JP2024002995A - Steel for induction hardening - Google Patents

Abstract

To provide a steel for induction hardening with superior cold forgeability, which enables the omission of annealing before cold forging and quenching and tempering before induction hardening in the production process of induction-hardened parts.SOLUTION: A steel for induction hardening comprises C: 0.36-0.55 mass%, Si: 0.10 mass% or less, Mn: 0.15-0.45 mass%, P: 0.050 mass% or less, S: 0.050 mass% or less, Al: 0.010-0.090 mass%, Mo: 0.05-0.35 mass%, Ti: 0.010-0.200 mass%, B: 0.0005-0.0100 mass% and N: 0.0150 mass% or less, with the balance being Fe and impurities. The total fraction of the ferrite and pearlite structures is 80% or more, and the fraction of the ferrite structure is 40% or more.SELECTED DRAWING: None

Description

The present disclosure relates to steel for induction hardening.

Generally, mechanical structural parts used in automobiles and the like are formed by hot forging or cold forging, and then cut into a final shape. In particular, cold forging has the advantage of being able to reduce the amount of cutting after forging because it has excellent dimensional accuracy. Therefore, in recent years, applications of cold forged products have been increasing.

For example, Japanese Patent No. 5,679,440 (Patent Document 1) discloses a steel for induction hardening which defines a spheroidized structure of cementite, has excellent cold forgeability, and has excellent torsional strength after induction hardening. In addition, JP 2020-100896 A (Patent Document 2) specifies the balance of alloying elements and describes steel for induction hardening and induction hardened parts that can obtain excellent hardness and surface fatigue strength through induction hardening after normalizing. is disclosed.

Patent No. 5679440 Japanese Patent Application Publication No. 2020-100896

Machine structural parts are sometimes subjected to induction hardening as a final heat treatment after molding. Most of these parts generally undergo softening annealing before cold forging and refining heat treatment (quenching and tempering) before induction hardening. In response to the recent intensification of parts price competition and the increasing momentum towards carbon neutrality (reducing CO ₂ emissions), there is an increasing need for steel materials that can omit heat treatment in the parts manufacturing process.

The induction hardening steel disclosed in Patent Document 1 requires spheroidizing annealing before cold forging. Further, the steel for induction hardening disclosed in Patent Document 2 requires tempering heat treatment before induction hardening. As described above, the problem with the conventional technology is that the cost of manufacturing parts increases due to the addition of heat treatment costs.

The present disclosure has been made in view of the above-mentioned circumstances, and the present disclosure has been made in view of the above-mentioned circumstances. The purpose is to provide steel for induction hardening.

The steel for induction hardening according to the present disclosure for achieving the above object is as follows.

(1) As a component composition,
C: 0.36 to 0.55% by mass,
Si: 0.10% by mass or less,
Mn: 0.15 to 0.45% by mass,
P: 0.050% by mass or less,
S: 0.050% by mass or less,
Al: 0.010 to 0.090% by mass,
Mo: 0.05 to 0.35% by mass,
Ti: 0.010 to 0.200% by mass,
Contains B: 0.0005 to 0.0100% by mass and N: 0.0150% by mass or less, the remainder being Fe and impurities,
A steel for induction hardening in which the total fraction of ferrite structure and pearlite structure is 80% or more, and the fraction of ferrite structure is 40% or more.

(2) The component composition further includes:
Cr: 0.65% by mass or less,
Cu: 1% by mass or less and
The steel for induction hardening according to (1) above, containing one or more types selected from Ni: 1% by mass or less.

(3) As the component composition, further,
Se: 0.3% by mass or less,
Ca: 0.05% by mass or less,
Pb: 0.3% by mass or less,
Bi: 0.3% by mass or less,
Mg: 0.05% by mass or less,
Zr: 0.2% by mass or less,
The steel for induction hardening according to (1) or (2) above, containing one or more selected from REM: 0.01% by mass or less and O: 0.025% by mass or less.

(4) As the component composition, further,
The steel for induction hardening according to any one of (1) to (3) above, containing one or more selected from Nb: 0.1% by mass or less and V: 0.3% by mass or less.

(5) The component composition further includes:
Sn: 0.1% by mass or less and
Sb: 0.1% by mass or less,
The steel for induction hardening according to any one of (1) to (4) above, containing one or more selected from the following.

According to the present disclosure, it is possible to provide a steel for induction hardening that has excellent cold forgeability and can omit annealing before cold forging and tempering heat treatment before induction hardening in the manufacturing process of induction hardened parts. .

It is a graph showing a pattern of induction hardening. It is a graph showing a pattern of tempering.

