JP2012219335A - Steel for induction hardening excellent in machinability and high-temperature strength and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel for induction hardening with characteristics excellent in machinability.SOLUTION: The steel for induction hardening comprises 0.40-0.65% of C, >0.5% but ≤2% of Si, 0.2-2.% of Mn, ≤0.03% of P (not including 0%), 0.002-0.1% of S, 0.01-0.3% of Cr, 0.06-0.5% of Al, 0.0005-0.01% of B, 0.002-0.02% of N and the balance being Fe and inevitable impurities and has a metallic structure comprising ferrite, pearlite and bainite, wherein ferrite, pearlite and bainite account for ≥95% in total of the entire area of the structure, ferrite and bainite account for 3-10% and 20-40%, respectively, of the entire area, the average aspect ratio of ferrite crystal grains is ≥5, and the distance between ferrite crystal grains is 3-30 μm.

Description

  The present invention relates to steel for producing steel parts by induction hardening after cutting, and particularly to induction hardening steel excellent in machinability and high-temperature strength, 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 the current 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 likely to be oxidized and worn. 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 carburizing treatment, it is necessary to increase the C content in the steel so that the strength is sufficiently improved by martensitic transformation, and the internal hardness is high frequency. Since it does not change before and after the quenching 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 hardness is ensured 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.

  In addition, when steel parts are used in places where the temperature becomes high during use, such as around the engine, there has been a problem that the wear resistance and hardness (Hv) of the steel parts decrease with the passage of the period of use. In other words, steel used for high-temperature locations also needs to be cut, but steel parts manufactured with increased steel machinability are subject to reduced wear resistance and strength when exposed to high temperature conditions for long periods. There was a problem of shortening the service life. In this way, the steel parts used in locations that become hot during use not only have excellent machinability as described above, but also wear resistance and hardness in a high temperature environment (hereinafter referred to as high temperature strength). Although demanded, a steel excellent in machinability and high temperature strength has not been proposed yet.

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

  The present invention has been made paying attention to such circumstances, and its purpose is to cut the shape of the part and to obtain the hardness (Vickers hardness) and fatigue characteristics (rotational bending) required for steel parts after induction hardening. Fatigue properties) and excellent machinability during cutting, as well as excellent high-temperature strength that can ensure hardness and fatigue properties even when used in locations exposed to high temperatures. It is to provide a steel for induction hardening and a method for manufacturing the same.

  The present invention that has solved the above problems is: C: 0.40 to 0.65% (meaning mass%, the same applies to chemical components), Si: more than 0.5 to 2%, Mn: 0.2 -2%, P: 0.03% or less (excluding 0%), S: 0.002-0.1%, Cr: 0.01-0.3%, Al: 0.06-0.5 %, B: 0.0005 to 0.01%, N: 0.002 to 0.02%, the balance is made of iron and unavoidable impurities, and the metal structure of the steel is ferrite, pearlite, and It has bainite, and the total area ratio of ferrite, pearlite, and bainite with respect to the entire structure is 95 area% or more, and each area ratio of ferrite and bainite with respect to the entire structure is 3-10 area% for ferrite, bainite Is 20-40 area%, and ferrite crystal grains A the average aspect ratio of 5 or more, and a high-frequency hardened steel having a gist that the distance between the particles of the ferrite crystal grains is 3 to 30 .mu.m.

  In the present invention, it is also preferable that Mo: 1% or less (not including 0%) is contained as another element, and Ti: 0.2% or less (including 0%) is included as still another element. No), Nb: 0.2% or less (not including 0%), and V: 0.2% or less (not including 0%), and at least one element selected from the group consisting of This 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 A preferred embodiment is one containing at least one element selected from the group consisting of 0.001% or less (excluding 0%).

  In the present invention, the steel satisfying the above component composition is hot worked in a temperature range of 850 to 1250 ° C., and then held in the temperature range for 5 seconds to 60 minutes, and then the temperature from the temperature range to 500 ° C. A method for producing a steel for induction hardening which has the gist of cooling the zone at an average cooling rate of 0.1 to 3 ° C./s is also a preferred embodiment.

