JP4797673B2 - Hot forging method for non-tempered parts - Google Patents

Description

  The present invention relates to a hot forging method for non-tempered parts. More specifically, the present invention relates to a hot forging method for non-tempered parts, which is excellent in machinability and has high strength and toughness and is suitable for manufacturing non-tempered parts for automobiles such as hubs and connecting rods. More specifically, by using a so-called “ferrite + pearlite” type non-heat treated steel composed of a mixed structure of ferrite and pearlite, the above-mentioned non-heat treated parts for automobiles have high strength and high toughness at a low manufacturing cost. The present invention relates to a hot forging method for non-tempered parts.

Usually, automotive parts such as hubs and connecting rods are formed into a complex shape by hot forging and then finished into a final shape by machining such as cutting. These automotive parts generally include
(B) High strength and high toughness,
(B) Good machinability,
(C) Low manufacturing cost,
Is required.

  The high strength and high toughness of the above (a) can be achieved by subjecting the part to a tempering treatment, that is, quenching-tempering. Further, (a) high strength and high toughness can be achieved by lowering the forging temperature of steel, that is, by “controlled forging” called “sub-hot forging” or “warm forging”. .

  However, when the tempering treatment is performed to ensure high strength and high toughness, the heat treatment cost increases, so that the requirement for the low production cost (C) cannot be satisfied.

  In addition, when ensuring high strength and high toughness by “controlled forging”, the forging temperature is low, leading to a decrease in mold life and an increase in mold cost. I can't. On the other hand, in order to prevent the mold cost from rising by suppressing the decrease in the mold life, the amount of C, Si, Mn, V, etc. contained in the steel to be “controlled forged” is reduced and deformed. It is effective to reduce the resistance. However, in this case, although high toughness can be ensured, the strength decreases.

  Therefore, when manufacturing automobile parts, it is very difficult to satisfy both requirements of (a) ensuring high strength and toughness and (c) low manufacturing cost.

  For this reason, in order to satisfy the requirement of low production cost of (C), the high strength and high toughness of (A) and ( B) There is a great demand for a technology capable of ensuring both good machinability, and Patent Documents 1 to 4 describe a so-called “ferrite + pearlite” type or “ferrite + pearlite + bainite” type non-heat treated steel and its manufacture. A method has been proposed.

  In Patent Document 1, in order to prevent a decrease in fatigue strength due to decarburization of the ferrite portion, decarburization is prevented by a decrease in the amount of C, and furthermore, by adding V and Si to strengthen the ferrite, the area ratio is 70%. “Non-tempered steel for forging excellent in fatigue strength and toughness and method for producing the same” has been disclosed that has the above ferrite and that can be prevented from decarburization even when heated at 1050 ° C. or higher where decarburization is a concern. ing.

  Patent Document 2 discloses a method for producing hot forged non-tempered steel having high fatigue strength and a forged product in which ferrite is strengthened by refinement of pearlite, solid solution strengthening by Si and N, and precipitation strengthening by V. It is disclosed.

  In Patent Literature 3, by controlling the amount and size of inclusions such as Ti oxy-nitride and MnS, it suppresses the coarsening of austenite grains during heating and forms intranuclear ferrite formation nuclei. Has disclosed a "method for producing a high-strength, high-toughness non-heat treated steel part".

Patent Document 4 discloses a “method for producing a non-tempered steel part having high strength and high toughness” in which fine ferrite + pearlite structure is obtained by forging in a temperature range from Ac ₁ point to Ac ₃ point, that is, warm forging. It is disclosed.

JP-A-8-120398 JP-A-9-143610 JP-A-8-92687 JP-A-5-255739

  Since the technique proposed in Patent Document 1 described above does not contain Ti, austenite coarsening occurs during heating, and the ferrite + pearlite structure obtained during cooling after forging also coarsens, which is not always sufficient. Toughness cannot be ensured. Furthermore, since the Si content is as low as 0.35% or less, sufficient strength cannot always be obtained.

