JP2012041601A - Steel material for machine structural use suitable for friction pressure welding, and friction pressure welding part superior in impact property and bending fatigue property - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel material for machine structural use that is suitable for friction pressure welding, and in which part properties such as a fatigue strength and an impact strength are increased, and to provide a friction pressure welding part.SOLUTION: The steel material is produced by a specific rolling condition to have a specified composition containing a solid solution [V] and also to have a steel structure, wherein an average area ratio of ferrite grains to pearlite grains and a ferrite-pearlite area ratio, are a specific mixed-phase structure of specific fine ferrite grains and pearlite grains, so as to increase the part properties such as the fatigue strength and the impact strength of a composite steel material or composite steel part that is formed by joining the steel material to other steel materials by friction pressure welding.

Description

  The present invention relates to a steel material for a machine structure suitable for a friction welding application and a friction welding component excellent in impact characteristics and bending fatigue characteristics.

  For example, engine parts such as piston pins used in automobile engines, transmissions, differentials, etc., and steel mechanical structural parts such as gears, shafts, connecting rods, etc. in recent years have been accompanied by a reduction in vehicle weight due to energy savings. Miniaturization is being pursued. 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, steel materials excellent in workability (hardened steel, mixed structure of ferrite and pearlite) are used for the steel materials that are the materials of these mechanical structural parts. This steel is usually processed into bars and wires by hot rolling or hot forging, and then cold-worked, such as cold forging, if necessary. -Finishing is being performed.

  Here, when the strength and toughness of the mechanical structural steel material, which is the raw material, is increased as the material of the mechanical structural component corresponding to the load increase as described above, the precise cutting process becomes extremely difficult. Therefore, there is a need for a steel material having both the above-mentioned high strength and high toughness of the component characteristics and machinability. However, the strength and the machinability are in a contradictory relationship. It is extremely difficult to achieve both machinability.

  For this reason, as one of the measures to achieve both the high strength and high toughness of the part characteristics and the machinability, the steel material used for the parts where the part characteristics such as strength and toughness are required, and the parts where the machinability is necessary. There is a method in which both steel properties and machinability are achieved by preparing steel materials to be used separately and joining both steel materials having different properties to each other to form composite steel materials or composite steel parts.

  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, this Patent Document 1 does not describe a 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, there is a problem in that the strength of the portion (HAZ portion) that is affected by frictional heat is reduced, and conversely, the strength of the joining portion is increased. Become. 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 this Patent Document 2, 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 In order to keep Mo and Nb carbonitrides precipitated at the time of friction welding after hot rolling the steel containing 0.60%, 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 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 this joined portion is rapidly cooled by the surrounding base material after being heated by frictional heat, it tends to become a martensite phase, and the strength tends to increase originally. If this precipitation strengthening is also added, the strength of the joined portion is conspicuously increased by synergistic hardening 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, the austenite crystal grains of the high carbon steel material are prevented from coarsening in rapid heating under high pressure of friction welding, and the hardness of the joined portion is increased. Brittleness is suppressed. In this case, the solute Nb precipitates as NbC after friction welding and contributes to prevention of coarsening of crystal grains.

  The use of such solute Nb is a technique for suppressing martensitic transformation due to coarsening of crystal grains when steel materials that have been quenched and tempered in advance are friction-joined. Therefore, although the present technology can suppress the deterioration of the component strength due to the variation in hardness, the structure has the same crystal grain size as that of the base material and is not sufficient for improving the joint surface. In other words, it exhibits performance for uniaxial loads such as compression and tension, but for multiaxial loads such as bending, fracture tends to proceed along the joint surface due to the nature of friction welding technology. . For this reason, the problem which deteriorates notably arises especially under the impact load which bends to a fraction of a base material.

  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.

  However, the bainite structure or the mixed structure of bainite and martensite as in Patent Document 5 cannot be applied at all to a mixed structure steel material of ferrite and pearlite having excellent workability.

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

  As described above, in steel materials, which are materials for ordinary machine structural parts, the strength reduction of the heat-affected zone and the strength increase of the joint zone are suppressed together (simultaneously) while still being friction-bonded. There has not yet been proposed a technique for minimizing the strength fluctuations of the joints and the joints and not reducing (improving) the component properties such as fatigue strength and impact strength of the composite steel material.

  As described above, mechanical structural parts for engine parts such as automobiles are required to have improved fatigue strength and impact strength in response to an increase in load on the parts due to downsizing and higher output. Therefore, in order to apply the composite steel material (composite steel part) by the friction welding method to such a use, it is naturally required to improve these characteristics.

  In this respect, fatigue strength and impact strength cannot be improved unless the toughness of the joint is improved not only by minimizing each strength variation of the heat affected zone and the joint, but also by friction welding. The composite steel material (composite steel part) by the friction welding method lacks reliability and cannot be used as an engine part of the automobile or the like.

  The present invention has been made in view of such a problem, and provides a steel material for mechanical structure and a friction welded part suitable for friction welding, which can improve part properties such as fatigue strength and impact strength while maintaining friction welding. With the goal.

  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: 0.25 to 0.65%, Si: 0.02 to 2.0%, Mn: 0.7 to 3.0%, P: 0.03% or less (excluding 0%), S: 0.005 to 0.1%, Cr: 1% or less (excluding 0%) ), Al: 0.005 to 0.1%, N: 0.02% or less (excluding 0%), V: 0.03 to 0.5%, respectively, and solid solution [V] = [ V] -3.6 [N] ([V] is V content, [N] is N content) The solid solution V calculated by V is -0.01% or more, while the solid solution N is 0.00%. 001% or less (including 0%), the balance is made of iron and inevitable impurities, and the steel structure has an average area ratio of ferrite grains to pearlite grains (average area of ferrite grains / average area of pearlite grains) of 0.00. It is 05 or more and 0.5 or less Moni, ferrite - total area ratio of pearlite is 90% or more relative to the total tissue is to consist of mixed phase of ferrite grains and pearlite grains.

