JP5314582B2 - Manufacturing method of machine parts - Google Patents

Description

  The present invention relates to a method for manufacturing a machine part by cold forging machine structural steel.

  Conventionally, machine parts used in vehicles such as automobiles are generally manufactured by hot forging and cutting a carbon steel material for machine structure and a low alloy steel material for machine structure. Examples of these mechanical parts include connecting rods, crankshafts, constant velocity joints, transmission gears, large screws (screws), gears, pulleys, bolts / nuts, pinion gears, steering shafts, valve lifters, common rails, torsion bars, etc. .

  However, in recent years, in the manufacture of the mechanical parts, switching from the hot forging process to the manufacturing by the cold forging process is directed due to the improvement of productivity and the problem of the global environment. That is, by switching from hot forging that requires a large amount of heat to cold forging, it is possible to reduce the amount of carbon dioxide emission in the mechanical component manufacturing process. In addition, since the dimensional accuracy of the molded product processed by cold forging is high, the cutting cost in the cutting process for finishing to the final part shape can be reduced, or the cutting process can be omitted, improving the productivity and manufacturing. Cost can be reduced.

  However, when machine parts such as connecting rods and large screws manufactured by hot forging are manufactured by switching to cold forging, cold forging has higher deformation resistance than hot forging. For this reason, it is necessary to reduce the deformation resistance and improve the deformability by reducing the alloy elements that contribute to spheroidizing annealing and strengthening. However, in normal cold forging steel, only the component strength corresponding to the deformation resistance can be obtained. Therefore, quenching and tempering and aging treatment are performed after cold forging to obtain the required component strength.

  On the other hand, in order to obtain the required component strength in cold forging, it is necessary to use steel materials with a high degree of deformation resistance. However, such steel materials are inferior in workability and thus greatly reduce the mold life. There is a concern that the machine parts during cold forging are likely to crack.

  That is, the deformation resistance and the component strength are in a contradictory relationship, and it has been necessary to add a process such as a heat treatment in order to make the deformation resistance and the component strength compatible in the prior art. For this reason, it is necessary to repeat the cold forging and the heat treatment for softening a plurality of times until the final shape (finished shape) of the machine part is completed. On the contrary, the number of manufacturing steps increases and the processing time increases. As a result, the manufacture of machine parts by cold forging has not necessarily led to the improvement of productivity, cost reduction, or reduction of carbon dioxide emission in the manufacturing process.

  On the other hand, conventionally, various steels having improved cold forgeability by controlling the steel components and the steel structure of the steel for cold forging have been proposed. For example, typically, the steel material, in particular, the content of S and O (oxygen), etc. is limited, and the specific composition or coarse inclusions or non-metallic inclusions present in the steel material are limited, It has been proposed to control the shape of MnS or the like. It has also been proposed to regulate the amount of cementite in the ferrite-pearlite structure of cold forging steel.

  However, these conventional techniques for improving cold forgeability of steel for cold forging are generally used in order to obtain the required component strength. Low carbon steel containing (carbon) or medium carbon steel containing 0.3 to 0.7% by mass of C is targeted. For this reason, even if the cold forgeability can be improved, it is still difficult to obtain the same low deformation resistance and high workability as the hot forge.

  In order to reduce the deformation resistance of steel and improve the deformability, it is known that additive elements such as C (carbon), Si, and Mn may be reduced. However, if the additive element is simply reduced and the deformation resistance is lowered, the tool life can be improved, but the problem arises that the required component strength cannot be obtained after cold forging. Therefore, conventionally, after cold forging steel into a predetermined shape, heat treatment such as quenching and tempering has been performed to ensure a predetermined hardness.

  However, if the heat treatment is performed after the parts are processed, the dimensions of the parts will change, so that the parts must be further processed. In order to improve productivity and save energy, there is a demand for a solution that can secure a predetermined hardness and at the same time omit the heat treatment after cold forging and the subsequent processing.

  Against such a background, depending on the presence of nitrogen of solid solution N (solid solution nitrogen) or nitride compound (nitride, N compound), cold forgeability (deformation resistance and deformability) and after cold forging Attempts have been made in the past to balance strength.

  Patent Document 1 discloses that a fine nitride compound is precipitated in ferrite grains and a carbide such as cementite is precipitated using this as a nucleus in order to suppress an increase in deformation resistance during cold forging. . From this, it is disclosed that solid solution N and solid solution carbon are fixed as nitride compounds and carbides to suppress an increase in deformation resistance by suppressing dynamic strain aging during processing.

  Patent Document 2 discloses that age hardening due to carbon and nitrogen is suppressed by controlling the amount of N and solute Al to fix N as AlN, and further precipitating carbon as a carbide by aging treatment. .

  In the methods of Patent Documents 1 and 2 described above, in order to suppress dynamic strain aging and suppress an increase in deformation resistance, solid solution N and solid solution carbon are fixed as nitride compounds and carbides in ferrite grains. . In order to fix solute N and solute carbon, it is necessary to add Al. If Al is 0.039 to 0.045% as in the examples, it is considered that there is almost no solid solution N even if the nitrogen content is 0.015%.

  Patent Document 3 discloses a method for reducing deformation resistance during cold working by lowering the hardness of a steel material due to softening by solid solution by adding Cr and fixing solid solution N by adding Al. However, even in this method, since solid solution N is fixed by adding Al, it is considered that there is almost no solid solution N as in Patent Documents 1 and 2 above.

  In cold-worked parts after cold working, hardening heat treatment, for example, quenching and tempering, may be performed to ensure a predetermined hardness, but as described above, quenching and tempering are performed from the viewpoint of productivity improvement and energy saving. There is a need to omit it. For example, Patent Document 4 discloses that aging treatment (quenching and tempering) after cold forging can be omitted by cooling from the exothermic temperature after cold forging to room temperature at a cooling rate of 50 to 70 ° C./hr. Yes.

