JP2013044030A - Non-heat treated steel for hot forging excellent in machinability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a non-heat treated steel for hot forging, having sufficient strength and excellent in machinability without needing a nitriding and an induction hardening.SOLUTION: The non-heat treated steel for hot forging, excellent in machinability, contains, by mass, 0.35-0.55% C, 0.40-0.80% Si, 0.90-1.60% Mn, 0.030-0.080% S, ≤0.30% Ni, ≤0.35% Cr, ≤0.05% Mo, 0.008-0.035% Al, 0.07-0.14% V, ≤0.0030% O, 0.0030-0.0200% N and the balance Fe with inevitable impurities, and the ratio of C/V is 2.80-6.00, and C equivalent shown in a formula (1): C equivalent=C%+Si%/7+Mn%/5+Cr%/9+V%/2 is 0.72-0.86, and the structure after hot forging is ferrite-pearlite.

Description

  TECHNICAL FIELD The present invention relates to a machine structural steel useful for producing parts such as automobiles and industrial machines, and particularly relates to a non-heat treated steel for hot forging excellent in machinability.

  Components of the non-heat treated steel for machine structure are adjusted so that hardness required for the parts can be obtained by air cooling mainly after hot forging. Therefore, in the subsequent cutting process, since it is necessary to cut in a state where the hardness is somewhat high, excellent machinability is desired. In order to improve the machinability, it is effective to add a machinability improving element such as S or Pb. However, when S is added excessively to steel, the strength of the parts made of this steel is reduced, so the amount of addition is naturally limited. Moreover, since Pb is also an environmentally hazardous substance, its use may be greatly restricted in the future. Against this background, there is a growing need for non-heat treated steel for machine structures with improved machinability without relying solely on the addition of machinability improving elements.

  Then, the method of manufacturing the non-tempered steel forging product excellent in the machinability and the fatigue strength after compression processing by increasing the ferrite fraction in steel is proposed (for example, refer patent document 1). . However, although this method has a beneficial effect that the wear of the turning flank of the carbide tool is small, the drill workability is not considered.

  Furthermore, a method for producing a hot forged non-heat treated steel part having a micro structure made of ferrite and pearlite and excellent in machinability has been proposed (for example, see Patent Document 2). However, this method is disadvantageous in terms of production efficiency because the heating temperature before forging needs to be raised to at least 1250 ° C. or more.

  On the other hand, in order to solve the above problems, the inventor uses V as an essential additive element, and further balances the C amount and the V amount so that the hardness when air-cooled from the hot forging temperature is approximately equal to the conventional one. In addition, in the non-heat treated steel adjusted so as to have a ferrite-pearlite structure, an intensive study was made on the balance of C and V amounts excellent in drill workability. As a result, contrary to the conventional view that drilling workability will be better as the ferrite fraction is higher (the C / V ratio is lower at this time), the pearlite fraction is higher (at this time the C / V ratio is higher). Was found to be superior in drill workability. On the other hand, the wear ratio on the flank of the carbide tool in turning and the proof stress ratio (ie 0.2% proof stress / tensile strength), which is important as a strength index for non-heat treated steel parts, decreases when the C / V ratio is high. I understood that. Furthermore, the C-equivalent, which is a factor that affects the hardness and machinability of the non-heat treated steel, is also clarified and the present invention is achieved.

JP 2000-239782 A JP 2005-82840 A

  This application is based on the findings of the inventors' extensive research and investigation, and does not require nitriding or induction hardening, has sufficient strength, and has excellent machinability for hot forging. Is to provide steel.

In the means of the present invention for solving the above-mentioned problems, the invention of claim 1 is, in mass%, C: 0.35 to 0.55%, Si: 0.40 to 0.80%, Mn: 0 90-1.60%, S: 0.030-0.080%, Ni: 0.30% or less, Cr: 0.35% or less, Mo: 0.05% or less, Al: 0.008-0 0.035%, V: 0.07 to 0.14%, O: 0.0030% or less, N: 0.0030 to 0.0200%, and the balance Fe and inevitable impurities. Further, this steel has a C / V ratio in the range of 2.80 to 6.00, a C equivalent represented by the following formula (1) in a range of 0.72 to 0.86, and after hot forging. Is a non-tempered steel for hot forging with excellent machinability, whose structure is ferrite-pearlite.
C equivalent = C% + Si% / 7 + Mn% / 5 + Cr% / 9 + V% / 2 (1)

Invention of Claim 2 is mass%, C: 0.35-0.55%, Si: 0.40-0.80%, Mn: 0.90-1.60%, S: 0.030- 0.080%, Ni: 0.30% or less, Cr: 0.35% or less, Mo: 0.05% or less, Al: 0.008 to 0.035%, V: 0.07 to 0.14% , O: 0.0030% or less, N: 0.0030 to 0.0200%, Ti: 0.006 to 0.020%, and the balance Fe and inevitable impurities. This steel has a C / V ratio in a range of 2.80 to 6.00, a C equivalent represented by the following formula (1) in a range of 0.72 to 0.86, and a structure after hot forging. Is a non-tempered steel for hot forging with excellent machinability, which is ferrite-pearlite.
C equivalent = C% + Si% / 7 + Mn% / 5 + Cr% / 9 + V% / 2 (1)

  The reasons for limiting the chemical components of the non-heat treated steel for machine structure of the above invention will be described below. In addition,% is the mass%.

