JP5642327B2 - Hardening method for steel - Google Patents

Description

  The present invention relates to a steel material quenching method, and more particularly to a method for quenching high-strength boron steel used for automobile bolts and the like.

  In recent years, steel (so-called boron steel) in which boron is added to structural steel is often used as a steel material for forming automotive parts that require high strength (for example, various tightening bolts, connecting rod bolts, cylinder head bolts). It has been. In boron steel, the addition of boron can significantly improve the hardenability of the steel, and can save expensive alloy elements such as nickel, molybdenum, and chromium in structural alloy steels. can do.

  Conventionally, quenching of such high-strength boron steel has been performed in the same manner as other steel materials such as chromium molybdenum steel. However, high-grade steels such as chromium molybdenum steel and nickel chromium molybdenum steel contain many hardenability-improving elements, so the surface layer of the steel material after quenching has a uniform quenching structure, whereas high-strength boron steel There is a problem that the surface layer contains a lot of incompletely quenched structure after quenching.

More specifically, in boron steel, a phenomenon occurs in which boron in the surface layer is diffused and released during quenching and the concentration decreases (so-called deboron phenomenon). Boron improves the hardenability of the material even when added in a very small concentration. On the other hand, if it is completely deboronated, the hardenability of the material is greatly reduced. For this reason, if a deboron phenomenon occurs in the member surface layer in the quenching process, an incompletely quenched structure is generated in the member surface layer portion.
“Iron and Steel” published by the Japan Iron and Steel Institute, No. 11, 1983, p. 1494-1501 (by Inoue, Yoshitaka Ochida, Kunio Tsuji, “Deboronation in Boron Steel and its Calculation Model”)

  As a method for improving such an incomplete quenching structure of the boron steel material surface layer, a surface layer decarburization prevention method by adjusting the atmosphere in the quenching furnace and a carburizing method are known. However, these methods are effective for chromium-molybdenum steel and the like whose surface layer alloy components do not vary, but cannot be said to be sufficiently effective as a countermeasure against the deboron phenomenon in boron steel. That is, in the case of boron steel, the incompletely quenched structure is not completely eliminated even if the atmosphere in the quenching furnace is adjusted. In addition, even if microcarburization is performed until the carbon content of the material is exceeded, an incompletely quenched structure remains, and carburization causes delayed fracture of the steel, so it cannot be employed particularly in the quenching of high-strength bolts. For this reason, there has been a demand for a quenching method that does not cause an incompletely quenched structure in the surface layer of high-strength boron steel.

  The present invention has been made in view of such problems, and in the quenching method for steel materials such as high-strength boron steel, by preventing incomplete quenching structure of the steel surface layer after quenching and obtaining a uniform structure An object of the present invention is to provide a quenching method capable of improving the mechanical properties of steel materials.

  In the present invention, in the quenching method of quenching steel using quenching oil, the vapor film collapse time, which is the elapsed time from the start of cooling in the quenching oil cooling curve to the time when the temperature of the steel suddenly drops, A quenching oil adjusted so that the vapor film collapse time is a predetermined time or less is used as a management item for determining characteristics.

The quenching oil may be adjusted such that the vapor film collapse time is 2.1 seconds or less in the cooling performance test method defined in JIS K 2242.
The quenching oil may be adjusted by using a mineral oil as a base oil and adding a polymer or a petroleum polymer oil as an additive to the quenching oil.

  The steel material may be boron steel.

  According to the present invention, the vapor film disintegration time (elapsed time from the time when cooling starts in the quenching oil cooling curve to the time when the temperature of the steel material suddenly drops) is not longer than a predetermined time (for example, cooling performance defined in JIS K 2242). Since quenching of steel is performed using quenching oil adjusted to be 2.1 seconds or less under the conditions of the test method), even when, for example, boron steel is quenched, an incompletely quenched structure occurs in the surface layer portion. You can avoid it. That is, in the present invention, the vapor film collapse time of the quenching oil, that is, the period during which the steel material does not rapidly cool because the vapor film of the quenching oil covers the steel material in the initial stage of quenching is adjusted to be sufficiently short. Generation of an incompletely quenched structure that tends to progress easily. For example, in the case of quenching boron steel, since the hardenability of the surface layer portion of the steel material is reduced by the deboron phenomenon, ferrite transformation is likely to occur near a temperature of 600 to 700 ° C. during the vapor film collapse time. In this method, since the period in which the quenching oil is in the vicinity of the temperature of 600 to 700 ° C. within the vapor film collapse time is adjusted to be sufficiently short in the quenching step, ferrite transformation is suppressed. Therefore, since the structure of the steel material surface layer can be made uniform, the mechanical properties of the steel material are improved, and a high-quality steel material can be obtained.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
In the present embodiment, in the quenching method for boron steel, as the quenching oil used, one whose characteristics are adjusted so that the vapor film collapse time is not more than a predetermined time is used. Here, the vapor film collapse time is the time from the start of cooling until the time when the vapor film collapses and the temperature of the processed material (steel material) suddenly drops when the heated processed material (steel material) is cooled and quenched. (That is, the time to reach the inflection point in the initial stage of the cooling curve). That is, the temperature of the treated material (steel material) is not rapidly cooled due to the presence of the vapor film in the initial cooling stage with quenching oil, but when the vapor film collapses, the treated material (steel material) is ) Is rapidly cooled, and a bending point occurs in the cooling curve. The time to the inflection point is the vapor film collapse time.

