JP2011184742A - Steel for machine structure suitable for friction pressure welding, and friction pressure-welded component - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a low carbon steel for a machine structure which is suitable for friction pressure welding, in which component properties such as fatigue strength and impact strength are improved together with cold workability such as cold forging and machinability, and a friction pressure-welded component. <P>SOLUTION: N in the steel of a stock slow carbon steel member is beforehand made to exist as a compound N; further, abundance as solid solution N is regulated, and MnS with a fixed size and elongated shape is increased; thus its cold workability and machinability are improved, and simultaneously, the compound N in the steel in a heat affected zone in the low carbon steel member is decomposed by frictional heat upon friction pressure welding to increase the amount of the solid solution N; thus ferrite or austenite (cementite) in the heat affected zone is solid solution-strengthened by dynamic strain aging owing to the solid solution N; further, MnS is finely parted, and its joining strength after the friction pressure welding is improved. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a steel material for machine structure suitable for applications subjected to friction welding and a friction welding component in which a steel material for machine structure is friction welded to another steel material to form a composite steel.

  For example, steel mechanical structural parts such as gears such as reduction gears, differential gears, and CVT pulleys used in automobile engines, transmissions, differentials, etc., often need to increase the hardness of the surface layer, Surface hardening treatment such as carburizing, nitriding, and carbonitriding is performed on the structural steel material. These mechanical structural parts are also parts that require precise cutting in order to guarantee the shape accuracy of the final part.

  In recent years, these mechanical structural parts used for vehicles such as automobiles have been pursued to be miniaturized as the weight of the vehicle body is reduced by energy saving. And with the increase in the output of engines such as automobiles, the load on these mechanical structural parts is increasing due to the synergistic effect with the downsizing. For this reason, these mechanical structural parts are required to improve various characteristics such as impact characteristics, bending fatigue characteristics, and surface fatigue characteristics, in addition to basic required characteristics such as strength and toughness.

  Usually, a low carbon steel material (case-hardened steel, mixed structure of ferrite and pearlite) excellent in workability is used as the steel material as the material of these mechanical structural parts. This low-carbon steel is usually precision processed into machine structural parts after cold working such as cold forging, if necessary, after processing into bars and wires by hot rolling or hot forging. Cutting and finishing are performed.

  Here, if the strength and toughness of the mechanical structural steel material, which is the raw material of the machine structural component corresponding to the load increase as described above, is increased, cold working such as cold forging and precise cutting work are remarkably performed. It becomes difficult. Therefore, there is a demand for a steel material that combines the above-mentioned high strength and high toughness component characteristics with cold workability and machinability. However, there is a conflict between strength and cold workability and machinability. It is extremely difficult to achieve both strength, cold workability, and machinability with steel materials for machine structural use.

  For this reason, as one of the measures to achieve both the high strength and high toughness of the component characteristics and the cold workability and machinability, the steel material used for the parts that require the component characteristics such as strength and toughness, Steel materials used for parts that require workability and machinability are prepared separately, and both steel materials having different characteristics are joined together to form a composite steel material or a composite steel part. There are ways to achieve both.

  Methods for joining such steel materials or steel members for producing such a composite steel material are roughly classified into a melt joining method and a solid phase joining method. Among these, in the fusion bonding, since the joining portions of the steel materials are in a high temperature state higher than the melting point, joining defects such as coarsening of crystal grains and generation of bubbles are likely to occur at the joining portions. In addition, the heat affected zone becomes large, and there is a problem that cracks are likely to occur at the interface between the base material and the heat affected zone.

  On the other hand, solid phase bonding is a welding method in which the joining surfaces of the steel materials are solid phase surfaces, and can be joined at a temperature below the melting point of the base material without using a filler material. As a typical solid phase bonding method, there is a friction welding method. This friction welding method generates frictional heat on the contact surface (contact surface) while pressurizing and rotating two steel materials (steel members) to each other, thereby joining each steel material (hereinafter referred to as a joint part). This is a method of joining (welding) by heating and softening, and then applying an upset force (pressure contact force) to the joint.

  In such a friction welding method, a portion heated to a semi-molten state is discharged from the joining surface as a burr by the action of the upset force, and the clean surfaces are joined at a temperature below the melting point. For this reason, compared with the said melt-bonding method, it has the characteristics that the coarsening of a crystal grain, the generation | occurrence | production of a bubble, the crack by the interface of a heat affected zone, etc. do not generate | occur | produce easily in a joining part.

  Such a friction welding method itself between steel materials is conventionally known. For example, Patent Document 1 discloses an improved technique for the friction welding method. That is, Patent Document 1 discloses that it is possible to suppress waste of input energy and materials in the friction welding method, and to suppress variations in product dimensional accuracy, bonding strength, and mechanical properties. However, in this patent document 1, there is no specific description regarding the material steel material suitable for the friction welding method.

  On the other hand, when the friction welding method having such characteristics is applied to the joining of steel materials, the strength of the portion (HAZ portion) that is affected by the heat of friction is reduced, though not as much as the fusion welding method. On the contrary, an increase in the strength of the joined portion becomes a problem. This is because the joint portion is rapidly cooled by the surrounding base material after being heated by frictional heat, so that it tends to become a martensite phase and the strength tends to increase. If the strength reduction of the heat affected zone and the strength increase of the joined portion are large, the strength fluctuation due to the friction welded portion of the base material, the heat affected zone, and the joined portion of the composite steel material (composite steel part) Is large and deteriorates the component characteristics such as fatigue strength and impact strength.

  For such a problem, only the strength reduction of the heat affected zone or only the strength increase of the joint portion can be dealt with individually, but the material steel side for friction welding has been improved conventionally. Various techniques have been proposed.

  For example, Patent Document 2 proposes a method for manufacturing a high-strength ERW steel pipe for friction welding, in which a decrease in strength of the heat-affected zone is suppressed. In Patent Document 2, a steel containing C: 0.08 to 0.23%, Si: 0.5% or less, Mn: 1.8% or less, Nb: 0.01 to 0.1%, Mo: 0.05 to 0.60% is subjected to friction after hot rolling. In order to keep the Mo and Nb carbonitrides precipitated during pressure welding in a solid solution state, the coiling temperature of the hot-rolled steel sheet is set to less than 450 ° C. These Nb and Mo in a solid solution state are precipitated as carbonitrides during friction welding, and the softening of the heat affected zone is suppressed by precipitation strengthening.

  However, the precipitation strengthening described above extends not only to the heat-affected zone but also to the joint portion of the steel materials. Since such a joint portion is rapidly cooled by the surrounding base material after being heated by frictional heat, it tends to become a martensite phase and tends to increase in strength originally. If this precipitation strengthening is also added, the strength of the joint portion of such a joint portion increases conversely due to a synergistic effect with the martensite phase.

  Such an increase in the strength of the joint portion significantly accelerates the embrittlement of the joint portion in use and makes it easy to generate cracks in mechanical structural parts with increased loads such as impact, bending fatigue, and surface fatigue. For this reason, the reliability as a machine structural component or steel for machine structures is reduced.

  Patent Document 3 discloses a technique for suppressing an increase in strength of a joint portion due to such friction welding on the high carbon hot rolled steel material side that is a material. In this Patent Document 3, by containing a small amount of solute Nb, coarsening of austenite crystal grains of a high carbon steel material in rapid heating under high pressure of friction welding is prevented, and the hardness of the joined portion is increased. Brittleness is suppressed. In this case, solute Nb precipitates as NbC after friction welding and contributes to prevention of crystal grain coarsening.

