JP2019143236A - Non-heat-treated forged component and non-heat-treated forging steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a non-treatable forged part capable of achieving both high strength and excellent manufacturability, particularly prevention of surface cracking during continuous casting, and a non-treatable forged steel useful for manufacturing the non-treatable forged part. do. SOLUTION: It has a predetermined chemical composition, satisfies all of the following formulas (1) to (4), has a bainite fraction of 5 area% or less with respect to the entire structure, and has a ferrite fraction of 25. Non-tempered forged parts characterized by an area of less than% and the balance being pearlite. 1.10 ≤ [C] +0.5 x [Mn] +0.3 x [Cr] +0.9 x [V] ≤1.28 ... (1), [Mn] / [Cr] ≤1.2 ... (2), [C] x ([V]-[N] x 50.94 / 14.0) ≧ 0.130 ... (3), [V] x ([N]-[Ti) ] × 14.0 / 47.9) × 10000 ≦ 35.0 ・ ・ ・ (4) However, in the above formulas (1) to (4), the [element name] is represented by the mass% of each element. It means the content. [Selection diagram] None

Description

  The present invention relates to a non-tempered forged part and a non-tempered forged steel. In particular, the present invention relates to a non-tempered forged part and a non-tempered forged steel that exhibit excellent manufacturability, particularly excellent continuous castability, while having high strength.

  Forged parts used as automotive parts such as connecting rods are required to have higher strength as the weight of automobiles is reduced. Specifically, 0.2% proof stress is required to be 850 MPa or more, and in terms of hardness, 350 HV or more is required. In addition, from the viewpoint of cost reduction and manufacturing efficiency, it is required to achieve the above strength with a non-tempered forged part that is not subjected to heat treatment after forging.

  As a non-tempered forged part with increased strength, for example, in Patent Document 1, elements such as C, Mn, Cr, and V include 2Mn + 5Mo + Cr ≦ 3.1, C + Si / 5 + Mn / 10 + 10P + 5V ≧ 1.8, and Ceq = C + Si. A hot forged non-tempered steel part having a hardness of 330 HV or higher by containing / 7 + Mn / 5 + Cr / 9 + V so as to satisfy the range of 0.90 to 1.10 is shown.

JP 2011-195862 A

  Patent Document 1 aims to achieve 330 HV or higher by performing parametric control as described above. However, generally, when elements such as C and Mn are added, the hardness can be improved, but cracks are likely to occur during production. In Patent Document 1, a certain amount of V is contained, and the carbonitride of this V is finely precipitated at the time of manufacture to increase the strength. Such V-reinforced non-tempered steel can ensure high strength, but on the other hand, decrease in manufacturability due to an increase in the amount of alloy added, and in particular, internal cracks and surface cracks of slabs during continuous casting are likely to occur. . Moreover, when the amount of alloy addition increases, a supercooled structure is generated by cooling after hot forging, and the strength may decrease instead. Hereinafter, the ability to cast without causing surface cracks during continuous casting is sometimes referred to as “excellent in continuous castability” or simply “excellent in manufacturability”.

  As described above, Patent Document 1 does not describe any knowledge about prevention of cracking during production, and there is no example that achieves both high strength and prevention of cracking during production. The present invention has been made in view of such a situation, and the purpose thereof is a non-tempered forged part capable of achieving both high strength and excellent manufacturability, in particular, prevention of surface cracking during continuous casting, An object of the present invention is to provide a non-tempered forging steel useful for manufacturing the non-tempered forged part.

Aspect 1 of the present invention
C: 0.40-0.60 mass%,
Si: more than 0% by mass, 1.0% by mass or less,
Mn: 0.01-0.70 mass%,
P: more than 0% by mass, 0.20% by mass or less,
S: more than 0% by mass, 0.20% by mass or less,
Cr: 0.01-1% by mass,
Al: more than 0% by mass, 0.1% by mass or less,
V: 0.30 to 0.38% by mass, and N: more than 0% by mass, 0.0080% by mass or less, with the balance consisting of iron and inevitable impurities,
All the following formulas (1) to (4) are satisfied, and
The non-tempered forged part is characterized in that the fraction of bainite with respect to the entire structure is 5 area% or less, the fraction of ferrite with respect to the entire structure is 25 area% or less, and the balance is pearlite.
1.10 ≦ [C] + 0.5 × [Mn] + 0.3 × [Cr] + 0.9 × [V] ≦ 1.28
... (1)
[Mn] / [Cr] ≦ 1.2 (2)
[C] × ([V] − [N] × 50.94 / 14.0) ≧ 0.130 (3)
[V] × ([N] − [Ti] × 14.0 / 47.9) × 10000 ≦ 35.0 (4)
However, in said formula (1)-(4), [element name] means content represented by the mass% of each element.

  Aspect 2 of the present invention is the non-tempered forged part according to Aspect 1, further comprising 0.001 to 0.030 mass% of Ti.

Aspect 3 of the present invention
C: 0.40-0.60 mass%,
Si: more than 0% by mass, 1.0% by mass or less,
Mn: 0.01-0.70 mass%,
P: more than 0% by mass, 0.20% by mass or less,
S: more than 0% by mass, 0.20% by mass or less,
Cr: 0.01-1% by mass,
Al: more than 0% by mass, 0.1% by mass or less V: 0.30 to 0.38% by mass, and N: more than 0% by mass, 0.0080% by mass or less, with the balance being iron and inevitable impurities Consists of
It is a non-tempered forging steel characterized by satisfying all of the following formulas (1) to (4).
1.10 ≦ [C] + 0.5 × [Mn] + 0.3 × [Cr] + 0.9 × [V] ≦ 1.28
... (1)
[Mn] / [Cr] ≦ 1.2 (2)
[C] × ([V] − [N] × 50.94 / 14.0) ≧ 0.130 (3)
[V] × ([N] − [Ti] × 14.0 / 47.9) × 10000 ≦ 35.0 (4)
However, in said formula (1)-(4), [element name] means content represented by the mass% of each element.