  However, as described above, cold forging workability and hardness after cold forging are contradictory properties, and conventionally, a steel for cold working that is good for both has not been obtained. On the other hand, in Patent Document 5, the material steel material contains a predetermined amount or more of solid solution N, the hardness after cold working is increased, and the harmful effects of solid solution N are suppressed by high-speed cold working. Thus, it has been proposed to maintain good cold workability and improve the hardness of steel parts after cold work even if quenching and tempering after cold work is omitted.

  Specifically, as steel for high-speed cold working, in mass%, C: 0.03-0.15%, Si: 0.005-0.6%, Mn: 0.05-2%, P: 0.05% or less (not including 0%), S: 0.05% or less (not including 0%), and N: 0.04% or less (not including 0%), the balance being iron It consists of unavoidable impurities, and the amount of solute N in the steel is 0.006% or more.

Japanese Patent No. 3515923 JP 60-82618 Japanese Patent Publication No.57-60416 JP 2003-266144 A JP 2008-163410 A

  However, even in Patent Document 5, if the amount of solute N in the entire steel material is increased in advance in order to obtain the necessary strength for omitting quenching and tempering after cold working, the cold forgeability also decreases. To do. This is because the influence of dynamic strain aging becomes significant over the entire material steel material with a high amount of solute N, so the deterioration of deformability becomes more significant than the increase in strength, and during cold forging This is because cracks are likely to occur in the product.

  For this reason, in order to improve cold forgeability, when a soft steel material whose C content is lowered to an extremely low carbon region of 0.06 mass% or less is used, the machine part (cold forged product) is used. In fact, there has never been a manufacturing technique that can sufficiently ensure the required strength.

  The present invention has been made in view of such a problem, and even if the soft steel material is used, the strength of the product can be sufficiently ensured, and cold forging can achieve both the deformation resistance and the component strength. A method of manufacturing a machine part using the

The gist of the manufacturing method of the machine part of the present invention for achieving this object is mass%, C: 0.005 to 0.06%, Si: 0.005 to 0.05%, Mn: 0.4 to 1%, P: 0.05% or less (excluding 0%), S: 0.005 to 0.05%, Al: 0.005 to 0.1%, N: 0.008 to 0.02% Each of which comprises a steel for structural machine made of iron and inevitable impurities, and when the machine part is manufactured by cold forging, corresponding to a partially strengthened region of the machine part, before the cold forging A partial high-strength region in the mechanical structural steel is determined in advance, and the solid solution N amount in the partial high-strength region is partially increased to an amount necessary for the high strength in advance. in, when imparting plastic strain at least this partial strengthening region 200 ° C. below ambient temperatures for preparative To perform a cold forging which imparts plastic strain against a portion other than the partial strengthening region, to increase the strength of the partial high intensity region of the machine component, said machine component shape It is to be.

  Here, the partial strength enhancement region of the mechanical component means a portion of the mechanical component that requires higher strength than other portions. In addition, the partial strength enhancement region in the material machine structural steel before cold forging is a region that becomes a partial strength enhancement region of the mechanical component by the cold forging. However, as long as this partial high-strength region does not impair the cold forgeability, it includes other parts (no need for high-strength) and has an area larger than that of the partial high-strength region. It may be large. On the other hand, if high strength in the machine part is guaranteed, the partial high strength region does not necessarily need to include all of the partial high strength region in the machine part. The area may be smaller than a typical high-strength region. In the present invention, with these meanings, it is described that the partially strengthened region in the mechanical structural steel before the cold forging corresponds to the partially strengthened region of the machine part. ing.

  Moreover, in the case of the cold forging, it is necessary to apply plastic strain (strain) at an atmospheric temperature of 200 ° C. or less to a partially strengthened region in the material machine structural steel before the cold forging. For other parts, this condition may be removed. That is, as long as it does not adversely affect the partial strength enhancement region, the ambient temperature may exceed 200 ° C., and there may be a portion where plastic strain is not applied. Therefore, in the present invention, with these meanings, it is described that plastic strain is applied at an atmospheric temperature of 200 ° C. or less to at least the partial high-strength region during the cold forging.

  In the present invention, as a preferred embodiment, the solid solution N amount in the partially strengthened region in the mechanical structural steel is in the range of 0.008 to 0.014%, and in this partially strengthened region. The applied cold forging plastic strain is in the range of 0.5-5. As a preferred embodiment, the solid solution N amount in the partially strengthened region is a model formula based on any one of deformation resistance (DR), component strength (Hv), and fatigue strength (F), or in advance. Obtain from the acquired experience. As a preferred embodiment, the amount of solute N in the partially strengthened region in the mechanical structural steel is increased by heat treatment in the partially strengthened region.

  In the present invention, when using an extremely low carbon and soft steel material as a material for cold forging, the amount of solid solution N (solid nitrogen amount) is increased only in a portion where the strength is required in the mechanical parts, and the plasticity of cold forging is increased. The strength is partially increased by strain. At this time, the other portions as a whole ensure the cold forgeability while maintaining the original solid solution N amount, that is, the original soft steel for cold forging. In other words, the solid solution N amount is partially increased to the solid solution N amount necessary for increasing the strength by, for example, heat-treating the portion requiring this strength. And the intensity | strength of this part (part which requires intensity | strength) is increased to the said required intensity | strength by provision of the cold forging plastic strain to the partial area | region where this amount of solute N increased. In the present invention, the amount of solid solution N of such a steel material for cold forging is partially increased. As in Patent Document 5, the amount of solid solution N of the whole material for cold forging material is set in advance. Higher, the cold forgeability is not sacrificed for increasing the strength.