  In order to achieve the above object, the gist of the friction welded part having excellent impact characteristics and bending fatigue characteristics of the present invention is the temperature at which the steel material for machine structural use according to the gist mentioned above or in a preferred mode described later includes normal temperature. After being formed into a desired shape by forging or cutting in a region, it is a composite steel material joined by the same mechanical structural steel materials or other steel materials by friction welding.

  The steel material for cold and hot working according to the present invention first requires V addition, contains solid solution V, and has a structure of a mixed phase of ferrite grains and pearlite grains with a specified area ratio. By this combination, the structure in the vicinity of the joint surface at the time of friction bonding can be refined with the ferrite-pearlite structure as it is, the fatigue strength of the friction bonded product can be improved, and the deterioration of the impact strength can be suppressed. Can do.

  Usually, in a joint portion of steel materials subjected to friction welding, rapid heating and cooling cause crystal grain coarsening and a sharp increase in hardness due to martensitic transformation. For this reason, the joint portion and the vicinity of the joint portion become brittle, and parts characteristics such as fatigue strength and impact strength cannot be satisfied.

  For this reason, in the prior art, the coarsening of crystal grains is suppressed by utilizing solid solution Nb. However, when normal ferrite-pearlite structure steel is used, the strength of the parts cannot be ensured, and therefore quenching and tempering in advance. It is necessary to use the finished steel. Therefore, there is a problem that the hardness of the steel material itself is high and the machinability is deteriorated. In addition, the joint after friction welding is characterized by a crystal grain size close to that of the base material, and the deterioration of toughness, which is a weak point of friction welding, cannot be sufficiently improved.

  On the other hand, the technique of the present invention is characterized in that the friction joint portion is miniaturized as compared to before joining. Such refinement of crystal grains in the friction joint is effective in improving toughness, and it is possible to suppress the deterioration of impact strength due to friction joining by the effect of improving toughness by refinement of crystal grains.

  In the present invention, strong plastic deformation is imparted to the pressure contact portion and the vicinity thereof by friction welding. In general steel materials, both ferrite and pearlite are similarly plastically deformed, so that crystal grains are easily coarsened after friction welding. Here, if the ferrite and pearlite grains have a size difference (ferrite is sufficiently small compared to pearlite), the relatively small ferrite grains are plastically deformed, and the relatively large pearlite grains are fragmented. Occurs.

  Usually, pearlite grains are austenitized due to a temperature rise and aggregate to lead to coarsening of the austenite grains, but are inhibited by ferrite grains that are fine and plastically deformed, so that coarsening does not easily occur. Since the austenite that has been divided is less likely to undergo martensitic transformation during cooling, the result is a fine ferrite-pearlite structure at the joint and in the vicinity of the joint, achieving a refinement of crystal grains without increasing the hardness too much. Let

  Solid solution V also plays an important role in this, and it is presumed that VC precipitated during friction welding acts effectively on the suppression of the austenite coarsening. The steel material in which the joint portion and the vicinity of the joint portion are miniaturized by friction welding as described above can be a component in which the deterioration of impact characteristics is suppressed even if the steel material is a friction-joined component.

  In order to deposit VC having such an effect at the time of friction welding, it is necessary to make V in the steel material exist in a solid solution state before the friction welding. In the case where VC is preliminarily set before friction welding, this preexisting VC does not effectively act to suppress the austenite coarsening. Further, since the solid solution V is also insufficient (small), the VC precipitated during friction welding is also insufficient, and the effect of suppressing the austenite coarsening is insufficient.

FIG. 1 is a drawing-substituting photograph showing the structure of the steel material of the present invention. FIG. 1 (a) is a steel material before joining, and FIG. 1 (b) is a steel material after friction welding. It is a drawing substitute photograph which shows the structure | tissue of the steel material for conventional mechanical structures, Fig.2 (a) is the steel material before joining, FIG.2 (b) is the steel material after friction welding.

Steel composition:
First, the reason for limiting the chemical composition of the steel of the present invention will be described. The chemical composition of the steel material for machine structure of the present invention (skin-hardened steel) is the strength and toughness characteristics required for machine structural parts such as the engine parts of automobiles, impact characteristics, bending fatigue characteristics, surface This is a precondition for improving the characteristics such as the pressure 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: 0.25-0.65%, Si: 0.02-2.0%, Mn: 0.7-3.0%, P: 0.03. % Or less (excluding 0%), S: 0.005 to 0.1%, Cr: 1% or less (excluding 0%), Al: 0.005 to 0.1%, N: 0 0.02% or less (excluding 0%), V: each containing 0.03 to 0.5%, solid solution [V] = [V] -3.6 [N] ([V] contains V The solid solution V calculated by the amount, [N] is N content) is 0.01% or more, the solid solution N is 0.001% or less (including 0%), and the balance is iron and inevitable impurities It is set as the chemical component composition which consists of. 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 further includes Ti: 0.2% or less (excluding 0%) as a selective additive element in addition to the specific chemical component composition. ), Nb: 0.2% or less (excluding 0%), B: 0.01% or less (excluding 0%), Mo: 1% or less (excluding 0%), Cu: 1% or less (however, not including 0%), Ni: 1% or less (however, not including 0%) may be included, or one or more may be included. Further, Ca: 0.02% or less (excluding 0%), REM: 0.02% or less (excluding 0%), Li: 0.005% or less (excluding 0%) ), Mg: 0.005% or less (but not including 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 the steel material for this type of mechanical structure (hardened steel) is used.