  The technique proposed in Patent Document 2 cannot always obtain sufficient strength because the Si content is as low as 0.01 to 0.1%. Moreover, since the upper limit is not provided in the heating temperature of the steel materials before forging, and content of Ti is not essential, favorable toughness cannot necessarily be ensured.

In the technique proposed in Patent Document 3, it is necessary to control the average particle size and content of inclusions. Specifically, 0.1 to 5 μm of inclusions is 1 × 10 ² to 1 × 10 ⁶ pieces / mm ^2. Therefore, when casting molten steel, it is necessary to control the cooling rate at 1500 to 900 ° C. to 1 ° C./min or more, which is problematic in terms of manufacturability. Moreover, since the Si content of the steel specifically disclosed in the “Example” is about 0.5% at most, the strength is low.

  The technique proposed in Patent Document 4 has a low forging temperature and a great decrease in mold life, so that high toughness is obtained, but the manufacturing cost increases.

  Accordingly, an object of the present invention is to ensure high strength and high toughness, as well as good machinability, while being forged in a relatively high temperature range in which the life of the mold does not decrease. The present invention provides a “hot forging method for non-tempered parts” suitable for producing non-tempered parts for automobiles such as the above.

  The present inventors have made various studies in order to ensure high strength, high toughness, and good machinability while remaining untempered. As a result, first, the following findings (a) to (d) were obtained.

  (A) The toughness of the “ferrite + pearlite” type non-tempered steel is superior to the toughness of other non-tempered steels such as the “ferrite + pearlite + bainite” type.

  (B) In general, V precipitates as nitrides or carbides. V nitride suppresses austenite grain growth during high-temperature heating and has the effect of improving toughness by forming a ferrite formation nucleus during the ferrite + pearlite transformation and refining the structure, while V carbide is When it precipitates in ferrite during forging ferrite + pearlite transformation, it has a great strengthening effect, but also causes a decrease in toughness.

  (C) However, V carbides have different effects on toughness depending on the matrix (base) structure to be precipitated. When V carbide is precipitated in austenite, the decrease in toughness is smaller than when it is precipitated in ferrite. Therefore, if the precipitation of V carbide in austenite is promoted and the precipitation of V carbide and V nitride is optimized, the strengthening action by V carbide and the toughness improving action by V nitride can be made compatible.

  (D) However, if a large amount of V nitride is precipitated, the amount of V necessary for the precipitation of carbides is reduced. Therefore, V is an element that forms precipitates that suppress austenite grain growth during high-temperature heating. In addition, Ti should be used.

  Accordingly, the present inventors have further studied the “ferrite + pearlite” type non-heat treated steel. As a result, the following knowledge (e) was obtained.

  (E) Precipitation and solid solution reaction of V carbide is a reversible reaction of “V + C → VC” and “VC → V + C”, between the equilibrium constant [% C] [% V] and the temperature t (K). Since it is known that the empirical formula of Log [% C] [% V] = (− 9500 / t) +6.72 holds, the content of C and V is set within an appropriate range, and the solid solution If forging is performed immediately above the temperature, precipitation of V carbide is promoted by the strain energy, and V carbide is precipitated in the austenite region in the cooling process after forging, so that a decrease in toughness due to V carbide can be suppressed.

  This invention is completed based on said knowledge, The summary exists in the hot forging method of the non-tempered part shown to following (1)-(3).

(1) A hot forging method for non-tempered parts, in mass%, C: 0.15-0.5%, Si: 0.6-1.0%, Mn: 0.6-1. 5%, P: 0.03% or less, S: 0.03-0.2%, N: 0.008-0.025%, Ti: 0.003-0.1%, V: 0.1 0.5%, Cr: 0.2 to 0.8% and Al: 0.002 to 0.1%, with the balance being Fe and impurities, [% C] / [% V] ≧ 0. 5 was heated to 1100 to 1250 ° C. and forged into a part shape in the temperature range of (T + 50) to (T + 100) ° C., and then the cooling rate at 800 to 500 ° C. was 0.2 to 5.0 ° C. / A hot forging method for non-tempered parts, characterized by cooling in seconds. However, T points out the temperature (degreeC) below 1000 degreeC calculated by the following (1) Formula and (2) Formula.
Log [% C] [% V] = (− 9500 / t) +6.72 (1),
T = t-273 (2).
[% C] and [% V] refer to the contents of C and V in the steel in mass%, respectively.