  However, the use of such solute Nb is effective in the case of steel materials or steel parts (tempering imparting materials) that are used in the state of friction welding, but carburizing the steel materials or steel parts after friction welding. When the surface hardening treatment is applied to further improve the component strength, the meaning is lost. That is, in such a surface hardening treatment, in order to heat a steel material or a steel part to a high temperature, NbC once precipitated starts to decompose again by this heating. For this reason, partial coarsening of crystal grains is likely to occur, and cooling after the surface hardening treatment results in a martensitic phase with a variation in prior austenite grain size. Such variation in crystal grain size also significantly reduces component characteristics such as fatigue strength and impact strength. For this reason, the reliability as a machine structural component or a steel material for machine structure having increased loads such as impact, bending fatigue, and surface pressure fatigue is lowered.

  By the way, as another technique, in Patent Document 4, C is 0.1% or more of medium and high carbon steel materials, and the hardened layer (oxide) generated at the joining portion (joining interface) by friction welding is discharged as burrs. In order to ensure the bending ductility of the joint portion, the structure of the material steel is controlled. That is, the hardened layer is made of a bainite structure or a mixed structure of bainite and martensite in which the area ratio of the material steel is less than 40% by combining ferrite and pearlite, and softening due to heating during friction welding is delayed. Is easily discharged as burrs, and the bending ductility of the joint is ensured.

  Moreover, in patent document 5, about the medium and high carbon steel materials with C 0.1% or more, the surface of the composite steel material joint part friction-welded is hit | damaged by an ultrasonic vibration terminal, the stress concentration of a junction part is relieve | moderated, Fatigue strength is improved.

  However, these Patent Documents 4 and 5 are all intended for medium and high carbon steel materials (bainite structure or a mixed structure of bainite and martensite), and are low carbon steel materials (ferrite and pearlite) that are materials of ordinary mechanical structural parts. It cannot be applied to the improvement of fatigue strength and impact strength of the mixed structure.

Japanese Patent Laid-Open No. 11-47958 JP-A-4-116123 JP 2002-294404 A JP 2003-183768 A JP 2006-297398 A

  In the conventional metallurgical methods in which Nb and Mo of the material side steel materials of Patent Documents 2 and 3 are previously dissolved, and the softening of the heat affected zone is suppressed by precipitation strengthening at the time of friction welding. There is a limit. Therefore, for low-carbon steel materials, an alternative metallurgical method is used to improve the joint strength after friction welding by suppressing an excessive decrease in strength of the heat-affected zone and an excessive increase in strength at the joint due to friction welding. There is a need for a means to do this.

  Moreover, even if low carbon steel is made into composite steel (composite steel parts) by the friction welding method, it is accompanied by the above-mentioned miniaturization and high output as friction welding parts for engine parts such as automobiles. There is also a demand for further improvements in cold workability and machinability of parts such as cold forging.

  However, the improvement of the joint strength after friction welding and the improvement of cold workability and machinability are difficult technical issues that contradict each other for low carbon steel materials. Therefore, metallurgical methods that improve the cold workability and machinability of such low carbon steel materials for friction welding as well as the joint strength after friction welding have not been proposed so far. .

  The present invention has been made in view of such problems, and has improved cold workability and machinability, and improved part characteristics such as fatigue strength and impact strength, and is a low carbon steel material for machine structures suitable for friction welding. Another object of the present invention is to provide a friction welded part having excellent impact characteristics and bending fatigue characteristics.

In order to achieve the above object, the gist of the steel for machine structure suitable for friction welding according to the present invention is mass%, C: 0.08 to 0.61%, Si: 0.08 to 0.5%, Mn: 0.4 to 1.5%, P: 0.03% or less (excluding 0%), S: 0.005 to 0.1%, Cr: 0.4 to 2%, Al: 0 0.005 to 0.1%, N: not more than 0.02% (but not including 0%), each of which consists of the balance Fe and unavoidable impurities, and the content of compound N in the steel is 0.006 to 0.02% and the amount of solute N is 0.0015% or less (including 0%), and MnS having a maximum length of 2 μm or more is present in steel in an amount of 100 to 4000 per mm ² , The average aspect ratio of these MnS shall be 2 or more.

In order to achieve the above object, the gist of the friction welded part excellent in bending fatigue characteristics of the present invention is mass%, C: 0.08 to 0.61%, Si: 0.08 to 0.5. %, Mn: 0.4 to 1.5%, P: 0.03% or less (excluding 0%), S: 0.005 to 0.1%, Cr: 0.4 to 2%, Al : 0.005 to 0.1%, N: 0.02% or less (excluding 0%), respectively, and the balance is composed of Fe and unavoidable impurities, and the content of compound N in the steel is 0.00. in 006-0.02%, and a dissolved amount of N is 0.0015% or less (except including 0%), the maximum length is more than 2 [mu] m MnS is present 100 to 4000 per 1 mm ² in the steel However, the mechanical structural steel having an average aspect ratio of MnS of 2 or more and other carbon steel or alloy steel are in friction. A friction welded part that is joined by pressure welding to form a composite steel of a desired shape, and further subjected to surface hardening treatment and tempering treatment, wherein the joint in the range of 1 mm width from the joint formed by the friction welding Restricting MnS with an aspect ratio of 2 or less and a maximum length of 1 μm or less to 25 or less (including 0) per mm ² in the steel in the heat-affected zone on the machine structural steel side It is.

  Here, the compound N (compound nitrogen) refers to other elements such as nitride and carbonitride except for the solid solution N dissolved in the steel matrix among all N (total nitrogen) in the steel. Is a generic name for N (nitrogen) present in steel as a compound of (form a compound).

  This inventor examined again the relationship (metallurgical method) with the solid solution of the alloy element of the raw material side low carbon steel materials, and the precipitation strengthening in the friction welding. As a result, changing the form of N in the steel before and after friction welding contradicts the low carbon steel materials, namely, improvement in joint strength after friction welding and improvement in cold workability and machinability. It was found that it is effective for solving technical problems.

  In addition, by changing the existence form of N before and after the friction welding, the shape of MnS existing in the steel can be changed before and after the friction welding, which is a synergistic effect with the N control. The present inventors have found that it is further effective in solving technical problems that contradict the improvement of joint strength after friction welding and the improvement of cold workability and machinability.

  That is, before friction welding, N in the steel of the raw material low-carbon steel is allowed to exist as the compound N, and the abundance as the solute N is regulated. This improves the cold workability and machinability of the low carbon steel material as a raw material and the low carbon steel material as a composite steel by friction welding. At the same time, by adding S to the raw material low-carbon steel (increasing the S content), the machinability is further improved by increasing MnS having a certain size of elongated shape in the steel.

  By preparing the composition and structure of the raw material low-carbon steel in this way, the compound N in the steel of the heat-affected zone in the low-carbon steel after friction welding is decomposed by friction heat and strain during friction welding. Increase the amount of solute N. And by dynamic strain aging by this solid solution N, the ferrite or austenite (cementite) of the heat affected zone is strengthened, and an excessive decrease in strength of the heat affected zone and an excessive increase in strength of the bonded portion are suppressed. , Improve the bonding strength after friction welding.

