JP7548951B2 - Hardened steel material and its manufacturing method - Google Patents

Description

The present invention relates to hardened steel material and its manufacturing method.

A conventional high-frequency hardening method is known in which the workpiece is preheated by induction heating to a temperature of 1000°C to 1200°C at a heating rate of 300°C/sec or more in a short time, then slowly cooled, and then the workpiece is heated by induction heating to a temperature of 900°C to 1050°C at a heating rate of 1000°C/sec or more in a short time, which is lower than the preheating temperature, and then rapidly cooled (see Patent Document 1).

Japanese Patent Application Publication No. 09-241749

Murai, Tsumura: Effects of alloying elements and carbon potential on the amount of retained austenite in carburized steel, Tetsu-to-Haganen, Vol. 84 (1998) No. 6, pp. 446-451

However, the induction hardening method described in Patent Document 1 aims to facilitate uniform austenitization of the material and perform good contour hardening without sacrificing machinability, etc., but is not intended to extend the life of the steel material.
The present invention has been made in light of the above-mentioned problems, and has an object to provide a hardened steel material that has sufficient durability as a steel material and can extend the service life of the steel material, and a manufacturing method thereof.

The hardened steel material according to the present invention is characterized by having a base part made of high carbon hypereutectoid steel, and a surface layer part having a mixed phase structure of martensite and carbides, a surface hardness of 700 HV0.3 or more, an average particle size of the carbides of 0.01 μm or more and 0.50 μm or less, and an amount of retained austenite of 5 vol. % or more and 30 vol. % or less.

The manufacturing method of the hardened steel material according to the present invention is characterized in that a heated product made of high carbon hypereutectoid steel is preheated by induction heating to a first temperature of 1000°C or more and 1300°C or less, then quenched to a second temperature that is higher than the Ms point and lower than the A1 point, and then the second temperature is maintained, and then the product is fully heated by induction heating to a third temperature of 750°C or more and 950°C or less, and then quenched.

The present invention provides a hardened steel material that has sufficient durability as a steel material and can extend the life of the steel material, as well as a manufacturing method thereof.

FIG. 1 is a conceptual diagram for explaining an embodiment of the present invention, in which (a) is a hardened steel material, and (b) is a conceptual diagram showing a heat pattern related to the manufacturing method of (a). FIG. 2 is a conceptual diagram of a test piece used in the examples. 4 shows the cross-sectional hardness measurement results of the heated product obtained in the examples, where (a) shows the measurement direction, and (b) shows the hardness distribution map in the depth direction from the surface of the heated product at the measured cross section. 1 shows the results of cross-sectional structure observation of a heated product obtained in an example, where (a) shows the measurement position and (b) shows a cross-sectional SEM image (magnification: 5000 times) of the surface layer of the heated product. FIG. 2 is a conceptual diagram showing positions at which the amounts of retained austenite shown in Table 2 are measured.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
1A and 1B are conceptual diagrams for explaining an embodiment of the present invention, in which (a) is a hardened steel material, and (b) is a conceptual diagram showing a heat pattern according to the manufacturing method of (a). Note that the dotted line shown in Fig. 1A is a cutting line.

The hardened steel material 1 according to this embodiment has a base material made of high carbon hypereutectoid steel, and a surface layer having a mixed phase structure of martensite and carbides, a surface hardness of 700 HV0.3 or more, an average particle size of the carbides of 0.01 μm or more and 0.50 μm or less, and an amount of retained austenite of 5 vol. % or more and 30 vol. % or less.

In the hardened steel material 1 according to this embodiment, the material portion 11 is made of high carbon hypereutectoid steel. The high carbon hypereutectoid steel contains 0.85 to 1.20 mass% C, 0.10 to 2.00 mass% Si, 0.10 to 2.00 mass% Mn, with the balance being Fe and unavoidable impurities.
It is preferable to use JIS-standard high carbon chromium bearing steel as the high carbon hypereutectoid steel. Since high carbon chromium bearing steel is standard steel, it is easy to obtain and is suitable as a material for the raceway. In addition, when the raceway volume is large and high hardenability is required, it is preferable to use SUJ2 or SUJ3 among the high carbon chromium bearing steels.
As described above, in the hardened steel material 1 according to the present embodiment, the material portion 11 is made of high carbon hyper-eutectoid steel, and therefore has sufficient durability as a steel material.