  According to the present invention, by defining the composition of steel and appropriately controlling the ratio of the metal microstructure and the dispersion state of ferrite, high temperature strength (including both hardness and fatigue properties) and during processing It is possible to provide a steel for induction hardening that is excellent in both machinability. That is, the steel for induction hardening according to the present invention has good machinability when cut, particularly a tool life when cut intermittently, and steel parts formed by induction hardening after cutting are as follows: Hardness and fatigue characteristics can be ensured for a long time even when used in locations exposed to high temperatures.

FIG. 1A is a metallographic photograph of 1A (Example). FIG. 1B is a metallographic photograph of 1G-18 (comparative example).

  The inventors of the present invention provide a steel for induction hardening that has good machinability and can secure the Vickers hardness and fatigue characteristics at high temperatures required for steel parts over a long period of time by induction hardening after cutting. It has been studied to provide. As a result, the steel composition is appropriately adjusted (especially, the combined addition of Al and B, and the amount of Si added), and the metal structure of the steel is appropriately controlled. Specifically, ferrite occupies the entire metal structure. Appropriately controls the area ratio of ferrite, bainite, and the area ratio of ferrite and bainite in the total metal structure, and the ferrite crystal grain shape (average aspect ratio) and dispersion state (inter-particle distance) within a specific range As a result of the control, it was found that a steel for induction hardening having both machinability and high temperature strength can be provided, and the present invention has been completed.

  In the present invention, being excellent in high-temperature strength means that the surface hardness (Hv) at normal temperature of a steel part after tempering (for example, after being exposed to a high temperature of 300 ° C. or higher for 3 hours) is 600 or higher.

  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.

  The present inventors believe that if the metal structure of steel is a mixed structure of ferrite and pearlite, the machinability when cutting (particularly the tool life when cutting intermittently) can be improved. Based on the investigation. However, if the metal structure of steel is a mixed structure of ferrite and pearlite, the strength required for steel parts may not be ensured. Especially when induction hardening is performed, although the hardness near the surface of the steel is improved, the internal hardness does not change before and after induction hardening, so to obtain the same surface hardness and internal hardness as when carburized, It is necessary to improve the internal hardness of the steel. As for the metal structure and strength of steel, it is generally known that bainite is effective in increasing the strength of steel. Therefore, we thought that the internal hardness and fatigue characteristics could be improved if the steel microstructure was mixed with ferrite and pearlite and mixed with bainite. However, since bainite is a hard phase, it contributes to increasing the strength while reducing the machinability, so that the required machinability cannot be ensured.

  Therefore, the present inventors have attempted to improve the hardness and fatigue characteristics of the surface and the interior of the steel by making the metal structure of the steel a mixed structure of ferrite, pearlite, and bainite. The investigation was repeated from the viewpoint of controlling the shape and form of the medium component and ferrite.

  The chemical composition of steel is known to improve machinability when Al is present in a solid solution state in the steel, but it can also be obtained when Al is present in a solid solution state in the steel having the above metal structure. It turned out that the machinability improvement effect is not sufficient and further improvement is necessary. Therefore, the present inventors examined the machinability in detail. Cutting is the action of cutting the surface by breaking and separating the steel material surface with which the tool blade edge comes into contact with a strong force, and when the metal structure of the steel of the cutting part (the part in contact with the tool edge) is the same, A constant force is always applied to the tool edge. Therefore, if the steel is hard (there are many hard phases), the tool edge is also subject to a strong force, and heat is generated due to friction during cutting, resulting in oxidative wear and deterioration of the tool edge at an early stage, resulting in reduced machinability. I understood. On the other hand, if the steel in the cutting part is softened by increasing the amount of ferrite, such as connecting the beads in a daisy chain as in Patent Document 3, it will be easy to cut, but the tool edge will adhere and wear due to the adhesion. It has been clarified that machinability decreases with time. Therefore, in the present invention, various studies have been made in order to prevent adhesion and oxidation wear while making steel easy to cut. As a result, it was found that machinability can be improved by controlling the shape and distribution of ferrite. Specifically, by dispersing the ferrite on the prior austenite grain boundaries and increasing the aspect ratio of the ferrite grains, the surface and internal hardness of the steel and the fatigue characteristics of the steel are secured while maintaining a high level. We found that sex can be improved dramatically.

