JP5706766B2 - Induction hardening steel excellent in machinability and manufacturing method thereof - Google Patents

Description

  The present invention relates to steel for producing steel parts by induction hardening after cutting, and more particularly to induction hardening steel excellent in machinability and a method for producing the same.

  Steel parts used in automobiles and various machinery (specifically, gears, shafts, pulleys, constant velocity joints, etc. used in various gear transmissions including automobile transmissions and actuators, and crankshafts) In general, a mechanical structural component such as a connecting rod is manufactured by subjecting hot-worked steel (for example, hot rolling or hot forging) to a final shape (part shape) by cutting. Steel parts after cutting are required to have high hardness (for example, Vickers hardness) and excellent fatigue properties such as rotational bending fatigue strength (hereinafter referred to as “strength” by combining hardness and fatigue properties). However, in order to increase the strength of the steel part, if the strength of the steel before cutting is increased, cutting becomes difficult. On the other hand, since the cost required for cutting is high in the total part production cost, the steel before cutting is required to have good machinability. Therefore, the steel before cutting improves the machinability by reducing its hardness, and increases the strength of steel parts by performing heat treatment such as quenching and tempering (tempering) and carburizing and quenching after cutting. Has been done.

  Here, the cutting process will be described in detail. In the cutting process for manufacturing gears among the above-mentioned mechanical structural parts, it is common to perform gear cutting with a hob. In this case, the cutting process is called intermittent cutting. It is. As a tool used for hobbing, a high-speed tool steel coated with AlTiN or the like (hereinafter, sometimes abbreviated as “high-speed tool”) is currently mainstream. Gear cutting by hobbing (intermittent cutting) using a high-speed tool is low speed (specifically, cutting speed of about 150 m / min or less) and low temperature (specifically, about 200 to 600 ° C.), but intermittent. Because of cutting, the tool is easy to come into contact with air and is subject to oxidative wear. For this reason, steel used for intermittent cutting such as hobbing is particularly required to extend the tool life.

  The present applicant has proposed steels for machine structural use with improved machinability (particularly, tool life) in intermittent cutting in Patent Documents 1 and 2. Among these, in patent document 1, the machinability in the continuous cutting with a high-speed tool is improved by adjusting each component of oxide inclusions appropriately to facilitate deformation of the entire inclusion with a low melting point. . On the other hand, in Patent Document 2, by adding an element that has a greater tendency to oxidize than Fe to solid solution, the mechanical structure steel is prevented from being rapidly oxidized during intermittent cutting, and oxidative wear of the tool is prevented. Suppresses and improves the machinability of steel. However, in Patent Documents 1 and 2, as described above, in order to increase the strength of the steel part, it is necessary to perform heat treatment such as quenching and tempering (tempering) or carburizing and quenching after cutting.

  By the way, in recent years, in order to reduce the burden on the global environment and improve the working environment, induction hardening is performed instead of heat treatment such as quenching and tempering (tempering) and carburizing and quenching. Induction hardening is a method in which only the vicinity of the steel surface layer is rapidly heated and cooled, and the hardness and fatigue characteristics of the surface layer portion of the steel part can be increased in a short time. On the other hand, in order to ensure the same surface and internal hardness as the carburizing treatment, it is necessary to increase the C content in the steel so that the strength is sufficiently improved by martensitic transformation. Further, since the internal hardness of the steel part does not change before and after the induction hardening treatment, it is necessary to increase the hardness of the steel before cutting.

  For example, Patent Document 3 is known as a steel for induction hardening with improved machinability while ensuring fatigue characteristics. In Patent Document 3, machinability is improved by optimizing the chemical composition and structure of steel, and the fatigue strength is secured by making the ferrite structure beaded around the pearlite structure. Technology is disclosed.

  However, in the said patent document 3, machinability is not fully improved by the production | generation of a low temperature transformation phase (bainite, martensite). Moreover, since the inside of steel has a ferrite-pearlite structure, the internal hardness is insufficient, and the required characteristics as steel parts are not fully met.

  By the way, in actual machining, for example, like a gear with a shaft, machining of the shaft portion is performed by continuous cutting, and machining of the gear portion is performed by intermittent cutting. It is required to have excellent machinability. However, since continuous cutting results in a high temperature (800 ° C or higher) compared to interrupted cutting, conventional steel materials for improving intermittent cutting performance generally suffer from damage due to high tool cutting edges in continuous cutting at high temperatures. There was a problem that machinability deteriorated, for example, the degree increased.

  Moreover, in the case of continuous cutting, the steel chips that have been machined are more likely to be connected and generated, but when the chips are connected and stretched, not only the chips get entangled with the machine but also the machine is damaged, There was a risk of injury. For this reason, chips must be separated by a chip breaker or the like, and safety is also a problem.

JP 2009-30160 A JP 2009-287111 A JP 2006-28598 A

  The present invention has been made paying attention to such a situation, and the purpose thereof is machinability during cutting (intermittent cutting and continuous machinability) and fragmentation of chips generated by continuous cutting. In addition to its excellent properties, it has been cut into a part shape to ensure high hardness (surface layer and internal Vickers hardness) and high fatigue characteristics (rotating bending fatigue characteristics) required for steel parts after induction hardening. An object of the present invention is to provide a steel for induction hardening and a method for producing the same.