The hardened steel material 1 according to this embodiment has a mixed phase structure of martensite and carbide in the surface layer 12, and has a surface hardness of 700 HV0.3 or more. The surface here specifically refers to a position 0.1 mm from the outermost surface 12a in the depth direction.
From the above, the hardened steel material 1 according to this embodiment has sufficient durability as a steel material.
The surface hardness is preferably 800 HV0.3 or more.

Moreover, in the hardened steel material 1 according to this embodiment, the average particle size of the carbides in the mixed phase structure of the surface layer portion 12 is 0.01 μm or more and 0.50 μm or less.
The average particle size of the carbides is determined by measuring the horizontal and vertical feret diameters of each carbide in an SEM image as shown in FIG. 4 described later, adding up the average values of the two feret diameters of each carbide, and dividing the sum by the number of carbides measured.
The maximum particle size (critical particle size) at which dislocations generated in the hardened steel material 1 cut through carbides and move is approximately 15 nm in the case of Fe ₃ C. Therefore, as described above, by making the average particle size of carbides in the mixed phase structure 0.01 μm or more, there is a possibility that this will contribute greatly to material strengthening.
Furthermore, by making the average particle size 0.50 μm or less, the carbides that are the source of stress concentration are finely dispersed, so that the stress concentration originating from the carbides is reduced, thereby making it possible to extend the life of the hardened steel material.
The average particle size is preferably 0.01 μm or more and 0.20 μm or less.

Furthermore, the hardened steel material 1 according to this embodiment has a retained austenite amount of 5 vol. % or more and 30 vol. % or less in the surface layer 12. Therefore, an improvement in the surface fatigue strength can also be expected.
The thickness of the surface layer 12 of the hardened steel material 1 according to this embodiment (the depth from the outermost surface 12a of the steel material 1 to the point where the hardness becomes the same as that of the material portion 11) is, for example, 1.5 mm.

In the manufacturing method 2 of the hardened steel material according to the present invention, a heated product made of high carbon hypereutectoid steel is preheated by induction heating to a first temperature of 1000°C or more and 1300°C or less, then quenched to a second temperature that is higher than the Ms point and lower than the A1 point, and the second temperature is maintained. Next, the product is heated to a third temperature of 750°C or more and 950°C or less by induction heating, and then quenched.

The manufacturing method 2 of the hardened steel material according to the present embodiment uses a heated product made of high carbon hypereutectoid steel as a raw material. The high carbon hypereutectoid steel contains 0.85 to 1.20 mass% C, 0.10 to 2.00 mass% Si, 0.10 to 2.00 mass% Mn, and the balance is Fe and unavoidable impurities.
It is preferable to use JIS-standard high carbon chromium bearing steel as the high carbon hypereutectoid steel. Since high carbon chromium bearing steel is standard steel, it is easy to obtain and is suitable as a material for the raceway. In addition, when the raceway volume is large and high hardenability is required, it is preferable to use SUJ2 or SUJ3 among the high carbon chromium bearing steels.

Next, the workpiece made of high carbon hypereutectoid steel is preheated by increasing the temperature to a first temperature (T ₁ ) of 1000° C. or more and 1300° C. or less by induction heating.
Using a heated article made of high carbon hypereutectoid steel, the decomposition of carbides in the high carbon hypereutectoid steel and the disappearance of ferrite are promoted by preheating to a first temperature (T ₁ ) of 1000° C. or higher, and a large amount of C is dissolved in the austenite. In addition, by setting the first temperature (T ₁ ) to be reached by preheating to 1300° C. or lower, the occurrence of defects such as melting of the heated article during preheating is suppressed.
The first temperature (T ₁ ) is preferably 1000° C. or higher and 1200° C. or lower.

In the preheating, it is preferable to maintain the first temperature (T ₁ ).
By maintaining the first temperature (T ₁ ), the decomposition of carbides and the disappearance of ferrite in the high carbon hypereutectoid steel are further promoted, and a large amount of C is easily dissolved in the austenite.
The holding time for holding the first temperature (T ₁ ) is preferably 1 second or more and 10 seconds or less, taking productivity into consideration.
The rate of temperature rise to the first temperature (T ₁ ) in the preheating is not particularly limited, but is preferably 20° C./sec or more and 1500° C./sec or less in consideration of productivity.