  A steel material for cold forging that is extremely low carbon and soft can usually obtain only a component strength corresponding to deformation resistance even when cold forging. However, according to the knowledge of the present inventors, when dynamic strain aging is caused by the solid solution N during cold forging, more mobile dislocations are generated than usual, and heat generation during cold forging and subsequent It became clear that the static strain aging occurs after processing by cooling. That is, since the allowance for static strain aging due to the solid solution N is synergistically added to the work hardening allowance of the steel material, the mechanical component strength can be increased more than the normal deformation resistance. In addition, the strength of the other portion, which is the original amount of solute N, is increased only by the ordinary deformation resistance, and the cold forgeability can be ensured.

  Therefore, according to the present invention, when a soft steel material having a C content of 0.06% by mass or less in an extremely low carbon region is made into a mechanical part by cold forging, the strength of the product can be sufficiently secured. Thus, it is possible to achieve both the deformation resistance and the component strength.

It is the flowchart which showed the process of the cold forging in this invention in order. It is explanatory drawing which showed the manufacturing process of the large sized screw (mechanical component) by this invention in order.

  The method for manufacturing a machine part according to the present invention will be specifically described below with reference to the drawings. The present invention can be applied to various machine parts that have been manufactured by hot forging or cold forging as described above. In the following description, a large screw will be described as a representative example.

  FIG. 1: has shown the flowchart which showed in order the process (procedure of the manufacturing method of a machine component) of the cold forging in this invention. FIG. 2 shows in sequence a manufacturing process of a large screw according to the present invention (procedure of a manufacturing method of a machine part).

  In the following description of the method for manufacturing a machine part based on FIGS. 1 and 2, the contents of the design stage are included, and S1: product shape determination process to S4: material property determination process, which will be described later, are in the design stage. (Pre-processing stage), and S5 and subsequent stages are specific processing stages.

S1: Product Shape Determination Step As shown in FIGS. 1 and 2, first, when manufacturing the large screw 1, the product shape A of the large screw 1 is determined as in the S1: product shape determination step. That is, machining is performed after cold forging (after strength increase), but the shape of the large screw 1 after the completion of the machining, that is, the product shape A of the large screw 1 when made into a product is in the production drawing. Determine from. This shape is often determined by user requests.

S2: Shape determination step before cold forging Next, S2: As a shape determination step before cold forging, considering the flow of the steel material during cold forging into the product shape A determined in the product shape determination step S1. The shape B before cold forging is determined. For example, in the cold forging process, the shape B before cold forging is determined by adding the dimensions of the amount (for example, calculated from the volume of the steel material) that the steel material of the screw part and the head part moves.

  More specifically, in this step S2, the dimensions of the machining allowance (for example, several millimeters) at each location are added to the product contour line (manufactured contour line) of the large screw 1 indicated by the dotted line, A contour line after cold forging (contour line after increasing strength) is obtained, and a shape B before cold forging that can be formed by cold forging is determined. Determine the shape before the strength increase (shape before strain), and determine the diameter, length, etc. of the round bar 3 from which the large screw 1 is based from the shape before the strength increase (width, height, volume, etc.) Determine the initial shape of the material.

Partially strengthened areas:
Here, the product shape A (mechanical part), which requires a higher strength than the other parts, corresponds to a partial strength-enhanced region, and is a partial in the material machine structural steel before cold forging. The high strength region is also determined in advance. If the machine part is the large screw 1, the parts that require high strength are the shaft portion and the screw portion region of the large screw 1. The material mechanical structure steel in FIG. 2 is the partial strength enhancement region in the steel for structural machinery before cold forging corresponding to the partial strength enhancement region of the shaft portion and screw portion of the large screw 1. This is a blackened portion in S3 in the shape B.

  The portion blacked out in S3 of FIG. 2 (partial high-strength region in the material machine structural steel) substantially corresponds to the shaft portion and the screw portion region of the large screw 1 on a one-to-one basis. In other words, the size and the position are set so that the region of the shaft portion and the screw portion of the large screw 1 is almost formed by cold forging. However, as described above, this partially strengthened region in the raw material machine structural steel may include other portions (which do not require increased strength) as long as the cold forgeability is not impaired. On the other hand, if high strength is guaranteed in the machine part, this partial high strength region does not necessarily need to include all of the partial high strength region in the mechanical component.

S3: Strength increase allowance determining step (Solution of N amount for increasing strength)
In the present invention, the amount of solid solution N in the partially strengthened region in the raw structural steel and the control of plastic strain (strain) by cold forging applied to the partially strengthened region. Thus, the required strength of the partially strengthened region in the machine part is obtained. Among these, first, as S3: strength increase allowance determining step, the plasticity imparted during cold forging to the pre-cold forging shape B determined in the pre-cold forging shape determination step S2 to the portion requiring strength. Based on the strain amount ε, the solute N amount is determined. This preferable plastic strain amount ε will be described later. Here, the amount of solute N should be increased only in the part of the partially strengthened region corresponding to the highly strengthened region of the machine part. The amount of solute N may be increased.

  For example, in the large screw 1, when strength is required for the shaft portion and the screw portion of the screw, the region of the shaft portion and the screw portion of the screw (high-strength region) is calculated by calculating the required reinforcement allowance. The amount of solute N required in the portion where high strength is required) is determined. In this S3: strength increase allowance determining step, only a material region corresponding to the high-strength region intentionally or forcibly in advance for a portion requiring strength (partial high-strength region), The amount of solute N is partially increased, and YS (yield strength) and TS (tensile strength) of this portion are increased by cold forging. That is, the necessary amount of solute N is calculated in advance with the intention of increasing the strength in this high strength region.

  The determination of the amount of solute N in the partially strengthened region of the material is based on a model formula based on one of deformation resistance (DR), component strength (hardness Hv), and fatigue strength (F), or in advance. Use the acquired experience. Here, as the model formula, for example, the solid solution N amount is determined using model formulas shown in the following formulas (1) to (4).