C: 0.35-0.55%
C is an element indispensable for ensuring the strength of the non-heat treated steel. For this purpose, C needs to be 0.35% or more. However, if C exceeds 0.55%, the machinability and strength decrease. Therefore, C is 0.35 to 0.55%, preferably 0.35 to 0.45%, and more preferably 0.36 to 0.42%.

Si: 0.40 to 0.80%
Si is an element necessary as a deoxidizer and is an essential element for securing the strength of non-tempered steel. For this purpose, Si needs to be 0.40% or more. However, if Si exceeds 0.80%, the ferrite phase becomes hard and machinability is lowered. Therefore, Si is 0.40 to 0.80%, preferably 0.50 to 0.70%.

Mn: 0.90 to 1.60%
Mn is an element indispensable for ensuring the strength of non-heat treated steel, and is an element necessary for producing MnS to improve machinability. For this purpose, Mn needs to be 0.90% or more. However, if Mn is more than 1.60%, bainite is generated and machinability is greatly reduced. Therefore, Mn is 0.90 to 1.60%, preferably 0.90 to 1.40%, and more preferably 1.00 to 1.40.

S: 0.030 to 0.080%
S is an element indispensable for ensuring machinability and chip disposal in drilling and turning. For this purpose, S needs to be 0.030% or more. However, if S is more than 0.080%, strength properties such as static strength and fatigue strength are lowered, and hot workability is further lowered. Therefore, S is 0.030 to 0.080%, preferably 0.040 to 0.070%.

Ni: 0.30% or less Ni is inevitably contained in the steel, but reduces the machinability of the non-tempered steel. Therefore, Ni is restricted to 0.30% or less.

Cr: 0.35% or less Cr may be added as necessary to secure the hardness of the non-heat treated steel, but the machinability is lowered by the addition of 0.35% or more. Therefore, Cr is made 0.35% or less.

Mo: 0.05% or less Mo is inevitably contained in the steel, but if it is contained in an amount of more than 0.05%, bainite is likely to be generated and machinability is lowered. Therefore, Mo is restricted to 0.05% or less.

Al: 0.008 to 0.035%
Al is an element that forms nitrides and is effective in suppressing grain coarsening during forging heating, and for this purpose, it needs to be 0.008% or more. However, Al ₂ O ₃ harmful to the machinability and fatigue life needs to be reduced, so the upper limit of Al is 0.035%. Therefore, Al is made 0.008 to 0.035%, preferably 0.016 to 0.030%.

V: 0.07 to 0.14%
V is an element necessary for ensuring the strength of the non-heat treated steel, and for this purpose, 0.07% or more is necessary. However, when V is more than 0.14%, V-based precipitates that become ferrite nuclei become excessive in the structure forming process after hot forging, and the amount of ferrite is greatly increased, and drill workability is impaired. Therefore, V is set to 0.07 to 0.14%, preferably 0.08 to 0.12%.

Ti: 0.006 to 0.020%
Ti has the effect of suppressing crystal grain coarsening during hot forging. Therefore, Ti may be added in an amount of 0.006% or more as necessary, such as when the heating time for hot forging is long. However, if Ti is added more than 0.020%, the machinability is impaired. Therefore, the upper limit of Ti is 0.020%.

O: 0.0030% or less O generates oxide inclusions that are harmful to machinability and fatigue life. Therefore, O needs to be limited to 0.0030% or less, and preferably limited to 0.0020% or less.

N: 0.0030 to 0.0200%
N is an element that forms nitrides with Al and is effective in suppressing grain coarsening during forging heating. Therefore, N needs to be added in an amount of 0.0030% or more. However, even if N is more than 0.0200%, the effect of suppressing grain coarsening is saturated. Therefore, N is 0.0030 to 0.0200%, preferably 0.0030 to 0.0150%.

  Moreover, the steel used by this invention contains P and Cu as an unavoidable impurity other than said element. However, even if the amount is large, P is 0.030% or less and Cu is 0.30% or less.

Reason for limiting the C / V ratio to 2.80 to 6.00 by mass% By limiting the C / V ratio to the range of 2.80 to 6.00, machinability, that is, drilling in the present invention. , And a tempered carbide flank wear amount in turning, and a non-tempered steel excellent in yield strength ratio determined from 0.2% yield strength / tensile strength. When the C / V ratio is smaller than 2.80, the ferrite fraction becomes excessive and drill workability is lowered. On the other hand, when the C / V ratio is larger than 6.00, the pearlite fraction becomes too large, and the drill workability is good, but the turning workability and proof stress ratio with a carbide tool are impaired. Therefore, the C / V ratio is set to 2.80 to 6.00.