  At the same time, due to the strengthening of ferrite or austenite (cementite) around MnS accompanying the dynamic strain aging due to the solid solution N, combined with the high strain processing by friction welding, the preexisting elongated size of MnS is obtained. Finely divide into fine pieces. And MnS refined | miniaturized (partitioning and decomposition | disassembly) in this way contributes to the granulation of the austenite particle size of the said heat affected zone, and improves the joint strength after friction welding. In addition, such miniaturization of MnS also has the effect of eliminating the influence (contribution) to the strength degradation of the heat-affected zone of the elongated shape of MnS, which also improves the joint strength after friction welding. Let

  According to the present invention, with respect to low carbon steel material (case-hardened steel) for mechanical structure, the strength of the part (HAZ part) affected by frictional heat at the time of friction welding is reduced, and the strength of the joint part of each steel material Each increase can be suppressed. Moreover, these effects are not impaired even if a subsequent surface hardening treatment such as carburizing, nitriding, carbonitriding, or the like is performed. Therefore, it can be a composite steel member that suppresses variation in strength due to friction welding, and furthermore, by performing surface hardening treatment and subsequent tempering heat treatment, characteristics such as fatigue strength and impact strength, which were disadvantages of friction bonded parts, are reduced. It can be set as the steel part which suppressed. In addition, the characteristics can be satisfied with respect to the parts requiring surface fatigue characteristics, bending fatigue characteristics, and machinability.

Steel composition:
First, the reason for limiting the chemical composition of the steel of the present invention will be described. The chemical component composition of the low carbon steel material (case-hardened steel) for the machine structure of the present invention is the strength and toughness characteristics required for machine structural parts such as the engine parts of automobiles described above, impact characteristics in addition to these, and bending fatigue characteristics. Therefore, it is a precondition for improving the characteristics such as surface fatigue characteristics and for obtaining the structure of the present invention for improving these characteristics.

  For this reason, this invention steel material is the mass%, C: 0.08-0.61%, Si: 0.08-0.5%, Mn: 0.4-1.5%, P: 0.03 % Or less (excluding 0%), S: 0.005 to 0.1%, Cr: 0.4 to 2%, Al: 0.005 to 0.1%, N: 0.02% or less ( However, 0% is not included), and the chemical composition is composed of the remaining Fe and inevitable impurities. In addition, although the unit of the following element content is all the mass%, it may only describe with%.

  Here, in order to improve various properties, the steel material of the present invention is further added as a selective additive element in addition to the above specific chemical component composition, and further, by mass, Ti: 0.2% or less (however, 0 %), Nb: 0.2% or less (excluding 0%), V: 0.2% or less (excluding 0%), B: 0.01% or less (excluding 0%) 1) or less of Mo: 1% or less (excluding 0%), Cu: 1% or less (excluding 0%), Ni: 1% or less (excluding 0%) It may contain seeds or more. In addition to or in place of these, Ca: 0.02% or less (excluding 0%), REM: 0.02% or less (excluding 0%), Li: 0.02 % Or less (excluding 0%), Mg: 0.02% or less (excluding 0%), or one or more of them may be contained.

  Other elements other than these are basically impurities, and the impurity content (allowable amount) level of a normal low carbon steel material (case-hardened steel) for this type of mechanical structure is used.

  Below, content of each main element and its limitation reason (meaning) are demonstrated.

C: 0.08 to 0.61%
C is a basic element for ensuring the necessary strength as a machine structural component. If the C content is too small, the strength required for the machine structural component targeted by the present invention cannot be ensured. However, when C is contained excessively, ductility is deteriorated, the steel material becomes brittle, and impact characteristics deteriorate. Therefore, the C content is in the range of 0.08 to 0.61%, and the lower limit is preferably 0.10%, more preferably 0.13%. The upper limit is preferably 0.58%, more preferably 0.53%.

Si: 0.08 to 0.5%
Si contributes to the deoxidizing action of steel during melting. Moreover, it has the effect | action which raises base material intensity | strength by solid solution strengthening. When there is too little Si content, deoxidation will become inadequate and it will become easy to generate | occur | produce a gas defect at the time of melting. Further, the strength required for the machine structural component targeted by the present invention cannot be ensured. However, when Si is contained excessively, an increase in deformation resistance and a decrease in deformability are caused. This tendency begins to be noticeable when the Si content exceeds 0.5%. Therefore, the Si content is in the range of 0.08 to 0.5%, and the lower limit is preferably 0.10%, more preferably 0.15%. The upper limit is preferably 0.5%, more preferably 0.4%.

Mn: 0.4 to 1.5%
Mn is effective as a deoxidizing and desulfurizing element for steel during melting, and has an effect of suppressing deterioration of workability during hot working on steel. Furthermore, it is effective to improve the deformability of the steel material by combining with S. If the Mn content is too small, these effects cannot be obtained, the deformability is deteriorated, and cracking is likely to occur. On the other hand, when Mn is contained excessively, an increase in deformation resistance and a decrease in deformability due to solid solution strengthening are brought about. Moreover, segregation to the grain boundary of P is promoted, and the fall of grain boundary strength and the fall of fatigue strength are caused. Therefore, the Mn content is in the range of 0.4 to 1.5%, and the lower limit is preferably 0.45%, more preferably 0.5%. The upper limit is preferably 1.2%, more preferably 1.0%.

P: 0.03% or less (excluding 0%)
P is an element inevitably mixed in and contained as an impurity, segregates at the ferrite grain boundary, and deteriorates deformability. Further, P strengthens the solid solution of ferrite and increases the deformation resistance. Therefore, it is desirable to reduce P as much as possible from the viewpoint of deformability, but extreme reduction leads to an increase in steelmaking cost. Therefore, the lower the P content is, the better the content is 0.03% or less. However, since it is difficult to make it 0%, it is defined as 0.03% or less (but not including 0%). The upper limit is preferably 0.025%, more preferably 0.02%.

S: 0.005-0.1%
S has an effect of improving machinability, and if the S content is small, the machinability is deteriorated. However, when S is combined with Fe, it precipitates in the form of a film on the grain boundary as FeS, so that the deformability is deteriorated. For this reason, S needs to be combined with Mn in its entirety, and be deposited harmlessly as MnS. However, as the amount of MnS deposited increases, the deformability also deteriorates. Accordingly, the S content is in the range of 0.005 to 0.1% considering the balance between deformability and machinability, and the lower limit is preferably 0.007%, more preferably 0.01%. The upper limit is preferably 0.08%, more preferably 0.06%.

Cr: 0.4-2%
Cr enhances the hardenability of steel materials, and gives the hardened layer depth by surface hardening treatment such as carburizing, nitriding, carbonitriding, and the necessary base material hardness, thereby providing static strength as a mechanical structural component such as gears. It is an important element in ensuring fatigue strength. If the Cr content is too small, such effects cannot be exhibited. On the other hand, even if the Cr content is excessive, segregation as carbides occurs in the prior austenite grain boundaries, which causes a decrease in fatigue strength and impact strength. Accordingly, the Cr content is in the range of 0.4 to 2%, the lower limit is preferably 0.5%, more preferably 0.6%, and the upper limit is preferably 1.8%, more preferably 1.%. 5%.

Al: 0.005 to 0.1%
Al is effective as a deoxidizing element for steel during melting. If the Al content is too small, deoxidation during melting becomes insufficient and gas defects are likely to occur, so that cracking is likely to occur. On the other hand, even if the Al content is excessive, non-metallic inclusions such as aluminum oxide-based oxides are generated, and cold workability and machinability are deteriorated. Therefore, the Al content is in the range of 0.005 to 0.1%, the lower limit is preferably 0.008%, more preferably 0.01%, and the upper limit is preferably 0.08%, more preferably 0.06%.

N: 0.02% or less (excluding 0%)
N is an element necessary for causing dynamic strain aging during friction welding and for decomposing MnS. However, when it exists in a solid solution state, it causes deterioration of hot ductility, cold workability and machinability due to dynamic strain aging, so it is necessary to preliminarily bond with Al and precipitate as AlN. is there. Accordingly, the N content is 0.02% or less (excluding 0%), but if N is 0.006% or less, cold workability and machinability can be improved, but MnS is used. The lower limit is preferably 0.006% or more, more preferably 0.0065% or more, and even more preferably 0.0070% or more, since there is a possibility that the strength deterioration of the joint and heat-affected zone may not be suppressed. . The upper limit is preferably 0.018%, more preferably 0.015% (both do not include 0%).