  Aspect 4 of the present invention is the non-tempered forging steel according to aspect 3, further comprising 0.001 to 0.030 mass% of Ti.

  According to the present invention, a non-tempered forged part capable of achieving both high strength and excellent manufacturability, in particular, prevention of surface cracking during continuous casting, and non-tempered forging useful for producing the non-tempered forged part. Steel can be provided.

FIG. 1 is a schematic explanatory view for explaining a sampling position of a test piece for evaluation of structure and Vickers hardness in a forged steel material used in Examples, (a) is a schematic top view of the test piece, and (b) is a schematic top view. The schematic sectional drawing of a test piece is shown.

  The present inventors have intensively studied to solve the above problems. In particular, earnest research to realize non-tempered forged parts that can achieve both high strength of 350HV or more and prevention of surface cracks during continuous casting, and non-tempered forged steels useful for the production of such non-tempered forged parts. Went. Hereinafter, the non-tempered forged parts may be simply referred to as “forged parts”.

  In order to achieve high strength of forged parts, (i) suppression of bainite, (ii) refinement of pearlite lamellar spacing, and (iii) precipitation strengthening by vanadium carbide (VC) are effective in the structure of the parts. It is. Although it is effective to contain Mn and Cr for the refinement of the lamellar spacing of (ii) above, bainite is likely to occur when a large amount of Mn and Cr is contained, making it difficult to realize (i) above. In addition, when V is added for precipitation strengthening by VC of (iii), bainite is likely to be generated when the content is increased, and it becomes difficult to realize (i).

Therefore, as a result of intensive studies on conditions for realizing a high strength of 350 HV or higher, it was found that all of the following formulas (1) to (3) should be satisfied. In particular, paying attention to the ratio of the contents of Mn and Cr, the mass ratio of Mn to Cr [Mn] / [Cr] is 1.2 or less as shown in the following formula (2), and the following formula (1) and formula It has been found that by satisfying (3), the lamella spacing of pearlite can be made fine while suppressing the formation of bainite; and further, precipitation is strengthened by precipitation of VC into fine lamella, and high strength can be realized.
1.10 ≦ [C] + 0.5 × [Mn] + 0.3 × [Cr] + 0.9 × [V] ≦ 1.28
... (1)
[Mn] / [Cr] ≦ 1.2 (2)
[C] × ([V] − [N] × 50.94 / 14.0) ≧ 0.130 (3)
However, in said formula (1)-(3), [element name] means content represented by the mass% of each element.

Moreover, it discovered that it was necessary to satisfy | fill following formula (4) with said Formula (1)-(3), in order to prevent the surface crack at the time of continuous manufacturing by the outstanding productivity with the said high intensity | strength.
[V] × ([N] − [Ti] × 14.0 / 47.9) × 10000 ≦ 35.0 (4)
However, in said formula (4), [element name] means content represented by the mass% of each element.

  Details of the above formulas (1) to (4) will be described below.

  In the above formula (1), [C] + 0.5 × [Mn] + 0.3 × [Cr] + 0.9 × [V] (hereinafter referred to as “value of formula (1)”) is hardenability, specifically Is a formula representing the ease of occurrence of bainite. When the value of Formula (1) is too high, a supercooled structure like bainite is likely to be generated, and the strength is lowered. Therefore, in this invention, the value of Formula (1) shall be 1.28 or less. The value of formula (1) is preferably 1.27 or less, more preferably 1.26 or less, and still more preferably 1.24 or less. On the other hand, if the value of the above formula (1) is too low, the effect of increasing the strength by solid solution strengthening and precipitation strengthening is insufficient. Therefore, the value of the above formula (1) is 1.10 or more, preferably 1.15 or more, more preferably 1.18 or more, and further preferably 1.20 or more.

  [Mn] / [Cr] in the above formula (2) (hereinafter referred to as “value of the formula (2)”) is an index for making the pearlite lamella spacing fine, and together with the above formula (1), the above formula By satisfying (2), bainite generation is sufficiently suppressed, the lamella spacing can be made fine, and it contributes to securing high strength. The value of the above formula (2) is 1.2 or less, preferably 1.1 or less, more preferably 1.0 or less. The smaller the value of the above formula (2), the better. The lower limit of the above formula (2) is not particularly limited from the viewpoint of ensuring high strength, and is about 0.01 from the range of each content of Mn and Cr.

  In the above formula (3), [C] × ([V] − [N] × 50.94 / 14.0) (hereinafter referred to as “value of formula (3)”) is precipitation strengthening by vanadium carbide (VC). It is an index representing the degree. Specifically, it is an index based on V that contributes to carbide formation except for V that constitutes vanadium nitride (VN) that does not contribute to precipitation strengthening among V present in steel. By satisfying the above formula (1) and formula (2) and satisfying the above formula (3), the bainite generation is sufficiently suppressed, and the ferrite-pearlite structure in which the pearlite lamella spacing is fine, VC precipitates in fine lamellae, and high strength of 350 HV or higher is obtained by precipitation strengthening. In order to express this effect, the value of the formula (3) is set to 0.130 or more. The value of the formula (3) is preferably 0.140 or more, more preferably 0.150 or more, and further preferably 0.160 or more. From the viewpoint of ensuring machinability, the value of the formula (3) is preferably 0.220 or less, more preferably 0.200 or less, and still more preferably 0.175 or less.

