JPWO2012147439A1 - Manufacturing method of steel product having fine ferrite grain boundary precipitation type martensite structure - Google Patents

Abstract

It is an object of the present invention to provide a steel product having a fine ferrite grain boundary precipitation type martensite structure as a structure having excellent toughness, high strength, and hardness excellent in heat treatment reliability, and a method for producing the steel product.
A method for manufacturing a steel product having a martensite structure in which fine ferrite and cementite and other carbides are precipitated in the form of nets at grain boundaries is obtained by bringing a steel material or a part of the steel material above the A3 or A1 transformation point by resistance heat due to primary energization. After heating, after a cooling time of 2 seconds or less, after reheating by secondary energization to raise or maintain the temperature above the pearlite precipitation line in the continuous cooling diagram, it is related to continuous cooling transformation It passes over the pearlite precipitation line of a CCT diagram, and also passes through a martensitic transformation point and cools.

Description

  The present invention relates to a steel product having a carbide grain boundary precipitation martensite structure such as fine ferrite and cementite, and a method for producing the same.

  Conventionally, steel materials having a martensite structure are generally used as a tough product by performing a tempering treatment because the toughness is greatly reduced. In this case, the tempering temperature is adjusted depending on which of toughness and strength is required and given priority, so that the steel product is used as a steel product having performance according to the application.

  Therefore, the current heat treatment is that a steel product that satisfies both toughness and strength cannot be obtained. That is, in the conventional heat treatment (tempering), the entire martensite structure is converted into a fine pearlite structure such as troostite or sorbite structure to eliminate the martensite structure, and thus the strength is unavoidable.

For example, Patent Document 1 describes that a ball stud subjected to electrical resistance welding is heated to 600 to 650 ° C. by current heating and subjected to current tempering, thereby reducing the hardness of the welded portion and imparting toughness. Has been.
Further, in Patent Document 2, the workpiece is quenched by induction heating method and the like, sent to a tempering station, tempered at a temperature of about 300 ° C. or less by induction heating, and product defects such as cracking of the workpiece are less likely to occur. It is described to do.

  In addition, the present applicant previously disclosed in Patent Document 3 a heat treatment method in press-fitting and a joining structure based thereon. In this method, using a jig such as an electrode, the shaft body is solid-phase bonded to the first steel material (having a hole) by press-fitting, further annealed, and subjected to heat treatment such as carburizing.

JP2004-278666 JP2008-223055 International Publication 2006/033316

The heat treatment such as tempering described in each of the above patent documents 1 to 3 converts the entire martensite structure, which is a quenched structure, into a very fine layered structure (fine pearlite) of cementite such as troostite and sorbite and ferrite. And eliminates the martensite organization.
For this reason, the conventional heat treatment has a problem that the toughness is restored while the strength is lowered, which is a characteristic of a generally known heat treatment.

  The present invention has been made in view of the above problems, and is excellent in heat treatment reliability, and carbides such as fine ferrite and cementite, which are structures having high toughness and high strength and hardness, are precipitated at grain boundaries in a net shape. An object of the present invention is to provide a steel product having a martensitic structure and a method for producing the steel product.

In order to solve the above technical problems, the steel product according to the present invention is characterized by having a martensitic structure in which fine ferrites and carbides such as cementite are each agglomerated and precipitated in the form of nets at grain boundaries.
Further, the steel product according to the present invention has a martensite structure in which fine carbides such as ferrite and cementite are respectively formed in a lump shape and precipitated at grain boundaries in a net shape at a joint portion between the steel materials.

The inventors of the present invention related to the resistance heating type press-fitting manufacturing method and heat treatment according to Patent Document 3 described above, and while repeating various experiments, there is little decrease in strength, rather strength is improved, and toughness and ductility. Has led to the development of a steel product having excellent characteristics and its manufacturing method.
In this production method, a steel product having a useful structure that could not be obtained by conventional heat treatment could be obtained by devising the joining conditions and the heat treatment method after joining.

That is, the steel product has a feature that a carbide such as fine ferrite and cementite is agglomerated and precipitated at the grain boundary in a net shape while leaving the martensite structure.
The carbide includes not only Fe ₃ C (cementite) but also Fe ₂₀ C ₉ , Fe ₇ C ₃ , Fe ₅ C ₂ , and the like.
As can be seen in the structure picture of the joints of steel products described later, this structure is precipitated in a net form by the precipitation of very ductile ferrite and a small amount of carbide precipitates such as cementite at the grain boundaries. And a martensite structure surrounded by these precipitates.

  The steel product according to the present invention is characterized by having a metal structure in which carbides such as ferrite and cementite are precipitated in a net shape and each is precipitated in a lump shape. Such a metal structure is different from a structure in which ferrite and carbide form a layered structure in a sandwich shape like conventionally known pearlite.

There are few brittle cementite-like carbides in the net-like precipitates, and martensite is present in the dough. For this reason, the structure of the steel product has strength (joining strength), ductility and toughness.
In general, a structure in which hard brittle cementite precipitates in the form of nets at the grain boundaries of martensite is known, but cementite has almost no ductility, so those with such a structure are prone to cracking, It is not used for members that require

  In the method for producing a steel product according to the present invention, a steel material or a part of the steel material is heated (rapidly) to the A3 or A1 transformation point or more to change the steel material structure to an austenite structure, and then rapidly cooled. During this cooling, the cooling rate is controlled so that the cooling curve passes on the pearlite precipitation line (Ps line) related to the continuous cooling transformation in the continuous cooling transformation diagram (CCT diagram) of the steel material. And then passing through the martensitic transformation point and cooling. As a method for controlling the cooling rate, reheating means are employed in the present invention.

  In the cooling process of a steel material, usually fine pearlite (a layered structure of ferrite and cementite) is first precipitated. Even in the cooling process according to the present invention, carbides such as fine ferrite and cementite are precipitated at the austenite grain boundaries, but this is not a layer, but is agglomerated and precipitated in a net shape, which is the normal one described above. Is a very big difference.

Thereafter, as the cooling proceeds, the entire substrate (untransformed portion) changes to martensite by martensitic transformation. By passing through such a cooling process, the structure of the steel product according to the present invention in which net-like precipitates exist at the martensite grain boundaries is obtained.
This steel product manufacturing process does not require a tempering process after diffusion bonding by primary energization, and in particular, it is possible to obtain a material having strength and toughness without adding a tempering process. This is one of the features.

  Moreover, the manufacturing method of the steel product according to the present invention is a method using electrical resistance heating, in which a steel material or a part of the steel material is heated to A3 or A1 transformation point or more by resistance heat by primary energization in order to form an austenitic structure ( After a rapid), after a cooling time of 2 seconds or less, reheat by secondary energization to raise or maintain the temperature above the pearlite precipitation line in the continuous cooling diagram, that is, from austenite to martensite. In the transformation process, the temperature of the steel material is raised (rapidly) and then cooled. At this cooling, the cooling curve passes over the pearlite precipitation line of the CCT diagram related to the continuous cooling transformation, and further the transformation point of martensite. To cool through.