  The mechanism that can improve the machinability while ensuring the strength is not limited to the following, but is considered as follows. That is, to improve the machinability while maintaining the strength, it is effective to control the area ratio of ferrite, pearlite, and bainite in the metal structure. As described above, the above-mentioned adhesion occurs. Therefore, if the soft phase ferrite is dispersed and the hard phase bainite is alternately cut at an appropriate interval, the burden on the tool edge due to the hard phase or the soft phase can be reduced. Further, by increasing the aspect ratio of the ferrite, it is considered that a part of the ferrite tip enters into the hard phase and acts as a notch, and the crack of the hard phase is likely to progress and the machinability is improved.

  In order to obtain a dispersed state of ferrite grains exhibiting such an effect, the present inventors need to add B as described later, and to make the aspect ratio and dispersed state of ferrite having the above-described action. Found that it was effective to control the manufacturing conditions.

  However, oxidation wear during intermittent cutting cannot be sufficiently suppressed only by controlling the metal structure as described above. Therefore, as a result of studies by the present inventors, it is effective to add Al in order to suppress oxidative wear without impairing the strength and machinability.

  Furthermore, as a result of examining the improvement in strength when used as a part in a high temperature environment, the strength deteriorates with the passage of time when used in a high temperature environment simply by specifying the metal structure and the like. It has been found that the deterioration of strength in a high temperature environment can be suppressed by containing a specific amount of Si.

  The present invention has been made on the basis of the above knowledge, and is intended to appropriately control the area ratio of ferrite, pearlite, and bainite, the average aspect ratio of ferrite crystal grains, and the distance between ferrite crystal grains within a specific range. It is characterized in that it is required to be controlled, the addition of specific amounts of Al and B as essential chemical components of steel, and the Si content is increased. Hereinafter, the present invention will be specifically described.

  First, the metal structure of steel will be described.

Metal structure: Having ferrite, pearlite, and bainite As described above, bainite is a metal structure that contributes to increasing the hardness of steel and improving fatigue properties. On the other hand, ferrite and pearlite are metal structures that contribute to improving the machinability of steel. Therefore, the strength and machinability can be improved by making the metal structure of steel a mixed structure of ferrite, pearlite, and bainite. However, as described above, the desired strength and machinability cannot be obtained simply by using a metal structure containing ferrite, bainite, etc., so as described in detail below, the area ratio of each structure and the average of ferrite It is necessary to satisfy the aspect ratio, the distance between particles, and the like.

The total area ratio of ferrite, pearlite, and bainite: 95 area% or more with respect to the entire structure As described above, the strength and machinability of the steel of the present invention is a mixed structure of ferrite, pearlite, and bainite. It is expressed by. In order to obtain such an effect, the total area ratio of ferrite, 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. Metal structures other than ferrite, pearlite, and bainite include, for example, martensite and retained austenite that are inevitably produced in production, but machinability deteriorates when the area ratio of these structures increases. Therefore, it may not be included at all. Therefore, the total area ratio of ferrite, pearlite, and bainite with respect to the entire structure is more preferably 100 area%.

Area ratio of ferrite: 3 to 10% by area with respect to the entire structure
Since ferrite is a softer phase than pearlite or bainite, it is a structure that acts as a starting point for fracture / separation preferentially over other structures during cutting, and effectively acts on crack initiation and propagation. In the present invention, since the component strength is increased by increasing the Si content and by solid solution strengthening with Si, in order to obtain such an effect, the area ratio of ferrite is 3% by area or more, preferably It is 3.5 area% or more, more preferably 4 area% or more. On the other hand, although the adhesion due to ferrite is suppressed to some extent by solid solution strengthening with Si, if the area ratio of ferrite occupying the whole structure becomes too high, not only does the strength decrease, but adhesion to the tool edge tends to occur. Machinability may deteriorate. Therefore, the upper limit of the area ratio of ferrite is 10 area% or less, preferably 9 area% or less, more preferably 8 area% or less with respect to the entire structure.