An induction hardening steel according to an embodiment of the present disclosure will be described. First, an overview of the induction hardening steel according to this embodiment will be explained.

The induction hardening steel according to the present embodiment has a composition of C (carbon): 0.36 to 0.55 mass%, Si (silicon): 0.10 mass% or less, Mn (manganese): 0.15 to 0.45 mass%, and P (phosphorus). ): 0.050 mass% or less, S (sulfur): 0.050 mass% or less, Al (aluminum): 0.010 to 0.090 mass%, Mo (molybdenum): 0.05 to 0.35 mass%, Ti (titanium): 0.010 to 0.200 mass%, Contains B (boron): 0.0005 to 0.0100% by mass and N (nitrogen): 0.0150% by mass or less, the remainder being Fe (iron) and impurities. In the induction hardening steel according to this embodiment, the total fraction of ferrite structure and pearlite structure is 80% or more, and the fraction of ferrite structure is 40% or more. In the following description, when "a to b" (where a and b are numerical values and a<b) is written, it means "a or more and b or less".

The induction hardening steel according to the present embodiment can omit annealing before cold forging and refining heat treatment before induction hardening in the production process of induction hardening parts, and has excellent cold forgeability.

An example of the steel for induction hardening according to the present embodiment is steel for induction hardening used for mechanical structural parts such as automobiles, specifically, steel bars and wire rods.

Parts in which the induction hardened steel according to the present disclosure is used include, in the automobile field, for example, engine crankshafts, camshafts, timing gears, and diesel common rails, propeller shafts, drive shafts, and CVJ outer races of drive systems, and suspension hubs. , steering pinions, worm and ball joints, electrical equipment rotor shafts and motor shafts, etc. Parts in which the induction hardened steel according to the present disclosure is used include, for example, shafts of ball screws and rails of linear bearings in the field of industrial machinery, and ring gears and swing gears of travel reducers in the field of construction machinery, for example. Examples include the inner and outer rings of the ring.

Hereinafter, the composition of the induction hardening steel according to this embodiment will be explained in detail.

C: 0.36-0.55% by mass
C needs to be added in an amount of at least 0.36% by mass in order to ensure the strength of the hardened layer after induction hardening. When the amount of C added exceeds 0.55% by mass, cold forgeability decreases and quench cracking during induction hardening is likely to occur. In addition, in order to improve cold forgeability and strength balance after induction hardening, the amount of C added is 0.36 mass% or more and 0.55 mass% or less, preferably 0.36 mass% or more and 0.50 mass% or less, and more preferably The range is 0.36% by mass or more and 0.40% by mass or less.

Si: 0.10% by mass or less
Si is effective as a deoxidizing agent and is preferably added in an amount of 0.01% or more. When the amount of Si added exceeds 0.10% by mass, cold forgeability decreases. The amount of Si added is 0.10% by mass or less, preferably 0.08% by mass or less, and more preferably 0.07% by mass or less.

Mn: 0.15-0.45% by mass
Mn has the effect of improving hardenability, and must be added in an amount of at least 0.15% by mass to ensure strength after induction hardening. When the amount of Mn added exceeds 0.45% by mass, cold forgeability decreases. The amount of Mn added is 0.15% by mass or more and 0.45% by mass or less, preferably 0.20% by mass or more and 0.40% by mass or less, and more preferably 0.20% by mass or more and 0.35% by mass or less.

P: 0.050% by mass or less P reduces the toughness of steel, so it is desirable to reduce the amount of P added, but extremely low P increases the cost of the steel manufacturing process. From the viewpoint of cost, the amount of P added is preferably 0.002% by mass or more. For use in mechanical structural parts, the amount of P added may be 0.050% by mass or less. The amount of P added is preferably 0.030% by mass or less, more preferably 0.015% by mass or less.

S: 0.050% by mass or less S is an element that exists as a sulfide inclusion and is effective in improving machinability. The amount of S added is 0.050% by mass or less. Addition of S exceeding 0.050% by mass does not increase deformation resistance during cold forging, but increases the incidence of cracking. Note that if it is necessary to improve machinability, S may be added in an amount of 0.005% by mass or more, and the amount of S added is preferably in the range of 0.005% by mass or more and 0.050% by mass or less. The amount of S added is more preferably 0.010% by mass or more and 0.035% by mass or less.