  The present invention that has solved the above problems is: C: 0.40 to 0.65% (meaning of mass%, the same applies to chemical components), Si: 0.010 to 0.50%, Mn: 1.0. 0 to 2.0%, P: 0.03% or less (excluding 0%), S: 0.002 to 0.10%, Cr: 0.010 to 0.3%, Al: 0.06 to 0.50%, B: 0.0050 to 0.010%, N: 0.010 to 0.030%, the balance is made of iron and unavoidable impurities, and 0.010% of nitride The steel has a pearlite and bainite, and may further have ferrite, and the total area ratio of pearlite and bainite to the entire structure is 95% by area or more, In addition, each area ratio of ferrite and bainite with respect to the entire structure is one side of ferrite. % Or less (including 0 area%), bainite is a steel induction hardening with the spirit to be 20 to 50 area%.

  In the present invention, it is also preferable that Mo: 1.0% or less (not including 0%) is contained as another element. Further, as another element, Ti: 0.2% or less (0% Nb: not more than 0.2% (not including 0%), and V: not less than 0.2% (not including 0%), containing at least one element selected from the group consisting of It is also a preferred embodiment.

  Furthermore, it is preferable that other elements include Cu: 3% or less (not including 0%) and / or Ni: 3% or less (not including 0%). , Ca: 0.005% or less (not including 0%), Mg: 0.005% or less (not including 0%), Li: 0.001% or less (not including 0%), and REM: 0 One preferred embodiment is one containing at least one selected from the group consisting of .0010% or less (excluding 0%).

  Moreover, the manufacturing method of the induction hardening steel which concerns on this invention which could solve the said subject, after hot-working the steel which satisfies the said component composition in the temperature range of 850-1250 degreeC, it is 700 degreeC from the said temperature range. Is cooled at an average cooling rate of 0.05 to 0.5 ° C./s, and then is cooled at an average cooling rate of 1.0 to 5.0 ° C./s. In particular, it has a gist.

  According to the present invention, by defining the composition of steel and the amount of nitride, and by appropriately controlling the ratio of the metal structure of the steel, the machinability at the time of cutting is excellent and the strength of the steel part (steel part) In addition, the steel for induction hardening excellent in the surface layer and the internal hardness and fatigue characteristics can be provided. The steel for induction hardening of the present invention has good machinability when it is cut, in particular, good tool life when it is cut intermittently or continuously, and also excellent in the ability to cut chips generated during continuous cutting. ing. Furthermore, steel parts formed by induction hardening after cutting can ensure hardness and fatigue characteristics.

1A is a metal structure photograph of steel type 1A in Table 1, and FIG. 1B is a metal structure photograph of steel type 1G-17 in Table 1.

  The inventors have good machinability for interrupted cutting and continuous cutting, and good chip breaking during continuous cutting, and ensure Vickers hardness and fatigue characteristics required for steel parts by induction hardening after cutting. Studies have been made to provide a steel for induction hardening. As a result, the steel composition is appropriately adjusted (especially the combined addition of N, Al, and B, and the addition amount of Mn and B is increased), and the metal structure of the steel is appropriately controlled (to the total metal structure). The area ratio of pearlite and bainite occupying, and when ferrite is included, the area ratio of ferrite is appropriately controlled), and the nitride in the steel is controlled to a specific range, so that all these characteristics are combined. The present inventors have found that a steel for hardening can be provided and completed the present invention.

  Hereinafter, the process of reaching the present invention will be described in order, and then the steel for induction hardening according to the present invention will be described.

  When induction hardening is performed to ensure the strength of steel parts, the hardness near the surface of the steel is improved, but the internal hardness does not change before and after induction hardening. In order to obtain the internal hardness, it is necessary to improve the internal hardness of the steel. As for the metal structure of steel, it is generally known that bainite is effective in increasing the strength of steel. Therefore, if the bainite ratio of the steel metal structure is increased, the internal hardness and fatigue characteristics can be improved. However, since bainite is a hard phase, it contributes to high strength while reducing machinability. Therefore, even if the required metal structure is used, continuous machinability and intermittent machinability are not improved.

  First, the present inventors examined continuous machinability and intermittent machinability. At the time of continuous cutting performed on a shaft or the like, it is generally known that the cutting edge is easily damaged because the cutting tool is always in contact with a steel material (workpiece) at high temperature and high pressure. When this point was examined in more detail, it was found that the continuous machinability deteriorates not only due to the initial hardness of the steel before cutting but also due to work hardening by cutting. Therefore, studies were repeated based on the idea that if the work hardening caused by cutting of the steel material can be suppressed, the machinability can be improved without reducing the hardness of the steel material. As a result, it was found that the work hardening of the steel material becomes remarkable due to the localization of the strain to the soft phase of the steel material. That is, when a hard phase (bainite, martensite, pearlite, retained austenite, etc.) and a soft phase (ferrite) are mixed, deformation tends to concentrate on the soft phase, and the steel material is formed by the work hardening of that part. It tends to be hard. For this reason, it was considered that work hardening can be suppressed by suppressing the formation of ferrite, which is a soft phase.