  The primary energization makes the metal structure an austenite structure, and the secondary energization is an energization for delaying the cooling of the metal structure so that the cooling curve passes through the Ps line. The structure is changed to a structure in which carbides such as fine ferrite and cementite are precipitated in a net shape.

When the cooling time is within 2 seconds, if this time is long (but 2 seconds or less), the steel product avoids the Ps wire without passing through the Ps wire, passes through the short time side (left side) of the Ps wire, and passes through the Ps wire. In this case, it is reheated to a temperature higher than the Ps line by secondary energization, and the cooling curve is moved in the direction of the time axis long time side (cooling delay). The cooling curve passes through the Ps line.
Also, if the cooling time is short (but 2 seconds or less), the cooling curve is above the Ps line temperature, so the heating curve is reheated to pass the Ps line and the cooling curve is moved in the time axis direction (cooling delay). The cooling curve passes through the Ps line.
If the cooling time exceeds 2 seconds, the temperature of the metal structure is too low, and it becomes difficult to operate the cooling curve so as to pass the Ps line even by reheating by secondary energization.

  As a method for obtaining such a structure, there is a method using high frequency induction heating in addition to the method using the electric resistance heat. This is because the steel material is placed in a coil connected to a high-frequency power source, the steel material is heated to A3 or A1 transformation point or more by induction heating (rapid), and then a cooling time of 2 seconds or less is allowed. After heating to raise or maintain the temperature above the pearlite precipitation line in the continuous cooling diagram, the cooling curve passes over the pearlite precipitation line related to the continuous cooling transformation, and further passes through the martensite transformation point. It is to cool.

  In addition, there is a method using electron beam heating, which irradiates the above-mentioned steel material with a fired (converged) electron beam in a vacuum and heats the steel material to A3 or A1 transformation point or higher (rapid) for 2 seconds. The cooling curve passes over the pearlite precipitation line related to the continuous cooling transformation after the electron beam is irradiated again to raise or maintain the temperature to a temperature higher than the pearlite precipitation line in the continuous cooling diagram. Furthermore, it is to cool through the transformation point of martensite.

Moreover, the manufacturing method of the steel product which concerns on this invention heats (rapidly) the junction part 2 of the 1st steel material 4 and the 2nd steel material 6 more than A3 or A1 transformation point with the resistance heat by primary electricity supply. After performing solid phase diffusion bonding, a cooling time of 2 seconds or less is allowed, and then the above-mentioned joint is reheated by secondary energization to raise the temperature to a temperature above the pearlite precipitation line in the continuous cooling diagram or In the transformation process from austenite to martensite, the steel material temperature is (rapidly) increased (or maintained) and then cooled, and during this cooling, the pearlite precipitation line (Ps line) in which the cooling curve is related to the continuous cooling transformation ) To pass through and further pass through the transformation point of martensite to cool.
From this, it can be said that the primary energization is an energization for performing diffusion bonding and an energization for making the metal structure of the bonded portion an austenite structure. The secondary energization changes the cooling curve through the Ps line, and the secondary energization changes the austenite structure to a structure in which carbides such as ferrite are precipitated in a net shape.

  Moreover, the manufacturing method of the steel product which concerns on this invention presses the 2nd steel material 6 in the hole 5 of the 1st steel material 4 with predetermined pressure, and performs primary electricity supply between these steel materials, and both steel materials Is heated (rapid) above the A3 or A1 transformation point to soften the joint 2 of both steel materials and press-fit the second steel material 6 into the hole 5 of the first steel material 4. After performing solid phase diffusion bonding, allow a cooling time of 2 seconds or less, and then reheat the joint 2 by secondary energization to raise the temperature above the pearlite precipitation line in the continuous cooling diagram. Or, during the transformation process from austenite to martensite, the steel material temperature is (rapidly) raised (or held) and then cooled, and during this cooling, the cooling curve passes over the pearlite precipitation line related to the continuous cooling transformation. And then pass through the martensite transformation point to cool down. Is Rukoto.

  The method for producing a steel product according to the present invention includes a steel material having a carbon content of 0.18 to 1.2% (preferably 0.32 to 0.9%), or a steel material that has been subjected to carburizing or carbonitriding as the steel material. Is used. In addition, such steel materials, such as carbon amount, are used for one or both of the first steel material and the second steel material.

  In the method for manufacturing a steel product according to the present invention, the cooling time is preferably set to 1.5 seconds or less.

  According to the steel product of the present invention, carbides such as fine ferrite and cementite each have a martensitic structure precipitated in the form of a net in the form of a lump in the form of a net, and the structure is used as a joint between steel materials. Since it has it, there exists an effect that the structure | tissue which is excellent in the reliability of heat processing and has high toughness, high intensity | strength, and hardness is obtained.

  In addition, according to the method for manufacturing a steel product according to the present invention, fine ferrite and cementite such as cementite are respectively formed in a lump in the steel material, a part of the steel material, or the joint between the first steel material and the second steel material. Since a martensite structure precipitated at grain boundaries in a shape is generated, there is an effect that a structure having excellent toughness, high strength, and hardness is obtained with excellent heat treatment reliability.

It is a figure which concerns on embodiment and shows the joining state of the steel products in manufacture. It is a figure which concerns on embodiment and shows the state which hold | maintained steel materials to the electrode. It is a systematic diagram of the electrode mechanism in the manufacturing apparatus according to the embodiment. (A) is a structure photograph of the joining part (the right side is the first steel material (carburized steel) side, the left side is the second steel material (S20C) side) of the steel product (with secondary energization heat treatment), (b) Is an enlarged view of part (a) of (a). (A) is a part extracted from the structure photograph of FIG. 4 (first steel (carburized steel) side), and (b) is a scanning electron micrograph (SEM photograph) of (a). . It is an electron beam microanalyzer photograph (EPMA photograph) of the structure | tissue of FIG. 5, and shows the carbon mapping result by FE-EPMA. (The elemental analysis of C (carbon) is performed to display the concentration distribution state). (The original photo is in color, but here I made it monochrome and supplemented the explanation of the organization.) (A) is a structure photograph of a steel product (without secondary energization heat treatment) at the joint (the right side is the first steel (carburized steel) side, the left side is the second steel (S20C) side), (b) Is an enlarged view of part (a) of (a). It is a graph which shows the result of having measured the temperature of the site | part about 0.5 mm away from the junction part of steel materials with the thermocouple. It concerns on embodiment and shows the continuous cooling diagram (CCT diagram) of the steel materials in the manufacturing process of steel products. The structure photograph of the junction part of a steel product is shown, (a) (b) is a 2nd manufacturing method, (c) is based on a 1st manufacturing method. It is a graph which shows hardness distribution (joining part vicinity) in steel products (the presence or absence of secondary electricity supply). About the cooling time in a 1st manufacturing method, regarding the influence of the cooling time exerted on the shear peel strength, the relationship between the shear peel strength of the steel product and the cooling time when the cooling time is changed is shown in a graph. In order to investigate how the presence or absence of secondary energization affects the amount of carbon, a structural photograph in the vicinity of the joint of a steel product manufactured using S35C and S45C as the second steel material is shown. It is a figure which shows the result (image processing photograph) which performed the image process about the net-like precipitate of FIG. 6, (a) is the image which extracted the part ("ferrite" and arrow in FIG. 6) with little carbon content. , (B) shows an image obtained by extracting a portion having a large amount of carbon (indicated by “carbide” in FIG. 6). (The original photo is extracted and distinguished by color display, but here it is converted to black and white and the relevant part is surrounded by a circle.)