Al: 0.010 to 0.090 mass%
Al is an effective element for deoxidation. Further, it has the effect of reducing the crystal grain size by combining with N to form fine nitrides. In order to obtain these effects, it is necessary to add Al in an amount of 0.010% by mass or more. On the other hand, even if Al is added in an amount exceeding 0.090% by mass, the above effect is only saturated. The amount of Al added is 0.010% by mass or more and 0.090% by mass or less, preferably 0.015% by mass or more and 0.050% by mass or less, and more preferably 0.015% by mass or more and 0.035% by mass or less.

Mo: 0.05 to 0.35% by mass
Mo has the effect of improving hardenability, and in order to ensure strength after induction hardening, it is necessary to add at least 0.05% by mass. On the other hand, when the amount of Mo added exceeds 0.35% by mass, cold forgeability decreases. The amount of Mo added is 0.05% by mass or more and 0.35% by mass or less, preferably 0.07% by mass or more and 0.30% by mass or less, and more preferably 0.07% by mass or more and 0.25% by mass or less.

Ti: 0.010 to 0.200 mass%
Ti has the effect of reducing solid solution N in steel by combining with N to form nitrides. Further, since Ti has a stronger tendency to form nitrides than B, the formation of nitrides of B is suppressed, and by increasing the solid solution B, the hardenability of steel can be improved. To obtain this effect, an addition of at least 0.010% by mass of Ti is required. On the other hand, if Ti is added in an amount exceeding 0.200% by mass, nitrides become coarse and cause fatigue failure. The amount of Ti added is 0.010 mass% or more and 0.200 mass% or less, preferably 0.010 mass% or more and 0.050 mass% or less, and more preferably 0.012 mass% or more and 0.035 mass% or less.

B: 0.0005 to 0.0100% by mass
B has the effect of improving the hardenability of steel by being dissolved in the steel. In particular, compared to Si, Mn, Cr, etc., it is possible to increase hardenability without increasing deformation resistance, and in order to achieve both excellent cold forgeability and sufficient hardenability, B Addition is essential. To obtain this effect, it is necessary to add at least 0.0005% by mass of B. On the other hand, if the amount of B added exceeds 0.0100% by mass, cracks are likely to occur after continuous casting, resulting in a decrease in yield. The amount of B added is in the range of 0.0005% by mass or more and 0.0100% by mass or less, preferably 0.0005% by mass or more and 0.0050% by mass or less. More preferably, it is 0.0010% by mass or more and 0.0035% by mass or less.

N: 0.0150% by mass or less N combines with Al, Ti, and B to form nitride. The amount of N added is 0.0150% by mass or less. When the amount of N added exceeds 0.0150% by mass, the formation of BN (boron nitride) becomes significant, and solid solution B decreases, resulting in a decrease in the hardenability of the steel. Further, since excessively low N content increases the cost of the steel manufacturing process, the preferable range of the amount of N added is 0.0010% by mass or more and 0.0150% by mass or less. The amount of N added is more preferably 0.0015% by mass or more and 0.0080% by mass or less, and optimally 0.0020% by mass or more and 0.0065% by mass or less.

In the induction hardening steel according to this embodiment, the remainder of the composition is Fe and impurities. In this embodiment, impurities are those that are mixed in from ore as a raw material, scrap, or the manufacturing environment when steel materials are manufactured industrially. Impurities are allowed within a range that does not adversely affect the properties of the induction hardening steel according to this embodiment.

The induction hardening steel according to this embodiment may further contain the following optional components.

Cr: 0.65% by mass or less
Cr (chromium) has the effect of improving hardenability, and may be added preferably in an amount of 0.05% by mass or more when it is necessary to improve the strength after induction hardening. However, Cr has a strong effect of concentrating in carbides and increasing the thermal stability of carbides, and when added in excess of 0.65% by mass, carbides remain undissolved during induction hardening, resulting in solid solution carbon in steel. This has the disadvantage that the strength after induction hardening decreases. Therefore, as long as the amount of Cr added is within the range of 0.65% by mass or less, it may be added as appropriate depending on the conditions of induction hardening, but in applications where the above-mentioned adverse effects must be stably avoided, Cr The amount added is preferably 0.35% by mass or less. The amount of Cr added is more preferably 0.25% by mass or less.

Cu: 1% by mass or less
Ni: 1% by mass or less
Cu (copper) and Ni (nickel) have the effect of improving hardenability, and may be added preferably in an amount of 0.01% by mass or more when it is necessary to improve the strength after induction hardening. However, if Cu or Ni is added in an amount exceeding 1% by mass each, cold forgeability decreases. The amounts of Cu and Ni added are each 1% by mass or less, preferably 0.5% by mass or less, and more preferably 0.35% by mass or less.