Area ratio of bainite: 20 to 40 area% with respect to the entire structure
Since bainite is a harder phase than ferrite and pearlite, it is a structure that contributes to improving the strength of parts after induction hardening. 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. In addition, since the strength of the parts is increased by solid solution strengthening with Si, it is desirable to reduce the area ratio of bainite in the present invention having a high Si content. Therefore, the upper limit of the area ratio of bainite is 40 area% or less, preferably 37.5 area% or less, more preferably 35 area% or less, with respect to the entire structure.

Average aspect ratio of ferrite crystal grains: 5 or more Since ferrite is a soft phase, it tends to be the starting point of fracture / separation during cutting. In order to exert such effects, the area ratio of ferrite is within the above range. In addition, it is necessary that the ferrite crystal grains have an elongated shape. That is, it is considered that if the ferrite crystal grains are elongated, they act as notches in the hard phase, and the machinability is improved. Such an effect requires the average aspect ratio of the ferrite crystal grains to be 5 or more. The average aspect ratio is preferably 6 or more, more preferably 7 or more. Since the larger the aspect ratio of the ferrite crystal grains, the more effectively the notch is operated, there is no particular upper limit.

Distance between grains of ferrite crystal: 3 to 30 μm
As described above, ferrite effectively acts on crack initiation and propagation during cutting, but if the ferrite crystal grains along the prior austenite grain boundary are continuous, the cutting edge of the tool is agglomerated as the amount of cutting increases. Wear and wear, and machinability deteriorates. Therefore, from the viewpoint of preventing such machinability deterioration, the distance between adjacent ferrite crystals is preferably 3 μm or more, preferably 5 μm or more, more preferably 7 μm or more. In this way, by properly securing the inter-particle distance and keeping the adjacent ferrite particles independent of each other, the hard phase existing between the ferrite crystal grains exerts the action of suppressing adhesion, and cuts the workpiece. Improved. On the other hand, in the present invention where the Si content is increased to enhance the solid solution, if the inter-particle distance of the ferrite crystal grains is too far, the burden on the tool due to the hard phase increases and the machinability deteriorates. There is. Therefore, the upper limit of the distance between the ferrite crystal grains is 30 μm or less, preferably 25 μm or less.

  The steel of the present invention having excellent machinability and strength needs to satisfy not only the metal structure of the steel but also the composition of 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: more than 0.5 to 2%
Si acts as a deoxidizing element, improves the internal quality of steel, and is an element necessary for suppressing deterioration of strength when a steel part is used in a high temperature environment for a long time. 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 more than 0.5%, preferably 0.55% or more, more preferably 0.60% or more. However, when the amount of Si becomes excessive, the steel becomes too hard and the machinability deteriorates. Therefore, Si is 2% or less, preferably 1.8% or less, more preferably 1.6% or less.

Mn: 0.2-2%
Mn is an element necessary for improving the hardenability and improving the strength of the steel, and is 0.2% or more, preferably 0.4% or more, more preferably 0.6% 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% 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 to 0.1%
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 0.1% or less, preferably 0.08% or less, more preferably 0.05% or less.

Cr: 0.01 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 0.01% 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 to 0.5%
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 0.07% or more, more preferably 0.08% or more. However, when Al is excessive, a large amount of AlN precipitates and the workability is lowered. Therefore, Al is 0.5% or less, preferably 0.4% or less, more preferably 0.3% or less.

B: 0.0005 to 0.01%
B is an important element that has the effect of further improving the machinability by improving the hardenability and dispersing ferrite with BN as a core. In order to exert such an effect, B is 0.0005% or more, preferably 0.0010% or more, more preferably 0.0015% or more. However, if B is excessive, the steel becomes too hard and the machinability deteriorates. Therefore, B is 0.01% or less, preferably 0.008% or less, more preferably 0.006% or less.

N: 0.002 to 0.02%
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. In order to exert such an effect, N is 0.002% or more, preferably 0.004% or more, more preferably 0.006% or more. However, when N is excessive, a large amount of AlN precipitates and the workability is lowered. Therefore, N is 0.02% or less, preferably 0.018% or less, more preferably 0.016% 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% 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, so that the content is preferably 1% 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 an action of preventing abnormal growth of crystal grains during hot working and preventing deterioration of the toughness and fatigue characteristics of steel. Such an effect is exhibited by containing 1 or more of these. Such an action increases as the content thereof increases, but Ti, Nb, and V are each preferably contained in an amount of 0.005% or more, more preferably 0.010% or more. 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 0.1% 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 001% 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, more preferably 0.0008% or less, and still more 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.