(2) Instead of a part of Fe of the steel described in (1) above, one or more of Mo: 0.7% or less, Nb: 0.1% or less, and Zr: 0.01% or less Is heated to 1100 to 1250 ° C. and forged into a part shape in a temperature range of (T + 50) to (T + 100) ° C., and then the cooling rate at 800 to 500 ° C. is 0.2 to 5.0 ° C. / A hot forging method for non-tempered parts, characterized by cooling in seconds. However, T points out the temperature (degreeC) below 1000 degreeC calculated by the following (1) Formula and (2) Formula.
Log [% C] [% V] = (− 9500 / t) +6.72 (1),
T = t-273 (2).
[% C] and [% V] refer to the contents of C and V in the steel in mass%, respectively.

(3) Instead of a part of Fe of the steel described in (1) or (2) above, Pb: 0.4% or less, Ca: 0.01% or less, Bi: 0.3% or less, and Te: A steel containing one or more of 0.1% or less is heated to 1100 to 1250 ° C. and forged into a part shape in a temperature range of (T + 50) to (T + 100) ° C., and then at 800 to 500 ° C. A hot forging method for non-tempered parts, wherein the cooling is performed at a cooling rate of 0.2 to 5.0 ° C / second. However, T points out the temperature (degreeC) below 1000 degreeC calculated by the following (1) Formula and (2) Formula.
Log [% C] [% V] = (− 9500 / t) +6.72 (1),
T = t-273 (2).
[% C] and [% V] refer to the contents of C and V in the steel in mass%, respectively.

  Hereinafter, the inventions related to the hot forging method for non-tempered parts (1) to (3) are referred to as “present invention (1)” to “present invention (3)”, respectively. Also, it may be collectively referred to as “the present invention”.

  According to the present invention, the refinement of the steel structure can be achieved without lowering the forging temperature as much as sub-hot forging, and the reduction in toughness due to V carbide can be reduced. It is possible to manufacture a non-tempered part that is excellent as a non-tempered part for automobiles, such as a hub and a connecting rod, and that has high strength and toughness.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the content of the chemical component means “mass%”.

(A) Chemical composition of steel:
C: 0.15-0.5%
C is an important element that greatly affects the strength and toughness of steel. If the content is less than 0.15%, an increase in mold life and high toughness can be obtained, but high strength is obtained. I can't. On the other hand, when the C content exceeds 0.5%, high strength can be obtained, but the mold life and toughness are reduced. Therefore, the content of C is set to 0.15 to 0.5%. In addition, it is preferable that content of C shall be 0.2 to 0.4%.

Si: 0.6 to 1.0%
Si has an action of strengthening ferrite and a deoxidizing action. However, if the content is small, the effect cannot be obtained. In particular, when the Si content is less than 0.6%, the mold life can be extended, but the predetermined strength cannot be obtained. On the other hand, if the Si content exceeds 1.0%, the strength increases and the mold life is significantly reduced. Therefore, the Si content is set to 0.6 to 1.0%. The Si content is preferably 0.6 to 0.9%.

Mn: 0.6 to 1.5%
Mn has an action of strengthening ferrite and an action of making pearlite fine and improving toughness. Mn also has the effect of improving the machinability by forming sulfides. However, if the content is less than 0.6%, a sufficient effect cannot be obtained. On the other hand, when the Mn content exceeds 1.5%, the strength increases and the mold life is reduced, and the machinability also decreases. Further, the cost is increased. Therefore, the Mn content is set to 0.6 to 1.5%. In addition, it is preferable that content of Mn shall be 0.6 to 1.4%.