  However, even in the metal structure of the hard phase, if the ratio of martensite to retained austenite is increased, the hardness becomes too high and the tool edge is liable to be damaged. Therefore, the mixed structure of bainite and pearlite is mainly used. Decided to suppress sites and retained austenite.

  Further, if the ferrite ratio is decreased, the machinability will decrease. As a result of studying techniques for improving the machinability, the hardness of the parts can be increased by increasing the amount of nitride in the steel even if the ferrite ratio is suppressed. It was found that machinability can be improved without lowering the steel, and that when the nitride in the steel is increased, the cutting performance of the chips when continuous cutting is improved and the chips are appropriately cut. .

  On the other hand, when performing intermittent cutting like gear gear cutting, the cutting tool repeats cutting and idling, but the hot tool tip is oxidized by oxygen in the atmosphere during idling, so the machinability is reduced by oxidative wear. It turns out that it gets worse. Therefore, when the means for suppressing the oxidation of the tool edge was studied, in order to suppress the oxidation wear of the tool edge, the tool was obtained by preventing the rapid oxidation of the steel material by making Al exist in the solid state in the steel material. The knowledge that the oxidation of the blade edge can be suppressed was obtained.

  Moreover, it has been found that if the amount of nitride in the steel is increased as described above, good machinability can be exhibited even in intermittent cutting.

  The mechanism that can improve the machinability while ensuring the strength is not limited to the following, but is considered as follows. That is, in order to improve strength and machinability, it is effective to control the area ratio of pearlite and bainite in the metal structure (the area ratio of ferrite when ferrite is included). Machinability is reduced. Therefore, it is considered that by increasing the amount of nitride in the steel, the steel material is embrittled, and the fracture and separability of the steel material by cutting can be improved, so that the machinability is improved. Moreover, it is considered that by making the steel material brittle in this way, the toughness is lowered, so that the chips after cutting are easily divided.

  In order to suppress the work hardening by reducing the ferrite area ratio, the present inventors are effective in increasing the amount of addition of B and Mn, and nitriding which is effective in machinability and chip breaking properties. In order to increase the amount of material, it has been found that it is effective to increase the amount of addition of N, Al, and B, and to control the production conditions to generate nitride while suppressing the formation of ferrite.

  The present invention has been made on the basis of the above-mentioned knowledge, the point of appropriately controlling the area ratio of pearlite and bainite (the area ratio of ferrite when ferrite is included), the point of positively introducing nitride, steel It is characterized in that the addition of specific amounts of Al, Mn, B, and N as essential chemical components is essential. Hereinafter, the present invention will be specifically described.

  First, the metal structure of steel will be described.

Metal structure: having pearlite and bainite As described above, bainite and pearlite are metal structures that contribute to increasing the hardness of steel and improving fatigue properties. Therefore, the strength can be improved by making the steel metal structure substantially a mixed structure of pearlite and bainite. However, as described above, the balance between desired strength and machinability cannot be obtained simply by using a metal structure containing pearlite or bainite. It is necessary to satisfy the introduction of the component and the component composition.

Total area ratio of pearlite and bainite: 95 area% or more with respect to the entire structure As described above, the internal strength of the steel of the present invention is manifested by a mixed structure of pearlite and bainite. In order to obtain such an effect, the total area ratio of pearlite and bainite with respect to the entire structure is 95 area% or more, preferably 97 area% or more, more preferably 99 area% or more. The metal structure other than pearlite and bainite includes, for example, ferrite, martensite, and retained austenite that are inevitably produced in production. However, when the area ratio of these structures is increased, the machinability is increased by work hardening. Since it may deteriorate, it may not be included at all. Therefore, the total area ratio of pearlite and bainite relative to the entire structure is more preferably 100 area%.

Area ratio of bainite: 20 to 50 area% with respect to the entire structure
Since bainite is a harder phase than pearlite, it is a structure that contributes to improving the strength of parts after induction hardening and fatigue characteristics. In order to obtain such an effect, the area ratio of bainite with respect to the entire structure is 20 area% or more, preferably 22.5 area% or more, more preferably 25 area% or more. On the other hand, when the area ratio of bainite occupying the whole structure becomes too high, although the component strength is improved, the machinability is lowered. Therefore, the upper limit of the area ratio of bainite is 50 area% or less, preferably 45 area% or less, more preferably 40 area% or less with respect to the entire structure.

Ferrite area ratio (only when ferrite is included): 1 area% or less (including 0 area%) with respect to the entire structure
Since ferrite is a soft phase compared to pearlite or bainite, when deformation stress is applied during cutting, it acts as a buffer material for the hard phase and promotes work hardening of the steel material. For this reason, from the viewpoint of suppressing work hardening, it is desirable that the amount of ferrite is as small as possible. The remaining structure is mainly pearlite, but may contain martensite, retained austenite, and the like that may be inevitably generated.

  The steel of the present invention having properties excellent in machinability and strength needs to satisfy not only the metal structure of the steel but also the composition of the steel and the amount of nitride in the steel.

C: 0.40 to 0.65%
C is an element necessary for ensuring the strength, and by containing 0.40% or more, it is possible to ensure the strength required for the components (the steel surface and internal hardness after induction hardening, and fatigue characteristics). C is preferably 0.43% or more, more preferably 0.45% or more. However, if the amount of C is excessive, the steel becomes too hard and the machinability and toughness deteriorate. Therefore, the C content is 0.65% or less. The amount of C is preferably 0.62% or less, and more preferably 0.60% or less.