  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 holding in the temperature range for 5 seconds to 60 minutes, and then from the temperature range to 500 ° C. Can be produced by cooling the temperature range up to 0.1 to 3 ° C./s.

  In order to obtain steel that satisfies the above metal structure, 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 working is a working process accompanied by the above heating, and plastic working such as hot rolling or hot forging is exemplified.

  After the hot working, by holding for a certain period of time at the hot working temperature, fine BN that becomes the core of ferrite precipitation can be precipitated on the austenite grain boundary. Can be dispersed in the inter-particle distance. In order to obtain such an effect, it is desirable that the holding time is 5 seconds or longer, preferably 10 seconds or longer, more preferably 15 seconds or longer. The longer the holding time, the more effective for the growth of BN. However, the longer the holding time, the more N bonded with B becomes bonded with Al, so that the above BN precipitation effect can be sufficiently obtained. Disappear. Accordingly, the holding time is 60 minutes or less, preferably 45 minutes or less, more preferably 30 minutes or less.

  Here, holding refers to a state in which a decrease in the temperature of the object is suppressed by a cover, a heater, or the like in order to avoid a state in which the object temperature naturally decreases such as air cooling. Therefore, even if the temperature decreases during holding, the growth of BN is not inhibited as long as it is less than 0.1 ° C./s.

  After holding for the predetermined time, the range from the holding temperature to 500 ° C. is cooled at a rate of 0.1 to 3 ° C./s, so that the total area ratio of ferrite, pearlite, and bainite to the metal microstructure of the steel is 95. The area ratio of ferrite and bainite relative to the entire structure can be 3 to 10 area% ferrite and 20 to 40 area% bainite, and the aspect ratio of ferrite is within the above range. be able to. When the cooling rate is less than 0.1 ° C./s, bainite is less than 20%, and the strength is insufficient. On the other hand, in the present invention in which the Si content is increased, it is desirable to control the cooling time in order to suppress the amount of bainite produced. When the cooling rate exceeds 3 ° C./s, martensite is likely to be formed and the workpiece is cut. Sex is reduced. Further, ferrite grains cannot be grown sufficiently, and the aspect ratio is outside the above range. A preferable cooling rate is 0.2 ° C./s or more, more preferably 0.3 ° C./s or more, preferably 2.5 ° C./s or less, and more preferably 2 ° C./s or less.

  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 with the chemical composition shown in Tables 1 to 3 below (the balance is iron and inevitable impurities) is melted in a vacuum induction furnace and cast into an ingot of φ245 mm (upper surface) × φ210 mm (lower surface) × 480 mm (length). Then, it was hot forged or rolled under any one of the following conditions ([Hot Rolled A], [Hot Forged B]) to obtain a φ45 mm round bar.

[Hot rolling A]
The ingot was heated to 1200 ° C. and then hot forged to obtain a billet (155 mm square) and then cooled. Further, the end of the billet was cut, and a dummy billet (155 mm square × length: 9 to 10 m) was welded. After welding, the billet was heated to 1200 ° C., hot-rolled into a φ45 mm round bar, and then air-cooled.

[Hot forging B]
The ingot was heated to 1200 ° C. and then hot forged to obtain a billet (155 mm square) and then cooled. Subsequently, after the billet was heated to 1200 ° C., it was hot forged to form a φ45 mm round bar and then air-cooled.

  Each round bar produced as described above was cut every 150 mm to prepare a sample. Each sample was heated and held at a temperature range of 800 to 1300 ° C. for 30 minutes, and then a round bar was hot forged (until the sample width became 12 mm). Then, after hold | maintaining for 0 to 5000 second at this hot forging temperature, it cooled to room temperature with the cooling rate of 0.05-10 degreeC / S from this holding temperature, and produced the test piece. In addition, the heating temperature (° C.) during the hot working of each specific sample, the holding time at the heating temperature (holding time (sec)), the average cooling rate up to 500 ° C. (cooling rate (° C./sec)) Is as described in Tables 1-3.