  [V] × ([N] − [Ti] × 14.0 / 47.9) × 10000 (hereinafter referred to as “value of the formula (4)”) in the above formula (4) is a continuous casting particularly during production. It is a formula showing the degree of the risk of surface cracks at the time. A large amount of vanadium nitride (VN) precipitates at the grain boundaries, so that the surface cracks are likely to occur. In the present invention, [V] × ([N] − [Ti] × 14.0 / 47.9) × 10000 is used as an index of the amount of VN generated, and this [V] × ([N] − [Ti] X14.0 / 47.9) x10,000 is suppressed to prevent surface cracks. Even if all of the above formulas (1) to (3) are satisfied, if the value of the formula (4) is too high, a large amount of the above VN precipitates at the grain boundaries and the high temperature ductility is lowered, and surface cracks occur during continuous casting. As a result, problems such as inability to manufacture occur. In this invention, in order to prevent this surface crack, the value of Formula (4) shall be 35.0 or less. The value of formula (4) is preferably 20.0 or less, more preferably 15.0 or less, and even more preferably 10.0 or less. The lower limit of the formula (4) is preferably as small as possible.

  The non-tempered forged part of the present invention is a ferrite-pearlite in which the fraction of bainite relative to the entire structure is suppressed to 5 area% or less, the ferrite fraction relative to the entire structure is 25 area% or less, and the balance is pearlite. This is a non-tempered forged part of the mold. Hereinafter, the area fraction of bainite with respect to the entire structure is referred to as “bainite fraction”, and the area fraction of ferrite with respect to the entire structure is referred to as “ferrite fraction”. When the bainite fraction exceeds 5 area% or the ferrite fraction exceeds 25 area%, the strength decreases and the hardness is not 350 HV or higher. The bainite fraction is preferably 3 area% or less, and most preferably 0 area%. The ferrite fraction is preferably 20 area% or less, more preferably 15 area% or less, and still more preferably 10 area% or less. The lower the ferrite fraction, the better, but the lower limit may be 3 area%. The higher the bainite fraction and the ferrite fraction, the higher the pearlite fraction is, for example, 75 area% or more, further 80 area% or more, and even more preferably 90 area% or more.

  In order to fully exhibit the effect by the control of the above formulas (1) to (4), the content of each element is set within the following range. The present invention also includes a non-tempered forging steel used for the production of the above-mentioned non-tempered forged part, and the contents of each element of the non-tempered forged steel and the formulas (1) to (4) are defined as follows: Same as non-tempered forged parts.

C: 0.40 to 0.60 mass%
C is an element necessary for ensuring the strength. If the amount of C is too small, the strength is lowered. From such a viewpoint, the C content needs to be 0.40% by mass or more. C content becomes like this. Preferably it is 0.45 mass% or more, More preferably, it is 0.48 mass% or more. However, when the C content is excessive, the strength becomes higher than necessary, and the machinability and manufacturability deteriorate. From such a viewpoint, the C content needs to be 0.60% by mass or less. C content becomes like this. Preferably it is 0.58 mass% or less, More preferably, it is 0.56 mass% or less. Furthermore, it is good also as 0.54 mass% or less, Furthermore, it is good also as 0.52 mass% or less.

Si: More than 0% by mass and 1.0% by mass or less Si is useful as a deoxidizing element during steel melting and also useful for increasing the strength of forged parts. From the viewpoint of securing strength, the Si content can be 0.05% by mass or more, further 0.10% by mass or more, and further 0.15% by mass or more. However, when the Si content is excessive, the strength becomes higher than necessary and the machinability deteriorates. In addition, the amount of scale generated by hot rolling and hot forging increases, which causes tool wear. Therefore, Si content needs to be 1.0 mass% or less. Si content becomes like this. Preferably it is 0.9 mass% or less, More preferably, it is 0.7 mass% or less. Furthermore, it is good also as 0.50 mass% or less, and also 0.30 mass% or less.

Mn: 0.01-0.70 mass%
Mn is an element useful for securing the strength of a steel material by solid solution strengthening or structure strengthening. Therefore, the Mn content is 0.01% by mass or more. Mn content becomes like this. Preferably it is 0.1 mass% or more, More preferably, it is 0.2 mass% or more. However, when the Mn content is excessive, a supercooled structure such as bainite is generated, and the yield strength is decreased. Therefore, the Mn content needs to be 0.70% by mass or less. Mn content becomes like this. Preferably it is 0.60 mass% or less, More preferably, it is 0.55 mass% or less, More preferably, it is 0.50 mass% or less.

P: more than 0% by mass and 0.20% by mass or less P is an element that can induce casting defects such as cracks during continuous casting. From such a viewpoint, the P content is 0.20% by mass or less. P content becomes like this. Preferably it is 0.10 mass% or less, More preferably, it is 0.030 mass% or less, More preferably, it is 0.020 mass% or less, More preferably, it is 0.010 mass% or less.

S: more than 0% by mass and 0.20% by mass or less S is an element useful for ensuring machinability. Specifically, S hardly dissolves in steel, for example, forms sulfides such as MnS, and the stress concentrates on the sulfides during cutting, so that chips are easily separated and machinability is improved. Has an effect. In order to fully exhibit this effect, it is preferable to make S content into 0.010 mass% or more, More preferably, it is 0.020 mass% or more. On the other hand, excessive S causes cracking during continuous casting, cracking during hot forging, a decrease in fatigue strength, and induction of chipping. Therefore, S content needs to be 0.20 mass% or less. S content becomes like this. Preferably it is 0.070 mass% or less, More preferably, it is 0.050 mass% or less, More preferably, it is 0.040 mass% or less.

Cr: 0.01-1 mass%
Cr is an element useful for securing the strength of a steel material by solid solution strengthening or structure strengthening. Therefore, Cr content shall be 0.01 mass% or more. Cr content becomes like this. Preferably it is 0.05 mass% or more, More preferably, it is 0.10 mass% or more. The Cr content can be further 0.20% by mass or more, further 0.30% by mass or more, and particularly 0.40% by mass or more. However, when the Cr content is excessive, a supercooled structure such as bainite is generated, and the yield strength is decreased. From such a viewpoint, the Cr content needs to be 1% by mass or less. Cr content becomes like this. Preferably it is 0.80 mass% or less, More preferably, it is 0.70 mass% or less, More preferably, it is 0.60 mass% or less.