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

C: 0.25 to 0.65%
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 0.25 to 0.65%, and the lower limit value is preferably 0.28%, more preferably 0.30%. The upper limit is preferably 0.62%, more preferably 0.60%.

Si: 0.02 to 2.0%
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. If the Si content is too small, deoxidation becomes insufficient, and gas defects are likely to occur during 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 occur. This tendency begins to be noticeable when the Si content exceeds 2%. Therefore, the Si content is in the range of 0.02 to 2.0%, and the lower limit is preferably 0.05%, more preferably 0.08%. The upper limit is preferably 1.5%, more preferably 1%.

Mn: 0.7-3.0%
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. For this reason, the Mn content is in the range of 0.7 to 3.0%, and the lower limit is preferably 0.75%, more preferably 0.8%. The upper limit is preferably 2.5%, more preferably 2.0%.

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 is also an element that is inevitably mixed and contained as an impurity, and when combined with Fe, FeS is deposited on the grain boundary as a film, so that the deformability is deteriorated. Therefore, it is necessary to combine S with Mn and deposit it as MnS harmlessly. However, as the amount of MnS deposited increases, the deformability also deteriorates. On the other hand, S has an effect of improving machinability, and when the S content is extremely reduced, the machinability is deteriorated. Accordingly, the S content is in the range of 0.005 to 0.1% in consideration of the balance between deformability and machinability, and the lower limit is preferably 0.010%, more preferably 0.015%, and the upper limit. The value is preferably 0.09%, more preferably 0.08%.

Cr: 1% or less (excluding 0%)
Cr is an element effective for securing the strength of the friction welded part and enhancing the toughness of the joint. However, if the Cr content is excessive, it segregates as carbides at the prior austenite grain boundaries, which causes a decrease in fatigue strength and impact strength. Accordingly, the Cr content is 1% or less (excluding 0%), the lower limit is preferably 0.05%, more preferably 0.10%, and the upper limit is preferably 0.8%, more preferably. Is 0.6%.

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 the machinability is 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 forms nitrides with nitride-forming elements such as Al and V, and contributes to refinement of the structure and grain size. However, when present in a solid solution state, N deteriorates hot ductility and deforms due to dynamic strain aging. In order to reduce the performance, it is necessary to combine the entire amount with V to eliminate the solid solution N and to precipitate it as VN. Therefore, the N content is 0.02% or less (excluding 0%), and the upper limit is preferably 0.015%, more preferably 0.012%.

Solid solution N: 0.001% or less (including 0%)
When N is present in a solid solution state, the hot ductility is deteriorated and impact characteristics are deteriorated due to dynamic strain aging. Therefore, it is combined with a nitride-forming element (for example, Al, V, etc.) in advance and precipitated as nitride. Therefore, it is necessary to reduce the solid solution N as much as possible. However, if the solute N is 0.001% or less, such an adverse effect can be suppressed, so the upper limit amount allowed is set to 0.001%. This upper limit amount is preferably 0.0008% or less, more preferably 0.0005% or less.

V: 0.03-0.5%
V forms carbides and nitrides, particularly fixes N, prevents deterioration of deformability due to solute N, and contributes to austenite grain refinement and grain sizing. Further, in the present invention, it is necessary to contain V as much as the solid solution V can remain. As described above, this solid solution V forms VC during friction welding. This VC contributes to the suppression of the increase in hardness of the friction bonded portion by making the austenite grains finer and sized. If the V content is too small, a sufficient amount of solute V cannot be applied during friction welding. On the other hand, if the V content is too large, a large amount of VC is generated, so that the hardness of the joint becomes too high due to precipitation strengthening of the VC, and the component characteristics deteriorate. Therefore, the V content is in the range of 0.03 to 0.5%, the lower limit is preferably 0.04%, more preferably 0.05%, and the upper limit is preferably 0.45%, more preferably 0.40%.

Solid solution V: 0.01% or more In the present invention, the joint of friction welding and the vicinity of the joint have a fine ferrite-pearlite structure, which is important for achieving refinement of crystal grains without significantly increasing the hardness. The solid solution V amount is secured. In order to maximize the effect of improving the toughness due to the refinement of the microstructure of the joint by VC, 0.01% or more of solid solution V is required. It is inferred that the solid solution V precipitates as VC during friction welding, and this effectively acts to suppress the austenite coarsening. The steel material in which the joint portion and the vicinity of the joint portion are miniaturized by friction welding as described above can be a component in which the deterioration of impact characteristics is suppressed even if the steel material is a friction-joined component.

  In order to deposit VC having such an effect at the time of friction welding, it is necessary to make V in the steel material exist in a solid solution state before the friction welding. In the case where the added V is VC in advance in the steel before friction welding, this pre-existing VC does not effectively act to suppress austenite coarsening during friction welding. Further, since the solid solution V is also insufficient (small), the VC precipitated during friction welding is also insufficient, and the effect of suppressing the austenite coarsening is insufficient. That is, even if V is added, the above effects cannot be sufficiently exhibited with a steel material in which a necessary amount is not previously dissolved before friction welding.

  In order for V to fix the entire amount of N, V and N need to satisfy the relational expression of 0.01 ≦ [V] −3.6 [N]. Moreover, in order to combine V and C at the time of friction welding and to maximize the effect of improving toughness by refining the structure of the joint by VC, 0.01% or more of solid solution V is required. If the solid solution V is less than 0.01%, deterioration of toughness during friction welding cannot be sufficiently suppressed. In addition, the upper limit of the solid solution V indicates a state in which all V contained in the steel is in solid solution. The solid solution V is calculated by solid solution [V] = [V] -3.6 [N] (where [V] is the V content and [N] is the N content). The lower limit value of the solid solution V is preferably 0.015%, more preferably 0.02%.