Solid solution N amount = f- ¹ (required yield strength-yield strength of steel) (1)
Solid solution N amount = g- ¹ (Required tensile strength-Tensile strength of steel) (2)
Solid solution N amount = h- ¹ (Required hardness-Hardness of steel) (3)
Solid solution N amount = i- ¹ (Required fatigue strength-Fatigue strength of steel) (4)
Here, f, g: stress-solid solution N amount curve or deformation resistance-solid solution N amount curve function, h: hardness-solid solution N amount curve function, i: fatigue strength-solid solution N amount curve Is a function of

Strength improvement mechanism by solute N:
The strength improvement mechanism by the solute N will be described. Solid solution N is generally present in steel as an interstitial solid solution element, and increases the deformation resistance of the steel material by generating dynamic strain aging during plastic deformation such as cold forging. There is a problem of degrading (workability). Therefore, in this type of cold forging steel field, it is treated as a harmful impurity and is usually fixed as a nitride compound with Al, Ti or the like.

  By the way, the inventors have clarified that the deterioration of the deformability in the steel for cold forging mainly occurs at the interface between the hard phase (for example, pearlite) and the soft phase (for example, ferrite) in the steel. . Therefore, as an ultra-low carbon steel, the deformability could be improved by significantly reducing the carbon content and reducing the hard phase as much as possible. In addition, regarding the increase in deformation resistance, it was considered that the strengthening of ferrite by dynamic strain aging with a solid solution N amount facilitates strain into the hard phase, resulting in a high deformation resistance. For this reason, in the ultra-low carbon steel as in the present invention, as a result of significantly reducing the carbon content and reducing the hard phase as much as possible, it is clear that the deformation resistance does not increase so much within a certain range of solute N content. became.

  On the other hand, with regard to component strength, ordinary cold forging steel can only obtain component strength according to deformation resistance. However, when dynamic strain aging is caused by solute N during cold forging, it is more than usual. It was found that many mobile dislocations were generated, and static strain aging occurred after machining due to heat generation during cold forging and subsequent cooling. That is, it became clear that the strength of static strain aging due to solute N is added to the work hardening allowance of the steel material, so that the component strength can be increased more than the deformation resistance. As a result, by increasing the amount of solute N only in the part where the strength of the steel material is to be increased, the deformation resistance of the part where the strength is not so required can be reduced as a result, and as a result, the strength of the component can be ensured and the cooling can be reduced. It is possible to achieve both improvement of the mold life. If the large screw 1 is simply cold forged without controlling the amount of solute N as in the prior art, as described above, there are places where the strength required for the product cannot be obtained.

Solid solution N amount required for strength improvement:
Here, the amount of solute N necessary for improving the strength of the partially strengthened region of the material (the amount of solute N adjusted in advance) is 0.008 to 0.014% by mass. By adjusting the solid solution N amount of the partially strengthened region of the material in this range in advance before cold forging, the deformation resistance is not increased so much and a strengthening allowance according to the solid solution N amount is given. It becomes possible.

  On the other hand, when the amount of the solid solution N is less than 0.008% by mass, the amount of the solid solution N is too small and a sufficient strengthening allowance cannot be obtained even by cold forging. In addition, when the amount of solute N exceeds 0.014% by mass, the effect of dynamic strain aging becomes remarkable, so that the deterioration of deformability becomes more significant than the increase in strength, and during cold forging, The product tends to crack.

Method for measuring the amount of solute N:
The method for measuring the amount of solute N in the present invention is based on JIS G1228, and the amount of solute N in steel is obtained by subtracting the total N (total nitrogen) compound from the total N (total nitrogen) content in the steel. calculate. First, (a) inert gas melting method-thermal conductivity method is used for measuring the total N content in steel. Cut the sample from the test steel material, put the sample in a crucible, extract it in an inert gas stream, extract N, transport it to the thermal conductivity cell, measure the change in thermal conductivity, Calculate the total N content.

  Next, (b) ammonia distillation separation indophenol blue absorptiometry is used for the amount of all nitride compounds in steel. 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). Then, about 0.5 g of the sample is dissolved, and the insoluble residue (nitride compound) is filtered through a polycarbonate filter having a hole size of 0.1 μm. This insoluble residue is decomposed by heating in sulfuric acid, potassium sulfate and pure Cu chips and combined with the filtrate. After making this solution alkaline with sodium hydroxide, steam distillation is performed, and the distilled ammonia is absorbed by dilute sulfuric acid. Phenol, sodium hypochlorite and sodium pentacyanonitrosyl iron (III) are added to form a blue complex, and the absorbance is measured using an absorptiometer to determine the total amount of nitrided compounds in the steel.

  And the amount of solid solution N can be calculated | required by subtracting the total amount of nitriding compounds calculated | required by the method of said (b) from the total amount of N calculated | required by the method of said (a).

Means for increasing the amount of solute N:
Means for increasing the amount of solute N necessary for improving the strength of the partially strengthened region of the material (to secure the necessary amount of solute N) is a control of N that is fixed as a nitride compound. That is, it is carried out by controlling the amount of precipitation of the nitride compound. Solid solution N tends to form nitride compounds (nitrogen compounds) with Al, Ti, Nb, V, B, Hf, Ta, Zr, and the like. However, in the heat treatment temperature range in which the necessary amount of solid solution N is ensured in the production method of the present invention, the amount of solid solution N is determined mainly by the bonding state between Al and nitrogen. Nitride compounds of elements other than Al, such as Ti, Nb, V, B, Hf, Ta, and Zr, are difficult to decompose in the heat treatment temperature range of the production method of the present invention, require good productivity, and require a solid solution N amount. It is more difficult to secure the amount than Al. Therefore, in the present invention, Al is mainly used for securing the necessary amount of the solute N amount.

  For this purpose, the shaft portion of the large screw 1 and the region of the screw portion determined by the S2: shape determination step before cold forging (in S3 in the steel shape B for material mechanical structure before cold forging in FIG. 2) Blackened portion) = Cold forging is performed by performing a heat treatment to be described later on a partially strengthened region (corresponding to the partially strengthened region) of the shape B before cold forging. It adjusts to the amount of solute N determined based on the plastic strain amount (epsilon) provided.