Ti: 0.2% or less (excluding 0%)
Ti forms carbides and nitrides, particularly fixes N, prevents deterioration of deformability due to solute N, and contributes to austenite grain refinement and grain sizing. Moreover, in this invention, it is necessary to contain Ti which can remain solid solution Ti. This solute Ti forms TiC during friction welding as described above. This TiC contributes to precipitation strengthening while making austenite grains finer and sized. Therefore, it is possible to suppress a decrease in strength in the heat affected zone after the friction welding.

  If the Ti content is too small, a sufficient amount of solute Ti cannot be applied during friction welding. On the other hand, when the Ti content is too large, a large amount of TiC is generated, and the strength is decreased. Therefore, the Ti content is in the range of 0.2% or less (excluding 0%), and the lower limit is preferably 0.001%, more preferably 0.01%, still more preferably 0.015%, The upper limit is preferably 0.15%, more preferably 0.1%.

One or more of Nb, V, Mo, Cu, and Ni Nb, V, Mo, Cu, and Ni are all described as equivalent elements in Patent Document 5 without any loss of toughness. It is effective for improving the strength of steel as a raw material and composite steel after friction welding.

  Nb: 0.2% or less (excluding 0%), V: 0.2% or less (excluding 0%): Nb and V both form carbides and heat after friction welding It is possible to suppress the strength reduction in the affected part and substantially improve the strength. Therefore, Nb: 0.2% or less (excluding 0%) and V: 0.2% or less (excluding 0%) are added as necessary.

  The lower limit of the Nb content when selectively added is preferably 0.01% or more, and more preferably 0.015% or more. On the other hand, if the Nb content is too large, a large amount of NbC is generated, and the strength is reduced. Therefore, the upper limit of the Nb content is preferably 0.15% or less, and more preferably 0.1% or less.

  The lower limit of the V content when selectively added is preferably 0.01% or more, and more preferably 0.015% or more. On the other hand, if the V content is too large, a large amount of VC is generated, and the strength is reduced. Therefore, the upper limit of the V content is preferably 0.15% or less, and more preferably 0.1% or less.

B: 0.01% or less (excluding 0%)
In addition to improving the hardenability of the steel material, B has the effect of increasing the impact strength by strengthening the grain boundaries. If the B content is insufficient, these effects cannot be obtained. On the other hand, if the B content is excessive, the grain boundary strength starts to decrease, so that the cold and hot workability deteriorates. Therefore, the B content is in the range of 0.01% or less (excluding 0%), the lower limit is preferably 0.0005%, more preferably 0.008%, and the upper limit is preferably 0.008. %, More preferably 0.005%.

Mo: 1% or less (excluding 0%)
Mo is an element effective for securing the hardenability of the steel material, suppressing the formation of an incompletely quenched structure, and improving the strength. Therefore, if necessary, Mo: 1% or less (but not including 0%) is added. On the other hand, if the Mo content is excessive, the hardness of the base material becomes unnecessarily hard and the toughness and impact properties deteriorate, so only 1% or less, preferably 0.8% or less, more preferably 0. Add less than 5%. In addition, in order to exhibit the said effect by Mo effectively, addition of 0.06% or more is preferable, More preferably, 0.08% or more is added.

Cu, Ni
Both Cu and Ni are effective in strengthening the solid solution of the steel material and improving the strength of the base material and the joint portion. Therefore, Cu: 1% or less (excluding 0%) and Ni: 1% or less (excluding 0%) are added as necessary. On the other hand, if the Cu and Ni contents are excessive, the hot ductility deteriorates, so each is limited to 1% or less, preferably 0.8% or less, more preferably 0.6% or less. In order to effectively exhibit the above-described effects due to Cu and Ni, addition of 0.2% or more is preferable, and more preferably 0.3% or more is added.

One or more of Ca, REM, Li, and Mg Ca, REM, Li, and Mg are commonly used to spheroidize sulfide compound inclusions such as MnS to improve the deformability of the steel material, It is an element that contributes to improving workability and machinability. Therefore, as necessary, Ca: 0.02% or less (excluding 0%), REM: 0.02% or less (excluding 0%), Li: 0.005% or less (providing 0%) 1) or 2 or more of Mg: 0.005% or less (excluding 0%).

  In order to effectively exhibit the above effects, Ca and REM are preferably added in an amount of 0.0005% or more, more preferably 0.001% or more, and still more preferably 0.0015% or more. Similarly, Li and Mg are preferably added in an amount of 0.0001% or more, more preferably 0.0002% or more, and still more preferably 0.0003% or more. On the other hand, even if these are added excessively, the effect is saturated, and an effect commensurate with the amount added cannot be expected, which is economically disadvantageous. Therefore, Ca and REM are each preferably added in an amount of 0.01% or less, more preferably 0.005% or less. Similarly, Li and Mg are each preferably added in an amount of 0.0025% or less, more preferably 0.001% or less.

In the present invention, there is a contradiction in that the presence form of N in the steel is changed before and after the friction welding to improve the joint strength after the friction welding and to improve the cold workability and machinability. Solve technical issues. That is, before friction welding, most of N in the steel of the raw material low carbon steel is present as the compound N, and the abundance as the solid solution N is regulated. This improves the cold workability and machinability of the low carbon steel material as a raw material and the low carbon steel material as a composite steel by friction welding.

  By preparing the raw material low carbon steel material in advance in this way, the compound N in the steel of the heat affected zone in the low carbon steel material after friction welding is decomposed by the friction heat at the time of friction welding, and the amount of solid solution N is reduced. Increase the dose. And, by dynamic strain aging with this solid solution N, ferrite or austenite (cementite) of the heat affected zone is solid solution strengthened, and excessive strength decrease of the heat affected zone and excessive strength increase of the joined portion are suppressed. Thus, the bonding strength after friction welding is improved.

  Also, by changing the form of N in this way before and after friction welding, the shape of MnS present in the steel can be changed before and after friction welding, with a synergistic effect with N control, It is possible to further improve the joint strength after friction welding and improve the cold workability and machinability.

Compound N
In the present invention, as described above, the amount of compound N (compound nitrogen) in the low carbon steel material before friction welding is increased. If the amount of compound N in the raw low carbon steel material is too small, the amount of solid solution N generated by decomposition of compound N in the steel in the heat-affected zone in the low carbon steel material after friction welding is also reduced. As a result, due to dynamic strain aging with solid solution N, ferrite or austenite (cementite) in the heat affected zone is solid solution strengthened, and excessive strength decrease in the heat affected zone and excessive increase in strength in the bonded portion are suppressed. The effect to do is also weakened. As a result, it becomes impossible to improve the bonding strength after friction welding.

  Also, if the amount of compound N in the low-carbon steel material is too small, ferrite or austenite (cementite) around MnS accompanying dynamic strain aging due to solid solution N as the amount of solid solution N decreases after friction welding. The solid solution strengthening is also weakened. For this reason, the effect of finely dividing MnS having a certain size and having an elongated shape to be finely divided is weakened. As a result, the effect of eliminating the influence (contribution) of the refined MnS on the austenite grain size adjusting effect of the heat affected zone and the strength deterioration of the heat affected zone is weakened.

  However, if the amount of compound N is too large, the bonding strength after friction welding is lowered. For this reason, content of the compound N in steel shall be 0.006 to 0.02% of range.