P: 0.03% or less P lowers toughness. In particular, when its content exceeds 0.03%, the toughness is significantly lowered. Therefore, the content of P is set to 0.03% or less. Note that the smaller the P content, the better.

S: 0.03-0.2%
S combines with Mn to form MnS and has an effect of improving machinability. However, if the content is less than 0.03%, sufficient effects cannot be obtained. On the other hand, if the S content exceeds 0.2%, not only the toughness is lowered, but also forging cracks are caused. Therefore, the content of S is set to 0.03 to 0.2%. In addition, it is preferable that content of S shall be 0.04-0.13%.

N: 0.008 to 0.025%
N combines with Ti to form TiN, and suppresses coarsening of the structure during heating during hot forging, thus contributing to improved toughness. In addition, it forms VN by combining with V, and not only suppresses the coarsening of the structure during heating during hot forging, but also serves as a ferrite nucleation, thereby making the structure finer and contributing to improved toughness. . However, if the content is less than 0.008%, the above effect cannot be obtained. On the other hand, if the N content exceeds 0.025%, hard TiN and VN are coarsely produced in large quantities, so that the toughness is lowered and the machinability is also lowered. Therefore, the content of N is set to 0.008 to 0.025%.

Ti: 0.003-0.1%
Ti combines with N to form TiN and suppresses coarsening of the structure during heating during hot forging, thus contributing to improved toughness. However, if the content is less than 0.003%, a sufficient effect for the above action cannot be obtained. On the other hand, if the Ti content exceeds 0.1%, hard TiN is coarsely produced in a large amount, so that the toughness is lowered and the machinability is also lowered. Therefore, the content of Ti is set to 0.003 to 0.1%. The Ti content is preferably 0.003% or more and less than 0.08%.

V: 0.1-0.5%
V is an extremely important element in the present invention. That is, V has an effect of strengthening ferrite by forming V carbide and an effect of preventing coarsening of austenite grains during heating by forming V nitride. However, if the content is less than 0.1%, the strengthening of ferrite is reduced and the toughness is increased, but high strength cannot be obtained. On the other hand, if the V content exceeds 0.5%, V carbides are excessively present and the ferrite strengthening becomes excessive, resulting in a large decrease in toughness. In addition, the machinability is lowered and the mold life is deteriorated. Therefore, the content of V is set to 0.1 to 0.5%. In addition, it is preferable that content of V shall be 0.15-0.33%.

Cr: 0.2 to 0.8%
Cr has an action of strengthening ferrite and an action of making pearlite fine and improving toughness. However, if the content is less than 0.2%, the above effect cannot be obtained. On the other hand, if the Cr content exceeds 0.8%, the strength becomes too high, resulting in a decrease in toughness, and the mold life is also deteriorated. Therefore, the Cr content is set to 0.2 to 0.8%. In addition, it is preferable that content of Cr shall be 0.3-0.8%.

Al: 0.002 to 0.1%
Al is an element necessary for deoxidizing steel. In addition, Al forms nitrides and suppresses austenite grain growth, contributing to improved toughness. However, if the Al content is less than 0.002%, the effect is not sufficiently obtained. On the other hand, even if Al is contained in an amount exceeding 0.1%, the above effect is saturated, and hard precipitates are excessively present, resulting in a decrease in forgeability. Therefore, the Al content is set to 0.002 to 0.1%.

[% C] / [% V]: 0.5 or more By adjusting the content of C and V to the above-mentioned range, and further setting the value of [% C] / [% V] to 0.5 or more High toughness can be obtained. That is, if the value of [% C] / [% V] is less than 0.5, V carbides are excessively present and the precipitation strengthening amount is excessively increased, resulting in a large decrease in toughness. Therefore, the value of [% C] / [% V] is set to 0.5 or more. In order to effectively obtain the strength increasing action by precipitation of V carbide, [% C] / [% V] is preferably 5 or less.