Al: more than 0% by mass and 0.1% by mass or less Al is an element useful for deoxidation during steel melting. Further, at the time of melting, an appropriate amount of Si and Ca together with Al is present in the molten steel, so that a composite oxide useful for ensuring machinability is formed. From these viewpoints, the Al content may be 0.001% by mass or more. However, when the Al content is excessive, a hard oxide is formed and machinability is hindered. From such a viewpoint, the Al content needs to be 0.1% by mass or less. Al content becomes like this. Preferably it is 0.05 mass% or less, More preferably, it is 0.030 mass% or less. The Al content is preferably 0.020% by mass or less, and more preferably 0.010% by mass or less. Most preferably, it is 0.005 mass% or less.

V: 0.30 to 0.38 mass%
Since V is an element necessary for ensuring strength, the V content needs to be 0.30% by mass or more. V content becomes like this. Preferably it is 0.31 mass% or more, More preferably, it is 0.32 mass% or more. However, when the V content is excessive, the above effect is saturated and the addition cost cannot be met. Moreover, it becomes easy to produce the continuous castability fall. From such a viewpoint, the V content needs to be 0.38% by mass or less. V content becomes like this. Preferably it is 0.37 mass% or less, More preferably, it is 0.36 mass% or less.

N: more than 0% by mass and 0.0080% by mass or less N is an unavoidable impurity, and about 0.0030% by mass or more can be mixed in a normal steelmaking technique. N may be added. However, when the N content is excessive, deterioration of manufacturability, particularly hot workability is hindered. From such a viewpoint, the N content needs to be 0.0080% by mass or less. N content becomes like this. Preferably it is 0.0070 mass% or less, More preferably, it is 0.0060 mass% or less.

  The basic components of the non-tempered forged part of the present invention are as described above, and the balance is iron and inevitable impurities. Inevitable impurities are elements that are brought in depending on the situation of raw materials, materials, manufacturing equipment and the like. Inevitable impurities include, for example, less than 0.001 mass% Ti in addition to O, Sb, and the like. In addition, like P and S, for example, the smaller the content, the better. Therefore, it is an unavoidable impurity, but there are elements separately defined as described above with respect to the composition range. For this reason, the above-mentioned “inevitable impurities” in this specification means an element excluding an element whose composition range is separately defined.

  The non-tempered forged part of the present invention only needs to contain the above elements in the chemical composition and satisfy the formulas (1) to (4). The selective elements described below may not be included, but if necessary, together with the above elements, higher strength and excellent continuous castability can be achieved more easily, and these characteristics can be further improved. It is possible to enhance or combine machinability. Hereinafter, selective elements will be described.

Ti: 0.001 to 0.030 mass%
Ti is an element useful for securing high strength by solid solution strengthening. Further, Ti forms N and TiN and precipitates, so that VN generated at the grain boundary is relatively suppressed, and the high temperature ductility can be remarkably improved, and the risk of surface cracking can be further avoided. In order to exert the above effect, the Ti content is preferably 0.001% by mass or more. The Ti content is more preferably 0.0012% by mass or more, and still more preferably 0.0015% by mass or more. However, when the Ti content is excessive, hard inclusions are formed and the machinability tends to deteriorate. From such a viewpoint, the Ti content is preferably 0.030% by mass or less. The Ti content is more preferably 0.025% by mass or less, and still more preferably 0.020% by mass or less. Most preferably, it is 0.015 mass% or less.

Cu: more than 0% by mass, 0.2% by mass or less,
Ni: more than 0% by mass, 0.2% by mass or less,
One or more elements selected from the group consisting of Mo: more than 0% by mass, 0.2% by mass or less, and Nb: more than 0% by mass, 0.2% by mass or less These elements are non-tempered forged parts It is an element useful for further improving the strength of steel materials constituting non-tempered forging steel. Hereinafter, each element will be described.

Cu: More than 0 mass%, 0.2 mass% or less By containing Cu, the hardenability of steel materials can be improved and the stable intensity | strength of steel materials can be obtained. In order to acquire this effect, it is preferable to make Cu content over 0 mass%, More preferably, it is 0.01 mass% or more, More preferably, it is 0.03 mass% or more. However, when the Cu content is excessive, the hot workability is hindered and the manufacturability deteriorates. From such a viewpoint, the Cu content is preferably 0.2% by mass or less, more preferably 0.15% by mass or less, and still more preferably 0.10% by mass or less.

Ni: More than 0% by mass and 0.2% by mass or less By containing Ni, the hardenability of the steel material can be improved, and the stable strength of the steel material can be obtained. In order to acquire this effect, it is preferable to make Ni content over 0 mass%, More preferably, it is 0.01 mass% or more, More preferably, it is 0.03 mass% or more. However, when the Ni content is excessive, the toughness of the steel material is excessively increased, and it becomes difficult to separate with good fit at the time of, for example, production of a fracture separation type connecting rod. From such a viewpoint, the Ni content is preferably 0.2% by mass or less, more preferably 0.15% by mass or less, and still more preferably 0.10% by mass or less.

Mo: More than 0 mass%, 0.2 mass% or less By containing Mo, the hardenability of steel materials can be improved and the stable strength of steel materials can be obtained. In order to acquire this effect, it is preferable to make Mo content over 0 mass%, More preferably, it is 0.01 mass% or more, More preferably, it is 0.03 mass% or more. However, when the Mo content is excessive, the strength is excessively increased and the machinability is deteriorated. From such a viewpoint, the Mo content is preferably 0.2% by mass or less, more preferably 0.15% by mass or less, and still more preferably 0.10% by mass or less.

Nb: More than 0 mass%, 0.2 mass% or less By including Nb, the strength of the steel material is improved. In order to acquire this effect, it is preferable to make Nb content more than 0 mass%, More preferably, it is 0.01 mass% or more, More preferably, it is 0.03 mass% or more. However, if the Nb content is excessive, the strength improvement effect is saturated, and the effect is not commensurate with the alloy cost. From such a viewpoint, the Nb content is preferably 0.2% by mass or less, more preferably 0.15% by mass or less, and still more preferably 0.10% by mass or less.

