere composed of RX gas .   2. The method for carburizing steel for high Si carburizing according to claim 1, wherein the heating temperature during the Fe oxide film forming process is 550 to 650 [deg.] C.   The method for carburizing high-Si carburizing steel according to claim 1 or 2, wherein the oxygen-containing atmosphere during heating for forming the Fe oxide film is air.