  In this embodiment, by performing quenching using a quenching oil having a short vapor film collapse time, ferrite transformation in the boron steel surface layer can be suppressed, and generation of an incompletely quenched structure can be effectively prevented. In the conventional quenching method, the vapor film disintegration time was not particularly considered in order to determine the characteristics of the quenching oil. On the other hand, the quenching method of the present invention adds the vapor film collapse time as a new management item and uses the quenching oil having a sufficiently short vapor film collapse time to greatly improve the properties of the steel material.

  As a specific numerical value of the vapor film disintegration time, it is adjusted to be 2.1 seconds or less in the measurement in the cooling performance test method defined in JIS K 2242 (2006 version). Such a setting can suppress the occurrence of an incompletely quenched structure in the steel surface layer. The experimental results that serve as the basis thereof will be described later with reference to FIG.

  The vapor film disintegration time is adjusted by selecting a refining method (type of base oil) and attaching an additive. Examples of the base oil include paraffinic mineral oils (for example, base oils belonging to group III of API classification (American Petroleum Institute classification) (sulfur content 0.03 or less, saturation content 90% or more, viscosity index 120% or more, The main component i-paraffin)) is used. Moreover, as an additive for shortening the vapor film collapse time, for example, various polymers having a high molecular weight or petroleum polymer oils are used.

  The basic characteristics other than the kind of quenching oil and the vapor film disintegration time are not particularly limited, and the basic characteristics other than the vapor film disintegration time of the quenching oil are based on, for example, one or two heat-treated oils defined in JIS K 2242 To set.

  The specific quenching process and equipment are not particularly limited, and the same conventional ones may be used. For example, a member that has been molded as necessary is heated to a transformation point temperature or higher, and then immersed in quenching oil and quenched to quench. In this case, the characteristics of the quenching oil are appropriately adjusted by performing a sampling test and adding additives as necessary.

  Next, the effect | action of this embodiment is demonstrated using FIG. The graph of FIG. 1 represents the quenching oil cooling curve (cooling time-temperature curve) in this embodiment and the conventional method, and the quenching oil cooling curve used in this embodiment (quenching cooling curve of the method of the present invention). Is indicated by a solid line, and a cooling curve of the quenching oil used in the conventional method (quenching cooling curve of the conventional method) is indicated by a broken line.

  As shown in the figure, in the quenching cooling curve of the method of the present invention, the temperature of the processed material (steel material) is not rapidly cooled from the cooling start point A to the bending point B, and gradually decreases. However, when it reaches the bending point B, it rapidly decreases with the collapse of the vapor film and reaches the quenching transformation line. The time from the cooling start time A to the bending point B is the vapor film collapse time according to the method of the present invention. Similarly, in the quenching cooling curve of the conventional method, the temperature of the processed material (steel material) is not rapidly cooled from the cooling start time A to the bending point C, and gradually decreases. When the inflection point C is reached, it rapidly decreases with the collapse of the vapor film and reaches the quenching transformation line. The time from the cooling start time A to the bending point C is the vapor film collapse time according to the conventional method.

  In this graph, in addition to these cooling curves, the ferrite transformation line of the boron steel material is indicated by a two-dot chain line, and the ferrite transformation line of the deboronized steel surface layer is indicated by a one-dot chain line. As can be seen from the figure, the quenching cooling curve of the conventional method does not intersect with the ferrite transformation line of the boron steel material that has not been deboronated, but intersects with the ferrite transformation line of the surface layer of the deboroned steel material. End up. For this reason, ferrite transformation occurs at this intersection, and the steel surface layer portion includes an incompletely quenched structure. In other words, since the hardenability of the steel surface layer is reduced by deboronation, ferrite transformation is likely to occur near a temperature of 600 to 700 ° C. during the vapor film collapse time, and this temperature region needs to be cooled faster than the conventional method. is there.