Plastic strain applied by cold forging:
In the present invention, as described above, the amount of solid solution N in the partially strengthened region of the material is controlled to the above-described range of 0.008 to 0.014%, and the partially strengthened region is included. By controlling the cold forging plastic strain applied to the steel, the necessary strength as a machine part is obtained. In order to increase the strength of the partially strengthened region of the material more than the deformation resistance by cold forging, giving the work hardening allowance of the steel material the strengthening allowance of static strain aging by solute N. The cold forging plastic strain is set in the range of 0.5 to 5 on the premise of the range of the solid solution N amount in the partially strengthened region of the material.

  By imparting plastic strain within this range, movable dislocations are appropriately introduced into the steel, so that the component strength can be improved by static strain aging during cooling after processing. When the amount of plastic strain applied to the partially strengthened region of the material is less than 0.5, it is not possible to provide sufficient strengthening allowance. In addition, when the amount of plastic deformation of a portion requiring reinforcement is less than 0.5 in a normal cold forging process, a plastic working allowance is added in advance (the amount of steel in that portion is increased in advance). Cold forging may be performed.

  On the other hand, when the amount of plastic strain applied to the partially strengthened region of the material exceeds 5, the processing limit of the steel material is exceeded, and the deformability starts to deteriorate, so that the part is likely to crack, Parts cannot be processed by forging.

  When the plastic strain amount is 0.5, the cylindrical steel material is compressed from the state where both ends of the cylindrical steel material are constrained until the height of the cylindrical steel material is reduced by 34% (66% of the original height). It corresponds to the amount of strain. The plastic strain amount 5 corresponds to the strain amount when compressed until the height is reduced by 99% when compressed in the same manner. Therefore, in the present invention, the unit of plastic strain is dimensionless. However, when strain is applied from various axes in various parts, it is different from a processing method in which the height is simply reduced, and thus a large strain may be applied even with a smaller amount of deformation.

  In addition, the amount of plastic strain to be applied to the steel material part other than the partial high strength region of the material may be the same as this partial high strength region (high strength region), but may be changed, In this case, it is preferable to select from the range of 1 to 8. If the amount of plastic strain is greater than 8, the strength increases too much and the workability of the steel material deteriorates, and parts cannot be processed by cold forging. Further, if the amount of plastic strain to be applied is less than 1, sufficient part strength cannot be obtained for the entire machine part.

S4: Material property determining step:
Based on the above, S4: As the material property determination step, the heat treatment conditions (heat treatment) for setting the partial strength-enhancing region of the material to the necessary solute N amount in relation to the N content of the steel material Means, heating rate, heating temperature, holding temperature / time, cooling rate, etc.) are selected and determined.

  As a method for obtaining the solid solution N amount-temperature curve, a calculation by Thermo-calc, a calculation by a solubility product, a method of actually measuring a steel material whose solid solution N amount is adjusted at each temperature by the extraction residue method described above, and the like are used.

  In this heat treatment, only a partial high-strength region (high-strength region) of the material is partially heated to decompose AlN, which is an existing form in the original steel material, There is a method for increasing the amount of solute N in the high strength region. In addition, the amount of solute N of the original steel material is increased by the necessary amount as a whole, and heat treatment is performed only on the portion where strength is not required, and by forming AlN, only the portion where strength is not required, There is a method for reducing the amount of solute N.

  The bonding state of AlN depends on temperature. The higher the heating temperature, the easier it is for AlN to decompose. By setting the cooling rate after heating and holding to a certain level (0.5 ° C / s or higher), AlN does not precipitate during cooling, and the amount of dissolved N can be reduced. It can be adjusted as designed.

  As specific heat treatment conditions, only the partially strengthened region (region corresponding to the shaft portion and the screw portion of the large screw 1 which is the strengthened region) of the steel material that has been preliminarily heated is heated to 700. By holding in the range of ˜1100 ° C. and cooling at the cooling rate, a part of the AlN produced in the steel is decomposed to ensure the required amount of solid solution N. Thus, the amount of solute N can be increased and adjusted to the appropriate amount only in the partially strengthened region of the steel material.

  A known high-frequency heating means is suitable as a means suitable for such a partial and rapid heat treatment or cooling heat treatment. In order to partially heat-treat only the high-strength region or only the portion that does not require strength as described above, it can be partially applied to steel materials, and can be rapidly heated (rapidly heated) and rapidly Therefore, a heat treatment means capable of cooling (rapid cooling) is required. If the heat treatment means is applied to the entire steel material (or extends), or if the heating rate or the cooling rate is slow, the entire steel material is heat-treated by heat conduction, making it difficult to control the partial amount of solute N. .

  Based on the above process design, in FIG. 1 and FIG. 2, the process of S5-S7 manufactures machine parts centering on cold forging.

S5: Shape processing step before cold forging (preliminary processing step)
The initial shape (diameter, length, etc.) of the round bar 3 that is the basis of the large screw 1 is the shape B before cold forging (the shape before increasing strength suitable for cold forging, the shape before strain). In S5, preliminary processing is performed. This preliminary processing is performed by hot forging or machining such as cutting.

S6: Material property processing step Next, in the material property processing step (the heat treatment step) of S6, the amount of solid solution N in the partially strengthened region (a portion requiring strength) of the material is determined according to the above-described conditions. Control by performing partial heat treatment such as by high-frequency heating.

S7: Strength increasing process (cold forging process)
After the material property processing step of S6 (the heat treatment step), cold forging is performed while applying the plastic strain, and in particular, the strength of the partially strengthened region of the material is increased, and mechanical parts ( Product) shape. In this cold forging (strength increasing step), the plastic strain amount is given to the material of the shape B before cold forging, and a partial high strength region of the raw material is set to a necessary high strength. At the same time, the product shape A shown in FIG.

