JP3671688B2 - Non-heat treated steel for hot forging for fracture split type connecting rods with excellent fracture splitting - Google Patents

Description

【００４４】
これらの結果から、以下の様に考察できる。
まずNo.４，５は本発明で規定する要件のいずれも満足する実施例であり、鍛造割れが発生することなく、衝撃値もその基準となる２０Ｊ／ｃｍ²よりも小さく、脆性破面率も必須条件である１００％を満たしており、良好な破断分割性が得られていることが分かる。
【００４５】
これに対して、本発明で特定する要件のいずれかを満足しないＮo.８〜１２は、夫々以下の不具合を抱えている。
【００４６】
Ｎo.８は、硫化物系介在物の個数が多い例であり、熱間での鍛造割れが認められる。一方、Ｎo.９は硫化物系介在物の個数が少ない例であり、衝撃値、脆性破面率ともに目標レベルに達していない。
【００４７】
また、Ｎo.１０は硫化物系介在物の個数は本発明の要件を満足するが、アスペクト比がその上限を超えており、衝撃値、脆性破面率ともに目標レベルに達していない。
【００４８】
更にＮo.１１は、硫化物系介在物の個数およびアスペクト比の両方が本発明の要件を満足しない例であり、衝撃値、脆性破面率ともに目標レベルに達していない。
【００４９】
また、Ｎo.１２はＳ含有量が高く、硫化物系介在物のアスペクト比が大きい例であるが、熱間での鍛造割れが認められた。
【００５０】
【発明の効果】
本発明は以上の様に構成されているので、高強度、低靱延性を満足し、破断分割性にも優れた熱間鍛造用非調質鋼を得ることができる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a non-heat treated steel for hot forging suitable for breaking and splitting automotive parts and the like cold. More specifically, the present invention relates to a connecting rod main body (abbreviation: connecting rod main body) and a connecting rod cap (abbreviation: connecting rod cap) constituting a connecting rod (abbreviation: connecting rod) used as a part of an automobile engine or the like by cold working. The present invention relates to a non-heat treated steel for hot forging that can be easily divided, has an improved fitting property on a fractured dividing surface, and can keep the amount of plastic deformation low.
[0002]
[Prior art]
Conventionally, for parts that require division after molding, such as connecting rods, the connecting rod main body and connecting rod cap are separately hot forged, then the mating surface is processed by cutting, and further, if necessary, a pin to prevent misalignment It was processed. Alternatively, even when the connecting rod is integrally hot forged, cutting by machining and finishing of the mating surface are performed. However, when the above processing is performed, there is a problem that the yield of the material is reduced and the cost is increased due to a large number of steps.
[0003]
In view of this, a method has been studied in which a connecting rod is integrally hot forged, machined without machining of the divided surface, then broken and broken in cold, and coupled to a crankshaft for assembly. However, when conventional connecting rod steel is fractured cold, the tough ductility is good, so the split surface cannot be fitted due to plastic deformation, or the amount of plastic deformation at the large inner diameter of the connecting rod is large. As a result, there were problems such as an increase in the amount of finishing.
[0004]
[Problems to be solved by the invention]
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a non-heat treated steel for hot forging for fracture split type connecting rods which has low toughness and ductility and is excellent in fracture splitability. .
[0005]
[Means for Solving the Problems]
The non-heat treated steel for hot forging for fracture split type connecting rods according to the present invention that has achieved the above-mentioned problems,
Steel component is mass% (hereinafter the same)
C: 0.15-0.6%,
Si: 1.5% or less (excluding 0%),
P: 0.15% or less (excluding 0%),
Cr: 2% or less (excluding 0%),
Al: 0.06% or less (excluding 0%),
N: 0.03% or less (excluding 0%),
Mn: 2.0% or less (excluding 0%),
S: 0.038-0.061%,
V: 0.45% or less (excluding 0%)
Ca: 0.0018% or less (excluding 0%)
And Zr: 0.2% or less (excluding 0%)
Ti: 0.1% or less (excluding 0%)
Mg: 0.01% or less (excluding 0%)
REM: 0.3% or less (excluding 0%)
Containing at least one selected from the group consisting of (provided that Zr is 0.08% or more when Zr is contained)
With
In longitudinal section of the 1/4 thickness position from the surface of the forging material, the sulfide inclusions in the above range 1μm is 100 to 4000 pieces / mm ² is present, the average aspect ratio of sulfides (length / width) Has a gist where it is 10 or less.
[0006]
The non-heat treated steel for hot forging (hereinafter simply referred to as “non-heat treated steel for hot forging”) of the fracture split type connecting rod of the present invention may further include (1) Nb: 0.15% or less if necessary. (2) Pb: 0.3% or less (not including 0%) and / or Bi: 0.3% or less (not including 0%), etc. are recommended. Is done.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
The present inventors have intensively studied to provide a non-heat treated steel for hot forging excellent in fracture splitting property. As a result, the inventors have found that the intended purpose can be solved by appropriately controlling the number and aspect ratio of sulfide inclusions, and have completed the present invention.