  On the other hand, the quenching cooling curve of the method of the present invention intersects with the ferrite transformation line of the deboronized steel surface because the vapor film collapse time is adjusted to be sufficiently short compared to the quenching cooling curve of the conventional method. Before reaching the bending point B, it is quickly cooled without intersecting the ferrite transformation line, and reaches the quenching transformation line. Therefore, an incompletely quenched structure does not occur in the boron steel surface layer, and high-quality boron steel can be generated.

  FIG. 2 shows a list of experimental results for confirming the ferrite generation state on the steel surface layer in each of the method of the present invention and the conventional method. In addition, this experiment result was made on the conditions of the cooling performance test method prescribed | regulated to JISK2242. The chemical composition of the test material used in this experiment is as shown below the list.

  As shown in the list, in samples 1 to 12 of the conventional method in which the vapor film collapse time is 2.4 seconds to 2.8 seconds, generation of ferrite is observed on the steel material surface layer. On the other hand, in Samples 1 and 2 according to the method of the present invention in which the vapor film collapse time was adjusted to 2.1 seconds or less, the generation of ferrite could be completely suppressed. Thus, if the vapor film collapse time is 2.1 seconds or less, the surface layer of the high-strength boron steel member can be completely quenched, and the surface layer structure becomes uniform.

  FIG. 3 is a graph showing the in-table difference (difference between surface hardness and internal hardness) of the hardness of steel in each of the method of the present invention and the conventional method. As shown in the drawing, according to the method of the present invention, the hardness in the surface layer structure of the steel material is higher in hardness and less in comparison with the conventional method, and the strength and homogeneity of the surface layer structure is increased. I understand that.

  As described above, according to the quenching method of the present embodiment, the quality of the quenching oil used for quenching is set so that the vapor film collapse time is a predetermined time or less (the cooling performance test method defined in JIS K 2242). In the measurement based on the above, it was set to 2.1 seconds or less), so that the quenching cooling curve was earlier than the ferrite transformation that caused the incomplete quenching structure in the boron steel surface where the deboron phenomenon occurred. Quenching is performed by reaching the inflection point and quenching. Therefore, the occurrence of an incompletely quenched structure in the surface layer of the high-strength boron steel material is prevented, the surface structure is made uniform, and a decrease in the hardness of the surface layer is suppressed. Therefore, the static strength and fatigue strength of high-strength boron steel materials such as bolt products are improved. In addition, even low-cost boron steel can produce steel with a high-quality surface structure, so it is possible to use boron steel instead of expensive chromium-molybdenum steel or nickel-chromium-molybdenum steel. Reduction and metal resource saving can be achieved.

  In the above embodiment, the case where the quenching method of the present invention is applied to high-strength boron steel has been described as an example, but the scope of the present invention is not limited to quenching of boron steel, for example, a member having a large mass or quenching In general quenching of structural members using low-strength steel materials, the present invention can also be applied to improve the quenching structure.

It is a graph which shows the relationship between the quenching oil in this embodiment and the cooling curve of the hardening oil in the conventional method, and the ferrite transformation line of boron steel. It is a table | surface which shows the ferrite generation | occurrence | production situation in the steel material surface layer in each of this invention method and the conventional method. It is a graph which shows the table | surface difference of the hardness of the steel materials in each of this invention method and the conventional method.

Claims (2)

In the quenching method of quenching boron steel using quenching oil,
The vapor film disintegration time, which is the elapsed time from the start of cooling in the quenching oil cooling curve to the time when the temperature of the boron steel material suddenly drops, is a management item for determining the characteristics of the quenching oil,
By performing quenching under normal pressure using quenching oil adjusted so that the vapor film collapse time is 2.1 seconds or less in the cooling performance test defined in JIS K2242 of the 2006 version , the boron steel It is characterized by preventing the occurrence of incomplete quenching in the surface layer part,
The boron steel has a C content of 0.22 to 0.28%, a Mn content of 0.40 to 0.60%, a Cr content of 0.60 to 0.80%, and quenching temperature is 880 ° C or higher, and wherein the further hardened oils defined in the JISK2242, a kinematic viscosity at 40 ° C is 26 mm ² / s or less in a relatively low viscosity quenching oil, quenching Method. The quenching method according to claim 1, wherein the quenching oil is prepared by using a mineral oil as a base oil and adding a polymer or a petroleum polymer oil as an additive.

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