One or more of Ti, Nb, B, Mo, Cu, and Ni Ti, Nb, B, Mo, Cu, and Ni are all tough as described in Patent Document 5 as effective elements. It is effective in improving the strength of the steel material as a raw material and the composite steel material after friction welding without impairing the strength.

Ti, Nb, B:
Among these, Ti, Nb, and B can all form carbides, suppress the strength reduction in the heat-affected zone after friction welding, and substantially improve the strength. Therefore, as required, Ti: 0.2% or less (excluding 0%), Nb: 0.2% or less (excluding 0%), B: 0.01% or less (provided that 0% is not included) 1 type or 2 types or more are added.

  The lower limit of the Ti content when selectively added is preferably 0.005% or more, more preferably 0.01% or more, and still more preferably 0.015% or more. On the other hand, if the Ti content is too large, a large amount of TiC is generated, and the strength is reduced. Therefore, the upper limit of the Ti content is preferably 0.15% or less, and more preferably 0.1% or less.

  When the Nb content is selectively added, the lower limit of the Nb content is preferably 0.005% or more, more preferably 0.01% or more, and still 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 B content when selectively added is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, if the B content is too large, the hardenability is improved and a large amount of martensite is generated, so that toughness and impact properties are deteriorated. Therefore, the upper limit of the B content is preferably 0.008% or less, and more preferably 0.006% or less.

Mo: 1% or less (excluding 0%)
Mo is an element effective for improving the toughness of the steel material. 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 higher than necessary 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 order to effectively exhibit the effect of Mo, 0.04% or more is preferably added, more preferably 0.06% or more, and still more preferably 0.08% or more.

1 type or 2 types of Cu and Ni Both Cu and Ni are effective in strain-aging the steel material and improving the strength of the base material and the joint portion. Therefore, one or two of 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 effects of Cu and Ni, addition of 0.1% or more is preferable, more preferably 0.2% or more, and still more preferably 0.3% or more.

One or more of Ca, REM, Li, and Mg Ca, REM, Li, and Mg commonly spheroidize sulfide compound inclusions such as MnS to increase the deformability of the steel material and cut the workpiece. It is an element that contributes to improving the properties. Therefore, as necessary, Ca: 0.02% or less (excluding 0%), REM: 0.02% or less (excluding 0%), Li: 0.02% or less (excluding 0%) 1) or 2 or more of Mg: 0.02% 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 0.02% or less, more preferably 0.01% or less, and still more preferably 0.005% or less. Similarly, Li and Mg are each preferably added in an amount of 0.02% or less, more preferably 0.0025% or less, and still more preferably 0.001% or less.

Steel structure:
The structure suitable for friction welding of the steel for machine structure of the present invention will be described as follows. FIG. 1 (a) shows a structure (drawing substitute photograph) of the steel material of the present invention observed with an optical microscope of 400 times. FIG. 2 (a) shows a structure (drawing substitute photograph) of a conventional (ordinary) mechanical structural steel material observed with an optical microscope at a magnification of 400 times.
Further, the structure after friction welding of the steel materials shown in FIGS. 1 (a) and 2 (a) is observed as a structure (photograph substituted for drawing) observed with a 400 × optical microscope as shown in paragraph 0091. (B) and FIG. 2 (b), respectively. Incidentally, the structures before friction welding are shown in FIGS. 1 (a) and 2 (a), and the structures after friction welding are shown in FIGS. 1 (b) and 2 (b). Although it is an observation result by an optical microscope, as shown in each of these drawings, the appearance (mode) of the structure is greatly changed by friction welding.

  FIG. 1 shows the structure of invention type steel 1G of Examples Tables 1 and 4 to be described later, the steel material composition is appropriate, it contains an appropriate amount of solute V, and the amount of solute N is regulated. FIG. 1A shows a steel material before joining (Table 1), and FIG. 1B shows a steel material after friction welding (Table 4). On the other hand, FIG. 2 shows the structure of Comparative Example Steel Type 2X in Examples Tables 2 and 5 to be described later, and is a structure in which the V content of the steel material is too small and the amount of solute V is too small. FIG. 2A shows a steel material before joining (Table 2), and FIG. 2B shows a steel material after friction welding (Table 5).

  In order to obtain a structure suitable for friction welding, the structure of the steel for machine structural use according to the present invention shown in FIG. 1 (a) has an average area ratio of ferrite grains / average area of pearlite grains, which is an average area ratio of ferrite grains to pearlite grains. It consists of a mixed phase of ferrite grains and pearlite grains that are 0.05 or more and 0.5 or less. As shown in FIG. 1 (a), the structure of the steel of the present invention is composed of a fine mixed phase of ferrite grains that appear whitish and pearlite grains that appear black. The pearlite grains are several times larger than the ferrite grains. Thus, if the ferrite grains are several times larger than the pearlite grains, the average area of the pearlite grains inevitably becomes larger than the average area of the ferrite grains. Moreover, the ferrite structure of the steel material of the present invention has a complicated shape around the pearlite grains. Therefore, the microstructure of the steel material of the present invention is a fine mixed phase of new ferrite grains and pearlite grains in which pearlite grains are several times larger than ferrite grains and small ferrite grains are arranged around large pearlite grains. It can be said that it is an organization.