(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 perpendicularly to the longitudinal direction (or rolling direction), and the D / 4 position (D is the plate thickness) was subjected to Nital corrosion, and observed with an optical microscope (observation magnification 400 times) / image (photograph) I took a picture. Of the images taken at 10 arbitrary locations (all 10 images), image analysis was performed on 100 arbitrary locations, and the area ratio of the ferrite, pearlite, bainite, and other (martensite, etc.) structures at each location was measured. The average value was obtained. For reference, FIG. 1A (1A in the table corresponding to the invention example) and FIG. 1B (1G-18 in the table corresponding to the comparative example) are shown. As shown in FIG. 1A, the white area in the tissue and the area without shading are ferrite, and the dark contrast area where the other shaded portions 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 martensite. is there. The area ratio of each tissue is shown below.

(Ferrite aspect ratio and ferrite interparticle distance)
After cutting the test piece perpendicular to the longitudinal direction, the cut surface was mirror-polished by emery paper, diamond buffing, and electrolytic polishing. The mirror-polished surface of the test piece was observed and photographed with a field emission scanning electron microscope (FE-SEM: observation magnification 1000 times, acceleration voltage 20 kV). Observation was performed at five arbitrary locations, and photographs of each observation location were taken (total of 5 images). The photographed image was analyzed using a crystal orientation analyzer (EBSP), and the aspect ratio of ferrite (BCC in the analysis) and the interparticle distance of ferrite were measured, and the average value was obtained.

(Machinability evaluation)
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:
・ Incision amount: 1.0mm
・ Feeding speed: 0.24mm / min
・ Cutting speed: 165m / min
・ Cutting atmosphere: dry type ・ Cutting length: 150 mm / 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. The results are shown in Tables 7-9. In the present invention, flank wear after interrupted cutting was evaluated as acceptable (◯) when the flank wear amount was 70 μm or less.

(Evaluation of component strength)
A No. 1 fatigue test piece (diameter between gauge points: φ6 mm) conforming to JIS Z2274 was collected from the vicinity of the center position of the test piece, and the fatigue test piece was induction-hardened (heating temperature: 850 ° C., cooling conditions: Water cooling) was performed 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, the measurement was performed at two locations, a surface layer portion (0.05 mm inner side from the outermost surface of the test piece) and D / 4 portion (D is the thickness of the test piece) of the mirror-finished test piece. At the time of measurement, the measurement load was set to 300 g, and measurement was performed three times to obtain an average value. 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). As for the Vickers hardness of the surface layer portion, 670 Hv or more was determined to be acceptable (high strength), and less than 670 Hv was determined to be unacceptable (insufficient strength).

  In addition, the Vickers hardness of the surface layer of the test piece after tempering the steel at 300 ° C. was measured in the same manner (temperature is room temperature), simulating long-term use in a high temperature environment, and passed 600 Hv or higher (high strength) , Less than 600 Hv was regarded as a failure (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, the fatigue limit stress was determined to be 230 MPa or more as acceptable (high strength) and less than 230 MPa as 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.

  1G-1, 1G-6, 1G-7, 1G-8, 1G-12, 1G-13, 1G-18, 1G- which do not satisfy the production conditions of the present invention (hot working temperature, holding temperature, cooling rate) No. 19 did not satisfy the requirements (area ratio, average aspect ratio of ferrite crystal grains, distance between ferrite crystal grains) defined by the present invention, and did not satisfy machinability and / or strength.

  Specifically, in 1G-1 where the hot working temperature is low, the area ratio of ferrite and bainite is out of the specified range, and the aspect ratio of the ferrite crystal grains and the distance between the grains are short, so the hardness and fatigue limit strength inside the steel are insufficient. did.

  Moreover, in 1G-6 where the hot working temperature is high, since the area ratio of the ferrite is out of the specified range and the aspect ratio of the ferrite crystal grains and the distance between the particles are out of the specified range, sufficient machinability cannot be obtained. Moreover, the fatigue limit strength was insufficient.

  In 1G-7 and 1G-8 where the holding time after heating was short, the ferrite area ratio was below the specified range, and the average aspect ratio of the ferrite crystal grains and the distance between the ferrite crystal grains were not satisfied. The machinability was not obtained.