  By preparing the raw material low carbon steel material in advance in this way, the compound N in the steel of the heat affected zone in the low carbon steel material after friction welding is decomposed by the friction heat at the time of friction welding, and the amount of solid solution N is reduced. Increase the dose. And by the dynamic strain aging by this solid solution N, the ferrite or austenite (cementite) of the heat affected zone is solid solution strengthened, and the strength reduction of the heat affected zone and the excessive increase of the strength of the joined portion are suppressed. , Improve the bonding strength after friction welding.

  As described above, the compound N (compound nitrogen) is a compound such as a nitride or carbonitride that excludes solid solution N out of all N (total nitrogen) in the steel and exists in the steel ( Nitrogen) and can be directly measured by the ammonia distillation separation indophenol blue absorptiometry described later.

Measurement of the amount of Compound N The amount of Compound N is measured by ammonia distillation separation indophenol blue absorptiometry. That is, about 0.5 sample cut from a steel specimen was dissolved in 10% AA electrolyte by constant current electrolysis, and the resulting insoluble residue (nitride compound) was polycarbonate having a hole size of 0.1 μm. Filter with a made filter. The obtained insoluble residue is decomposed by heating in a chip made of sulfuric acid, potassium sulfate and pure copper, and the decomposition product is combined with the filtrate. After making this solution alkaline with sodium hydroxide, steam distillation is performed, and the distilled ammonia is absorbed in dilute sulfuric acid. Further, phenol, sodium hypochlorite and sodium pentacyanonitrosyl iron (III) are added to form a blue complex, and the absorbance is measured using an absorptiometer to determine the amount of the compound. Incidentally, the 10% AA electrolyte solution is a non-aqueous solvent electrolyte solution consisting of 10% acetone, 10% tetramethylammonium chloride, and the remaining methanol, and does not generate a passive film on the steel surface.

Solid solution N
With respect to the compound N, in the present invention, by sufficiently reducing the amount of solute N in the steel, the cold workability of the low carbon steel material as a raw material or the low carbon steel material as a composite steel by friction welding, Improves machinability. For this reason, the amount of solute N in the steel is regulated to 0.0015% or less (including 0%). When the amount of solute N in the steel exceeds 0.0015%, the cold workability and machinability of the low carbon steel material cannot be improved, and the joint strength cannot be improved.

  Thus, even if the solute N amount of the low carbon steel material is reduced in advance, in the heat-affected zone in the low carbon steel material after friction welding, the compound N in the steel can be decomposed to increase the solute N amount. It is possible to improve the bonding strength after friction welding, which is a great feature of the present invention.

  The content of the solid solution N can be obtained indirectly by subtracting the total amount of compound N obtained by the above method from the total amount of N obtained by the following method in accordance with JISG1228.

  The total amount of N is measured by inert gas melting method-thermal conductivity method. A sample cut from a steel specimen is put in a crucible and melted in an inert gas stream to extract N, and this extract Is transferred to the thermal conductivity cell and the change in thermal conductivity is measured. Then, the total N amount is determined from the relationship between the thermal conductivity determined in advance and the total N amount.

Low carbon steel MnS
In the present invention, in order to improve the machinability of the low carbon steel material, elongated MnS having a certain large size is present. That is, 100 to 4000 MnS having a maximum length of 2 μm or more per 1 mm ² are present in the steel of the low carbon steel material, and the average aspect ratio of these MnS is set to 2 or more. Here, the aspect ratio refers to the maximum length (longest axis or longest side axis length) and minimum length (shortest or shortest side axis length) of amorphous MnS particles. Ratio, maximum length / minimum length. As the aspect ratio increases, the MnS has an elongated shape (flat or elongated) rather than an equiaxed aspect ratio of 1.

  In order to improve the machinability of the low carbon steel material, the shape and content of MnS are important. In order to secure the strength (toughness) of the joint and the heat-affected zone, it is effective to reduce the amount of MnS. However, if the amount of MnS is reduced, sufficient machinability cannot be obtained.

In this regard, S is added to a low carbon steel material for mechanical structures, and the aspect ratio, which is the ratio of the maximum length to the minimum length in terms of shape, is greater than the certain size (length). If the MnS having the above shape is increased, the machinability is improved without reducing the strength (toughness) of the joint and the heat affected zone. An increase in stretched MnS with a certain size (length) or more and a large aspect ratio is effective in improving machinability because it reduces cutting resistance during cutting and improves chip breaking. is there. During cutting, the number of MnS with a maximum length (longest side length) of 2 μm or more is 100 or more per mm ² , and the average aspect ratio of MnS with a maximum length of 2 μm or more is 2 or more. It becomes possible to easily sever the chips.

  If the aspect ratio is less than 2 and the maximum length (longest axis or longest side axis length) is less than 2 μm, the increase in MnS makes it difficult for the chips to be cut and the cutting efficiency deteriorates. That is, the increase in MnS having a small aspect ratio is not effective in improving the machinability but rather lowers the machinability. Accordingly, in the steel of the low carbon steel material, MnS that is increased in advance prior to the friction welding is the maximum length in the amorphous MnS particles that are observed by a measurement method using an optical microscope with a magnification of 400 times described later. The MnS is a stretched (flat or elongated) MnS having an average aspect ratio of 2 or more.

  It should be noted that the machinability of these MnS shapes is improved as much as they are extended, but there is naturally a production limit of the maximum length depending on the production conditions, so the upper limit of the maximum length is preferably 100 μm or less, and the average aspect ratio The upper limit of the ratio is preferably 10 or less.

Further, even if MnS is a certain size (length) or more and has a large aspect ratio, if it is too much, MnS is discharged as burrs during friction welding when steel is friction welded. In the process, the light is deflected in the direction perpendicular to the pressure contact direction. Such MnS significantly deteriorates the toughness at the joint or heat-affected zone. Therefore, in the present invention, in order to ensure the strength (toughness) of the joint and the heat-affected zone, and to prevent the deterioration of the toughness of the steel material and cracking during the rolling of the steel material, the elongated shape having an aspect ratio of 2 or more. And the upper limit of the amount (number) to be present in the steel of MnS having a maximum length of 2 μm or more. That is, the upper limit of the amount (number) of MnS present in the steel is 4000 pieces / mm ² .

MnS for friction welding parts
In this way, the low-carbon steel material (before friction welding) causes MnS having an elongated shape to exist, but in the friction welding parts, MnS that affects (contributes) to the strength deterioration of the heat-affected zone is restricted. Is also a feature of the present invention.

Specifically, the MnS having an aspect ratio of 2 or less and a maximum length of 1 μm or less in the steel of the heat affected zone on the low carbon steel material side in the range of 1 mm width from the joint formed by the friction welding. Is regulated to 25 or less (including 0) per 1 mm ² . However, with such a fine MnS, there is of course a measurement limit in the measurement with a 400 × optical microscope, which will be described later, so the lower limit of the maximum length is measured with this optical microscope, and the number is measured in length. The possible limit length. Moreover, if it is fine MnS which cannot be measured with this optical microscope, it does not influence (contribute) to the strength deterioration of the heat affected zone.

  And as described above, the MnS regulation of such a heat-affected zone includes compound N in advance in a raw material (before friction welding), and decomposes this compound N by frictional heat during friction welding. Thus, it is possible to increase the amount of solute N in the heat-affected zone in the low carbon steel material after friction welding. The MnS having a maximum length of 1 μm or less is preliminarily present in combination with strong strain processing by friction welding due to the strengthening of ferrite or austenite (cementite) around MnS accompanying dynamic strain aging by the solid solution N. An elongated shape of MnS having a certain size is finely divided.