  Because of such a fine mixed phase structure, the inventive steel types 1G in Tables 1 and 4 are formed by friction welding of the steel material after friction welding as shown in FIG. In the depth range of 1 mm from the bonded interface, the ferrite grains have a fine structure with an average grain size of 5 μm or less.

  On the other hand, the structure of Comparative Example Steel Type 2X (conventional steel for machine structural use) shown in Example Tables 2 and 5 shown in FIG. 2 (a) is the same as the structure of the steel material of the present invention, ferrite grains that appear whitish, and pearlite grains that appear dark. The ferrite grains are approximately the same size as the pearlite grains. Moreover, the ferrite grains and the pearlite grains have a similar simple granular shape and are arranged next to each other. For this reason, although the ferrite grain average area / pearlite grain average area, which is the average area ratio of ferrite grains to pearlite grains, satisfies 0.05 or more and 0.5 or less, the average grain size is coarse.

  For this reason, the comparative example steel type 2X in Examples Tables 2 and 5 is 1 mm from the joint interface formed by friction welding of the steel material after friction welding as shown in FIG. 2B and Table 5 described later. In the depth range, the average grain size of the ferrite grains does not become 5 μm or less, and the entire surface is martensite. As a result, as shown in Table 5 described later, the comparative example steel type 2X is significantly inferior to the invention example in impact characteristics and bending fatigue characteristics required for automobile engine parts and the like.

Average area of ferrite grains / average area of pearlite grains:
Usually, the structure of the joint by friction welding is mainly composed of a martensite phase by rapid heating and rapid cooling. When the area ratio of ferrite grains to pearlite grains is close to 1 as in the conventional steel for machine structural use, the joint is heated to austenite temperature by friction welding, pearlite is austenitized, and they agglomerate. Easy to coarsen. And when it becomes a martensite phase by subsequent cooling, since the intensity | strength of a junction part will increase too much and embrittlement will be accelerated | stimulated, it will become easy to generate | occur | produce a crack. For this reason, characteristics such as fatigue strength and impact strength as a friction bonded part are lowered, and reliability as a machine structural part is lost.

  On the other hand, in the present invention, the average area ratio of ferrite grains / pearlite grains (average area of ferrite grains / average area of pearlite grains) is 0.5 or less, preferably 0.4 or less, compared to pearlite grains, Make ferrite grains significantly smaller. As a result, when plastic flow occurs due to friction welding of the joint, the fine ferrite grains hinder the austenite coarsening, and the austenite grains are refined by the subsequent dynamic recrystallization during cooling. Becomes a mixed phase structure of fine ferrite and pearlite. The fine mixed phase structure is effective for improving toughness, and can suppress an increase in strength of the joint portion, so that impact characteristics and fatigue characteristics can be improved.

  As a guideline for the fine mixed phase structure of joints in friction welded parts (composite steel materials) that ensure such characteristics as impact characteristics and fatigue characteristics, a depth range of 1 mm from the joint interface formed by friction welding The average grain size of the ferrite grains is 5 μm or less. As a result, the average particle size of pearlite, and hence the mixed phase of ferrite and pearlite, is also refined. If the average grain size of the ferrite grains becomes larger than this, there is a possibility that characteristics such as impact characteristics and fatigue characteristics of the joint cannot be guaranteed.

  In order to obtain a joint portion composed of such a fine mixed phase structure, in the present invention, the ferrite grain average area / pearlite grain average area is set to 0.5 or less. However, as the average area ratio approaches 1 (the sizes of ferrite and pearlite approach), the effect of suppressing the coarsening by ferrite is lost, so it is necessary to set it to 0.5 at the maximum. On the other hand, if the average area ratio of ferrite / pearlite is less than 0.05, the ferrite coarsening suppression effect does not function, so the lower limit is preferably 0.06 or more, more preferably 0.07 or more. The upper limit of ferrite / pearlite is preferably 0.45 or less, more preferably 0.40 or less.

  Even the steel of the present invention may contain other structures such as bainite as long as the amount is small, but in order to obtain the necessary overall characteristics, the total area ratio (fraction) of ferrite-pearlite is such as bainite. It is 90% or more, preferably 95% or more, and more preferably 97% or more with respect to the whole tissue including other tissues (area ratio in the tissue measured by the optical microscope observation). The area ratio of ferrite and pearlite in the entire structure is mainly determined by the amount of C. In order to obtain necessary overall characteristics in a structure composed of a mixed phase of ferrite grains and pearlite grains, the ferrite area ratio range is 30 to 80%, the pearlite area ratio range is 10 to 60%, and the ferrite grains And pearlite grains are preferable.

Production method of the steel of the present invention:
In order to form the above-described structure of the present invention, the following conditions are necessary. First, the steel for machine structure itself can be manufactured by a normal manufacturing process of the steel for machine structure for automobile parts. That is, the cast steel slab is processed into a steel material such as a wire rod by hot rolling or hot forging. However, as the temperature history of the mechanical structural steel material having the above specific component composition, the temperature of hot rolling or hot forging is kept at 1000 ° C. or higher, and then held at that temperature for 5 to 60 minutes. It is necessary to set the cooling rate of 0.2 to 1.5 ° C./s.

  In addition, the steel material for machine structure itself can be manufactured by the manufacturing process of the steel material for machine structure for the said said motor vehicle parts normally. That is, the cast steel slab is processed into a steel material such as a wire rod by hot rolling or hot forging. The steel of the present invention is a steel material that has been hot-rolled or hot forged (as hot-worked material) or a steel material that has been formed into a part shape by cold forging, etc. It may be a steel material.