Embodiments according to the present invention will be described below with reference to the drawings.
FIG. 1 relates to an embodiment, and shows a steel product 1 having a carbide grain boundary precipitation martensite structure such as fine ferrite and cementite in a joint portion 2 between steel materials, and a manufacturing state thereof (at the end). . Moreover, FIG. 2 shows the state in the middle of manufacturing the said steel products.
The steel product 1 includes a plate-shaped (plate) first steel material 4 and a shaft-shaped (shaft) second steel material 6 joined to the hole 5 of the first steel material 4.

The first steel material 4 has a predetermined thickness, and the hole 5 provided in the first steel material 4 has a circular shape with a constant cross-sectional diameter, and is perpendicular to the plate surface of the first steel material 4. The inner wall surface of the hole 5 is formed in
Here, the plate | board thickness of the 1st steel material 4 shall be 3.2 mm, and the internal diameter of the hole 5 is 11.15 mm. The plate thickness of the first steel material 4 is suitably 1 mm or more, but the upper limit of the plate thickness depends on the inner diameter of the hole 5 (the thickness of the second steel material 6) or the capacity of the power transformer 12.

The second steel material 6 has a columnar shape (or cylindrical shape) with a constant cross section, and has a flat upper surface portion and lower surface portion. Further, a corner portion is cut off around the lower surface portion of the second steel material 6 to form a chamfered portion 26.
Here, the second steel material 6 has an outer diameter of 11.55 mm. The outer diameter of the second steel material 6 is suitably at least twice the thickness of the first steel material 4.

  The outer diameter (diameter) of the insertion portion 25 of the second steel material 6 is slightly larger than the inner diameter of the hole 5 of the first steel material 4, and the press-fitting allowance is the difference between these. By this press-fitting allowance, the outer peripheral portion of the insertion portion 25 of the second steel material 6 is rubbed in contact with the inner wall surface of the hole 5 of the first steel material 4 to form a joining interface, and press-fitting joining over the entire circumference is performed. Done. At this time, the joining portion may be press-fit joining constituted by a plurality of partial joints instead of the entire circumference. Further, chamfering is performed on either or both of the upper edge portion of the hole 5 of the first steel material 4 and the edge portion of the lower surface portion of the second steel material 6.

  As a press-fitting condition, a predetermined press-fitting allowance (d) and a press-fitting depth (h) are set. This press-fitting allowance (d) is relative to the diameter, where d = the outer diameter (D2) of the second steel material 6−the inner diameter (D1) of the hole 5 of the first steel material 4. Further, the press-in depth (h) is h = the depth of press-in (intrusion) of the insertion portion of the second steel material 6.

In this embodiment, the press-fit allowance (d) is 0.4 mm (11.55-11.15 mm). The press-fitting allowance may be in a range where press-fitting is possible. The range of the press-fitting allowance is practically 0.1 mm to 0.7 mm, but if it is in the range of 0.3 mm to 0.5 mm, it is good because there are few burrs and high strength is obtained.
Also, the press-fit depth (or the thickness of the first steel material) is practically good in the range of 1 mm to 6 mm, and the inner diameter of the hole 5 (substantially the outer diameter of the second steel material 6) is 4 mm to 50 mm. The range is preferable from the viewpoint of the capacity of the power source.

  In addition, instead of the first steel material 4, steel materials (various shapes such as a rectangular parallelepiped) in which the holes 5 are formed in the vertical direction can be used. The second steel material 6 preferably has a flat upper surface portion and a lower surface portion. However, the lower surface portion (insertion portion) of the second steel material is a surface of a planned joining portion (hole portion) of the first steel material. It may have a curved surface shape that matches the shape, and the upper surface portion may have a shape other than a flat surface as long as it can be energized.

In addition, the second steel material 6 (insertion portion) adopts a second steel material that is not formed in a similar shape although the entire circumference is joined when the cross-section is similar to the hole 5. It is also possible. In this case, as the second steel material, for example, one having a polygonal cross section (such as a square having a circular arc shape at the corners) is used, and the joint with the first steel material is not composed of the entire circumference but is composed of a plurality of partial joints. It is good also as a form.
In any case, the above press-fitting allowance is necessary between the hole 5 of the first steel material 4 and the second steel material 6 (entire circumference or part).

FIG. 3 is a system diagram of the steel product manufacturing apparatus.
The manufacturing apparatus includes a receiving electrode 9, a lower platen 11 that holds the receiving electrode 9, a pressurizing electrode 8, an upper platen 10 that holds the pressing electrode 8, a power supply transformer 12 (TR) for supplying power, and an electrode The thyristor 14 (SCR) and the like for controlling the power supply interruption and the like. In addition, the manufacturing apparatus has a positioning mechanism, a pressure mechanism (not shown), and the like.
The receiving electrode 9 and the pressing electrode 8 are both made of chrome copper, and the lower platen 11 and the upper platen 10 are both made of brass.

The receiving electrode 9 has a cylindrical shape, and a circular hole 32 (or a through hole) having a predetermined depth is formed in the center of the upper surface portion 30. The hole 32 is formed so that the periphery of the hole 5 of the first steel material 4 is deformed in the press-fitting direction when the second steel material 6 is press-fitted and joined to the first steel material 4. Is provided.
The size (D: diameter) of the hole 32 is slightly larger than that of the second steel material 6. This is to prevent the second steel material 6 from contacting the hole 32 of the receiving electrode 9. Moreover, it is desirable that the size (D: diameter) of the hole 32 is slightly larger than the inner diameter of the hole 5 of the first steel material 4.

  A cooling circuit 34 through which cooling water passes is formed inside the receiving electrode 9. The cooling circuit 34 is formed in a U shape so as to surround the hole 32 when the receiving electrode 9 is viewed from above, and is formed horizontally at a position near the upper part between the upper and lower sides of the receiving electrode 9. Has been.

The lower platen 11 has a holding part 40 for placing and holding the receiving electrode 9 on the upper surface part, and a conduction part 42 extending from the holding part 40. The conduction part 42 is electrically connected to the output terminal of the power transformer 12. Moreover, the lower surface part of the holding part 40 is mounted and fixed to the support part of the manufacturing apparatus.
A cooling circuit 44 through which cooling water passes is formed inside the lower platen 11. The cooling circuit 44 is formed in a shape that extends from the conducting portion 42 to the holding portion 40, goes around the holding portion 40 in a U shape, and returns to the conducting portion 42. The lower platen 11 also contributes to cooling of the receiving electrode 9.