[0008]
Focusing on sulfide inclusions in steel as in the present invention, the inventor of the present application has found that the fracture splitting property of hot forged products in cold can be remarkably enhanced by forming predetermined sulfide inclusions. Are not found in the conventional method at all.
[0009]
For example, as conventional methods, (1) JP-A-8-291373 and JP-A-9-176786 are cited as those aiming to improve fracture splitting by positively adding an embrittlement element. The former is to improve fracture splitting by adding P, and the latter is by adding As, Sb, and Sn.
[0010]
Furthermore, as disclosed in JP-A-9-3589 and JP-A-9-176787, examples of the method for promoting brittle fracture by increasing the free solid solution N and improving the fracture splitting property are disclosed. Conditions for increasing dissolved N have been specified.
[0011]
Further, (3) for example, JP-A-9-31594 is cited as one that promotes brittle fracture by strengthening ferrite and improves fracture splitting ability, Si for strengthening ferrite by solid solution strengthening, precipitation strengthening Discloses a method for enhancing fracture splitting by adding V, which strengthens ferrite.
[0012]
As described above, all of the above conventional methods are intended to embrittle a predetermined metal structure by adjusting an additive element in the steel, and “break fracture by forming a predetermined sulfide-based inclusion. The technical idea of the present invention “to obtain a non-heat treated steel for hot forging with excellent properties” is not suggested and is different from the present invention.
[0013]
As described above, the non-heat treated steel for hot forging excellent in fracture splitting property according to the present invention is a sulfide inclusion having a width of 1 μm or more in a longitudinal section at a 1/4 thickness from the surface of the forged material. 100 to 4000 pieces / mm ² , and the average aspect ratio (length / width) of the sulfide inclusions is 10 or less.
[0014]
Thus, in the present invention, the reason for specifying the number of sulfide inclusions and the aspect ratio is as follows.
[0015]
Sulfide inclusions such as MnS are stretched in the rolling direction or the forging direction by rolling or hot forging. If this stretched sulfide inclusion is present in the longitudinal direction (stretched in the direction perpendicular to the fracture splitting surface) at the time of fracture splitting of forgings, etc., as the crack progresses, the sulfide inclusions and the metal matrix Peels and stress relaxation occurs. As a result, brittle fracture is inhibited, the toughness ductility value is improved, and the fracture splitting property is lowered. In contrast, when the sulfide inclusions are restrained from being stretched and the aspect ratio is reduced to a spheroidized shape, the cracks at the tip of the cracks generated around the sulfide inclusions are divided when the fracture is broken by the vertical line. Stress increases and brittle fracture is promoted. As a result, the fitting property of the fracture split surface is enhanced, and the amount of plastic deformation can be reduced.
[0016]
In order to effectively exhibit the effect of the above-described formation of sulfide inclusions, the average aspect ratio needs to be 10 or less. As described above, when the aspect ratio exceeds 10 and there are sulfide inclusions extending in the longitudinal direction, the break splitting property is lowered. Furthermore, by reducing the aspect ratio and making the sulfide inclusions as spherical as possible, the hot workability is improved, and the effect of preventing cracking during rolling or hot forging can be obtained. In order to further enhance such effects, it is recommended that this aspect ratio be as close to 1 as possible, preferably 6 or less, more preferably 4 or less.
[0017]
These effects are effective in the case of sulfide inclusions having a width of 1 μm or more. This is because, when the width is less than 1 μm, the above-described peeling does not occur at the time of break division, and the sulfide inclusions themselves break.
[0018]
Furthermore, in the present invention, it is necessary that the above-described sulfide inclusions having a width of 1 μm or more exist in a longitudinal section at a position of ¼ thickness from the surface of the forged material, in an amount of 100 to 4000 / mm ² . When the number of sulfide inclusions is less than 100 / mm ² , cracks propagating through the sulfide inclusions are reduced, and a desired effect associated with the progress of the cracks cannot be obtained. Preferably it is 300 pieces / mm ² or more, more preferably 400 pieces / mm ² or more. On the other hand, when the number of the sulfide inclusions exceeds 4000 / mm ² , problems such as cracks occur during rolling or hot forging. The number is preferably 3000 pieces / mm ² or less, and more preferably 2500 pieces / mm ² or less.
[0019]
The “sulfide inclusion” in the present invention mainly means MnS, but also other sulfides such as Mn, Zr, Ti, Mg, Ca, Se, Te, REM, and the like. Complex sulfides and the like are also included within the scope of the present invention.
[0020]
Next, the chemical composition of the hot forging die non-tempered steel according to the present invention will be described. As described above, in the present invention, the most important point is the presence of specific sulfide inclusions. For this purpose, the following elements are basically added.
[0021]
Mn: 2.0% or less (excluding 0%)
Mn is an element that is effective as a deoxidizing and desulfurizing element at the time of melting, and also increases the pearlite hardenability of the forged product to increase the amount of pearlite, and by reducing the lamellar spacing in the pearlite, proof stress, fatigue strength, etc. It is an element that contributes to the increase in strength. Furthermore, it combines with S to form sulfide inclusion MnS, and a notch effect at the time of fracture splitting can also be obtained. In order to exhibit such an action effectively, addition of 0.1% or more is preferable. However, if the Mn content is excessive, bainite is generated in the metal structure and adversely affects the machinability. Therefore, the upper limit is preferably suppressed to 2.0% or less. More preferably, it is 1.7% or less.