  For example, as described in the design stage, since the strength of the shaft portion 2 is increased in the large screw 1, cold forging is performed while straining the shaft portion 2 in the strength increasing step S6. Specifically, in this strength increasing step S6, compression or the like (partial reduction) is performed so that the pre-strained contour line L4 corresponding to the shaft portion 2 matches the post-forging contour line (contour line after increasing strength) L2. Strain is applied, thereby increasing the TS (tensile strength) of the shaft 2.

  The method for performing cold forging is not limited, and compression in the axial direction (compression partial compression) may be performed, extrusion molding (extrusion die forging) may be performed, or die forging may be performed. .

  At the time of cold forging (cold working), it is necessary to apply plastic strain at an atmospheric temperature of 200 ° C. or lower for the partial high-strength region, and the temperature before processing of the material is less than 100 ° C. It is preferable. Moreover, in order to suppress the process heat generation at the time of cold forging, the partially strengthened region may be forcibly cooled. For other parts, this condition may be removed. That is, as long as it does not adversely affect the partial strength enhancement region, the ambient temperature may exceed 200 ° C., and there may be a portion where plastic strain is not applied.

  In the above description, the strength increasing step (cold forging step) S7 is performed after the preliminary processing step S5. However, as shown by the arrow extending from the upper side of S2 in FIG. Without performing, you may perform cold forging process S7 directly with respect to the round bar 2 determined by the said shape determination process S2 before cold forging. In this case, while performing cold forging so as to have a shape with machining allowance, a strain is applied to a portion (for example, a shaft portion) requiring strength according to the plastic strain amount ε.

Steel composition:
The composition of the steel material that is the material (material) of the machine part will be described below. The material steel material must have a property of improving TS and YS when the plastic strain is applied by cold forging. In particular, the property that TS improves in proportion to the applied plastic strain is required. Further, the total N content that can be adjusted in advance to the amount necessary for increasing the strength of the solid solution N in the partially increased strength region is required. Furthermore, it is necessary to satisfy various characteristics such as strength, workability, machinability, and corrosion resistance for machine parts.

  The composition of the steel material for this purpose is mass%, C: 0.005 to 0.06%, Si: 0.005 to 0.05%, Mn: 0.4 to 1%, P: 0.05% or less. (Excluding 0%), S: 0.005 to 0.05%, Al: 0.005 to 0.1%, N: 0.008 to 0.02%, the balance being iron and inevitable impurities It is a steel for machine structural use. The element contents described here are all mass%.

C: 0.005-0.06%
C is an element that greatly affects the formation of the structure of the steel material, and it is necessary to reduce it as much as possible in order to make the structure of the steel material a ferrite single phase structure. Further, it is an element that greatly affects deformation resistance and deformability during cold forging, and it is desirable to reduce it as much as possible for cold forgeability. When it contains excessively, pearlite will produce | generate in the structure | tissue of steel materials, deformation resistance will become excessive by the work hardening of pearlite, and cold forgeability will deteriorate. Therefore, the C content needs to be 0.06% by mass, preferably 0.055% or less, more preferably 0.045% by mass or less. However, if the C content is extremely low, deoxidation during steel melting becomes difficult, so the lower limit is made 0.005%, preferably 0.008% or more, more preferably 0.013% or more. is there.

Si: 0.005 to 0.05%
Si is effective as a deoxidizing element during melting. If the Si content is less than 0.005%, deoxidation is insufficient and blow holes are generated during melting. However, if the Si content is excessive and exceeds 0.05%, the deformation resistance is increased and the deformability is lowered due to the solid solution strengthening of the ferrite phase of Si. For this reason, generation | occurrence | production of a crack becomes remarkable and cold forgeability deteriorates. Therefore, the Si content is preferably 0.007% or more, more preferably 0.012% or more, preferably 0.044% by mass or less, more preferably 0.031% or less.

Mn: 0.4 to 1%
Mn is effective as a deoxidizing and desulfurizing element during melting. In addition, when the N content in the steel material is increased, cracking is likely to occur due to dynamic strain aging due to heat generation during cold forging, but Mn is MnS, and the workability at that time (deformability) is improved. It has the effect of improving and suppressing cracking. In order to effectively exhibit such an effect, it is necessary to contain 0.4% or more, preferably 0.42% or more, more preferably 0.48% or more. On the other hand, if Mn is contained excessively, not only the deformation resistance becomes excessive, but also the structure non-uniformity due to segregation occurs, so it is necessary to make it 1% or less, preferably 0.92% or less, more preferably 0.83% or less.

P: 0.05% or less (excluding 0%)
Phosphorus (P) is an inevitable impurity. When this is contained in the ferrite of the steel structure, it segregates on the ferrite grain boundary, deteriorates the deformability, and lowers the cold forgeability. It is also an element that causes solid solution strengthening of the ferrite to increase deformation resistance and deteriorate cold forgeability. Therefore, in order to improve cold forgeability, the P content is 0.05% or less. Moreover, Preferably it is 0.022% or less. Setting the P content to 0% is industrially difficult and not economical, so it is specified in the proviso that 0% is not included.

S: 0.005-0.05%
Sulfur (S), like P, is an inevitable impurity. S is deposited as a film on the grain boundary as FeS, and deteriorates workability such as cold forgeability. It also has the effect of causing hot brittleness. Therefore, from the viewpoint of improving the deformability, the S content is set to 0.05% or less, preferably 0.019% or less. However, it is industrially difficult to reduce the S content to 0%, and S also has an effect of improving machinability. For this reason, the lower limit of the S content is set to 0.005% or more in consideration of the balance between the deformability, the machinability, and the amount that can be economically reduced. Preferably it is contained 0.008% or more.