The pressurizing electrode 8 has a cylindrical shape, and a circular holding hole 46 having a predetermined depth is formed at the center of the lower surface portion.
In addition, a cooling circuit 48 through which cooling water passes is formed inside the pressure electrode 8. The cooling circuit 48 is formed in a U shape so as to surround the holding hole 46 when the pressurizing electrode 8 is viewed from above, and is horizontally formed in the central portion between the upper and lower sides of the pressurizing electrode 8. Is formed. The holding hole 46 of the pressure electrode 8 holds the second steel material 6.

The upper platen 10 has a pressure holding part 50 for attaching the pressure electrode 8 to the lower surface part, and a conduction part 52 extending from the pressure holding part 50. The conduction portion 52 is electrically connected to the output terminal of the power transformer 12.
A cooling circuit 54 through which cooling water passes is formed inside the upper platen 10. The cooling circuit 54 is formed in a shape that extends from the conducting portion 52 to the pressurizing and holding portion 50, goes around the pressurizing and holding portion 50 in a U shape, and returns to the conducting portion 52. The upper platen 10 also contributes to cooling the pressure electrode 8.

  Further, the upper surface portion of the pressure holding unit 50 is fixed to an actuator unit of a press mechanism (hydraulic type or the like) of the manufacturing apparatus, and this actuator unit moves the pressure holding unit 50 up and down by an instruction from the control unit. And move downward with a constant applied pressure.

  The cooling circuits are connected in series, and the cooling water that has passed through the water supply valve 56 from the water supply device is, in order, the cooling circuit 48 for the pressure electrode 8, the cooling circuit 54 for the upper platen 10, and the cooling circuit for the receiving electrode 9. 34 and the cooling circuit 44 of the lower platen 11 to cool each part, and the water is discharged through the drain valve 58. Further, the power transformer 12 and the thyristor 14 also cool each part through the cooling circuit in which the cooling water is formed.

  As the power source used for the energization, direct current, alternating current, or direct current using a large-capacity capacitor can be used. In addition, by control from the control unit (not shown), adjustment of the pressurizing force of the press mechanism, start / stop control of energization to the receiving electrode 9 and the pressing electrode 8 from the power transformer 12 (by the thyristor 14), and It is possible to adjust the flow rate of the cooling water in the cooling circuit, control opening and closing, and the like.

Here, the process is demonstrated regarding the manufacturing method of the steel product 1 using the said manufacturing apparatus.
First, a steel material that has been carburized as the material of the first steel material 4 and carbon steel (S20C) as the material of the second steel material 6 are used as the steel materials for manufacturing the steel products.

In manufacturing, joining conditions are set in advance for a manufacturing apparatus (control unit). The joining conditions include pressure, pressurization time, heating time by primary and secondary energization, primary and secondary energization current values, cooling time, and current energization pattern.
In this embodiment, the applied pressure is 9.0 kN. This pressurizing force maximizes the pressurizing force that does not cause press-fitting when pressurized before energization, and minimizes the pressurizing force that does not cause a spark discharge due to a short circuit immediately after energization. For this reason, the optimum pressing force is appropriately 60% to 90% of the maximum pressing force.

Here, the heating time by the primary energization is 0.25 seconds, the primary energization current value is 22 kA, the heating time by the secondary energization is 0.2 seconds, the secondary energization current value is 15 kA, and after the primary energization is stopped. The cooling time until the start of secondary energization is set to 0.2 seconds.
Prior to the start of the manufacturing process, the water supply valve 56 and the drain valve 58 are opened based on an instruction from the control unit, and the supply of the cooling water sent from the water supply device to each cooling circuit is started.

  And as shown in FIG. 2, the 1st steel material 4 and the 2nd steel material 6 are hold | maintained at each electrode, respectively. For the receiving electrode 9, the first steel material 4 is placed on the upper surface portion 30. At this time, positioning is performed so that the center of the hole 5 of the first steel material 4 is positioned at the center of the hole 32 of the receiving electrode 9. At this time, a positioning member or the like may be used so that the first steel material 4 is placed in a predetermined position.

  The pressurization electrode 8 holds the second steel material 6. This second steel material 6 is inserted into the holding hole 46 of the pressurizing electrode 8 and is physically held. Thus, by holding the second steel material 6 by the holding hole 46, the influence of the electrical resistance of the second steel material 6 itself is reduced.

  Next, the second steel material 6 is moved to a predetermined positioning position by the upper platen 10 while being held by the pressure electrode 8 by a pressurizing process by a press mechanism. Then, the press mechanism presses the upper platen 10 and lowers the second steel material 6 held by the pressure electrode 8 together with the applied pressure, and the second steel material 6 eventually engages with the hole 5 of the first steel material 4. As a result, the second steel material 6 is maintained in a state with a constant pressure against the hole 5 of the first steel material 4.

  Then, in accordance with an instruction from the control unit, the thyristor 14 is activated (power is supplied), and primary energization is started between the pressurizing electrode 8 and the receiving electrode 9. As a result, a large-capacity current flows through the joint portion 2 between the second steel material 6 and the hole 5 of the first steel material 4, and the joint portion 2 softens as the electric resistance heat is generated, so that the second steel material 6 is press-fitted. Is started, and the insertion portion 25 of the second steel material 6 moves down in the hole 5 of the first steel material 4.

  In this case, the second steel material 6 is press-fitted into the hole 5 of the first steel material 4, and at this time, the ironing action occurs at the joint interface between the two steel materials, and the press-fit joining is performed. A joined product (steel product being manufactured) obtained by joining the steel materials 6 is obtained. This is because the surfaces of the first steel material 4 and the second steel material 6 are squeezed between the wall surfaces of the first steel material 4 and the second steel material 6 in the sliding direction, whereby the surface impurity layer is scraped and the surface is clean. Then, solid phase diffusion bonding (solid phase diffusion bonding) is performed on this clean tissue.

  In the above manufacturing method, as described above, press-fit joining is performed with a constant applied pressure, the joint is heated instantaneously, and the tip of the second steel 6 is press-fitted into the hole 5 of the first steel 4 in a short time. To complete the joining.

At the time of the press-fitting, the joint portion 2 between the hole portion 5 of the first steel material 4 and the second steel material 6 becomes high temperature due to primary energization. After the primary energization is stopped, the joint 2 between the first steel material and the second steel material is cooled. After the primary energization is stopped, heat treatment by secondary energization is performed immediately after the cooling time while maintaining the state of the joined product.
Here, the stop of the primary energization is the same time as the end of the press-fit bonding, and the primary energization is an energization necessary for the diffusion bonding, and the energization after the end of the diffusion bonding by the primary energization is the secondary energization.