Si: 0.010 to 0.50%
Si acts as a deoxidizing element and is an element necessary for improving the internal quality of steel. When there is too little Si, deoxidation will become inadequate and it will become easy to generate | occur | produce a gas defect at the time of melting. Therefore, Si is made 0.010% or more, preferably 0.03% or more, more preferably 0.05% or more. However, when the amount of Si becomes excessive, the steel becomes too hard and the machinability deteriorates. Therefore, Si is 0.50% or less, preferably 0.45% or less, more preferably 0.40% or less.

Mn: 1.0-2.0%
Mn is an element necessary for improving the hardenability and improving the strength of the steel. The Mn is to reduce the A ₃ temperature, which is an element effective for suppressing the formation of ferrite. In order to obtain such an effect, Mn is 1.0% or more, preferably 1.2% or more, more preferably 1.4% or more. However, when Mn is excessive, the hardenability is excessively improved and bainite is excessively generated or martensite is easily generated, and the machinability is lowered. Therefore, Mn is 2.0% or less, preferably 1.8% or less, more preferably 1.6% or less.

P: 0.03% or less (excluding 0%)
P is an impurity element that is inevitably contained in steel, and if the amount of P is excessive, it promotes the occurrence of cracks during processing, so it needs to be reduced as much as possible. Therefore, P is 0.03% or less, preferably 0.02% or less, more preferably 0.015% or less. In addition, it is industrially difficult to make P amount 0%.

S: 0.002-0.10%
S is an impurity inevitably contained in the steel, but is an element that effectively acts to improve the machinability of the steel by combining with Mn in the steel to form MnS inclusions. 002% or more, preferably 0.005% or more, more preferably 0.008% or more. However, if the amount of S becomes excessive, the amount of MnS inclusions increases, and the inclusions extend in the processing direction during processing (for example, hot rolling or hot forging), so the toughness in the direction perpendicular to the processing direction. (Train toughness) will be deteriorated. Therefore, the S content is 0.10% or less, preferably 0.08% or less, more preferably 0.05% or less.

Cr: 0.010 to 0.3%
Cr is an element that effectively acts to enhance the hardenability of the steel and improve the strength. Moreover, it is an element which acts effectively also for improving the machinability (especially intermittent machinability) of steel by combined addition with Al. In order to exhibit such an effect, Cr is 0.010% or more, preferably 0.03% or more, more preferably 0.05% or more. However, if the amount of Cr is excessive, coarse carbides are generated, or an excessively cooled structure is excessively generated to deteriorate the machinability. Therefore, the amount of Cr is 0.3% or less, preferably 0.27. % Or less, more preferably 0.25% or less.

Al: 0.06-0.50%
Al is an element necessary for improving the machinability when intermittent cutting is performed by suppressing the wear on the tool surface by being present in a solid solution state in steel. Further, Al is an element that binds to N and precipitates AlN to prevent the crystal grains from growing abnormally during processing and lowering the strength. Al also acts as a deoxidizer. In order to exert such an effect, Al is made 0.06% or more, preferably 0.08% or more, more preferably 0.10% or more. However, when Al is excessive, a large amount of AlN precipitates and the workability is lowered. Therefore, Al is 0.50% or less, preferably 0.4% or less, more preferably 0.3% or less.

B: 0.0050 to 0.010%
B is an important element having an effect of suppressing the formation of ferrite. In order to exert such an effect, B is 0.0050% or more, preferably 0.0055% or more, more preferably 0.0060% or more. However, if B is excessive, the steel becomes too hard and the machinability deteriorates. Therefore, B is 0.010% or less, preferably 0.0095% or less, more preferably 0.0090% or less.

N: 0.010 to 0.030%
N is an element that precipitates AlN and prevents crystal grains from growing abnormally during processing to lower the strength, and contributes to improving the machinability by depositing BN. Further, it is an essential element for nitride formation described later. In order to exert such an effect, N is 0.010% or more, preferably 0.012% or more, more preferably 0.014% or more. However, when N is excessive, the amount of dissolved N increases and dynamic strain aging occurs, and the work hardening of the steel material progresses to deteriorate the machinability. Therefore, N is 0.030% or less, preferably 0.028% or less, more preferably 0.026% or less.

  The component composition of the high-strength steel according to the present invention is as described above, and the balance is iron and inevitable impurities. As the inevitable impurities, mixing of trace elements (for example, As, Sb, Sn, etc.) brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. is allowed.

  In addition, Mo, Ti, Nb, V, Cu, Ni, Ca, Mg, Li, REM, and the like may be actively included as other elements within a range not impairing the effects of the present invention.

Mo: 1.0% or less (excluding 0%)
Mo is an element that acts to increase the hardenability of steel and suppress the formation of a structure that has not been quenched. Such an action increases as the content thereof increases, but is preferably 0.04% or more, more preferably 0.06% or more, and further preferably 0.08% or more. However, when Mo is excessively contained, an excessively cooled structure is excessively generated and machinability is lowered. Therefore, the content is preferably set to 1.0% or less. More preferably, it is 0.8% or less, More preferably, it is 0.5% or less.