  For the above reasons, the chemical composition of the material steel of the non-tempered part, which is the object of the present invention (1), contains elements from C to Al in the above-mentioned range, and the balance consists of Fe and impurities. % C] / [% V] ≧ 0.5.

In addition, in the material steel of the non-tempered part which is the object of the present invention, if necessary, instead of a part of Fe,
First group: Mo: 0.7% or less, Nb: 0.1% or less and Zr: 0.01% or less,
Second group: Pb: 0.4% or less, Ca: 0.01% or less, Bi: 0.3% or less and Te: 0.1% or less,
One or more of at least one group of elements can be contained. That is, one or more elements of at least one of the first group and the second group may be contained as an optional additive element instead of a part of Fe.

  Hereinafter, the above optional additive elements will be described.

First group: Mo: 0.7% or less, Nb: 0.1% or less, and Zr: 0.01% or less Mo has an effect of increasing strength by precipitation strengthening. The effect becomes remarkable when the content is 0.01% or more. On the other hand, if the Mo content exceeds 0.7%, the strength becomes too high, resulting in a decrease in toughness and machinability. Therefore, the content when Mo is contained is set to 0.7% or less. Note that the Mo content is preferably 0.01 to 0.7%, and more preferably 0.05 to 0.2%.

  Nb has the effect of increasing strength by precipitation strengthening. When the content is 0.001% or more, the above effect becomes remarkable. On the other hand, when the content of Nb exceeds 0.1%, the strength becomes too large, resulting in a decrease in toughness, and further, the machinability is also decreased. Therefore, the content when Nb is contained is set to 0.1% or less. The Nb content is preferably 0.001 to 0.1%, and more preferably 0.005 to 0.05%.

  Zr has the effect of increasing strength by precipitation strengthening. When the content is 0.001% or more, the above effect becomes remarkable. On the other hand, if the content of Zr exceeds 0.01%, the strength becomes too high, resulting in a decrease in toughness and a machinability. Therefore, the content when Zr is contained is set to 0.01% or less. Note that the Zr content is preferably 0.001 to 0.01%, and more preferably 0.001 to 0.005%.

  In addition, said Mo, Nb, and Zr can contain only any 1 type in them, or 2 or more types of composites.

Second group: Pb: 0.4% or less, Ca: 0.01% or less, Bi: 0.3% or less, and Te: 0.1% or less Pb has an effect of improving machinability. The effect becomes remarkable when the content is 0.01% or more. On the other hand, if the Pb content exceeds 0.4%, the hot workability is deteriorated, and cracks are generated during hot forging. Therefore, the content when Pb is contained is set to 0.4% or less. In addition, it is preferable that content of Pb shall be 0.01 to 0.4%.

  Ca has the effect | action which improves machinability. The said effect becomes remarkable when the content is 0.0002% or more. On the other hand, if the Ca content exceeds 0.01%, hot forging cracks are caused. Therefore, the content when Ca is contained is set to 0.01% or less. The Ca content is preferably 0.0002 to 0.01%.

  Bi has an effect of improving machinability. The effect becomes remarkable when the content is 0.01% or more. On the other hand, if the Bi content exceeds 0.3%, the hot workability is deteriorated and cracking occurs during hot forging. Therefore, the content when Bi is contained is set to 0.3% or less. The Bi content is preferably 0.01 to 0.3%.

  Te has the effect | action which improves machinability. When the content is 0.002% or more, the above effect becomes remarkable. On the other hand, when the content of Te exceeds 0.1%, the hot workability is deteriorated, and cracks are generated during hot forging. Therefore, the content when Te is contained is set to 0.1% or less. The Te content is preferably 0.002 to 0.1%.

  In addition, said Pb, Ca, Bi, and Te can be contained only in one of them, or 2 or more types of composites.

  For the above reasons, the chemical composition of the material steel of the non-heat treated part which is the object of the present invention (2) is part of the Fe of the material steel of the non-heat treated part which is the object of the present invention (1). Instead, it was specified to contain one or more of Mo: 0.7% or less, Nb: 0.1% or less, and Zr: 0.01% or less.