Pb: more than 0% by mass, 0.20% by mass or less,
Te: more than 0% by mass, 0.20% by mass or less,
Sn: more than 0% by mass, 0.20% by mass or less,
Zr: more than 0% by mass, 0.20% by mass or less,
Ca: more than 0% by mass, 0.01% by mass or less,
One or more elements selected from the group consisting of Mg: more than 0% by mass, 0.01% by mass or less, and B: more than 0% by mass, 0.02% by mass or less These elements ensure machinability Is a useful element. Hereinafter, each element will be described.

Pb: more than 0% by mass, 0.20% by mass or less, Te: more than 0% by mass, 0.20% by mass or less, Sn: more than 0% by mass, 0.20% by mass or less, Zr: more than 0% by mass, 0 .20 mass% or less Pb, Te, Sn and Zr are elements useful for ensuring machinability, hardly dissolve in steel, and enhance machinability by effects such as melt embrittlement and spheroidization of MnS. Has an effect. When the above elements are included in order to exert this effect, the content of each element is preferably more than 0% by mass, more preferably 0.01% by mass or more, and further preferably 0.03% by mass. That's it. However, excess Pb, Te, Sn, and Zr cause cracks in the slab that occur during continuous casting, cracks in the forged part that occur during hot forging, and a decrease in fatigue strength. Therefore, the content of each element is preferably 0.20% by mass or less, more preferably 0.10% by mass or less, and still more preferably 0.05% by mass or less.

Ca: more than 0% by mass and 0.01% by mass or less Ca is an element useful for ensuring machinability, and has an effect of improving machinability by an effect such as generation of a belag (tool protective film). It also has the effect of increasing the machinability by spheroidizing sulfide inclusions to promote embrittlement. In order to exert these effects, the Ca content is preferably more than 0% by mass. However, even if Ca is added excessively, the above effect is saturated, resulting in an increase in cost. From this viewpoint, the Ca content is preferably 0.01% by mass or less, more preferably 0.005% by mass or less, still more preferably 0.004% by mass or less, and still more preferably 0%. 0.003 mass% or less.

Mg: more than 0% by mass, 0.01% by mass or less Mg is a deoxidizing element and hardly dissolves in steel, but dissolves in MnS and promotes spheroidization, thereby increasing the anisotropy of mechanical properties. To reduce. However, excess Mg causes slab cracking caused by continuous casting, cracking of forged parts caused by hot forging, and a decrease in fatigue strength. Therefore, the Mg content is preferably 0.01% by mass or less, more preferably 0.005% by mass or less, still more preferably 0.004% by mass or less, and still more preferably 0.003% by mass or less. It is.

B: More than 0% by mass and 0.02% by mass or less B forms BN when N is sufficiently present, and this BN brings about a lubricating action with the tool to enhance machinability. In order to obtain good machinability, B may be contained in an amount of 0.0001% by mass or more. More preferably, it is 0.0005 mass% or more. However, when B is excessively contained, B becomes a solid solution and bainite is easily generated. Therefore, the B content is preferably 0.02% by mass or less, more preferably 0.015% by mass or less, and still more preferably 0.010% by mass or less.

  The non-tempered forging steel of the present invention is obtained by melting a steel satisfying the above chemical composition by a normal method, and then performing a casting step and, if necessary, a hot rolling process, It can manufacture through a rolling process in order. The non-tempered forging steel of the present invention can be obtained, for example, as a steel bar by the hot rolling. Moreover, the non-tempered forged part of this invention can be manufactured by passing through a hot forging process after the said hot rolling process. In order to obtain a non-tempered forged part having a desired structure, it is necessary to control the cooling conditions after the hot forging step, particularly after hot forging, among the above steps. Hereinafter, each process is demonstrated in order.

  First, steel satisfying the above-described chemical composition is melted and cast. The casting method is not particularly limited, and a commonly used method may be employed. For example, an ingot casting method or a continuous casting method can be employed. In the case of the continuous casting method, casting can be performed with a bloom continuous casting machine.

  After casting, hot rolling may be performed as necessary. Partial rolling may include soaking before the partial rolling. The batch rolling conditions are not particularly limited, and generally used methods can be employed. For example, the block rolling can be performed at 1000 ° C to 1250 ° C. The conditions in the hot rolling process are not particularly limited, and usually used methods can be employed. For example, hot rolling can be performed at 850 ° C to 1200 ° C.

  In order to obtain the non-tempered forged part of the present invention, hot forging is performed after the hot rolling. As described above, in order to obtain a non-tempered forged part having a desired structure, it is necessary to control the cooling conditions after hot forging in this hot forging step. Specifically, in the cooling after hot forging, the average cooling rate in the temperature range from 800 ° C. to 600 ° C. is set to 1.2 ° C./sec or more and 3.0 ° C./sec or less. By controlling the average cooling rate in this temperature range, a pearlite-based structure with a narrow lamella spacing can be obtained. If the average cooling rate is too slow, the ferrite fraction increases and the pearlite lamella spacing becomes coarse. Therefore, in this invention, the said average cooling rate shall be 1.2 degrees C / sec or more. Preferably it is 1.25 ° C./sec or more, more preferably 1.30 ° C./sec or more, and further preferably 1.35 ° C./sec or more. The above temperature and average cooling rate are both based on the surface temperature of the steel.

  On the other hand, if the average cooling rate in the temperature range is too high, bainite is likely to be generated, and a high strength of 350 HV or higher cannot be achieved. Therefore, in this invention, the said average cooling rate shall be 3.0 degrees C / sec or less. Preferably it is 2.8 degrees C / sec or less, More preferably, it is 2.6 degrees C / sec or less. Cooling from the end of hot forging to 800 ° C. and cooling from 600 ° C. to room temperature are not particularly limited, and can be allowed to cool, for example.