Selected from the group consisting of Ti: 0.2% or less (not including 0%), Nb: 0.2% or less (not including 0%), and V: 0.2% or less (not including 0%) At least one of the elements Ti, Nb, and V is an element that has the effect of preventing abnormal growth of crystal grains during hot working and preventing deterioration of the toughness and fatigue characteristics of steel. In addition, it is an element that effectively acts on the chip breaking property during continuous cutting. Such an effect is exhibited by containing at least any one or more kinds, and increases as the content increases. Ti, Nb, and V are each preferably 0.005% or more, more preferably 0.010. % Or more is desirable. However, if these elements are contained excessively, a large amount of hard carbides are formed and the machinability of the steel is lowered. Therefore, Ti, Nb, and V are each 0.2% or less, preferably 0.15%. Below, more preferably 0.10% or less. In addition, Ti, Nb, and V may be contained independently and may contain 2 or more types chosen arbitrarily.

Cu: 3% or less (not including 0%) and / or Ni: 3% or less (not including 0%)
Cu and Ni are elements that effectively act to improve the hardenability and increase the strength. Such an action increases as the content of these elements increases, but in order to effectively exhibit them, Cu and Ni are each preferably 0.05% or more, more preferably 0.10% or more. However, if excessively contained, an excessively cooled structure is excessively generated, and ductility and toughness are lowered. Therefore, Cu and Ni are each preferably made 3% or less. More preferably, it is 2% or less, and further preferably 1% or less. In addition, Cu and Ni may each be contained independently, and both may be contained, and when both are contained, the content may be any content within the above range.

Ca: 0.005% or less (not including 0%), Mg: 0.005% or less (not including 0%), Li: 0.001% or less (not including 0%), and REM: 0.00. At least one element selected from the group consisting of 0010% or less (excluding 0%) Ca, Mg, Li, and REM spheroidize sulfide compound inclusions such as MnS and improve machinability It is an effective element. These effects increase as the content thereof increases, but in order to exert them effectively, Ca and Mg are each preferably 0.0005% or more, more preferably 0.0010% or more, and Li and REM are each preferably. Is 0.0001% or more, more preferably 0.0002% or more. However, even if contained excessively, the effect is saturated and an effect commensurate with the content cannot be expected, so Ca and Mg are each preferably 0.005% or less, more preferably 0.0040% or less, and still more preferably 0.00. 0030% or less, Li and REM are each preferably 0.001% or less (REM is 0.0010% or less), more preferably 0.0008% or less, and further preferably 0.0005% or less. In addition, Ca, Mg, Li, and REM may be contained independently and may contain 2 or more types chosen arbitrarily. REM means a lanthanoid element (15 elements from La to Lu) and Sc (scandium) and Y (yttrium), and may contain one or more selected from these. Good.

Nitride: 0.010% or more In addition to the above component composition, the steel of the present invention contains 0.010% or more of nitride. When the nitride content increases, the cut steel can be cut before the work is hardened by the tool edge, so that the cutting energy can be reduced. Further, since nitrides embrittle steel, machinability is improved even if the ferrite area ratio is low.

  The nitride in the present invention may be a nitride with an element contained in the steel material, and examples thereof include AlN, BN, TiN, NbN, etc., and these nitrides and carbon composite nitrides (carbonitrides) are also included. included. The specific constituent elements of the nitride are not limited to these.

  Chip separation is improved by the nitride. In order to exert such an effect, the nitride is 0.010% or more, preferably 0.011% or more, more preferably 0.012% or more. Since the machinability becomes higher as the amount of nitride increases, there is no particular upper limit, but nitride is preferably 0.030% or less, more preferably 0.025% or less, and still more preferably 0.020% or less. Is desirable.

  Such a steel of the present invention is obtained by hot working a steel satisfying the above component composition in a temperature range of 850 to 1250 ° C., and then changing the temperature range from the temperature range to 700 ° C. to 0.05 to 0.5 ° C. / After cooling at an average cooling rate of s, it can be produced by cooling a temperature range from 700 to 500 ° C. at an average cooling rate of 1.0 to 5.0 ° C./s.

  In order to obtain a steel that satisfies the above metal structure and nitride, it is desirable to appropriately control manufacturing conditions such as hot working and subsequent cooling rate.

  That is, steel can be processed under low deformation resistance by setting the hot working temperature in the range of 850 to 1250 ° C. When the temperature is less than 850 ° C., the deformation resistance of the steel is not sufficiently lowered, so that desired processing becomes difficult. Preferably it is 875 degreeC or more, More preferably, it is 900 degreeC or more. From the viewpoint of improving workability by reducing deformation resistance, the upper limit of the heating temperature is not particularly limited. However, if the temperature is too high, dripping occurs at the end of the steel, resulting in poor handleability of the steel or low deformation resistance. The upper limit is set to 1250 ° C. or lower, preferably 1225 ° C. or lower, more preferably 1200 ° C. or lower because excessive processing may occur. In addition, hot processing is processing with heating, and plastic processing, such as hot rolling and hot forging, is exemplified.