After stopping the energization of the secondary energization, the joined product is air-cooled to obtain a steel product.
As described above, the method for manufacturing a steel product according to the above-described embodiment (herein referred to as “first manufacturing method”) presses the second steel material 6 into the hole 5 of the first steel material 4 with a predetermined pressure. At the same time, primary energization is carried out between these two steel materials, and in order to austenite the structure of the joint portion 2 of both steel materials, it is rapidly heated to a temperature above the A3 or A1 transformation point, and the joint portion 2 of both the steel materials is softened. After the second steel material 6 is press-fitted into the hole 5 of the first steel material 4 and solid phase diffusion bonding is performed, a cooling time of 0.2 seconds is set, and then the energization portion 2 is subjected to secondary energization. The temperature is rapidly raised to a temperature equal to or higher than the pearlite precipitation line (Ps line) in the continuous cooling diagram (FIG. 9), and then air-cooled.
During this air cooling, the cooling curve passes through the Ps line related to the continuous cooling transformation, and further passes through the martensitic transformation point (Ms line) for cooling.

In addition to the first manufacturing method in which heat treatment by secondary energization is performed, for comparison, steel is manufactured by a manufacturing method (herein referred to as “second manufacturing method”) in which secondary energization is not separately performed. Product 1 was manufactured. The manufacturing conditions (excluding secondary energization), materials, and the like in this second manufacturing method are all the same as in the first manufacturing method in which secondary energization was performed.
In the manufacturing process in which the secondary energization is not performed, the joined product (the steel product being manufactured) is cooled as it is after the primary energization is stopped. And the pressurization by a pressurization mechanism is unloaded, and also steel products are removed from each electrode.

4 (a) (b) and FIGS. 5 (a) and 5 (b) are structural photographs of the joint (near) of the steel product obtained by the first manufacturing method (heat treatment by secondary energization).
FIG. 4 (b) is an enlarged view of part (inside the frame) of FIG. 4 (a), and the right side from the boundary (joint part 2) seen on the left side is the first steel material 4 (carburizing treatment). The left side is the portion of the second steel material 6 (S20C shaft). Fig.5 (a) shows the part of the 1st steel material 4 in FIG.4 (b), FIG.5 (b) is a SEM photograph of Fig.5 (a).

  Here, according to the structure photograph of the steel product 1 described above, the structure of the first steel material 4 is such that fine carbides such as ferrite and cementite are precipitated in the form of a lump in the form of nets and at the grain boundaries while leaving the martensite structure. It is characterized by having a steel structure (herein referred to as “ferrite and carbide precipitation structure”) composed of a martensite structure.

  FIG. 6 is an EPMA photograph (originally a color photograph) of the tissue shown in FIG. 5 and shows the mapping result of the carbon concentration distribution by FE-EPMA. In this picture, the part with a small amount of carbon is the black part indicated by “ferrite” (hereinafter referred to as “ferrite part”), and the part with a large amount of carbon indicated by “carbide” (originally red) (hereinafter referred to as “carbide”). Part ")).

Here, since the material of the structure of FIG. 5 is obtained by carburizing a steel material, this structure is a compound of iron and carbon, and the other elements are trace amounts.
The net-like precipitate shown in FIG. 6 is a precipitate of ferrite and iron carbide precipitated at the austenite grain boundary, and was divided into a “carbide portion” in which a large amount of carbon is present and a “ferrite portion” in which no carbon is present. Is. For this reason, the “ferrite part” is a structure made of ferrite produced by decomposition of martensite.

  In addition, as can be seen from the photograph in FIG. 6, the portion constituting the net-like structure is mainly composed of a ferrite in which carbon is not detected and carbon such as cementite having a high carbon concentration. It can be seen that the structure of ferrite accounts for the majority. Further, it is also shown that ferrite and carbides do not form a layered structure in a sandwich shape as in the conventionally known pearlite. The gray mottled portion surrounded by the net-like precipitate is the martensite structure.

  In order to prove that the “carbide portion” is a carbide such as cementite in the photograph of FIG. 6 described above, an analysis was performed using this EPMA data. According to this photograph, the area of the ferrite portion displayed in the black portion occupies the entire screen is 5.0%, and the portion of the portion estimated to be the carbide displayed in the carbide portion occupies the entire screen of 1.1%. . When the carbon concentration of the carbide part was analyzed using this result, the carbon concentration was about 8%. From this value, it was confirmed that the precipitate in this portion was not graphite but iron carbide such as cementite. The EPMA surface analysis data was analyzed by a third party.

FIGS. 14A and 14B show the results of image processing performed on the net-like precipitate shown in FIG. The figure (a) is an image obtained by extracting the “ferrite part” in which the carbon does not exist (a part surrounded by a circle), and the figure (b) shows the “carbide part” in which the carbon is abundant (by The enclosed images are shown respectively.
Also from this analysis result, it was found that the area ratio was 5.0% for the “ferrite part” and 1.1% for the “carbide part”.
Here, the carbon concentration of the carburized material in FIG. 5 is 7.7%, which is substantially the same as the eutectoid steel concentration, and it is considered that this carbon has moved to the “carbide portion”. Is about 8% as described above. The value of this carbon concentration is the same as that of iron carbide (iron carbide) such as Fe3C. Therefore, the “carbide portion” is a structure in which iron carbide is deposited.

  Therefore, as shown in FIG. 5 (b), the joint portion of the steel product 1 has a ferrite structure in which most of the precipitates are rich in toughness and ductility, and there are few precipitates of brittle carbides, and a plate shape. It turns out that it has a shape which precipitates in a lump shape and is unlikely to cause cracks. The substrate has a martensite structure as it is.

Thus, the structure in the vicinity of the joint portion (1.0 mm or less) of the steel product 1 is maintained in strength due to the presence of martensite, and strain and stress are relieved by precipitation of fine ferrite at the grain boundaries, and toughness and Ductility is restored and this structure is compatible with conflicting performance.
In a normal tempered structure, the entire martensite structure is converted to a fine pearlite structure such as troostite and sorbite by reheating, and since martensite is lost, the ductility is restored. Will decline.

  4 (a) and 4 (b), it can be seen that the portion of the second steel material 6 has a thin structure of the present invention, although there is little structure in which the fine ferrite and the like are precipitated at the grain boundaries. This is because carbon steel S20C (carbon content 0.2%) with a small amount of carbon is used as the first steel material 4.

  FIGS. 7 (a) and 7 (b) are photographs of the structure of the steel product joint obtained by the second manufacturing method. In this structure, no ferrite or carbide precipitation structure is observed, and most of the structure is a martensite structure formed by quenching, which is hard and brittle.