  The other production conditions in the hot forging are not particularly limited, and usual conditions can be adopted. For example, the heating temperature before forging can be 1100 ° C. or higher and 1320 ° C. or lower. The temperature during forging, that is, the forging temperature can be 1100 ° C. or higher. The upper limit of the forging temperature is not particularly limited, and may be set to the heating temperature or lower.

  After forging, a forged part can be obtained by performing, for example, machining such as cutting as necessary to form a desired part shape.

  The non-tempered forged part and the non-tempered forged steel of the present invention are excellent in manufacturability, particularly continuous castability, as described above. In the present invention, “excellent in continuous castability” means that the high-temperature ductility evaluated in Examples described later, specifically, the drawing value in a tensile test at 800 ° C. is 13% or more. The aperture value is preferably 15.0% or more, more preferably 18.0% or more, and further preferably 20.0% or more.

  In addition, the non-tempered forged part of the present invention exhibits a high strength with a Vickers hardness of 350 HV or more. The Vickers hardness is preferably 360 HV or more, more preferably 370 HV or more. The 0.2% proof stress is 895 MPa or more, preferably 900 MPa or more, more preferably 910 MPa or more, and further preferably 950 MPa or more.

  Specific examples of the non-tempered forged parts of the present invention include forged parts such as connecting rods, lower arms, crankshafts, and the like used for engines and suspensions of transport machines such as automobiles and ships.

  Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited by the following examples, and can be implemented with appropriate modifications within the scope that can meet the above-mentioned and later-described gist, and they are all within the technical scope of the present invention. Is included.

  The steel was melted in accordance with a normal melting method, adjusted to satisfy the chemical composition shown in Table 1 below, cast, and then rolled in the range of 1100 ° C to 1250 ° C. In Table 1, “-” in the column of Ti indicates no addition, which is less than 0.001% by mass. In the present example, after the above-mentioned block rolling, the hot forging was performed after simulating the aforementioned hot rolling and heating to a heating temperature of 1200 ° C. Then, a square bar corresponding to non-tempered forging steel having a cross section perpendicular to the longitudinal direction and having a substantially square shape with a side of 20 mm and a length of 1100 mm was obtained.

  The square bar was cut perpendicular to the longitudinal direction to obtain 10 square bar pieces having a cross section of approximately 20 mm on a side and a length of 100 mm. In some examples, one of the square bar pieces was used to measure high temperature ductility, which is an index of continuous castability, as described below.

  The remaining square bar pieces were forged as follows to obtain a forged steel material. Specifically, Test No. In Nos. 1-14 and 16-22, the square bar pieces were held at 1250 ° C. for 10 minutes, then removed from the furnace, subjected to 60% compression press forging, then blast-cooled or cooled to room temperature by cooling. Thus, a forged steel material corresponding to a non-tempered forged part was obtained. The average cooling rate from 800 ° C. to 600 ° C. is approximately in the range of 2.3 to 2.5 ° C./sec for blast cooling, and approximately in the range of 1.3 to 1.6 ° C./sec for cooling. there were.

  In addition, Test No. No. 15, a steel piece having a diameter of 8 mm and a length of 12 mm was cut out from a square bar piece, and a forged steel material was obtained using a hot work reproduction test apparatus (TERMEC MASTER-Z) manufactured by Fuji Electric Koki. Specifically, first, the temperature is raised from room temperature to 1230 ° C. at an average temperature rising rate of 10 ° C./sec, and the temperature is raised from 1230 ° C. to 1250 ° C. at an average temperature rising rate of 1 ° C./sec. And held at 1250 ° C. for 10 minutes. Then, assuming hot rolling, the temperature was lowered from 1250 ° C. to 1100 ° C. at an average cooling rate of 5 ° C./sec, and then hot forging was simulated to compress at a compression rate of 60% and a compression rate of 10 / sec. Thereafter, the temperature was maintained at 1100 ° C. for 5 seconds, the temperature was decreased from 1100 ° C. to 800 ° C. at an average cooling rate of 1.8 ° C./sec, and further the temperature was decreased from 800 ° C. to 300 ° C. at an average cooling rate of 0.1 ° C./sec. Thereafter, it was rapidly cooled to room temperature to obtain a forged steel material corresponding to a non-tempered forged part. This test No. Under the production conditions of 15, the average cooling rate from 800 ° C. to 600 ° C. was also 0.10 ° C./sec as shown in Table 4 described later.

  Test No. obtained as described above. Using each of the forged steel materials 1 to 22 and using test pieces prepared by cutting as follows, the structure and the Vickers hardness were evaluated. Moreover, about some examples, 0.2% yield strength was also measured as follows. These results are shown in Table 2.

(1) Structure Evaluation Method FIG. 1 is a schematic explanatory view for explaining a sampling position of a test piece used for evaluation of the structure and evaluation of Vickers hardness described later in the forged steel material. A schematic top view, (b) shows a schematic cross-sectional view of the test piece. In FIG. 1, first, the central portion in the longitudinal direction, that is, the cutting perpendicular to the longitudinal direction is cut so that all of the central portion in the longitudinal direction L, the central portion in the width direction W, and the central portion in the thickness direction D can be observed. Cut along line XX. And in the cut surface Q obtained by cut | disconnecting by the cutting line XX, the test piece containing the observation area | region R of (b) which can observe the center part of the width direction and the center part of the thickness direction was prepared. The maximum size of the observation region R is 20 mm in the width direction W and 4 mm in the thickness direction D. The surface including the observation region R of the test piece was mirror-polished and then corroded with nital to obtain a structure observation test piece. And using the optical microscope, the said observation area | region R was image | photographed by 100 times-400 times, the metal structure was analyzed from the obtained photograph, and each area of a ferrite structure, a pearlite structure, and a bainite structure with respect to all the structures. The fractions, ie the ferrite fraction, the pearlite fraction, and the bainite fraction were measured. In Table 2, “F ratio” indicates the ferrite fraction, “P ratio” indicates the pearlite fraction, and “B ratio” indicates the bainite fraction.