  After the hot working, by cooling the range from the hot working temperature to 700 ° C. at an average cooling rate of 0.05 to 0.5 ° C./s or less, the nitride is suppressed while suppressing the generation amount of ferrite. Can be generated. If the cooling rate becomes too fast, the amount of nitride produced decreases, so the average cooling rate is 0.5 ° C./s or less, preferably 0.45 ° C./s or less, more preferably 0.40 ° C./s or less. It is. On the other hand, if the cooling rate is too slow, the amount of ferrite produced increases and work hardening tends to occur, so the average cooling rate is 0.05 ° C./s or higher, preferably 0.1 ° C./s or higher, more preferably. Is 0.15 ° C./s or more.

  Furthermore, by cooling the temperature range from 700 to 500 ° C. at an average cooling rate of 1.0 to 5.0 ° C./s, the total area ratio of pearlite and bainite relative to the metal structure of the steel is 95 area% or more. In addition, the area ratio of bainite relative to the entire structure can be set to 20 to 50 area%, and the area ratio of ferrite to the entire structure can be set to 1 area% or less (note that the remaining structure is mainly pearlite. May contain martensite, retained austenite, etc. that may inevitably be generated). When the average cooling rate is less than 1.0 ° C./s, the bainite is less than 20 area% and the strength is insufficient. On the other hand, when the average cooling rate exceeds 5.0 ° C./s, martensite is easily generated and machinability is lowered. A preferable average cooling rate is 1.5 ° C./s or more, more preferably 2.0 ° C./s or more, preferably 4.5 ° C./s or less, more preferably 4.0 ° C./s or less. Note that the temperature from 500 ° C. to room temperature may be appropriately selected according to the production equipment, production conditions, such as air cooling and air cooling, and is not particularly limited.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  150 kg of steel having the chemical composition shown in Tables 1 to 3 below (the balance being iron and inevitable impurities) was melted in a vacuum induction furnace and cast into an ingot (upper surface: φ245 mm × lower surface: φ210 mm × length: 480 mm). Subsequently, the ingot was heated to 1200 ° C., then hot forged into billets (155 mm square) and then cooled. Thereafter, hot rolling or forging was performed under any of the following conditions ([hot rolling A], [hot forging B]).

[Hot Rolling A] (Tables 1-3: 1A-2Z)
The end of the billet was cut and a dummy billet (155 mm square × length: 9 to 10 m) was welded (two created (A-1, A-2)).

  A-1 (first): After welding, the billet was heated to 1200 ° C., then hot-rolled into a φ45 mm round bar, and then air-cooled to room temperature. The obtained A-1 steel was subjected to hot forging.

  A-2 (second): After welding, the billet was heated to 800 ° C. to 1300 ° C. for 30 minutes, and then hot forged into a round bar of φ80 mm. Then, after cooling to 700 ° C. at an average cooling rate (“cooling rate to 700 ° C. (° C./sec)”) of 0.005 to 5 ° C./s, an average cooling rate from 700 ° C. to 500 ° C. (“700 The cooling rate (° C./s) ”was 0.1 to 10 ° C./s, and then air-cooled to room temperature. Then, it cut | disconnected every 350 mm in length, produced the test piece, and performed continuous cutting.

[Hot forging B] (Table 3: 3A to 3Z)
B-1 (1st): The billet (155 mm square) was heated to 1200 ° C., then hot forged into a φ45 mm round bar, and then air-cooled to room temperature. The obtained B-1 steel was subjected to hot forging.

  B-2 (second): The billet (155 mm square) was heated to 1100 ° C. for 30 minutes, and then hot forged into a round bar of φ80 mm. Then, after cooling to 700 ° C. at an average cooling rate (“cooling rate to 700 ° C. (° C./s)”) 0.3 ° C./s, an average cooling rate from 700 ° C. to 500 ° C. (“700 ° C. or less” Cooling rate (° C./s) ”) was cooled at 1 ° C./s, and then air-cooled to room temperature. Then, it cut | disconnected every 350 mm in length, produced the test piece, and performed continuous cutting.

  Samples A-1 and B-1 prepared as described above were cut every 150 mm in length. Each sample was heated to the temperature shown in Tables 1 to 3 (“heating temperature (° C.)” and held for 30 minutes, and then the round bar was hot forged (until the thickness of the sample reached 12 mm) to form a plate. Molded into. Then, after cooling from the hot forging temperature to 700 ° C. at the average cooling rate shown in Tables 1 to 3 (“cooling rate to 700 ° C. (° C./s)”), from 700 ° C. to 500 ° C. 1 to 3 (“cooling rate of 700 ° C. or lower (° C./s)”) and then cooled to room temperature (cooled) (average cooling different from cooling rate to 700 ° C.) Adopted speed). Intermittent cutting was performed with the obtained test piece.