Here, according to the first manufacturing method, a change in temperature in the vicinity of the joint portion of the steel material in the manufacturing process of the steel product 1 due to the primary energization and the secondary energization, and a continuous cooling diagram of the steel material based thereon explain.
FIG. 8 shows an example of the result of measuring the temperature of the part about 0.5 mm away from the joint of the steel material with a thermocouple. The measurement conditions here are 22 kA × 0.25 seconds for primary energization, 22 kA × 0.25 seconds for secondary energization, and 0.5 seconds for cooling time.
Dimensions of joint part First steel material Thickness: 3.2 mm Hole diameter: 17.2 mm Material: Carburized steel Second steel material Outer diameter: 17.5 mm Inner diameter: 10.7 mm Material: Carbon steel (S20C)
Note that, during the time during which the primary energization and the secondary energization are applied, the temperature cannot be measured by a thermocouple due to the occurrence of noise or the like. For this reason, temperature measurement by a thermocouple was started immediately after the primary energization and the secondary energization were stopped.

In FIG. 8, the dotted line shows the change in the temperature of the joint when only the primary energization is performed. A solid line indicates a change in temperature when temporary energization, cooling time, and secondary energization are performed. From this, the temperature of the junction exceeds 800 ° C. due to the primary energization, and the temperature of the junction due to the secondary energization reaches nearly 700 ° C.
In addition, since heating time is short, it seems that the temperature measurement by the said thermocouple could not fully follow actual temperature, and it can be estimated that it was actually higher temperature.

FIG. 9 is a CCT diagram of eutectoid steel. The Ps line shows the temperature at which pearlite first precipitates at the austenite grain boundaries when a steel product heated to the austenite state is continuously cooled. , Pf line is a line indicating the temperature at which the precipitation of pearlite is completed. The pearlite is deposited at a temperature between the Ps line and the Pf line. When the pearlite exceeds the Pf line, the precipitation of the pearlite is completed. Accordingly, the precipitation of fine carbides such as ferrite and cementite in the present invention occurs in the temperature region between these lines. Fig. 2 shows a continuous cooling line of steel in the manufacturing process of the steel product 1 by the first manufacturing method (with secondary energization), the second manufacturing method (without secondary energization), and the third manufacturing method with a slow cooling rate. Added to 9.
Here, the cooling curve (A) shows the first method, the cooling curve (B) shows the second method, and the cooling curve (C) shows the cooling state in the third method. This cooling curve is temperature data measured using a thermocouple at a position about 0.5 mm away from the joint, and shows the transition of heat generation and cooling of the steel product (near the joint) at the time of joining.

In the manufacturing process described above, secondary energization after press-fit joining means that the cooling curve (A) of the joint 2 of the steel product 1 is not a single cycle of heat generation and cooling, but the heat generation and cooling cycles are repeated twice. It will be.
As a result, the cooling curve (A) is not cooled as it is from the heating temperature by the primary energization, but rises again by the secondary energization. As the cooling curve rises due to this secondary energization, the subsequent cooling curve (A) follows a cooling path as shown in the figure, and the cooling curve passes through the Ps line and falls as it is.

  In the path of this cooling curve (A), the structure transformed to austenite precipitates carbides such as fine ferrite and cementite at the grain boundaries instead of pearlite at the point of passing through the Ps line, and the temperature further decreases. At the point of passing on the Ms line, the remaining austenite undergoes martensitic transformation and changes to martensite.

  In the conventional continuous cooling diagram, when the cooling curve passes through the Ps line, there is an example in which ferrite first precipitates and then fine pearlite precipitates, but as in the steel product according to this embodiment, There are no cases where precipitates containing carbides in ferrite are generated at the martensite grain boundaries.

  As shown in FIG. 9, when the heat treatment by the secondary energization is not performed, the cooling curve (B) is lowered as it is and does not pass on the Ps line, and reaches the Ms line. In this case, as shown in FIGS. 7 (a) and 7 (b), the ferrite and carbide precipitate structures do not precipitate and all become martensite structures.

In the manufacturing process of the steel product 1, heat treatment by secondary energization is performed after the primary energization and after a cooling time of 0.2 seconds, and the cooling curve (A) is time axis direction (right direction) by this secondary energization. And a curve passing through the Ps line is drawn.
However, if the cooling time after the primary energization is long, as shown in the cooling curve (D), the temperature rise is low even if the secondary energization is performed, and a curve passing through the Ps line is not drawn. The cooling curve is not much different from the shape without secondary energization.

Thus, as a heat treatment of the steel product (steel material), by performing secondary energization after primary energization, the cooling curve (A) moves in the time axis direction (right direction) and passes through the Ps line, and the ferrite and carbide A martensite structure in which a precipitated structure is deposited is generated.
Further, since the secondary energization for the steel product (steel material) moves the cooling curve, the secondary curve may be performed once or a plurality of times, or instead of the secondary energization. As shown in the cooling curve (C), the cooling rate after the primary energization is controlled slightly moderately, and if the cooling curve is moved in the time axis direction, the Ps line is passed, and the ferrite and carbide precipitation structures are precipitated. Will be.

  As a matter specifying the present invention, the conventional pearlite precipitation line (Ps line) is used in the present invention in the same manner as in the prior art in that ferrite and carbide precipitate in the Ps line. This is because the precipitation is agglomerate, but conventionally, it is only due to the difference in the precipitation shape of depositing in layers. If a detailed investigation is performed, it is considered that there is a slight difference between the Ps line of the present invention and the conventional Ps line. However, since it is determined that there is no essential problem, the conventional Ps line was used.

Here, in order to verify the toughness and strength of the steel product 1, a shear peel strength test and verification by a structure photograph were performed, and the results will be described. This shear peel test is an indentation peel test in which a load is applied in the direction opposite to the joining direction of the steel products (the direction in which the second steel material 6 is press-fitted into the hole of the first steel material 4) to peel the second steel material 6. Is due to.
For comparison, this test was performed on a steel product manufactured by the first manufacturing method and a steel product not subjected to secondary energization by the second manufacturing method.

The test conditions are as follows.
Dimensions of joint part 1st steel material Thickness: 3.2mm Hole diameter: 17.2mm Material: Carburized steel Second steel material Outer diameter: 17.5mm Inner diameter: 10.7mm (only hollow shaft)
Material: Carbon steel (S20C)
Joining conditions Primary energization: 22 kA x 0.25 seconds
Secondary energization: 18 kA x 0.17 seconds
Cooling time: 0.5 seconds

FIGS. 10A, 10B, and 10C are structural photographs of the joint portion of the steel product produced by the above-described manufacturing method, and an overall photograph is shown in the upper part and an enlarged photograph of the part is shown in the lower part, respectively. In each photograph, the structure on the left side of the joining line indicates the first steel material, and the structure on the right side indicates the second steel material.
10 (a) shows a second manufacturing method (using a solid shaft as the second steel material 6), FIG. 10 (b) shows a second manufacturing method (using a hollow shaft as the second steel material 6), FIG. 10C is based on the first manufacturing method (using a hollow shaft as the second steel material 6).
From the structure photograph, the ferrite and carbide precipitation structures are precipitated in the joint of the steel product produced by the first manufacturing method in FIG. Moreover, the martensitic structure is formed in the steel part by the 2nd manufacturing method of Fig.10 (a) (b) by quenching in the junction part.