Reason for limiting the C equivalent to 0.72 to 0.86 In addition to the above-described limitation of the chemical composition of the steel of the present invention and the limitation of the C / V ratio, the present invention sets the C equivalent to 0.72 to 0.86. limit. The reason for this is that when the C equivalent is less than 0.72, the hardness required for non-tempered steel parts is insufficient due to low hardness. On the other hand, when the C equivalent is larger than 0.86, the hardness is too high in normal hot forging, and bainite is generated, so that drillability is greatly impaired. Therefore, the C equivalent is limited to a range of 0.72 to 0.86.

  The steel of the present invention relies on machinability improving elements in parts made of non-tempered steel produced by ordinary hot forging by limiting the steel components, limiting the C / V ratio, and limiting the C equivalent. In addition, it has excellent drillability, has a small amount of carbide tool wear in turning, has a high yield ratio, and has a beneficial effect.

  In the steel of the present invention, V is indispensably added. When nitriding is performed, a V-based hard compound is formed on the surface and the strength is greatly impaired. Therefore, the steel of the present invention is not nitrided. . In addition, since the steel of the present invention can be sufficiently hardened by air cooling after hot forging, the present invention does not require induction hardening.

  Embodiments of the present invention will be described below. Ingots were obtained by melting 100 kg of steels having chemical components of the present invention and comparative examples shown in Table 1 in a vacuum melting furnace. Subsequently, the ingot was heated to 1250 ° C. and held for 5 hours, and then forged into a 30 mm square steel bar and a 35 mm diameter round bar. Subsequently, assuming normal hot forging, after heating to 1200 ° C., heat treatment for air cooling was performed.

  Subsequently, drill workability was evaluated using a 30 mm square steel bar obtained by forging as described above. About drill workability, it implemented on the conditions of Table 2.

  Furthermore, the turning workability with a carbide tool was evaluated using the round bar with a diameter of 35 mm obtained by forging as described above. About the turning workability by a carbide tool, it implemented according to the conditions described in Table 3.

  In addition, a tensile test piece having a parallel part diameter of 6 mm was prepared from the longitudinal direction of the middle peripheral part of a round bar having a diameter of 35 mm, and a 0.2% proof stress and tensile strength were measured by performing a room temperature tensile test. The yield ratio obtained from% yield strength / tensile strength was determined. The results are shown in Table 4. The hardness of these 30 mm square steel bars, which were air-cooled after heating to 1200 ° C., was the same as that of the 35 mm diameter round bars. Therefore, the hardness of a round bar having a diameter of 35 mm is shown in Table 4 as a representative of both.

  As seen in Table 4, No. of the invention example which is an example of the present invention. 1 to 10 each satisfy the range of the predetermined chemical component, and the C / V ratio is in the range of 2.80 to 6.00, and the C equivalent is in the range of 0.72 to 0.86. It is in. On the other hand, at least one of the chemical component, the C / V ratio, and the C equivalent is a comparative example No. 1 that falls outside the scope of the present invention. Nos. 11 to 20 are inferior to the inventive examples in at least one of drilling workability indicated by the number of holes until drill drilling is impossible, carbide tool flank wear amount and proof stress ratio in turning.

Claims (2)

In mass%, C: 0.35 to 0.55%, Si: 0.40 to 0.80%, Mn: 0.90 to 1.60%, S: 0.030 to 0.080%, Ni: 0.30% or less, Cr: 0.35% or less, Mo: 0.05% or less, Al: 0.008-0.035%, V: 0.07-0.14%, O: 0.0030% Hereinafter, N: 0.0030 to 0.0200% is contained, the balance is Fe and inevitable impurities, the C / V ratio is 2.80 to 6.00, and the C equivalent represented by the following formula (1) is A non-tempered steel for hot forging excellent in machinability, characterized in that it has a structure of 0.72 to 0.86 and the structure after hot forging is ferrite-pearlite.
C equivalent = C% + Si% / 7 + Mn% / 5 + Cr% / 9 + V% / 2 (1) In mass%, C: 0.35 to 0.55%, Si: 0.40 to 0.80%, Mn: 0.90 to 1.60%, S: 0.030 to 0.080%, Ni: 0.30% or less, Cr: 0.35% or less, Mo: 0.05% or less, Al: 0.008-0.035%, V: 0.07-0.14%, O: 0.0030% Hereinafter, N: 0.0030 to 0.0200%, Ti: 0.006 to 0.020%, comprising the balance Fe and inevitable impurities, C / V ratio is 2.80 to 6.00, The C equivalent represented by the following formula (1) is 0.72 to 0.86, and the structure after hot forging is ferrite-pearlite. Quality steel.
C equivalent = C% + Si% / 7 + Mn% / 5 + Cr% / 9 + V% / 2 (1)