  Further, the chemical composition of the material steel of the non-heat treated part which is the object of the present invention (3) is the same as that of Fe of the material steel of the unheated part which is the object of the present invention (1) or the present invention (2). Instead of part, it is specified that Pb: 0.4% or less, Ca: 0.01% or less, Bi: 0.3% or less, and Te: 0.1% or less are contained. did.

(B) Hot forging:
In the present invention, [% C] and [% V] are respectively the contents of C and V in the steel in mass%, and T is the following formulas (1) and (2) already described. The steel having the chemical composition described in the above section (A) is heated to 1100 to 1250 ° C. as the calculated temperature (° C.) of 1000 ° C. or less, and the part shape in the temperature range of (T + 50) to (T + 100) ° C. It is necessary to cool at a cooling rate of 800 to 500 ° C. at 0.2 to 5.0 ° C./second after forging.
Log [% C] [% V] = (− 9500 / t) +6.72 (1),
T = t-273 (2).

  First, when the heating temperature is lower than 1100 ° C., the solid solution of V carbide becomes insufficient, and the amount of V carbide precipitated during cooling after forging decreases, so that a predetermined strength cannot be obtained. Moreover, undissolved V carbide does not contribute much to strengthening of the forged product obtained after forging. However, since V carbide itself is hard, if it exists at the time of forging, the deformation resistance at the time of forging increases, and the life of the mold is reduced. Incurs a decline. Therefore, the die life is reduced during hot forging. On the other hand, when the heating temperature exceeds 1250 ° C., the austenite grains become extremely coarse and the toughness is reduced.

  Further, if the forging temperature to the part shape is lower than (T + 50) ° C., high toughness can be obtained, but the deformation resistance is increased, so that the die life is greatly reduced. On the other hand, when the forging temperature to the part shape exceeds (T + 100) ° C., the deformation resistance becomes small and the mold life is improved, but the strain energy during forging is released, and V carbide precipitates in the ferrite region. Therefore, the toughness is reduced.

  Further, if the cooling rate at 800 to 500 ° C. is less than 0.2 ° C./second after forging, the structure and V carbides are coarsened, so both strength and toughness are reduced. On the other hand, if the cooling rate at 800 to 500 ° C. exceeds 5.0 ° C./second after forging, hard martensite and bainite are generated, and problems such as deterioration of toughness and machinability and fire cracking occur. There is a case.

  When T exceeds 1000 ° C., the amount of V carbide is excessive and the precipitation strengthening amount is excessively increased, so that the toughness and the mold life are greatly reduced.

  Therefore, in the present invention, the steel having the chemical composition described in the section (A) is heated to 1100 to 1250 ° C. and forged into a part shape in the temperature range of (T + 50) to (T + 100) ° C., and then 800 It was specified that the cooling rate at ˜500 ° C. was 0.2 to 5.0 ° C./second.

  If the steel having the chemical composition described in the section (A) is hot-forged under the above-described conditions, a “ferrite + pearlite” type non-tempered part can be obtained.

  In addition, after forging into a part shape, it is preferable that the cooling rate in 800-500 degreeC shall be 0.3-3.0 degree-C / sec. However, the cooling conditions in the temperature range below 500 ° C. need not be specified, and may be cooled in the atmosphere, for example.

  The lower limit of the T is 839 ° C. when the C content is 0.15% and the V content is 0.5%.

  Hereinafter, the present invention will be described in more detail with reference to examples.

  Steels 1 to 21 having the chemical composition shown in Table 1 were melted in a vacuum furnace to produce a 150 kg steel ingot.

  Steels 1 to 13 and Steel 19 in Table 1 are steels whose chemical compositions are within the range defined by the present invention. On the other hand, Steels 14 to 18, Steel 20 and Steel 21 are steels of comparative examples whose chemical compositions deviate from the conditions defined in the present invention. Among the steels of the comparative examples, the steel 21 is a steel used as an example of a conventional “ferrite + pearlite” type non-tempered steel with general V addition.