  Table 1 shows the results of the shear peel strength test. In the table, (a) is a second production method (using a solid shaft as the second steel material 6), (b) is a second production method (using a hollow shaft as the second steel material 6), ( c) is based on the first production method (using a hollow shaft as the second steel material 6). Here, the amount of deformation represents the amount of deformation (mm) from the start of deformation to the maximum joint strength in the shear peel test.

  From the test results shown in Table 1, the steel product subjected to the heat treatment by the secondary energization has higher shear peel strength and at the same time the deformation is greatly improved compared to the steel product not subjected to the secondary energization. From these results, it can be seen that the bonded portion 2 of the steel product 1 subjected to the heat treatment by the secondary energization produces not only strength and toughness but also a structure excellent in ductility.

Next, since the test regarding variation in shear peel strength (product uniformity) was performed, the results will be described.
Table 2 shows the results of manufacturing a plurality of steel products (six in this case) for each of the above-described second manufacturing methods using only primary energization and the above-described first manufacturing method using primary and secondary energization. Is shown.

The test conditions are as follows.
Dimensions of joint part First steel material Thickness: 3.2 mm Hole diameter: 15.8 mm Material: Carburized steel Second steel material Outer diameter: 16.1 mm Inner diameter: 10.0 mm (hollow shaft)
Material: Carbon steel (S20C)
Joining conditions Cooling time: 0.5 seconds

From Table 2 above, in the case of only primary energization, there are many variations in strength. This is because the joint structure is mostly a martensite structure, and is therefore poor in ductility and toughness.
On the other hand, in the case where the secondary energization is performed, there is almost no variation in strength. This is because the joint structure is a martensite structure surrounded by a net-like structure having ductility and toughness. As a result, the product quality is also stable.
From these facts, what has been subjected to the secondary energization contributes to uniform product quality, in addition to an excellent performance structure that combines both strength and toughness.

  FIG. 11 shows the distribution of hardness (near the joint) in the vicinity of the joint of the steel product. This measurement was performed on a steel product produced by the first production method (with secondary energization) and a steel product produced by the second production method (without secondary energization). Moreover, in this measurement, the steel material (SPHC) which carried out the carbonitriding process was used for the 1st steel material 4, and carbon steel (S20C) was used for the 2nd steel material 6. The first steel material 4 (plate) has a plate thickness of 3.2 mm, and the second steel material 6 (shaft) has an outer diameter (φ) of 11.5 mm. Other test conditions are the same as the conditions according to Table 1 above.

  The measurement points are the hardness (Hv) at the tens of points up to 2.0 mm toward the second steel material side (right direction) around the boundary (0.0 mm) of the joint 2 of the steel product 1, the first steel material The hardness (Hv) at a dozen points up to 2.0 mm to the side (left direction) is measured.

From the above measurement results, the hardness increases in the absence of secondary energization. This is because a martensitic quenched structure is generated at the joint.
On the other hand, when secondary energization is performed, the hardness is slightly lower than when there is no secondary energization, but this is the hardness in a state where a troostite structure is precipitated by general tempering (indicated by a dotted line). The hardness is higher than.
From this, it is shown that the steel product 1 (ferrite and carbide precipitation structure) according to this embodiment has a structure that is harder and stronger than a general tempered structure. .

Next, a test on the influence of the cooling time on the shear peel strength was performed for the cooling time between the primary energization and the secondary energization in the first manufacturing method, and the results will be described.
Here, steel products were manufactured by changing the cooling time in the first manufacturing method, and the shear peel strength of the steel products manufactured by each cooling time was measured.

The test conditions are as follows.
Dimensions of joint part 1st steel material Thickness: 3.2mm Hole diameter: 17.0mm Material: Carburized steel (SPHC)
Second steel material Outer diameter: 17.4 mm Inner diameter: 10.7 mm Material: Carbon steel (S20C)
Joining conditions Primary energization: 22 kA x 0.25 seconds
Secondary energization: 18 kA x 0.17 seconds

FIG. 12 is a graph showing the relationship between each cooling time (0.01 sec to 10.0 sec) and the shear peel strength (kN) of the steel product produced by the cooling time when the cooling time is changed. It is shown.
Here, the same shear peel strength test was also performed on steel products not subjected to secondary energization.

First, as shown in the graph, the steel product that has been heat-treated by the secondary energization has the shear peel strength (17.1 kN) when the secondary energization is not performed, regardless of the cooling time. The effect of secondary energization is obtained.
Further, if the cooling time is within 2 seconds, the shear peel strength is high (25.0 kN), and the strength is good. For this reason, in the joint portion of the steel product, ferrite and ferrite in the martensite grain boundary are present. It is considered that a carbide precipitation structure is generated.

When this cooling time exceeds 2 seconds, the shear peel strength is 22 kN or less, and the effect of heat treatment by secondary energization is not sufficiently obtained.
This is because when the cooling time after the primary energization exceeds 2 seconds, the electrical resistance of the joint decreases due to the temperature decrease. It is considered that the cooling curve is not so different from the curve form without secondary energization and does not pass on the Ps line.

  In particular, if the cooling time is within 1.5 seconds, the shear peel strength is sufficiently high (around 25.0 kN to 30.0 kN) and excellent in strength. Furthermore, when the cooling time is in the range of 0.5 to 1.5 seconds, the shear peel strength is high (around 30.0 kN), and therefore, the degree of precipitation of the ferrite and carbide precipitation structures is remarkable in the joint portion of the steel product. It is considered effective. These results are accompanied by improvements in ductility and toughness even from the results in Table 1.

  According to the reference materials, “In the case of a steel material with a carbon equivalent of 0.2% or more, the welded part is baked and hardened. The tempering process gives sufficient cooling time after welding and completes the martensitic transformation. “If it is not done, the tempering effect is small.” As an example, the optimum cooling time for tempering treatment is 4 seconds (when the plate thickness is 3.2 mm) (welding book “resistance welding”, industry publications P70-72). ) When the carbon equivalent is high as in the present invention, the cooling time is generally extended, and there is almost no case for shortening the cooling time as in the present invention.

In addition, the primary energization current value and the secondary energization current value in the first manufacturing method are usually set such that primary current value> secondary current value.
This is because the primary energization raises the temperature of the metal structure in the vicinity of the joint and increases the electrical resistance, so that sufficient electrical resistance heat can be obtained even if the secondary current value is small.

Next, regarding the first manufacturing method, since the relationship between the level of the temperature in the vicinity of the joint portion of the steel material by the primary energization and the shear peel strength is examined, the test result will be described. In addition, since the test here is related to the temperature of primary energization, secondary energization is not performed.
Table 3 shows the test results such as the primary energization current value, the measured temperature value of the bonded portion, and the shear peel strength of the bonded product.