(2) Evaluation method of Vickers hardness Vickers hardness was evaluated as follows using the test piece prepared by cutting each forged steel material in the same manner as the evaluation of the structure. That is, according to the Vickers hardness test-test method of JIS Z 2244 (2009), the Vickers hardness near the observation region R of the structure shown in FIG. 1 is measured with a load of 5 kgf using a Vickers hardness tester. did. In this evaluation, 5 points of Vickers hardness were measured and an average value was calculated. In addition, the Vickers hardness set 350HV or more as the pass.

(3) Measuring method of 0.2% yield strength About some examples, the tensile test was done as follows and 0.2% yield strength was measured.
(Tensile test)
Each forged steel material is cut, and from the part including all of the center portion in the longitudinal direction, the center portion in the width direction, and the center portion in the thickness direction of the forged steel material, a 14B plate shape shown in JIS Z 2241 (2011) A tensile specimen was obtained. In the collection of the tensile test piece, the longitudinal direction of the tensile test piece and the longitudinal direction of the forged steel material were matched. Further, the tensile force applied in the tensile test was also set in the same direction as the longitudinal direction. The tensile test was performed at normal temperature according to JIS Z 2241 (2011). The results are shown in Table 2.

  As described above, in some examples, high temperature ductility, which is an index of continuous castability, was measured as follows.

(4) High temperature ductility evaluation method The square bar piece is cut, and the parallel part has a diameter from a part including all of the central part in the longitudinal direction, the central part in the width direction, and the central part in the thickness direction of the square bar piece. A tensile test piece having a length of 6 mm × 15 mm and a total length of 68 mm was obtained. In collecting the tensile test pieces, the longitudinal direction of the tensile test pieces and the longitudinal direction of the square bar pieces were matched. Further, the tensile force applied in the tensile test was also set in the same direction as the longitudinal direction. In the high temperature ductility test, the specimen is once heated and held at 1300 ° C. in an Ar atmosphere, then cooled to 800 ° C. at 5 ° C./sec and kept at 800 ° C., and the tensile strength is measured at a tensile speed of 0.01 mm / sec. Was given until it broke, and after the break, it was cooled rapidly, and the aperture value after breakage of the test piece was measured. And as an index of continuous castability, a drawing value of 13% or more was accepted. The results are shown in Table 3.

  Of the above examples, the test No. For Nos. 1-6, 13, and 20, the temperature history during cooling after the press forging was measured, and test Nos. For No. 15, the temperature history when the temperature was lowered from 800 ° C. to 300 ° C. at an average cooling rate of 0.1 ° C./sec was measured. And the average cooling rate of the temperature range of 800-600 degreeC was calculated | required from this temperature history. Specifically, Test No. For 1-6, 13 and 20, after press forging, using DUAL THERMO AR-1600, the surface temperature of the test piece was measured at a distance of 500 mm between the test pieces and a temperature measurement spot diameter of 12 mm, and the temperature history Asked. Test No. For No. 15, a thermocouple was attached to the surface of the test piece, and the temperature history was obtained by actual measurement. And from this temperature history, the time required for cooling from 800 ° C. to 600 ° C. was obtained, and the average cooling rate in the temperature range of 800 to 600 ° C. was calculated. The results are shown in Table 4 together with the values of the formulas (1) to (3) shown in Table 1 and the structure ratio and Vickers hardness shown in Table 2.

  From Tables 1-4, Test No. Nos. 1 to 5 are examples of the present invention that satisfy all the requirements defined in the present invention. The Vickers hardness shows a high strength of 350 HV or more, and the high temperature ductility is high and the continuous castability is excellent. Test No. No. 4 is a test No. using the same steel type C. Perlite fraction is lower than 3, but Vickers hardness is high. The reason for this is that the method of cooling to room temperature after hot forging is blast cooling, and the lamellar spacing is the same as in Test No. 1 above. It is conceivable that the strength was increased because it became finer than 3.

  Furthermore, test no. No. 1 and steel No. B test No. From the comparison of 2, it was found that when 0.009% by mass of Ti was contained, the drawing value at 800 ° C. shown in Table 3 reached 20% or more, and more excellent continuous castability could be realized. Note that the effect of improving the continuous castability by the addition of Ti is the test No. 1 as a comparative example. 6 and test no. It was also seen in 8 contrasts.

  On the other hand, test No. 6-22, and test No. Both 23 and 24 are comparative examples that do not satisfy the requirements defined in the embodiment of the present invention, and the results are low in strength or inferior in continuous castability.

  Test No. Examples 6 to 8 are examples using steel types E to G in which the amount of Mn is excessive and the value of the formula (1) exceeds the upper limit. In addition, Test No. In No. 6, cooling in the temperature range of 800 to 600 ° C. after hot forging was performed at a fast average cooling rate exceeding the recommended range. Due to these reasons, Test No. In all of 6-8, a lot of bainite was formed and the Vickers hardness was low.

  Test No. 9-11, the content of each element is within the specified range, but because the steel types H to J in which the value of the formula (1) exceeded the upper limit were used, a lot of bainite was formed and the Vickers hardness was lowered. .

  Test No. In Nos. 12 and 13, the amount of Mn was excessive, and steel types K and L in which the values of the formulas (1) and (2) exceeded the upper limit were used, so a lot of bainite was formed and the Vickers hardness was low.

  Test No. Nos. 14, 16 to 18 and 22 use steel types M to P and T in which the amount of C is insufficient, the amount of Mn is excessive, and all the values of formulas (1) to (3) are out of the specified range. A lot of bainite was formed, and the Vickers hardness was low.