  In Table 1, the value of T calculated by the above equations (1) and (2) is also shown.

  The steel ingot thus obtained was heated to 1100 to 1200 ° C., then hot forged into a round bar having a diameter of 60 mm, and further peeled to obtain a round bar having a diameter of 45 mm and a length of 110 mm. The cooling after hot forging was allowed to cool in the atmosphere.

  Next, the round bar having a diameter of 45 mm and a length of 110 mm was heated at 1250 ° C. for 50 minutes, and the temperature was lowered to the temperature shown in Table 2, followed by hot press forging to finish a forged product having a thickness of 15 mm. The hot press forging is performed at a strain rate of 10 / sec. After the press forging, the cooling rate at 800 to 500 ° C. is controlled to 0.5 ° C./sec, and the temperature range below 500 ° C. is in the atmosphere. Was allowed to cool.

  The forging temperature of 1100 ° C. as the hot press forging condition for steel 21 in Table 2 is generally adopted as the hot forging temperature.

  Table 2 also shows the value of T calculated by the equations (1) and (2).

  Various test pieces were cut out from the forged product having a thickness of 15 mm obtained as described above, and subjected to a tensile test, a Charpy impact test, and a machinability test.

  The tensile test was performed using a 14A test piece defined in JIS Z 2201 (1998) from the position of 1/4 in the width direction and 1/4 in the width direction of the forged product having a thickness of 15 mm (however, the diameter of the parallel part: 7 mm). ) Was taken, and the gauge distance was set to 35 mm.

In the Charpy impact test, a U-notch test piece having a width of 10 mm as defined in JIS Z 2202 (1998) was taken from a position in the thickness direction 1/2 and width direction 1/4 of the forged product having a thickness of 15 mm. The impact value (J / cm ² ) was determined at room temperature.

  The machinability test was performed by drilling 30 holes 10mm deep from the surface under the following conditions using a straight shank drill with a material of SKH51 and a drill diameter of 7mm in the thickness direction of a forged product with a thickness of 15mm. The amount of corner wear was measured to evaluate machinability.

・ Rotation speed: 600 rpm,
-Feed rate: 0.15 mm / rev. ,
・ Lubrication: Wet (uses water-soluble lubricant).

  Moreover, after heating the steel ingot of the component of the steel 1-21 shown in Table 1 to 1100-1200 degreeC and hot forging to a diameter of 60 mm, 30 mm in diameter and 70 mm in length were produced by peeling processing. . This round bar was heated at 1250 ° C. for 50 minutes, and after the temperature was lowered to the temperature shown in Table 2, the deformation load (ton) when hot forging into a 10 mm thick forged product at a strain rate of 10 / second was measured. did. As described above, the forging temperature of 1100 ° C. in the hot press forging conditions of the steel 21 is generally adopted as the hot forging temperature.

  Table 2 also shows the above test results (TS (tensile strength), impact value, drill corner wear amount and deformation load).

  The value of “(TS × impact value) / deformation load” calculated from the TS, impact value, and deformation load in the above unit is an index indicating “balance between mechanical properties and cost”, and this value is large. The “balance between mechanical properties and cost” is better. For this reason, the value of “(TS × impact value) / deformation load” is also shown in Table 2.

  In addition, each said characteristic value of the steel 21 in the test number 21 was made into the reference | standard of evaluation.

  From Table 2, when the conditions specified in the present inventions (1) to (3) are satisfied, TS, impact value, drill corner wear amount, and an index indicating “balance between mechanical properties and cost” “(TS It is clear that the value of “x impact value) / deformation load” exceeds the test number 21 that is the evaluation reference value (test numbers 1 to 13).