(Observation of metal structure)
About each said test piece, the metal structure and the area ratio of the metal structure were measured in the procedure shown below.
Each test piece was cut perpendicular to the longitudinal direction, the D / 4 position (D is the plate thickness) was corroded with 3% nital solution, and observed and imaged (photographed) taken with an optical microscope (observation magnification 400 times). did. Of the images taken at 10 arbitrary locations (all 10 images), image analysis was performed on 100 arbitrary locations, and the area ratio of ferrite, pearlite, bainite, and other (martensite, retained austenite, etc.) at each location was determined. The average value was measured. For reference, FIG. 1A (No. 1A) and FIG. 1B (No. 1G-17) are shown. As shown in FIG. 1 (a), a white area in the structure and no shaded area is bainite, and a dark contrast area in which other shaded parts are dispersed and mixed is pearlite. Further, as shown in FIG. 1B, in the dark contrast region, the region where the white portion is mixed in a needle shape is bainite, the black portion is pearlite, and the other white portion is a needle-like region. For example, martensite. The area ratio of each tissue is shown below.

(Measurement of nitride content)
After irradiating the test piece with X-rays to confirm the presence or absence of nitride, the nitride was separated and extracted by the extraction residue method, and the amount of nitride was calculated.

  The total amount of nitrogen compounds in the steel is calculated from the sample cut out from the test piece. For the total amount of nitrogen compounds in steel, ammonia distillation separation indophenol blue absorptiometry is used. A sample is cut out from the test piece, 10% AA electrolyte (nonaqueous solvent electrolyte that does not produce a passive film on the steel surface, specifically 10% acetylacetone, 10% tetramethylammonium chloride, the balance: Constant current electrolysis in methanol). About 0.5 g of the sample is dissolved, and the insoluble residue (nitrogen compound) is filtered through a polycarbonate filter having a hole size of 0.1 μm. The insoluble residue is decomposed by heating in sulfuric acid, potassium sulfate and pure Cu chips and combined with the filtrate. After making this solution alkaline with sodium hydroxide, steam distillation is performed, and the distilled ammonia is absorbed by dilute sulfuric acid. Phenol, sodium hypochlorite and sodium pentacyanonitrosyl iron (III) are added to form a blue complex, and its absorbance is measured using a photometer.

(Machinability evaluation)
(Intermittent cutting)
The test piece was cut into a plate material (test piece) of length: 150 mm × width: 100 mm × thickness: 10 mm. In order to evaluate the machinability of the plate material, a hob cutting test was performed to measure the amount of tool wear when the plate material was cut intermittently. Using a TiAlN coated high-speed hob (no rake face coating) as a cutting tool, intermittent cutting was performed under the following cutting conditions.
Cutting conditions Cutting depth: 1.0mm
Feeding speed: 42mm / s
Cutting speed: 165m / min
Cutting atmosphere: Dry Cutting length: 150mm / cut

  After 50 intermittent cuts (1 cut length: 150 mm), the tool surface was observed with an optical microscope (observation magnification 100 times), the flank wear amount (tool wear amount) was measured, and the average value was calculated. Asked. In the present invention, flank wear after interrupted cutting was evaluated as acceptable (◯) when the flank wear amount was 70 μm or less.

(Continuous cutting ability)
The test piece was subjected to 3000 mm longitudinal turning (continuous cutting) under the following cutting conditions, and then the flank wear amount was measured.
Cutting conditions Cutting tool: P10
Cutting speed: 200 m / min
Cutting depth: 1.5m
Feed amount: 0.25mm
Cutting oil: None

  After continuous cutting for 3000 m, the tool surface was observed with an optical microscope (observation magnification of 100 times), and the flank wear amount (tool wear amount) was measured to obtain an average value. In the present invention, the flank wear after continuous cutting was 70 μm or less, and the chip was excellent in chip separation, and was evaluated as acceptable (◯). In addition, the chip division property evaluates the chip when performing continuous cutting, and even if the chip is not divided by a chip breaker etc. Evaluated as excellent.

(Evaluation of component strength)
A No. 1 fatigue test piece (diameter between gauge points: φ6 mm) was collected from the vicinity of the center of the test piece based on JIS Z2274, and induction hardening (heating temperature: 850 ° C., cooling condition: water cooling) ) To obtain a strength test piece. Using this strength test piece, Vickers hardness and fatigue characteristics were evaluated under the following conditions.

(Vickers hardness)
The strength test piece was cut vertically at the center between the marks and embedded in cold resin so that the cross-section was the measurement surface. After polishing and finishing the cross section of the hardness test piece in a mirror state, it was measured using a Vickers hardness tester.

  Specifically, in order to evaluate the hardness of the entire steel material, the surface layer part (0.05 mm inner side from the outermost surface of the test piece) and D / 4 part (D is the thickness of the test piece) of the above-mentioned mirror-finished test piece are used. Measurements were made. In the measurement, the measurement load was set to 300 g, and the average value was obtained by measuring three times. In this invention, the Vickers hardness of D / 4 part evaluated 250Hv or more as a pass (high intensity | strength) and less than 250Hv as a disqualification (strength insufficient). Further, the surface layer portion Vickers hardness was determined to be 670 Hv or more as acceptable (high strength) and less than 670 Hv as unacceptable (insufficient strength).

(Fatigue properties)
The fatigue characteristics of the above-mentioned strength test pieces were evaluated using a rotary bending tester. Specifically, the frequency (20 Hz) and the load stress were changed between 700 MPa and 100 MPa, the stress (MPa) corresponding to 10 ⁷ times life was obtained, and this value was used as an index of fatigue characteristics. In this example, a fatigue limit stress of 220 MPa or more was determined to be acceptable (high strength), and less than 220 MPa was determined to be unacceptable (insufficient strength).