  The MnS thus divided and decomposed contributes to the austenite grain size regulation of the heat-affected zone as described above. However, if it is too much, the bonding strength after friction welding is lowered. For this reason, it regulates according to the above-mentioned regulation.

  Of course, the width of the heat-affected zone varies depending on the friction welding conditions. However, according to the normal friction welding conditions, the width of 1 mm or more from the joint of the friction welding is usually always sufficient. . Therefore, also from the point of reproducibility of measurement, it was defined as a heat affected zone on the low carbon steel material side in a range of 1 mm width from the joint portion of the friction welding.

Relationship between N and MnS Presence Form and Properties Here, the relationship between N and MnS existence form and properties will be described again. As a result of the inventors performing a friction welding experiment using steel materials with various components adjusted, a steel material having a high solute N amount in the vicinity of the joint after friction welding (in the range of 1 mm width from the joint) is It was revealed that the strength (toughness) of the heat affected zone and the heat affected zone is sufficiently secured.

  In the structure, no elongated MnS as in the initial stage was observed in the vicinity of the joint, and the crystal grains were also sized. This could be attributed to the fact that solute N contributes to the decomposition of MnS during friction welding. However, such a steel material with a high solute N content has deteriorated cold workability and machinability, and achieves both the cold workability and machinability and the strength of the joint and the heat affected zone. It was difficult.

  Based on the above experimental results, we learned that it is effective to change the form of N before and after friction welding, and adjusted the steel composition and structure before friction welding to be excellent in cold workability and machinability. And of course, by optimizing the friction welding conditions as usual, the amount of solute N was increased during the friction welding, and the strength of the joint and the heat affected zone could be improved.

  That is, friction welding is performed under conditions where heat is sufficiently input to the joint and heat-affected zone of the steel material during friction welding to decompose compound N (N compound) and increase the amount of solid solution N. It was also found that the dynamic strain aging caused by the solute N strengthens the ferrite or austenite (cementite) around the MnS, and the elongated MnS tends to be divided along with the high strain processing by friction welding. MnS sufficiently decomposed in this way contributes to the austenite grain size regulation and has the effect of not affecting (contributing) to the strength deterioration.

  As described above, by performing carburization, tempering heat treatment, etc., on a member that has suppressed toughness degradation by friction welding, it is possible to obtain a component that suppresses a decrease in bending characteristics and impact characteristics, which was a drawback of friction bonded parts. . Moreover, regarding the surface fatigue characteristics, cold workability and machinability as case hardening steel, the respective characteristics can be satisfied.

The structure of the steel material The structure of the steel material of the present invention is composed of a mixed phase of ferrite grains and pearlite grains in order to obtain a structure suitable for friction welding and to satisfy the characteristics as a friction bonded part. Incidentally, the structure of the joint by friction welding is composed of a martensite phase that is partially bainite by rapid heating and rapid cooling. Since such bainite has a lower hardness than the martensite, it can suppress an increase in strength of the joint and can improve impact characteristics and fatigue characteristics.

Manufacturing method of steel material In addition, the low-carbon steel material itself for machine structure can be manufactured in the manufacturing process of the said steel material for machine structures for motor vehicle parts including the mixed phase structure of a ferrite grain and a pearlite grain. That is, a cast ingot (ingot) is hot-forged into a billet (steel piece) and then processed into a steel material such as a wire rod or rod (round bar, square bar) by hot rolling or hot forging.

However, the steel material of the present invention is a steel material that has been hot-rolled or hot-forged (as it is hot-worked), or a steel material that has been formed into a part shape by cold forging. It may be a machine-structured part (steel material made into a machine-structured part) that has been finished.

  Here, in order to secure the content of the compound N in the steel defined in the present invention and control the amount of solid solution N, the number of elongated MnS and the aspect ratio, the components are adjusted appropriately, and The heating temperature before hot rolling of the billet and the hot rolling temperature to the wire or bar are controlled together. That is, the heating temperature of the billet is in the range of 1000 to 1250 ° C., and the hot rolling temperature from the start of rolling the billet to the end of rolling to the wire or rod is in the range of 800 to 1000 ° C.

  By appropriately controlling the heating temperature and hot rolling temperature of the billet, it becomes easy to become elongated MnS, and the number and aspect ratio of elongated MnS can be ensured.

  However, in the high-temperature heating and high-temperature rolling as described above, since N is present in a solid solution state, there arises a problem that the machinability is deteriorated after cooling. Therefore, by appropriately controlling the cooling rate, solid solution N is combined with Al during cooling and precipitated as AlN, so that solid solution N can be fixed and reduced. As a cooling rate therefor, it is preferable that the cooling rate to room temperature is 2.0 ° C./second or less after completion of the hot rolling or hot forging. By cooling at this cooling rate, solid solution N is combined with Al during cooling, and precipitated as AlN, so that solid solution N can be reduced. The steel material structure can also be a mixed phase of ferrite grains and pearlite grains more suitable for cutting.

If this cooling rate becomes too fast, sufficient time for AlN to precipitate cannot be secured, and a large amount of solid solution N remains after cooling, so that cold workability and machinability deteriorate. . On the other hand, since the strength of the steel material increases excessively and the machinability decreases due to an increase in the generation ratio of hard phases such as bainite and martensite, the temperature is preferably 2.0 ° C./second or less. On the other hand, the slower the cooling rate, the better because AlN is likely to be generated. However, considering the actual operation, 0.001 ° C./s or more is preferable.

Friction welding Friction welding itself may be a conventional method and may be carried out in a known condition range. However, as described above, compound N is decomposed during friction welding according to the composition of the low-carbon steel material or the counterpart steel material to be composite steel. Therefore, it is preferable that the frictional heat (the amount of frictional heat) that can increase the solute N amount is obtained. In this regard, depending on the composition of the low carbon steel material, friction pressure (MPa), upset pressure (pressure applied from both ends of the round bar to the joint, MPa), friction time (sec), upset time (joint The optimum value of main conditions such as pressurization time to the part, sec), and rotation speed (rpm) is selected from a known condition range.

Composite steel material by friction welding The composite steel material by friction welding targeted by the present invention is a low carbon steel material according to the present invention, depending on the intended mechanical structure component, as long as friction welding can be performed by a commercially available friction welding machine. On the other hand, various steel types can be selected. Moreover, the low carbon steel shape and composite steel shape of this invention can also select various shapes according to the said machine structural component made into the objective. For example, the low carbon steel materials of the present invention may be friction welded together, and the mating material may be a carbon steel for mechanical structures such as S45C or SCr420H, alloy steel, V-added steel, B-added steel, etc. These may be selected and combined based on various characteristics. In addition, the shape of the steel materials to be friction welded may be different or the same or similar. Of course, a combination of bars, a head (round material, square material, umbrella material, ring material) Etc.) and a bar material to be used as a shaft.

  These composite materials by friction welding are mainly subjected to surface hardening treatment such as carburizing, nitriding, carbonitriding on the machine structural steel side of the present invention, and then tempering the entire composite material or only the mechanical structural steel side of the present invention. Processed into machine structural parts. In addition, according to the use as a machine structural component, suitable surface treatments, such as a well-known antirust process and antirust coating, may be given.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated more concretely, this invention is not limitedly interpreted by this Example.

  After melting low carbon steel having various composition shown in Tables 1 to 4, hot rolling or hot forging under the conditions shown in Tables 5 to 8, compound N in the steel, amount of solute N, and A round bar (steel material) having a diameter of 80 mm was manufactured in common with the aspect ratio and the maximum length of MnS. In Tables 1 to 4, when each element is not detected, it is blank, and the downward arrow indicates the same content as the upper column.