Next, the material is rapidly cooled from the first temperature (T ₁ ) to a second temperature (T ₂ ) that is higher than the Ms point and lower than the A1 point.
By rapidly cooling to the second temperature (T ₂ ), the precipitation of coarse pro-eutectoid carbides at the austenite grain boundaries in the high carbon hypereutectoid steel is suppressed.
Here, the Ms point is the temperature at which austenite transforms into martensite, and when austenite is cooled to a temperature below the Ms point, martensite is generated.
The Ms point at which austenite transforms into martensite varies depending on the carbon concentration in the steel. In other words, the higher the carbon concentration in the steel, the lower the Ms point tends to be. The Ms point can be calculated using the following formula (1) (see Non-Patent Document 1).

Ms(K)=667-195C-44.9Mn-19.6Ni-21.4Cr-20.7Mo...Formula (1)

Point A1 is the austenite transformation point (727°C). Note that this temperature (727°C) is just an example. The temperature of point A1 may vary depending on the composition of the steel, etc.

In addition, by setting the second temperature ( _T2 ) reached by quenching to a temperature above the Ms point, the austenite in which a large amount of C has been dissolved by preheating is prevented from transforming into martensite. Furthermore, by setting the second temperature ( _T2 ) reached by quenching to a temperature below the A1 point, the austenite in which a large amount of C has been dissolved by preheating is transformed into ferrite and carbides.
The second temperature (T ₂ ) is preferably 500°C or higher and 700°C or lower.

The temperature drop rate for quenching to the second temperature (T ₂ ) is preferably 10° C./sec to 100° C./sec. By using such a temperature drop rate, it becomes easier to suppress the precipitation of coarse pro-eutectoid carbides at the austenite grain boundaries in the high carbon hypereutectoid steel.

Next, the second temperature (T ₂ ) is further maintained.
By maintaining the second temperature (T ₂ ), the austenite in which a large amount of C has been dissolved by preheating is prevented from being transformed into martensite, and the austenite is transformed into ferrite and carbides.
The holding time for holding the second temperature (T ₂ ) is preferably 20 seconds or more and 300 seconds or less, taking productivity into consideration.

Next, the temperature is increased from the second temperature (T ₂ ) to a third temperature (T ₃ ) of 750° C. or more and 950° C. or less by induction heating, and main heating is performed.
By setting the third temperature ( _T3 ) reached by heating to 750°C or higher, the ferrite and some of the carbides transformed by maintaining the second temperature ( _T2 ) are transformed into austenite, while only some of the carbides transformed by maintaining the second temperature ( _T2 ) are melted and finely dispersed. In addition, by setting the third temperature ( _T3 ) reached by heating to 950°C or lower, excessive melting of the carbides transformed by maintaining the second temperature ( _T2 ) is suppressed.

The rate of temperature rise from the second temperature (T ₂ ) to the third temperature (T ₃ ) is not particularly limited, but is preferably 50° C./sec or more and 500° C./sec or less in consideration of productivity.
In addition, in the main heating, the third temperature (T ₃ ) may be maintained.
By maintaining the third temperature ( _T3 ), the ferrite and some of the carbides transformed by maintaining the second temperature ( _T2 ) are further transformed into austenite, while the carbides transformed by maintaining the second temperature ( _T2 ) are partially melted and easily finely dispersed.
The holding time for holding the third temperature (T ₁ ) is preferably 1 second or more and 60 seconds or less, taking productivity into consideration.

Finally, it is rapidly cooled from the third temperature (T ₃ ).
By performing this main heating and then rapid cooling, the austenite transformed from ferrite and some carbides during the main heating is transformed into martensite, thereby obtaining hardness.
The temperature drop rate for this quenching is preferably 10° C./sec to 1000° C./sec. By using such a temperature drop rate, the austenite transformed from ferrite and some carbides during main heating can be easily transformed into martensite.

The hardened steel material (heated product) manufactured by the manufacturing method of the hardened steel material according to the present embodiment described above has the above-mentioned base portion 11 and surface layer portion 12.
Therefore, as described above, the hardened steel material (heated product) has sufficient durability as a steel material, and can be expected to have a long service life as well as improved surface fatigue strength.

FIG. 2 is a conceptual diagram of a test piece used in the examples.
The test pieces (heated items) were JIS standard SUJ3 having the chemical components shown in Table 1 and a cylindrical shape as shown in FIG. 2 (diameter a: 26 mm, length b: 40 mm).