  Test No. 15 is the test no. 14 is an example using the same steel type M as in the above test No. 14. Unlike 14, since cooling in the temperature range of 800 to 600 ° C. after hot forging was performed at a slow average cooling rate below the recommended range, a large amount of ferrite was formed and the Vickers hardness was low.

  Test No. No. 19 has an insufficient amount of C, an excessive amount of Mn, an insufficient amount of V, and steel type Q that does not satisfy the formulas (2) and (3). Became lower. Test No. No. 19 was excellent in continuous castability because the value of the formula (4) was within the specified range as shown in Table 3.

  Test No. In No. 20, the amount of C was insufficient, the amount of Mn was excessive, and the steel type R not satisfying the formulas (2) and (3) was used, so that a lot of ferrite was formed and the Vickers hardness was low. Test No. No. 20 was excellent in continuous castability because the value of formula (4) was within the specified range as shown in Table 3.

  Test No. No. 21 has insufficient C content, excessive Mn content, excessive N content, and steel type S that does not satisfy the formulas (2) and (3) is used. Became lower. Test No. No. 21 was excellent in continuous castability because the value of formula (4) was within the specified range as shown in Table 3.

  Test No. No. 23, C amount is insufficient, Mn amount is excessive, N amount is excessive, and the steel type U in which all of the formulas (1) to (4) are out of the specified range is used, and the continuous castability is increased. It is an evaluated example. Since the formula (4) is not satisfied as shown in Table 3, the high temperature ductility is low, and excellent continuous castability is not obtained. Further, since this steel type does not satisfy the formulas (1) to (3), it is considered that the desired structure cannot be obtained and the strength is low.

  Test No. No. 24 is an example in which continuous castability was evaluated using a steel type V in which the amount of C is insufficient, the amount of Mn is excessive, the amount of N is excessive, and the formulas (2) and (3) are not satisfied. . This test No. In No. 24, since the formula (4) was satisfied, excellent continuous castability was obtained. However, test no. Since the steel type V used in 24 does not satisfy the expressions (2) and (3), it is considered that a desired structure cannot be obtained and the strength is low.

  From the above, the following can be said particularly from Table 3 regarding the continuous castability. That is, test No. using steel type U. In No. 23, V was added in a large amount of 0.341% by mass, and the value of the formula (4) exceeded the upper limit. Therefore, the drawing value at 800 ° C. was as low as 12.2%, and the risk of surface cracking was high. In contrast, test no. In 1 and 2, high temperature ductility could be improved and the risk of surface cracking could be reduced by satisfying formula (4). The effect of improving the continuous castability by satisfying this formula (4) is the test No. 1 as a comparative example. Also seen in 6, 8, 19-21 and 24. However, these comparative examples have low strength because they do not satisfy the values of expressions other than expression (4).

  Moreover, the following can be said from Table 4 about the recommended manufacturing conditions. Test No. 1-5 and test no. From the comparison with 6, 13, 15 and 20, in order to obtain a predetermined structure for ensuring high strength, the chemical composition, in particular, the formulas (1) to (3) are satisfied, and at the time of cooling after hot forging. It can be seen that the average cooling rate in the temperature range from 800 ° C. to 600 ° C. is preferably 1.2 ° C./sec or more and 3.0 ° C./sec or less.

d1 ½ width of test piece d2 ½ length of test piece X cutting line W width direction L longitudinal direction D thickness direction Q cut surface R observation region

Claims (4)

C: 0.40-0.60 mass%,
Si: more than 0% by mass, 1.0% by mass or less,
Mn: 0.01-0.70 mass%,
P: more than 0% by mass, 0.20% by mass or less,
S: more than 0% by mass, 0.20% by mass or less,
Cr: 0.01-1% by mass,
Al: more than 0% by mass, 0.1% by mass or less,
V: 0.30 to 0.38% by mass, and N: more than 0% by mass, 0.0080% by mass or less, with the balance consisting of iron and inevitable impurities,
All the following formulas (1) to (4) are satisfied, and
A non-tempered forged part characterized in that the fraction of bainite with respect to the entire structure is 5 area% or less, the fraction of ferrite with respect to the entire structure is 25 area% or less, and the balance is pearlite.
1.10 ≦ [C] + 0.5 × [Mn] + 0.3 × [Cr] + 0.9 × [V] ≦ 1.28
... (1)
[Mn] / [Cr] ≦ 1.2 (2)
[C] × ([V] − [N] × 50.94 / 14.0) ≧ 0.130 (3)
[V] × ([N] − [Ti] × 14.0 / 47.9) × 10000 ≦ 35.0 (4)
However, in said formula (1)-(4), [element name] means content represented by the mass% of each element.   Furthermore, the non-tempered forged part of Claim 1 containing 0.001-0.030 mass% of Ti. C: 0.40-0.60 mass%,
Si: more than 0% by mass, 1.0% by mass or less,
Mn: 0.01-0.70 mass%,
P: more than 0% by mass, 0.20% by mass or less,
S: more than 0% by mass, 0.20% by mass or less,
Cr: 0.01-1% by mass,
Al: more than 0% by mass, 0.1% by mass or less V: 0.30 to 0.38% by mass, and N: more than 0% by mass, 0.0080% by mass or less, with the balance being iron and inevitable impurities Consists of
Non-tempered forging steel characterized by satisfying all of the following formulas (1) to (4).
1.10 ≦ [C] + 0.5 × [Mn] + 0.3 × [Cr] + 0.9 × [V] ≦ 1.28
... (1)
[Mn] / [Cr] ≦ 1.2 (2)
[C] × ([V] − [N] × 50.94 / 14.0) ≧ 0.130 (3)
[V] × ([N] − [Ti] × 14.0 / 47.9) × 10000 ≦ 35.0 (4)
However, in said formula (1)-(4), [element name] means content represented by the mass% of each element.   The non-tempered forging steel according to claim 3, further comprising 0.001 to 0.030 mass% of Ti.

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