Al: 0.005 to 0.1%
Al has a strong deoxidation effect and can improve the internal quality of the steel material. Moreover, it combines with N in steel to form AlN, and has the effect of regulating the ferrite crystal grains. Further, it is an important element for controlling the amount of dissolved N by the heat treatment. In order to effectively exhibit these effects, 0.005% or more of Al is necessary, 0.008% or more is preferable, and 0.015% or more is more preferable. When the Al content is less than 0.005%, the solid solution N amount cannot be controlled by the heat treatment. In addition, gas defects are likely to occur during melting, and cracks are likely to occur during cold forging. On the other hand, if it exceeds 0.1%, the bonding strength with solute N becomes remarkable, so that the amount of solute N cannot be controlled by the heat treatment. For this reason, the amount of solute N is reduced, and a predetermined component strength cannot be obtained. Therefore, the Al content is 0.1% or less, preferably 0.032% or less, more preferably 0.026% or less.

N: 0.008 to 0.02%
Nitrogen (N) needs to be added in a predetermined amount in order to secure a solid solution N amount necessary for obtaining a desired component strength after cold forging. It is also an important element for obtaining a predetermined strength by static strain aging after cold forging. In order to exert such effects, the total N content needs to be 0.008% or more. However, when the N content becomes excessive and exceeds 0.02%, the influence of dynamic strain aging during processing becomes more significant than static strain aging, and the deformation resistance increases. The preferable lower limit of the N content is 0.010% or more, more preferably 0.012% or more, and the preferable upper limit is 0.0155%, more preferably 0.0140% or less.

Other elements:
In the present invention, for example, elements such as Ti, Nb, V, B (boron), Hf, and Ta are unavoidable impurities as elements that easily form a solid solution N and a nitrogen compound. It may contain 2% or less (including 0%).

  Further, for example, other elements such as Cr, Mo, Cu, Ni, Ca, REM, Mg, and Li are also elements that are easily mixed from dissolved raw materials as impurities. As long as the characteristics are not impaired, the following inevitable amounts may be included as inevitable impurities. Cr: 2% or less (including 0%), Mo: 1% or less (including 0%), Cu: 5% or less (including 0%), Ni: 5% or less (including 0%), Ca, REM: each 0.2% or less (including 0%), Mg, Li: each 0.01% or less (including 0%).

  By manufacturing the large screw 1 by the forging method shown in the present invention using the above-mentioned materials, conventionally, the large screw 1 can be manufactured only by using hot forging. It can be manufactured. Note that the manufacturing method of the present invention can be applied not only to the large screw 1 but also to automotive parts such as crankshafts, connecting rods, transmission gears, and other machine parts that have been processed by hot forging so far. it can.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and may be implemented with appropriate modifications within a range that can meet the gist of the preceding and following descriptions. Of course, any of these is also included in the technical scope of the present invention.

  Steel having the chemical composition shown in Table 1 (the content of each element is mass%) was melted by a converter, made into a steel piece by continuous casting, and then rolled into a wire with a diameter of 12 mm. Thereafter, the rolled wire rod was subjected to a normalizing heat treatment at 800 ° C. for 2 hours to form a part of the solid solution N in the entire rolled wire rod in AlN. Here, “N” in Table 1 indicates the amount of total nitrogen including all nitride compounds and solid solution N.

  The obtained wire (machine structural steel before cold forging) is a partial corresponding to the shaft portion of the large screw 1 and the region of the screw portion that require high strength in the large screw 1 (machine part). A high-strength region was determined in advance, and the cold forging was performed after the amount of solid solution N in this partial high-strength region was increased to a necessary amount for increasing the strength. That is, the large screw 1 was manufactured through the steps S1 to S7.

  At this time, the amount of solid solution N in the partially increased strength region (the portion requiring strength) is partially changed as shown in Table 2 by high-frequency heating depending on S6: material property processing step (the heat treatment step). Heat treatment was performed and controlled. Here, only the partial high-strength region (portion where strength is required) is heated to each temperature shown in Table 2 rapidly and in a short time (about 100 ° C./sec with the φ12 mm screw diameter). Thus, the nitride compound such as AlN in the material was decomposed to increase the amount of solute N in the high strength region. At this time, the total time for heating and holding was set to a short time of about 300 seconds in common with each example, and other portions where strength was not required were not heated as much as possible. That is, particularly in the inventive examples, the portions other than the high-strength region that do not require strength were not heat-treated (no heat treatment).

  In cooling after heating and holding, including the invention example, the cooling rate of the partially strengthened region is controlled at 1 ° C./s, and the solid solution N generated in the partially strengthened region is It was precipitated again in the cooling process, so that it did not decrease below the amount necessary for the strength increase in cold forging.

  The “strengthened N amount” of the partial strength-enhancing region of the raw steel wire and the other shaft-corresponding portion was determined by the analysis method. The results are shown in Table 2. In Table 2, “Solution N amount” is expressed in mass%.

  Also, during cold forging, in common with each example, plastic strain was applied at an ambient temperature of 200 ° C. or lower to the other portions including the partial strength enhancement region. Then, while trying to partially increase the strength of the mechanical part that requires high strength, the large screw 1 was formed.

  In the cold forging, the wire rod subjected to the heat treatment was subjected to 10% area-reduction drawing, and then M12 was threaded using a parts former. After that, finishing threading was performed by continuous four-stage cold forging to finish the screw shape of the large screw part shown in FIG. The processing speed of this four-stage cold forging is 60 pieces / minute.