Hot rolling or hot forging:
In the process of hot rolling or hot forging, after rolling and forging at 1000 ° C. or higher, hold for 5 to 60 minutes. In the present invention, in order to completely fix the solute N which is harmful, it is necessary to first heat to 1000 ° C. or higher and to roll or forge at this temperature to decompose VN and AlN. Moreover, after this rolling or forging, VN and AlN can be uniformly precipitated again by holding for 5 to 60 minutes, and solid solution N can be completely fixed.

  Since such an effect cannot be obtained when the rolling or forging temperature becomes lower than 1000 ° C., the temperature is preferably 1025 ° C. or higher, more preferably 1050 ° C. or higher. On the other hand, from the viewpoint of decomposing VN and AlN, the higher the rolling or forging temperature, the better. However, when the temperature exceeds 1250 ° C., the problem that the end of the billet begins to sag due to heat starts, so the upper limit is about 1250 ° C. And

  If the holding time after this rolling or forging becomes as short as 5 minutes or less, the fixation of the solid solution N by VN and AlN becomes insufficient, so it is preferably 8 minutes or more, more preferably 10 minutes or more. Incidentally, even if the holding time exceeds 60 minutes, there is no particular problem, but since the manufacturing process is wasted, the upper limit was set to 60 minutes. Further, by this holding (heat treatment), the solid solution V is calculated as a solid solution [V] = [V] -3.6 [N] ([V] is the V content, and [N] is the N content). 0.01% or more can be ensured by the amount of solid solution V.

  Normally, in manufacturing this type of steel material, it is not performed to keep the high temperature as in the present case after hot rolling. That is, from the viewpoint of the production efficiency of rolling, it is generally cooled immediately after hot rolling, and the holding time is a short time of about 1 minute or less.

Cooling rate to 500 ° C: 0.2 to 1.5 ° C / s
The rolled or forged steel material needs to have a mixed phase structure of ferrite and pearlite in order to refine the joint after friction welding. For this reason, the steel material which the said holding was completed cools in the range whose cooling rate to 500 degreeC is 0.2-1.5 degree-C / s. It should be noted that if the average cooling rate up to 500 ° C. is too high, bainite begins to be generated, and the effect of miniaturizing the joint portion cannot be obtained. For this reason, the cooling rate to 500 ° C. is preferably 1.4 ° C./s or less, more preferably 1.3 ° C./s or less. On the other hand, if the cooling rate is slower than 0.2 ° C./s, pro-eutectoid ferrite tends to grow, and the ferrite cannot be refined, so that it is preferably 0.3 ° C./s, more preferably 0.4 ° C. / S or more.

Friction welding, composite material:
The composite material by friction welding which is the subject of the present invention can be made of various steel types according to the target mechanical structure component, as long as friction welding can be performed by a commercially available friction welding machine. The other steel can be selected. In addition, various shapes can be selected for the steel material shape and the composite material shape of the present invention in accordance with the target mechanical structural component. For example, the steel materials of the present invention may be friction-welded, and the counterpart material may be a carbon steel for mechanical structures such as S45C or SCr420H, alloy steel, V-added steel, B-added steel, etc. You may select and combine on the basis of 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 friction welding composite materials are mainly used as friction welding, but when the steel material to be joined is alloy steel, it is subjected to surface hardening treatment such as carburizing, nitriding, carbonitriding, and then the alloy steel side. Only the tempering process can be made 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.

  As shown in Tables 4 to 7, the steel materials having various composition shown in Tables 1 to 3 are controlled in terms of the temperature of hot rolling or hot forging, the holding time, the cooling rate to 500 ° C., and the like. And the average area ratio of ferrite grains and pearlite grains was made separately. Then, by simulating machine structural parts, these steel materials were made into composite materials by friction welding with these steel materials and the counterpart materials as steel materials of S45C and SCr420H.

  Table 1 shows the solid solution V amount of the manufactured material steel, and Tables 4 to 7 show the solid solution N amount, the average area ratio of ferrite grains and pearlite grains, and the like. In addition, Tables 4 to 7 show the ferrite grain size of the steel material part after friction welding, the measurement and evaluation of impact characteristics and rotational bending fatigue characteristics of composite steel materials (friction welding parts), respectively. Here, Table 1 corresponds to Tables 4 and 5, Table 2 corresponds to Table 6, and Table 3 corresponds to (follows) Table 7.

Steel production conditions:
The steel materials of Tables 1, 2, and 4, 5, and 6 were manufactured as round bars by hot rolling under the following manufacturing conditions.
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: This ingot was heated to 1200 ° C., hot forged into a billet (155 mm square), and cooled.
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 was heated to 950 to 1250 ° C., rolled into a round bar of Φ80 mm, held at that temperature for 1 to 50 minutes, and then cooled at 0.05 to 12 ° C./s.

  On the other hand, the steel materials of Tables 3 and 7 were manufactured as round bars by hot forging under the following manufacturing conditions. Manufacturing up to melting / casting, hot forging of billet and dummy billet welding under the same conditions as the above hot rolled material, heating billet to 1050 ° C, hot forging to 80mm round bar, at that temperature After holding for 10 minutes, it was cooled at 0.6 ° C./s.