  Then, by simulating machine structural parts, these low carbon steel materials were friction welded to each other with these low carbon steel materials and the counterpart material as steel materials of S45C to obtain composite steel materials. And as shown to Tables 9-12, the impact characteristic and bending fatigue characteristic of these composite steel materials were evaluated, respectively. Here, in each table, the same steel type No. shows the same example, each example in Table 1, Table 5, and Table 9 corresponds to each example, and each example in Table 2, Table 6, and Table 10 respectively. Corresponding, each example of Table 3, Table 7, and Table 11 corresponds, and each example of Table 4, Table 8, and Table 12 corresponds respectively.

Production conditions for low carbon steel:
Specifically, in order to obtain the steel structure of the present invention, the following manufacturing process was performed.
Melting / casting: 150 kg of the test steel was melted in a vacuum induction furnace and cast into an ingot having an upper surface: φ245 mm × lower surface: φ210 mm × length: 480 mm.
Billet forging: The ingot was heated to 1200 ° C., hot forged into billets (155 mm square), and cooled.

Here, among the tables, the low carbon steel materials of steel types 1A to 2Z in Tables 5, 6, and 7 (Tables 1, 2, and 3) were manufactured as round bars by hot rolling under the following manufacturing conditions.
Cutting and welding: The end of this forged billet was cut and a dummy billet (155 mm square × 9 to 10 m long) was welded.
Hot rolling: The billet after the dummy billet welding is heated in the range of 950 to 1250 ° C., then hot-rolled to a Φ80 mm round bar in the range of 750 to 1050 ° C., and after the rolling is completed, 0.5 to 2.0 Cooling was performed at a cooling rate range of ° C / s.
Among these, the low carbon steel materials of 1C-5, 1C-6, 1C-9, and 1D-5, which are comparative examples, show billet heating temperature, hot rolling temperature, or cooling rate after rolling in Table 5. As shown, it was manufactured out of the above preferred conditions.

Moreover, the low carbon steel materials of steel types 3A to 3M in Table 8 (Table 4) in the table were manufactured as round bars by hot forging under the following manufacturing conditions.
Hot forging: The forged billet was heated in the range of 1200 ° C., then hot forged into a round bar of Φ80 mm at 900 ° C., and cooled at a cooling rate range of 1.0 ° C./s after completion of forging.

Steel structure As a result of observation of the structure, each round bar (low carbon steel) of φ80 confirms that both the invention example and the comparative example other than 1D-5 are mixed phases in which only two phases of ferrite and pearlite are mixed. did. On the other hand, 1D-5, which has a faster cooling rate after rolling than the preferred conditions, partially produced bainite. For tissue observation, each round bar is cut at the center in the longitudinal direction, and the cut surface (radial cross section at 90 ° to the longitudinal direction) is embedded in resin, and the sample surface is mirror-polished with emery paper or diamond buff. The surface was etched with nital. This was observed using an optical microscope at a D / 4 position at a magnification of 400 times.

Method for measuring shape (aspect ratio, number) of MnS Each round bar sample of φ80 was cut at the center of the rolling or forging direction, embedded in resin, and the sample surface was mirror-polished with emery paper or diamond buff. After this polished sample surface was etched with nital, the D / 4 position was observed at 400 times magnification using an optical microscope, and five photographs were taken. Then, the number of MnS (sulfide inclusions) having a maximum length of 2 μm or more was counted to obtain an average value at five locations, which was used as the average number of MnS. Further, the aspect ratios of MnS having a maximum length of 2 μm or more were measured, and the average value was taken as the aspect ratio of MnS in the steel material. Confirmation and identification of whether or not the inclusion was MnS observed with the optical microscope was identified by an X-ray spectrometer (EDX).

Method for measuring machinability of steel material The machinability was evaluated by a turning test of each round bar of φ80. An NC lathe was used as a testing machine, and a machinability evaluation test piece (φ80 × 350 mmL) was turned. The change over time in the amount of tool wear (Vb) on the flank of the tool used at this time was measured, and Vb after 3000 m of cutting was measured under the following conditions. The cutting test conditions are as follows.
Tool: TiAlN coated chip Cutting speed: 200m / min, constant peripheral cutting oil: None (dry type)
Cutting depth: 1.5mm
Feed amount: 0.25mm / rev
Then, a steel material having a Vb wear amount of 60 μm or less after cutting 3000 m was judged to be acceptable as having excellent machinability.

Cold forgeability evaluation method Cold forgeability was evaluated by the end face constrained compression test of each round bar of φ80. A 1600-ton press was used as a testing machine, and a test piece for cold forgeability evaluation (φ10 × 15 mmL) was compressed. The test piece was cut out from the D / 4 position (D: diameter) of the rolled material. 80% compression processing was performed at room temperature and a strain rate of 10 / s. The surface of the test specimen after compression was observed with a stereomicroscope at a magnification of 20 times to confirm the presence or absence of cracks. It was determined that the steel material without cracks was excellent in cold forgeability.

Friction welding test A rod (test piece) of φ20 mm × 100 mmL was cut out from the D / 4 position along the rolling or forging direction of each φ80 round bar. A product name FF-4511-C manufactured by Nitto Seiki Co., Ltd. was used as an automatic friction welding machine, and friction welding was performed by a brake method. That is, the cut bar materials (friction welded steel materials of the test materials) and the cut bar material as the steel material of S45C (friction pressure welded steel material with S45C), each end in the longitudinal direction. As a round bar composite steel material (steel part), the friction welding was performed.

Friction welding was performed in accordance with the following conditions in common with each example.
Friction pressure: 100 MPa
Upset pressure (pressure applied from both ends of the round bar to the joint): 180 MPa,
Friction time: 10 sec,
Upset time (pressurization time to the joint): 10 sec,
Rotation speed: 1600 rpm,
Total margin: 8-15mm (shrinkage from the original round bar length)

Method for measuring shape (aspect ratio, number) of MnS of friction welded part In the steel in the heat affected zone on the low carbon steel material side in the range of 1 mm width from the joint formed by the friction welding, the aspect ratio is 2 or less The number of MnS having a maximum length of 1 μm or less was measured. The heat affected zone on the low carbon steel material side within the 1 mm width range is cut at any five locations, treated under the same conditions as the low carbon steel material, and the D / 4 position is 400 times magnification using an optical microscope. Observed and photographed. Then, the number of MnS having an aspect ratio of 2 or less and a maximum length of 1 μm or less was counted to obtain an average value at 5 locations. Confirmation and identification of whether or not the inclusion was MnS observed with the optical microscope was identified by an X-ray spectrometer (EDX).

Impact characteristics evaluation From the center position of the friction welding steel material (round bar composite steel material) of the friction test welding steel material and S45C between the test materials of Φ20 mm × about 200 mmL, one side is 10 mm so that the joint portion becomes a notch bottom. A Charpy test piece having a square cross section x 55 mmL was prepared. The notch shape was R10 (mm). Cu plating was performed on three surfaces other than the notch introduction surface (TP processing). And after making this preparation test piece 930 degreeC carburizing-oil quenching (carburizing process), the tempering process was performed at 170 degreeC.

  Subsequently, the impact property evaluation of the test piece after the tempering treatment was performed with a Charpy impact tester. The test conditions were room temperature and a load speed of 5 m / s. The Charpy impact test was performed five times and the Charpy impact value (absorbed energy) was measured. And in all joining components, the composite steel material from which absorbed energy became 10J (joule) or more was set as the pass. All Charpy values listed in Tables 9 to 12 indicate the value of the absorbed energy (unit: J).