Se: 0.3 mass% or less, Ca: 0.05 mass% or less, Pb: 0.3 mass% or less, Bi: 0.3 mass% or less, Mg: 0.05 mass% or less, Zr: 0.2 mass% or less, REM: 0.01 mass% or less, O :0.025 mass% or less
Se (selenium), Ca (calcium), Pb (lead), Bi (bismuth), Mg (magnesium), Zr (zinc), REM (rare earth metal) and O (oxygen) all improve machinability. It is effective and may be added within the above range if necessary, but even if added beyond the above range, the effect will be saturated.

Nb: 0.1% by mass or less V: 0.3% by mass or less
Nb (niobium) and V (vanadium) have the effect of precipitation strengthening steel by combining with C to form fine carbides. In order to obtain this effect, each of Nb and V may preferably be added in an amount of 0.005% by mass or more. The amount of Nb added is preferably 0.1% by mass or less. The amount of V added is preferably 0.3% by mass or less. Addition of Nb and V in excess of these decreases continuous castability and leads to a decrease in yield.

Sn: 0.1% by mass or less
Sb: 0.1% by mass or less
Sb (antimony) and Sn (tin) have the effect of making it easier to remove scale in descaling treatments such as shot blasting and pickling, and in order to obtain this effect, they may preferably be added in an amount of 0.003% by mass or more. . The amount of Sb and Sn added is preferably 0.1% by mass or less. However, even if Sb and Sn are added in an amount exceeding 0.1% by mass, the descaling property improvement effect is saturated.

The components of the present disclosure have been described above, but in order to soften the steel material and improve cold forgeability, it is also necessary to control the microstructure.

Fraction of ferrite structure is 40% or more In order to make it possible to omit annealing before cold forging with the steel of the present invention, the fraction of ferrite structure needs to be 40% or more. If the fraction of ferrite structure is 40% or more, the deformation resistance is significantly reduced and annealing can be omitted. For this purpose, the heating temperature during hot rolling is preferably 1050°C or lower. When the heating temperature during hot rolling is lowered, austenite becomes finer and the subsequent nucleation of ferrite transformation is promoted, thereby increasing the ferrite fraction.

The total fraction of ferrite structure and pearlite structure is 80% or more. The microstructure of the steel according to the present disclosure contains at least ferrite, and also contains any one of pearlite, bainite, and martensite, depending on the cooling rate after hot rolling. may include. Bainite and martensite, which are produced when the cooling rate is high, are relatively hard microstructures that increase the hardness of the steel material and thus the deformation resistance. On the other hand, ferrite and pearlite, which are produced when the cooling rate is low, have relatively soft microstructures, so they are suitable microstructures for reducing the hardness of the steel material and thus the deformation resistance. Therefore, in the steel according to the present disclosure, in order to improve cold forgeability, the total fraction of ferrite structure and pearlite structure is set to 80% or more. More preferably, it is 90% or more. Of course, it may be 100%.

In addition, if it is necessary to promote austenitization during high-frequency heating, such as to make the hardened layer thicker, the total fraction of ferrite structure and pearlite structure should be 90% or more, and the cementite in pearlite or ferrite should be It is preferable that the aspect ratio is 2 or more. This is because if the aspect ratio of cementite is 2 or more, the dissolution rate of cementite is high and it becomes possible to cause austenitization at an early stage.

In addition, in order to obtain a desired microstructure, water cooling (including mist), blast cooling, and other cooling promotions are not recommended in cooling after hot rolling. However, once the temperature of the steel material has decreased to 400°C or less, there is no problem in applying water cooling (including mist), blast cooling, or other cooling promotion methods. The cooling rate shall be 5.0℃/s or less in the temperature range of 1000 to 400℃ after hot rolling. Preferably it is 3.0°C/s or less, optimally 1.0°C/s or less. Moreover, any cooling rate may be used at temperatures below 400°C.

Hereinafter, the configuration and effects of the present disclosure will be described in more detail according to Examples. The present disclosure is not limited by the following examples, and may be modified as appropriate within the scope of the spirit of the present disclosure, and any of these may be included within the technical scope of the present disclosure. It will be done.

Steel with the composition shown in Table 1 is formed into a round bar with a diameter of 30 mm by hot rolling at the heating temperature shown in Table 2, and after being air cooled (some parts are water cooled or air cooled), the center of the round bar is rolled. A cylinder with a diameter of 20 mm and a height of 30 mm was collected by machining and used as a test piece for cold forging. For steels No. 1 to 41, No. 44, and No. 45, test pieces for cold forging were cold forged. As part of this cold forging, upsetting was performed with a height reduction rate of 30%. Thereafter, induction hardening and tempering was performed according to the patterns shown in FIGS. 1 and 2 to obtain induction hardened steel. In addition, the microscopic test piece for microstructure observation mentioned later was taken from the test piece for cold forging. Note that in Tables 1 and 2, underlined portions indicate that they are outside the scope of the present disclosure.