  Further, with 1G-12, which had a long holding time after heating, ferrite did not form, so sufficient machinability could not be obtained.

  In 1G-13 where the cooling rate was slow, the area ratio of ferrite and bainite was outside the specified range, and the aspect ratio of the ferrite crystal grains was also below the range of the present invention, and the hardness and fatigue limit strength inside the steel were insufficient. .

  In 1G-18 and 19 where the cooling rate was high, ferrite was not generated, so that sufficient machinability could not be obtained. In particular, in 1G-19 where bainite was not generated, internal hardness was also unpredictable.

  Even 2K to 2Z, which does not satisfy the chemical composition of the steel of the present invention, did not satisfy the machinability and / or strength characteristics.

  Specifically, at 2K where the C content is small, the area ratio of ferrite and bainite is below the specified range, and the aspect ratio of the ferrite is also outside the specified range, and the strength characteristics of the steel could not be secured. Further, in 2L having a large C content, the total area ratio of ferrite, pearlite, and bainite with respect to the entire structure was low, the steel became too hard and the machinability deteriorated, and the fatigue limit strength was insufficient.

  In 2M with a low Si content, the hardness after high-temperature tempering was inferior. Moreover, in 2N with much Si content, steel became too hard and machinability was bad.

  With 2O having a low Mn content, the ferrite area ratio is high, and the internal hardness and fatigue characteristics are inferior. Further, in 2P having a large Mn content, the total area ratio of ferrite, pearlite, and bainite in the entire structure was low, and machinability was poor.

  In 2Q with a large P content, fatigue characteristics were inferior.

  In 2R with a low S content, the machinability was poor. Moreover, 2S with much S content had inferior fatigue characteristics.

  In 2T with a low Cr content, the area ratio of ferrite was high, and the aspect ratio of ferrite crystal grains was below the specified range, resulting in poor fatigue characteristics. Moreover, in 2U with much Cr content, the bainite area ratio was high and steel became too hard, and machinability was bad.

  At 2V with a low Al content, machinability was poor. Moreover, in 2W with much Al content, the machinability and fatigue characteristics were poor.

  In 2X with a low B content, the ferrite area ratio was high but the bainite area ratio was low, and the average aspect ratio of the ferrite crystal grains was also small, so that the internal hardness and fatigue characteristics were inferior. In 2Y with a high B content, the bainite fraction was high and the distance between ferrite particles was short, so that the steel became too hard and the machinability deteriorated, and the surface hardness and the surface hardness after high-temperature tempering were poor. .

  In 2Z where the N content is off, the ferrite area ratio is high and the average aspect ratio of the ferrite grains is not satisfied, so the machinability and fatigue characteristics are not satisfied.

Claims (6)

C: 0.40 to 0.65% (meaning mass%, the same applies to chemical components),
Si: more than 0.5 to 2%,
Mn: 0.2-2%
P: 0.03% or less (excluding 0%),
S: 0.002 to 0.1%,
Cr: 0.01 to 0.3%
Al: 0.06 to 0.5%,
B: 0.0005 to 0.01%
N: 0.002 to 0.02%
And the balance consists of iron and inevitable impurities,
The steel metal structure has ferrite, pearlite, and bainite, and the total area ratio of ferrite, pearlite, and bainite with respect to the entire structure is 95% by area or more, and each area ratio of ferrite and bainite with respect to the entire structure Is 3 to 10 area% for ferrite, 20 to 40 area% for bainite, the average aspect ratio of ferrite crystal grains is 5 or more, and the distance between grains of ferrite crystal grains is 3 to 30 μm. Induction hardening steel with excellent machinability and high temperature strength. As other elements,
The steel for induction hardening according to claim 1, containing Mo: 1% 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%),
2. At least one element selected from the group consisting of Li: 0.001% or less (not including 0%) and REM: 0.001% or less (not including 0%). Steel for induction hardening according to any one of? 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., hold in the temperature range for 5 seconds to 60 minutes, and then the temperature range from the temperature range to 500 ° C. is 0.1 to 3 ° C./s average cooling A method for producing steel for induction hardening excellent in machinability and high-temperature strength, characterized by cooling at a speed.