  On the other hand, the chemical composition of the steel to be forged is within the range specified by the present invention, and the temperature T calculated by the above equations (1) and (2) is 1000 ° C. or less. However, when the forging temperature in the hot press deviates from the conditions specified in the present invention, “(TS × impact value) / deformation load” which is an index indicating “balance between mechanical properties and cost”. It is clear that the value is low, and further TS and toughness (impact value) may be low (test numbers 22 to 29).

  Even if the chemical composition of the material to be forged is within the range specified by the present invention, in the case of steel 19, the temperature T calculated by the above formulas (1) and (2) exceeds 1000 ° C. Therefore, the values of toughness (impact value) and “(TS × impact value) / deformation load” in Test No. 19, Test No. 34, and Test No. 35 are low.

  In addition, when the chemical composition of the steel to be forged is out of the range specified in the present invention, it is an index indicating "balance between mechanical properties and cost" regardless of the forging temperature in the hot press. It is apparent that the value of “(TS × impact value) / deformation load” is low and inferior in at least one characteristic of TS, toughness (impact value) and corner wear amount of the drill (test numbers 14 to 18, test number 20, test number 21, and test numbers 30-33).

According to the present invention, since the forging temperature during hot forging is lower than usual while being higher than sub-hot forging, the steel structure can be refined and the toughness reduction due to V carbide can be reduced. Thus, it is possible to produce a non-heat treated part that is excellent in machinability, has high strength and high toughness, and is suitable as a non-heat treated part for automobiles such as a hub and a connecting rod.

Claims (3)

It is a hot forging method for non-tempered parts, in mass%, C: 0.15 to 0.5%, Si: 0.6 to 1.0%, Mn: 0.6 to 1.5%, P: 0.03% or less, S: 0.03-0.2%, N: 0.008-0.025%, Ti: 0.003-0.1%, V: 0.1-0.5 %, Cr: 0.2 to 0.8% and Al: 0.002 to 0.1%, the balance is made of Fe and impurities, and satisfies [% C] / [% V] ≧ 0.5 Steel is heated to 1100 to 1250 ° C and forged into a part shape in the temperature range of (T + 50) to (T + 100) ° C, and then cooled at a cooling rate of 800 to 500 ° C at 0.2 to 5.0 ° C / sec. A hot forging method for non-tempered parts, characterized by:
However, T points out the temperature (degreeC) below 1000 degreeC calculated by the following (1) Formula and (2) Formula.
Log [% C] [% V] = (− 9500 / t) +6.72 (1)
T = t-273 (2)
[% C] and [% V] refer to the contents of C and V in the steel in mass%, respectively. A steel containing one or more of Mo: 0.7% or less, Nb: 0.1% or less, and Zr: 0.01% or less instead of a part of Fe of the steel according to claim 1. Is heated to 1100 to 1250 ° C. and forged into a part shape in a temperature range of (T + 50) to (T + 100) ° C., and then cooled at a cooling rate of 800 to 500 ° C. at 0.2 to 5.0 ° C./second. A hot forging method for non-tempered parts.
However, T points out the temperature (degreeC) below 1000 degreeC calculated by the following (1) Formula and (2) Formula.
Log [% C] [% V] = (− 9500 / t) +6.72 (1)
T = t-273 (2)
[% C] and [% V] refer to the contents of C and V in the steel in mass%, respectively. Instead of a part of Fe of the steel according to claim 1 or 2, Pb: 0.4% or less, Ca: 0.01% or less, Bi: 0.3% or less and Te: 0.1% or less A steel containing one or more kinds is heated to 1100 to 1250 ° C. and forged into a part shape in a temperature range of (T + 50) to (T + 100) ° C., and then the cooling rate at 800 to 500 ° C. is 0.2. A hot forging method for non-tempered parts, characterized by cooling at ˜5.0 ° C./sec.
However, T points out the temperature (degreeC) below 1000 degreeC calculated by the following (1) Formula and (2) Formula.
Log [% C] [% V] = (− 9500 / t) +6.72 (1)
T = t-273 (2)
[% C] and [% V] refer to the contents of C and V in the steel in mass%, respectively.