For comparison, steel No. 42 was formed into a round bar with a diameter of 30 mm by hot rolling, air cooled (some parts were water cooled or air cooled), and then kept at 760°C for 4 hours before being furnace cooled. Softening annealing was performed. Thereafter, a cylinder with a diameter of 20 mm and a height of 30 mm was taken from the center of the round bar by machining and used as a test piece for cold forging. Cold forging and induction quenching and tempering were performed in the same manner as in the case of steels No. 1 to 41, No. 44, and No. 45.

Steel No. 43 was cold forged in the same manner as No. 42, then held at 900°C for 30 minutes and then rapidly cooled, then held at 150°C for 30 minutes and then air cooled. Heat treatment (refining heat treatment before induction hardening and tempering) was performed. Then, the steel No. 43 after the tempering heat treatment was subjected to induction hardening and tempering in the same manner as the steels Nos. 1 to 41, No. 44, and No. 45.

Here, the microstructure was observed using an optical microscope after mirror polishing a cross section perpendicular to the cylindrical axis direction of a cold forging test piece and etching it with 3% nital. Using an image taken through an optical microscope, the area ratio of the ferrite structure and the area ratio of the pearlite structure are determined using the image analysis software Image-Pro PLUS, and based on these area ratios, the total area of these structures is calculated. It was evaluated as a percentage.

In addition, regarding cold forgeability, we measured the load during upsetting with the above-mentioned height reduction rate of 30%, and the measured values were published in the paper journal of the Japan Society for Plasticity Working, "Plasticity and Processing", Vol. 22, No. 241. It was converted into a deformation resistance value in accordance with the cold upsetting test method described on page 139 (February 1981), and comparative evaluation was performed. If this deformation resistance value is 730 MPa or less, annealing can be omitted. Furthermore, the strength after induction quenching and tempering was evaluated by measuring the Vickers hardness (load: 300 g) at 10 arbitrary points on the treated test piece at a depth of 0.1 mm from the surface, and comparing the average values. The above evaluation results are also shown in Table 2.

Steel No. 40 to 44 correspond to the commonly used JIS standard steel S38C. From the results shown in Table 2, the steels according to the present disclosure (example steels, No. 1 to 27) all have S38C (No. 42 and No. 43) after annealing even if annealing before cold forging is omitted. It can be seen that it has cold forgeability comparable to that of In addition, it can be seen that all the steels according to the present disclosure have strength comparable to that of S38C (No. 43) after the tempering heat treatment even if the tempering heat treatment before induction hardening is omitted. Furthermore, it can be seen that the alloy composition of the steel and the annealing before cold forging are closely related. Therefore, according to the present disclosure, both the annealing before cold forging and the refining heat treatment before induction hardening and tempering can be omitted at the same time.

As described above, it is possible to provide a steel for induction hardening that has excellent cold forgeability and can omit annealing before cold forging and refining heat treatment before induction hardening in the manufacturing process of induction hardened parts. .

Claims (1)

As a component composition,
C: 0.36 to 0.55% by mass,
Si: 0.10% by mass or less,
Mn: 0.15 to 0.45% by mass,
P: 0.050% by mass or less,
S: 0.050% by mass or less,
Al: 0.010 to 0.090% by mass,
Mo: 0.05 to 0.35% by mass,
Ti: 0.010 to 0.200% by mass,
B: 0.0005 to 0.0100% by mass,
N: 0.0150% by mass or less, and
Cr: 0.05-0.65% by mass
including, and further,
(i) Cu: 1% by mass or less, or Ni: 1% by mass or less (ii) Se: 0.3% by mass or less,
Ca: 0.05% by mass or less,
Pb: 0.3% by mass or less,
Bi: 0.3% by mass or less,
Mg: 0.05% by mass or less,
Zr: 0.2% by mass or less,
REM: 0.01% by mass or less, and
O: one or more selected from 0.025% by mass or less,
(iii) Nb: 0.1% by mass or less, and
V: one or more types selected from 0.3% by mass or less, and (iv) Sn: 0.1% by mass or less, and
Sb: Contains one or more groups selected from one or more types selected from 0.1% by mass or less, the remainder consisting of Fe and impurities,
A steel for induction hardening in which the total fraction of ferrite structure and pearlite structure is 80% or more, and the fraction of ferrite structure is 40% or more.

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