  From the above results, examples satisfying the requirements of the present invention were excellent in strength and machinability. On the other hand, examples that do not satisfy the requirements of the present invention have the following problems.

  In 1G-1, 1G-6, 1G-7, 1G-10 to 1G-12, 1G-16, and 1G-17 that do not satisfy the manufacturing conditions (hot working temperature, holding temperature, cooling rate) of the present invention, The requirements (area ratio) of the metal structure specified in the invention were not satisfied, and the machinability and / or strength were not satisfied.

  Specifically, in 1G-1 where the hot working temperature is low, the area ratio of ferrite and bainite is out of the specified range, so that the intermittent cutting property and the continuous cutting property are poor, and the strength of the steel material after the heat treatment is also deteriorated.

  In 1G-6 and 1G-7 where the cooling rate to 700 ° C. was slow, the area ratio of ferrite was high, and both the interrupted cutting ability and the continuous cutting ability were poor.

  In 1G-10 where the cooling rate to 700 ° C. was fast, the amount of nitride in the steel material was small and the continuous machinability was poor.

  In 1G-11 and 1G-12, which had a slow cooling rate of 700 ° C. or lower, the ferrite area ratio increased, and the bainite area ratio also deviated from the provisions of the present invention (1G-12). Both were bad and the strength after heat treatment was insufficient.

  In 1G-16 and 1G-17, which had a high cooling rate of 700 ° C. or less, the total area ratio of pearlite and bainite was low, both interrupted and continuous machinability were poor, and fatigue limit strength after heat treatment (1G− 17) was also bad.

  Further, even 2J to 2Z, which does not satisfy the chemical composition of the steel of the present invention, did not satisfy the machinability and / or hardness.

  Specifically, in 2J with a low C content, the bainite area ratio is below the specified range, the ferrite area ratio is also outside the specified range, the intermittent machinability is deteriorated, and the strength after heat treatment is also poor. It was. Moreover, in 2K with much C content, machinability was bad.

  In 2L with a small Si content, the total area ratio of pearlite and bainite deviated from the regulation, and the fatigue characteristics were inferior. Moreover, in 2M with much Si content, machinability was bad.

  In 2N with a low Mn content, the ferrite area ratio was high, and the machinability and fatigue properties after heat treatment were inferior. Moreover, in 2O with much Mn content, machinability was bad.

  With 2P having a large P content, the fatigue characteristics were inferior.

  In 2Q with a small S content, continuous machinability and fatigue limit strength were poor. Moreover, in 2R with a large S content, the fatigue characteristics were inferior.

  In 2S with a small Cr content, the continuous machinability was poor and the fatigue limit strength was also inferior. Moreover, in 2T with much Cr content, machinability was bad.

  With 2U having a low Al content, the intermittent cutting performance was poor. Further, at 2V with a large Al content, the continuous machinability was poor.

  At 2 W with a low B content, the intermittent cutting performance was poor. In 2X with a large B content, the continuous machinability was poor.

  In 2Y with a small N content, the nitride content in the steel material was also small, and the continuous machinability was poor. Further, in 2Z having a large N content, the machinability was not satisfied.

Claims (6)

C: 0.40 to 0.65% (meaning mass%, the same applies to chemical components),
Si: 0.010 to 0.50%,
Mn: 1.0-2.0%,
P: 0.03% or less (excluding 0%),
S: 0.002 to 0.10%,
Cr: 0.010 to 0.3%
Al: 0.06-0.50%,
B: 0.0050 to 0.010%,
N: 0.010 to 0.030%,
And the balance is made of iron and inevitable impurities, and has a nitride of 0.010% or more, and
The steel metal structure has pearlite and bainite, and may further have ferrite, and the total area ratio of pearlite and bainite with respect to the entire structure is 95% by area or more, and ferrite with respect to the entire structure, And each area ratio of bainite is 1 area% or less (including 0 area%) for ferrite, and 20 to 50 area% for bainite. Steel for induction hardening with excellent machinability. As other elements,
The steel for induction hardening according to claim 1, containing Mo: 1.0% or less (not including 0%). As other elements,
Selected from the group consisting of Ti: 0.2% or less (not including 0%), Nb: 0.2% or less (not including 0%), and V: 0.2% or less (not including 0%) The steel for induction hardening according to claim 1 or 2, which contains at least one kind of element. As other elements,
The steel for induction hardening according to any one of claims 1 to 3, wherein Cu: 3% or less (not including 0%) and / or Ni: 3% or less (not including 0%). As other elements,
Ca: 0.005% or less (excluding 0%),
Mg: 0.005% or less (excluding 0%),
It contains at least one selected from the group consisting of Li: 0.001% or less (not including 0%) and REM: 0.0010% or less (not including 0%). The steel for induction hardening as described in any of the above. Steel satisfying the component composition according to any one of claims 1 to 5,
After hot working in the temperature range of 850 to 1250 ° C., the temperature range from the temperature range to 700 ° C. is cooled at an average cooling rate of 0.05 to 0.5 ° C./s, and then from 700 to 500 ° C. A method for producing a steel for induction hardening excellent in machinability, wherein the temperature range is cooled at an average cooling rate of 1.0 to 5.0 ° C./s.