Next, the test piece was subjected to high-frequency hardening by induction heating in which an induction heating coil was placed around the test piece and a high-frequency current of 200 kHz was output to the induction heating coil to generate an induction magnetic field.
The test pieces were induction hardened according to the heat pattern shown in FIG.
Specifically, the test piece was heated to 1150°C (first temperature _T1 ) at a heating rate of 380°C/sec in an air atmosphere by induction heating, and after holding at 1150°C (first temperature _T1 ) for 1 second, compressed air was supplied and quenched to 610°C (second temperature _T2 ) at a heating rate of 36°C/sec. Then, the supply of compressed air was stopped and the test piece was allowed to cool and held at 610°C (second temperature _T2 ) for 30 seconds. Then, in an air atmosphere, the test piece was heated to 850°C (third temperature _T3 ) at a heating rate of 120°C/sec by induction heating, and immediately after reaching 850°C (third temperature _T3 ), it was quenched to room temperature (25°C) at a heating rate of 160°C/sec to obtain a heated product (hardened steel material).

FIG. 3 shows the cross-sectional hardness measurement results of the heated product obtained in the examples, where (a) shows the measurement direction and (b) shows the hardness distribution map in the depth direction from the surface of the heated product in the measured cross section.
That is, the heated product obtained in the example was cut along the dotted line in FIG. 3(a), and the Vickers hardness in the depth direction from the outermost surface of the cross section was measured in accordance with JIS Z 2244 with a test force of 1.96 N.
It can be seen from FIG. 3(b) that a hardened layer having a Vickers hardness of 800 HV0.3 or more is present in a depth direction (surface layer portion) of about 1.5 mm from the surface.

FIG. 4 shows the results of cross-sectional structure observation of the heated product obtained in the example, where (a) shows the measurement position and (b) shows a cross-sectional SEM image (magnification: 5000 times) of the surface layer of the heated product.
That is, the heated product obtained in the example is cut along the dotted line in FIG. 4(a), and a sample for metal structure observation including the cut surface of the surface layer of the heated product is taken. Next, the sample is embedded in resin and mirror-polished so that the cut surface becomes the observation surface. After polishing, the observation surface is etched with a nital solution. An arbitrary point on the surface layer of the etched observation surface is observed with a scanning electron microscope (SEM, Scanning Electron Microscope (observation magnification 5000 times, scanning electron beam acceleration voltage 20 kV) to obtain a cross-sectional SEM secondary electron image.
From FIG. 4, it can be seen that the cross section (surface layer portion) has a mixed-phase structure in which martensite (e.g., the region indicated by the arrow A) and finely dispersed carbides (white parts that appear as ovals or rods: particle size of 0.50 μm or less, e.g., the white dots indicated by the arrow B) are mixed together.

Table 2 shows the measurement conditions for measuring the amount of retained austenite in the heated product obtained in the example. Figure 5 is a conceptual diagram showing the position for measuring the amount of retained austenite in the heated product obtained in the example.

The amount of retained austenite in the surface layer of the heated product obtained in the example was measured and found to be 23 vol. %. Therefore, since retained austenite is also present in the surface layer of the heated product obtained in the example, it is expected that the surface fatigue strength will be improved.

1 Hardened steel material 2 Heat pattern 11 Material portion 12 Surface layer portion a Diameter b Length

Claims (2)

A heated product containing 0.85 to 1.20 mass% C, 0.10 to 2.00 mass% Si, 0.10 to 2.00 mass% Mn as a raw material, with the balance being Fe and unavoidable impurities, is preheated by heating to a first temperature of 1000°C or more and 1300°C or less, and then quenched to a second temperature that is higher than the Ms point and lower than the A1 point, and further, the second temperature is maintained, and subsequently, the product is heated by induction heating to a third temperature of 750°C or more and 950°C or less, and then quenched , thereby producing a material part made of the raw material, a material part having a mixed phase structure of martensite and carbides, a surface hardness of 700HV0.3 or more, an average particle size of the carbides being 0.01 μm or more and 0.50 μm or less, and an amount of retained austenite being 5 vol. % to 30 vol. %. % or less . A method for producing a hardened steel material, comprising the steps of: preheating a heated item made of a material specified by JIS standard SUJ3 by induction heating to a first temperature of 1000°C or more and 1300°C or less, quenching it to a second temperature that is higher than the Ms point and lower than the A1 point, maintaining the second temperature, and subsequently heating it to a third temperature of 750°C or more and 950°C or less by induction heating , followed by quenching, thereby producing a hardened steel material having a material part made of the material and a surface layer part having a mixed phase structure of martensite and carbides, a surface hardness of 700HV0.3 or more, an average particle size of the carbides being 0.01μm or more and 0.50μm or less, and an amount of retained austenite being 5vol.% or more and 30vol.% or less .

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