Measurement of the amount of solute N:
The value of “solid solution N amount” in the present invention was calculated in accordance with JIS G 1228 by subtracting all N compounds from the total N amount in steel.
(a) The total amount of N in steel uses an inert gas melting method-thermal conductivity method. A sample was cut from the test steel material, put in a crucible, melted in an inert gas stream, extracted N, transported to a thermal conductivity cell, and the change in thermal conductivity was measured.
(B) The amount of total N compounds in steel is determined by ammonia distillation separation indophenol blue absorptiometry. A sample is cut out from the test steel material, 10% AA electrolyte (non-aqueous solvent electrolyte that does not produce a passive film on the steel surface, specifically 10% acetylacetone, 10% tetramethylammonium chloride, Constant current electrolysis is performed in the remainder: methanol). About 0.5 g sample is dissolved, and the insoluble residue (N compound) is filtered through a polycarbonate filter having a hole size of 0.1 μm. The insoluble residue is decomposed by heating in sulfuric acid, potassium sulfate and pure Cu chips and combined with the filtrate. After making this solution alkaline with sodium hydroxide, steam distillation is performed, and the distilled ammonia is absorbed by dilute sulfuric acid. Phenol, sodium hypochlorite and sodium pentacyanonitrosyl iron (III) were added to form a blue complex, and the absorbance was measured using a photometer.
The amount of solute N in the steel was calculated by subtracting the amount of all N compounds from the total amount of N in the steel determined by the above method.

Method for measuring the average area ratio of ferrite and pearlite:
Cut each round bar after manufacture at the center in the longitudinal direction, embed the cut surface (radial cross section in the 90 ° direction relative to the longitudinal direction) in resin, and mirror-polish the sample surface with emery paper and diamond buff; The surface was etched with nital. Using an optical microscope, the D / 4 position was observed at a magnification of 400 times and five photographs were taken. The image was binarized using Image Pro Plus, and the ferrite grains (phase) were white and the pearlite grains (phase) were black. FIG. 1A and FIG. 2A show an example of this.

  Then, the average crystal grain size of ferrite and pearlite in each visual field is obtained from the maximum diameter of each grain (phase) of these images, and the average area of ferrite grains and pearlite grains is obtained from this average crystal grain size. It was. And the average area ratio of a ferrite grain and a pearlite grain and the average area of a ferrite grain / the average area of a pearlite grain were computed, and the average value of these 5 fields of view was made into the average area ratio of a ferrite / pearlite. Here, regardless of invention examples and comparative examples, the structure of all examples in this example is a mixed phase of only ferrite grains and pearlite grains shown in FIGS. 1 (a) and 2 (a). It consisted of.

Friction welding test:
A bar (test piece) of φ20 mm × 100 mmL was cut out from the D / 4 position along the longitudinal direction of each round bar after the heat treatment. 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, as the round bar composite steel material (steel part) in which the ends are butted against each other in the longitudinal direction as S35C, hot forged non-heat treated steel, and the cut bar material, and the counterpart material of the cut bar material, Each was friction welded. Friction welding was performed in accordance with the following conditions in common with each example.

Friction welding conditions:
Friction pressure: 80 MPa, friction time: 7 sec,
Upset pressure (pressure applied from both ends of the round bar to the joint): 160 MPa,
Upset time (pressurization time to the joint): 7 sec.
Rotation speed: 1600 rpm,
Total margin: 5-12mm (shrinkage from the original round bar length)

Evaluation of structure after friction welding:
Identification of the ferrite phase and evaluation of the size were performed as follows. The sample after friction welding was cut at the center in the horizontal direction of the joint surface (pressure contact surface), embedded in cold resin, and the sample surface was mirror-polished with emery paper, diamond buffing, and electrolytic polishing. Then, the surface of the sample was made to be within a depth range of 1 mm from the joint interface joined by friction welding as a surface on which the structure was observed and measured. The sample surface was observed and photographed with an optical microscope having a magnification of 400 times. Further, using a crystal orientation analyzer (EBSP), an image obtained by this optical microscope was analyzed, and BCC was measured as ferrite, and the equivalent circle diameter was measured as the diameter of ferrite grains.

Impact property evaluation:
Charpy test piece having a square cross section of 55 mmL and a square side of 10 mm in accordance with JIS Z 2242 so that the joining portion becomes a notch bottom from the center position of the friction bonded product (round bar composite steel material) of Φ20 mm × about 200 mmL Was made. The notch shape was a U notch, a notch radius of 1 mm, and a notch depth of 2 mm. Subsequently, the impact characteristic evaluation of the test piece was performed with the Charpy impact tester. The test conditions were room temperature and a load speed of 5 m / s. The test was performed 5 times, and the Charpy impact value (absorbed energy) was measured. And in all joining components, the composite steel material from which absorbed energy became 30J (joule) or more was set as the pass. All Charpy values listed in Tables 4 to 7 indicate the value of the absorbed energy (unit: J).

Fatigue property evaluation A No. 1 test piece having a diameter between gauges of φ6 mm was prepared from the center position of the friction bonded product (round bar composite steel material) of Φ20 mm × about 200 mmL according to JIS Z2274.
Subsequently, the rotating bending fatigue characteristic evaluation of the test piece was performed with the rotating bending fatigue testing machine. The test conditions were changed at a frequency of 20 Hz between a load stress of 700 MPa and 200 MPa, and a stress (MPa) corresponding to 10 ⁷ times life was obtained, which was used as an index of fatigue strength. In this example, a composite steel material having a fatigue limit stress of 300 MPa or more was regarded as acceptable for all joined parts. The 10 ⁷ times life described in Tables 4 to 7 all indicate this fatigue limit stress (unit: MPa).

  Table 1 and some of the rolled materials in Tables 4 and 5 (except for steel types 1F-1, 5, 9, 12, and 13), Table 2 and the corresponding rolled material steel types 2A to 2K in Table 6 and Table 3 All the forging steel types in Table 7 corresponding to are invention examples. In these invention examples, the steel component composition containing solute V and the structure including the average area ratio of ferrite grains and pearlite grains satisfy the conditions of the present invention. As a result, the friction-welded composite material has excellent impact characteristics and bending fatigue characteristics required for automobile engine parts and the like.