  As an evaluation of the strength of the obtained large-sized screw component, the Vickers hardness (Hv) of the region of the shaft portion and the screw portion, the other portion (the lower neck portion), where the material is the partial high-strength region The Vickers hardness (Hv) of each was measured with a Vickers hardness tester, and the difference in hardness between these parts (increase in the hardness of the shaft portion and the screw portion) was also obtained. At this time, it was determined that a component having a Vickers hardness of 300 Hv or more in the region of the shaft portion and the screw portion was excellent in strength, and that less than 300 Hv was inferior in strength. The results are also shown in Table 2. The Vickers hardness of the component is cut so that the screw is equally divided into two along the longitudinal direction at the axial center (in the vertical direction). The hardness was measured at each longitudinal position and at a depth position of D / 4 (D: diameter of threaded part) of each cross section. The Vickers hardness was measured under the common conditions of load: 1000 g and measurement frequency: 5 times, and the average was taken.

  The cold forgeability of the large screw parts obtained was also evaluated. That is, test pieces each having a diameter of 10 mm and a length of 15 mm were cut out from the axial portion of the screw and the central portion in the longitudinal direction under the neck, and these test pieces were strain rate: 10 / S (average strain rate during processing) Value), processing temperature: room temperature, compression ratio: 80%, and upsetting was performed using a 1600 ton press with the ends restrained. After this processing, the surface was observed with an actual microscope at an observation magnification of 20 times, the presence or absence of cracks was confirmed, and the cold forgeability was evaluated. The results are also shown in Table 2. In this example, the case where cracks are not observed in any of the tests on the shaft and the neck of the screw is excellent in cold forgeability (O), either one or both of the shaft and / or the neck of the screw. In the test, even if it was small, the case where cracks were observed in the part was determined to be inferior in cold forgeability (x).

  Inventive examples in Tables 1 and 2, steel types A4-A5, B3-B5, C, D, E, F, G, and H satisfy the steel composition, and cold forging corresponding to parts that require high strength. The previous partially strengthened region is determined in advance, and the amount of solute N in this strengthened region is partially increased by heat treatment in advance to an amount necessary for increasing strength. Then, cold forging is performed, and at the time of this cold forging, plastic strain is applied at an atmospheric temperature of 200 ° C. or lower to at least the partial high strength region, thereby increasing the strength of the machine part. The strength of only the necessary parts is partially increased. As a result, the inventive example is excellent in cold forgeability, the hardness of the screw shaft portion exceeds 300 HV, and is excellent in component strength.

  On the other hand, those that do not satisfy the requirements defined in the present invention are inferior in cold workability or component strength, as described below.

  Steel grades A1-A3, B1-B2 have the heating temperature of the partially strengthened region (portion requiring strength) too low, and decompose the nitride compound such as AlN in the strengthened region, The amount of solute N cannot be increased sufficiently, and the amount of solute N in this high strength region is insufficient. As a result, although the cold forgeability is naturally good, the hardness of the screw shaft portion is less than 300 HV even by cold forging, the strength cannot be increased, and the strength is insufficient.

  Steel type I lacks the total N amount of the raw steel wire rod. For this reason, coupled with the fact that the heating temperature of the partially strengthened region (portion requiring strength) is too low, the amount of solute N in the strengthened region is insufficient, and even by cold forging, The hardness of the screw shaft portion is less than 300 HV, the strength cannot be increased, and the strength is insufficient.

  Steel types J-1 to J-5 have too little Al content. For this reason, even if the heating conditions are the same as those of the invention example or the heating temperature is too low, the amount of solid solution N cannot be controlled by the heat treatment due to the bonding state of Al and nitrogen. As a result, in common, the amount of solute N in the original material steel wire remains as it is, and the amount of solute N in the entire material steel wire is too large, including the high strength region and other regions. Cold forgeability is inferior.

  Steel type K-1 is not subjected to any heat treatment for increasing the amount of dissolved N, including the high-strength region and other regions. For this reason, especially the amount of solute N in the high strength region is insufficient, and even by cold forging, the hardness of the screw shaft portion is less than 300 HV, the strength cannot be increased, and the strength is insufficient. .

  In steel type K-2, the heat treatment for increasing the amount of solute N described above is performed on the entire wire including the high-strength region and other regions. For this reason, there is too much solid solution N amount of the whole wire, and cold forgeability is inferior.

  According to the present invention, even when a soft steel material whose C content is reduced to an extremely low carbon region of less than 0.06 mass% is used, the strength of the product can be sufficiently ensured, and the deformation resistance and the component strength can be ensured. It is possible to provide a method of manufacturing a machine part using cold forging that can achieve both of the above. Therefore, it can be widely used as a method for manufacturing machine parts used in vehicles such as automobiles.

1: Large screw, 2: Shaft, 3: Round bar

Claims (4)

In mass%, C: 0.005 to 0.06%, Si: 0.005 to 0.05%, Mn: 0.4 to 1%, P: 0.05% or less (not including 0%), S: 0.005 to 0.05%, Al: 0.005 to 0.1%, N: 0.008 to 0.02%, respectively. When producing a machine part by cold forging, a partially strengthened region in the mechanical structural steel before the cold forging corresponding to a partially strengthened region of the machine part is determined in advance, after having previously partially increased to the amount required for the amount of dissolved N in the partial high intensity region of the high strength, the 200 ° C. or less for at least this partial strengthening region with imparting plastic strain at ambient temperature, the plastic His against a portion other than the partial strengthening region Perform cold forging of imparting, to increase the strength of the partial high intensity region of the machine component producing process of mechanical components, characterized in that the said mechanical part shape.   The amount of solute N in the partially strengthened region in the mechanical structural steel is in the range of 0.008 to 0.014%, and cold forging plastic strain applied to the partially strengthened region. The method for manufacturing a machine part according to claim 1, wherein the range of 0.5 to 5 is set.   The amount of solute N in the partially strengthened region is obtained from a model formula based on any one of deformation resistance (DR), component strength (Hv), and fatigue strength (F), or from an empirical value acquired in advance. The manufacturing method of the machine component of Claim 1 or 2.   The method for manufacturing a machine part according to any one of claims 1 to 3, wherein the solid solution N amount in the partially strengthened region in the mechanical structural steel is increased by heat treatment in the partially strengthened region. .

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