The test conditions are as follows.
Dimensions of joint part First steel material Thickness: 3.2 mm Hole diameter: φ17.2 mm Material: SPHC carburized material Second steel material Outer diameter: φ17.5 mm Inner diameter: φ10.5 mm Material: S20C
Press-fitting allowance: 0.3mm
Temperature measurement position: Measured by placing a thermocouple at a position about 0.5 mm away from the joint and at the center of the first steel material.
Joining condition Change the primary current value to 16-21KA x 0.25 seconds.

  From Table 3 above, it can be seen that the peel strength of the joint has reached saturation since the primary energization amount exceeds 20 kA. The temperature in the vicinity of the joint at this time is 710 ° C., which is a temperature close to about 730 ° C., which is the austenitizing temperature of the carburized portion, and the structure of the carburized portion is the same steel product (FIG. 10A). As shown in (b)), it is transformed into martensite. Therefore, it can be estimated that the actual temperature is 730 ° C. or higher.

Further, the S20C material after joining is partially martensitic. The fact that the S20C material having a carbon content of 0.2% has a martensitic transformation in a part thereof indicates that this part has reached a high temperature of about 820 ° C. or more.
The temperature data of the joint is a result of measurement with a thermocouple at a position of about 0.5 mm from the joint line. It should be noted that since the heating of the joint is a short heating time of 1 second or less, it seems that the temperature measurement by the thermocouple could not sufficiently follow the actual temperature, and was actually a higher temperature. Can be guessed.

  Further, in the case of the primary current value of 20 kA, a martensitic structure in which a net-like precipitate due to fine ferrite and carbide was present at the joint could be confirmed. From this, in order to austenite the structure of the joint, the temperature of the joint by primary energization needs to be higher than the A3 or A1 transformation point that can be heated to the austenite state.

Next, a steel product manufactured using S35C and S45C as carbon steel for the second steel material 6 will be described. Carburized steel was used for the first steel material 4.
In addition, in order to investigate how the presence or absence of secondary energization affects the carbon content of this steel product, the first production method (with heat treatment by secondary energization) and the second production method (secondary energization) None).
The test conditions are the same as those in the test according to Table 2 (hollow shaft + secondary energization).
FIG. 13 shows structural photographs of S35C and S45C as the second steel material 6 in the vicinity of the joint of the steel product, with and without secondary energization.

  According to the structure photograph, it can be understood that the ferrite and carbide precipitation structures are formed in any case (S35C, S45C) when the heat treatment is performed by secondary energization. Further, in the case where neither (S35C, S45C) is subjected to secondary energization, the ferrite and carbide precipitation structures are not seen.

Carbon steel S45C (carbon content 0.42 to 0.48%) is more prominent in the formation of the ferrite and carbide precipitation structures than carbon steel S35C (carbon content 0.32 to 0.38%). It is.
In the case of the carbon steel S35C, the ferrite and carbide precipitation structures are precipitated, and as described above, the ferrite and carbide precipitation structures are slightly observed even in the carbon steel S20C (carbon amount 0.18 to 0.23%). .

From this, in order to precipitate the ferrite and carbide precipitation structure in the steel product (joined part), the carbon content is 0.18% or more, preferably 0.32% (corresponding to S35C) or more. Moreover, as for the carburized (nitrocarburized) treated steel, as described above, precipitation of the ferrite and carbide precipitation structures is observed.
Moreover, if the carbon content of the carbon steel exceeds 1.2% (hypereutectoid steel), there is a problem that weld cracks are likely to occur. If the carbon content is within 0.9%, there is little risk of weld cracking.
From this, carbon steel (carbon content 0.18 to 1.2% (preferably 0.32 to 0.9%) is used as one or both of the first steel material 4 and the second steel material 6 as the material of the steel product. ) Or by using carburized steel or carburized and nitrocarburized steel, the ferrite and carbide precipitation structures are generated at the joint.

  The above manufacturing method and steel product can be used for element parts such as automobiles, motorcycles, industrial machines and the manufacture thereof. For example, transmission control lever components, shift lever components, sprockets, gears (with shafts), etc. It is suitable for a part in the form in which the second steel material is joined to the steel material, or an engine part and its manufacture.

DESCRIPTION OF SYMBOLS 1 Steel product 2 Joint part 4 1st steel material 5 Hole 6 Second steel material

The present invention relates to a method for manufacturing a steel article having a carbide grain boundary precipitation type martensite structure, such as fine ferrite及cementite.

The present invention has been made in view of the above problems, and is a martensite in which fine ferrite and iron carbide, which are excellent in heat treatment reliability and have a high toughness, high strength, and hardness, are precipitated in the form of nets at grain boundaries. and to provide a method for manufacturing a steel article having a site structure.

Claims (7)

  A steel product characterized by having a martensitic structure in which fine ferrite and cementite such as cementite are precipitated in the form of nets at grain boundaries.   A steel product characterized by having a martensite structure in which fine carbides such as ferrite and cementite are precipitated in the form of nets at grain boundaries at the joint between the steel materials.   After heating a steel material or a part of the steel material to the A3 or A1 transformation point or higher by resistance heat due to primary energization, allow a cooling time of 2 seconds or less, and then reheat by secondary energization to keep the temperature at a continuous cooling line. The cooling curve passes over the pearlite precipitation line related to the continuous cooling transformation, and further passes through the martensite transformation point for cooling after being raised to or maintained at a temperature equal to or higher than the pearlite precipitation line in the figure. Or the manufacturing method of the steel product of 2.   After the solid-phase diffusion bonding is performed by rapidly heating the joint between the first steel material and the second steel material to the A3 or A1 transformation point or higher by resistance heat due to primary energization, a cooling time of 2 seconds or less is set, After this, the joint is reheated by secondary energization to raise or maintain the temperature above the pearlite precipitation line in the continuous cooling diagram, and then the cooling curve passes over the pearlite precipitation line related to the continuous cooling transformation. The steel product manufacturing method according to claim 2, further comprising cooling through a martensitic transformation point.   The second steel material is pressed into the hole of the first steel material at a predetermined pressure, and a primary energization is performed between both the steel materials to rapidly heat the joint portion of both steel materials to the A3 or A1 transformation point or more. After softening the joint of the steel material and press-fitting the second steel material into the hole of the first steel material and performing solid phase diffusion bonding, a cooling time of 2 seconds or less is allowed, and thereafter After reheating by secondary energization to raise or maintain the temperature above the pearlite precipitation line in the continuous cooling diagram, the cooling curve passes over the pearlite precipitation line related to the continuous cooling transformation, and further the martensitic transformation point The method for producing a steel product according to claim 2, wherein the steel product is cooled by passing through the steel.   6. The method for producing a steel product according to claim 3, wherein a steel material having a carbon content of 0.18 to 1.2% or a carburized steel material is used as the steel material.   The method for producing a steel product according to any one of claims 3 to 6, wherein the cooling time is 1.5 seconds or less.