  On the other hand, a part of the rolling material of Table 4 corresponding to Table 1, steel types 1F-1, 1F-5, 1F-9, 1F-12, 1F-13, and rolling of Table 6 corresponding to Table 2 The steel types 2L to 2Z are comparative examples, and the impact characteristics and bending fatigue characteristics required for automobile engine parts and the like are significantly inferior to those of the invention examples.

Comparative Examples of Rolled Materials in Table 4 Steel types 1F-1, 1F-5, 1F-9, 1F-12, 1F-13, as shown in Table 1, the steel component composition including the V amount satisfies the conditions of the present invention. . However, these steel types are inadequate in rolling conditions (manufacturing conditions) for obtaining an appropriate amount of solute N as compared with steel type 1F of other invention examples, and do not satisfy the structural requirements of the present invention. Therefore, impact characteristics and bending fatigue characteristics are remarkably inferior compared with the steel type 1F of other invention examples.
Since the rolling temperature of steel type 1F-1 is too low, the amount of solute N is outside the upper limit.
In Steel Type 1F-5, the retention time after rolling is too short, so the amount of solute N is outside the upper limit.
In Steel Type 1F-9, the cooling rate after holding is too slow, so the particle size after friction welding is coarsened.
In steel types 1F-12 and 1F-13, the cooling rate after holding is too fast, so the fraction of ferrite grains and pearlite grains is too low.

  As shown in Table 2, the steel types of Comparative Example Steel Types 2L to 2Z are out of the ranges defined by the present invention (upper and lower limits) in terms of the content of main elements, the amount of solute V, the amount of solute N, and the like. Yes. As a result, as shown in Table 6, the comparative examples are significantly inferior to the invention examples in terms of impact characteristics and bending fatigue characteristics required for automobile engine parts and the like as the friction-welded composite materials.

Among these, the steel types 2X, 2Y, and 2Z have appropriate rolling conditions (manufacturing conditions), but the original V amount and solute V amount are too much outside the upper limit values defined in the present invention.
Steel type 2Y has an excessive amount of V of the steel material, and the toughness and impact characteristics required for automobile engine parts and the like are low as the friction-welded composite material.
In steel type 2X, the original V amount of the steel material is too small, and the solute V amount is too small. For this reason, the ferrite / pearlite ratio is close to 1, resulting in martensite, and the toughness and impact characteristics are low.
In steel type 2Z, the solid solution V amount of the steel material does not satisfy the judgment formula and is too much. Therefore, the ferrite / pearlite ratio is close to 1, resulting in martensite, and the toughness and impact characteristics are low.

Furthermore, in the steel types 2L to 2Z, the content of main elements is outside the range (upper and lower limit values) defined in the present invention.
In steel types 2L and 2M, the C content deviates from the upper and lower limits.
In the steel types 2N and 2O, the Si content deviates from the upper and lower limits.
In steel types 2P and 2Q, the Mn content deviates from the upper and lower limits.
In the steel type 2R, the P content is outside the upper limit.
In the steel type 2S, the S content is outside the upper limit.
In steel type 2T, the Cr content is outside the upper limit.
In steel types 2U and 2V, the Al content deviates from the upper and lower limits.
In the steel type 2W, the N content is 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.

  ADVANTAGE OF THE INVENTION According to this invention, the steel materials and friction welding components for machine structures suitable for friction welding which improved component characteristics, such as fatigue strength and impact strength, can be provided. For this reason, it can be suitably used as an engine component such as a piston pin used in an automobile engine, a transmission, a differential, or the like, or a mechanical structure component such as a gear, a shaft, or a connecting rod, which is friction-welded.

Claims (5)

  In mass%, C: 0.25 to 0.65%, Si: 0.02 to 2.0%, Mn: 0.7 to 3.0%, P: 0.03% or less (including 0%) S): 0.005 to 0.1%, Cr: 1% or less (excluding 0%), Al: 0.005 to 0.1%, N: 0.02% or less (provided 0%) V: 0.03 to 0.5% each, solid solution [V] = [V] -3.6 [N] ([V] is V content, [N] is N content) The amount of solid solution V calculated in (quantity) is 0.01% or more, while the amount of solid solution N is restricted to 0.001% or less (including 0%), the balance is made of iron and inevitable impurities, and the steel structure However, the average area ratio of ferrite grains to pearlite grains (average area of ferrite grains / average area of pearlite grains) is not less than 0.05 and not more than 0.5, and the total area ratio of ferrite-pearlite is based on the total structure. Over 90% That the machine structural steel suitable for friction welding, characterized by comprising a mixed phase of ferrite grains and pearlite grains.   In the steel for machine structure, Ti: 0.2% or less (excluding 0%), Nb: 0.2% or less (excluding 0%), B: 0.01 as other elements % Or less (excluding 0%), Mo: 1% or less (excluding 0%), Cu: 1% or less (excluding 0%), Ni: 1% or less (excluding 0%) The steel for machine structural use suitable for friction welding according to claim 1, comprising one or more of the following.   In the steel for machine structure, as other elements, 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 (not including 0%), or any one or more of them is suitable for friction welding according to claim 1 or 2. Steel for machine structure.   The steel for machine structure according to any one of claims 1 to 3 is formed into a desired shape by forging or cutting, and then a composite steel material joined to the same steel for machine structure or other steel materials by friction welding. Friction welded parts with excellent impact and bending fatigue properties.   5. The impact characteristics and bending fatigue characteristics according to claim 4, wherein the average diameter of the ferrite grains is 5 μm or less in a depth range from the joint interface where the friction welding parts are joined by the friction welding to 1 mm. Excellent friction welding parts.

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