Fatigue property evaluation From the center position of the friction welding steel material (round bar composite steel material) with the friction welding steel material of S20C and the above-mentioned test materials of Φ20 mm × about 200 mmL, one side is 13 mm so that the joint portion becomes a notch bottom. A four-point bending fatigue test piece with a square cross section × 100 mmL was prepared. The notch shape was R1.5 (mm). Cu plating was performed on three surfaces other than the notch introduction surface (TP processing). And after making this preparation test piece 930 degreeC carburizing-oil quenching (carburizing process), the tempering process was performed at 170 degreeC.
Next, the fatigue characteristics of the test piece were evaluated with a 4-point bending fatigue tester. The test conditions were performed at 11 levels with a load of 4000 N (stress 609 MPa) to 14000 (stress 2132 MPa) at a frequency of 20 Hz, and a stress (MPa) corresponding to a life of 20,000 times was obtained. It was used as an index. In this example, a test piece (composite steel material) having a fatigue limit stress of 1000 MPa or more was accepted. All the 20,000 times life described in Tables 9 to 12 show this fatigue limit stress (unit: MPa).

  In Tables 1 to 12, invention examples 1A to 1Z, 2A to 2J, 3A to 3M (however, 1C-5, 1C-6, 1C-9, 1D-5 are comparative examples) are steel. The component composition, the content of compound N in steel, the amount of solute N, the number of MnS having a maximum length of 2 μm or more and the average aspect ratio of 2 or more satisfy the conditions specified in the present invention.

  As a result, as a low carbon steel material, cold workability such as cold forging and machinability are excellent. Also, the friction welded parts (composite steel) subjected to the friction welding are excellent in impact characteristics and bending fatigue characteristics in which the joining strength is required (required) for automotive engine parts.

  On the other hand, in the comparative examples from 1C-5 to 1C-5, 1C-6, 1C-9, 1D-5 and 2K to 2Z in Tables 1 to 12, whether the content of the main elements deviates from the upper and lower limits, Although the steel component composition satisfies the conditions of the present invention, the production conditions deviate from the preferred range described above. As a result, these comparative examples are low carbon steel materials, which are inferior in cold workability and machinability such as cold forging, or as friction welded parts (composite steel) that are friction welded, such as automotive engine parts. For example, the joint strength is required (required), and the impact characteristics and bending fatigue characteristics are significantly inferior as shown in Tables 9 and 11 as compared with the above-described invention examples.

As shown in Table 5, the low carbon steel materials of 1C-5, 1C-6, 1C-9, and 1D-5, which are comparative examples, have a billet heating temperature, a hot rolling temperature, or a cooling rate after completion of rolling. Manufactured outside the above preferred conditions.
Steel type 1C-5 has a billet heating temperature too high.
Steel type 1C-6 has a too high rolling temperature.
Steel type 1C-9 has too low rolling temperature.
In steel type 1D-5, the cooling rate after the end of rolling is too fast.

As shown in Table 1, the low carbon steel materials of 2K to 2Z, which are comparative examples, have a manufacturing method within a preferable condition range, but the content of main elements is out of the upper and lower limits.
In steel types 2K and 2L, the C content deviates from the upper and lower limits.
In steel types 2M and 2N, the Si content deviates from the upper and lower limits.
In steel types 2O and 2P, the Mn content deviates from the upper and lower limits.
In steel type 2Q, the P content is outside the upper limit.
In steel types 2R and 2S, the S content is outside the upper limit.
In the steel types 2T and 2U, the Cr content deviates from the upper and lower limits.
In steel types 2V and 2W, the Al content deviates from the upper and lower limits.
Steel type 2X has a compound N content outside the lower limit.
In the steel type 2Y, the total N content and the solute N amount are out of the upper limit.
Steel content 2Z has a P content outside the upper limit.

  Therefore, from the results of the above examples, the critical significance or effect for obtaining the impact characteristics and bending fatigue characteristics required for the friction-welded composite of the composition, structure, and manufacturing method of the steel material according to the present invention. Is supported.

  According to the present invention, it is possible to provide a low carbon steel material and a friction welded part for machine structure suitable for friction welding, which have improved part characteristics such as fatigue strength and impact strength. For this reason, it can be suitably used as a mechanical structure part that is friction-welded, such as a reduction gear, a gear such as a differential gear, and a CVT pulley, which are used in automobile engines, transmissions, and differentials.

Claims (6)

In mass%, C: 0.08 to 0.61%, Si: 0.08 to 0.5%, Mn: 0.4 to 1.5%, P: 0.03% or less (excluding 0%) S): 0.005 to 0.1%, Cr: 0.4 to 2%, Al: 0.005 to 0.1%, N: 0.02% or less (however, not including 0%) Each of them includes the balance Fe and inevitable impurities, the content of compound N in the steel is 0.006 to 0.02%, and the amount of solid solution N is 0.0015% or less (however, 0% is included) ), And MnS having a maximum length of 2 μm or more is present in steel in an amount of 100 to 4000 per mm ² , and the average aspect ratio of these MnS is 2 or more. Steel material.   Further, the steel for machine structural use in terms of mass%, Ti: 0.2% or less (excluding 0%), Nb: 0.2% or less (excluding 0%), V: 0.2 % Or less (excluding 0%), B: 0.01% or less (excluding 0%), Mo: 1% or less (excluding 0%), Cu: 1% or less (excluding 0%) 2), Ni: 1% or less (excluding 0%), or one or more of them, containing the steel material for mechanical structure suitable for friction welding according to claim 1.   Further, the steel for machine structural use is, in mass%, Ca: 0.02% or less (excluding 0%), REM: 0.02% or less (excluding 0%), Li: 0.005 % Or less (however, not including 0%), Mg: 0.005% or less (however, not including 0%) one or more kinds of machines suitable for friction welding according to claim 1 or 2 Structural steel. In mass%, C: 0.08 to 0.61%, Si: 0.08 to 0.5%, Mn: 0.4 to 1.5%, P: 0.03% or less (excluding 0%) S): 0.005 to 0.1%, Cr: 0.4 to 2%, Al: 0.005 to 0.1%, N: 0.02% or less (however, not including 0%) Each of them includes the balance Fe and inevitable impurities, the content of compound N in the steel is 0.006 to 0.02%, and the amount of solid solution N is 0.0015% or less (however, 0% is included) ), And a steel material for machine structural use in which MnS having a maximum length of 2 μm or more is present in steel in an amount of 100 to 4000 per mm ² , and the average aspect ratio of these MnS is 2 or more, and other carbon steel materials or alloy steel materials Are joined by friction welding to form a composite steel of a desired shape, and further, surface hardening treatment and tempering treatment The aspect ratio in the steel of the heat affected zone on the machine structural steel side in the range of 1 mm width from the joint formed by the friction welding is 2 or less, and A friction welding part excellent in impact characteristics and bending fatigue characteristics, characterized in that MnS having a maximum length of 1 μm or less is regulated to 25 or less (including 0) per 1 mm ² .   Further, the steel for machine structural use in terms of mass%, Ti: 0.2% or less (excluding 0%), Nb: 0.2% or less (excluding 0%), V: 0.2 % Or less (excluding 0%), B: 0.01% or less (excluding 0%), Mo: 1% or less (excluding 0%), Cu: 1% or less (excluding 0%) 5), Ni: 1% or less (excluding 0%), or one or more of them, the friction welded part having excellent impact characteristics and bending fatigue characteristics according to claim 4.   Further, the steel for machine structural use is, in mass%, Ca: 0.02% or less (excluding 0%), REM: 0.02% or less (excluding 0%), Li: 0.005 % Or less (however, not including 0%), Mg: not more than 0.005% (however, not including 0%), or two or more kinds of impact properties and bending fatigue properties according to claim 5 Excellent friction welding parts.