[0022]
S: 0.38-0.061%
S is an element that forms sulfide-based inclusions MnS and the like, exhibits a notch effect at the time of fracture division, and contributes to improvement of machinability. In order to exhibit such an action effectively, 0.038% or more is added. However, when the S content is excessive, harmful effects such as cracking occur during rolling or hot forging, so the content is suppressed to 0.061% or less.
[0023]
Furthermore, in order to reduce the aspect ratio by spheroidizing sulfide inclusions, at least one selected from the group consisting of Zr, Ti, Mg, Ca and REM is positively added. By adding these elements, the fracture splitting property can be further improved, the hot workability can be improved, and the crack preventing effect at the time of rolling or hot forging can be obtained. However, if the amount is too much, the effect is saturated and the cost is increased, so Zr: 0.2% or less, Ti: 0.1% or less, Mg: 0.01% or less, Ca: 0.01% or less, REM: Controlled to 0.3% or less (all elements do not contain 0%).
[0024]
Furthermore, in the steel of the present invention, the following C, Si, P, Cr, Al, and N are positively added.
[0025]
C: 0.15-0.6%
C is an element that contributes to increasing the amount of pearlite in the metal structure of the forged product after hot forging and cooling to ensure a desired strength. In order to effectively exhibit such an action, it is preferable to add at least 0.15% or more. More preferably, it is 0.2% or more. However, if the C content is excessive, the machinability is lowered, so the upper limit is preferably suppressed to 0.6% or less. Considering the balance between strength and machinability, the more preferable C content is 0.5% or less.
[0026]
Si: 1.5% or less (excluding 0%)
Si is an effective element for deoxidation when steel is melted, and is an element effective for strengthening a forged product after hot forging and cooling by solid solution in the ferrite ground of the steel. In order to effectively exhibit such an action, it is preferable to add 0.1% or more. However, when the Si content is excessive, there is an adverse effect on machinability and hot workability, so the upper limit is preferably made 1.5%. More preferably, it is 1.0% or less.
[0027]
P: 0.15% or less (excluding 0%)
P is an element effective for lowering toughness due to segregation at grain boundaries. For that purpose, it is preferable to add 0.01% or more. However, when the P content is excessive, the hot workability deteriorates, so the upper limit is preferably made 0.15%. More preferably, it is 0.10% or less.
[0028]
Cr: 2% or less (excluding 0%)
Cr, like Mn, is an element that improves pearlite hardenability and contributes to an increase in strength such as yield strength and fatigue strength. In order to effectively exhibit such an action, it is preferable to add 0.2% or more. However, if the Cr content is excessive, the hardness will increase significantly, or bainite will form in the metal structure and adversely affect the machinability, so the upper limit should be made 2% or less. preferable. More preferably, it is 1.7% or less.
[0029]
Al: 0.06% or less (excluding 0%)
N: 0.03% or less (excluding 0%)
All of these elements contribute to improvement of fatigue characteristics by refining crystal grains. In order to effectively exhibit such an action, it is preferable to add Al: 0.001% or more and N: 0.002% or more. However, even if added excessively, the effect is saturated and adversely affects hot workability. Therefore, the upper limit is Al: 0.06%, N: 0.03%. It is preferable. More preferably, Al: 0.05% and N: 0.020% or less.
[0030]
Further, in the steel of the present invention, V is positively added.
[0031]
V: 0.45% or less (excluding 0%)
V is an element that forms fine carbides or nitrides and precipitates on ferrite ground, thereby contributing to an increase in strength such as yield strength and fatigue strength. In order to effectively exhibit such an action, it is preferable to set V: 0.02% or more. However, since the effect is saturated and the cost is increased even if contained in excess, the upper limit is preferably set to V: 0.45%. More preferably, V: 0.35% or less.
[0032]
The basic components for obtaining the desired sulfide inclusions are as described above, but in order to further reduce the aspect ratio by making the sulfide inclusions spherical, Se and Te are positively added. It is recommended. By adding these elements, the fracture splitting property can be further improved, the hot workability can be improved, and the crack preventing effect at the time of rolling or hot forging can be obtained. However, if the amount is too large, the effect is saturated and the cost is increased. Therefore, it is preferable to control Se: 0.1% or less and Te: 0.1% or less (both elements do not include 0%). .
[0033]
Further, in the steel of the present invention, Nb can be positively added.
[0034]
Nb: 0.15% or less (excluding 0%)
Nb is an element that forms fine carbides or nitrides and precipitates on ferrite ground, thereby contributing to an increase in strength such as yield strength and fatigue strength. In order to effectively exhibit such an action, it is preferable to set Nb: 0.005% or more. However, even if contained excessively, the effect is saturated and the cost is increased, so the upper limit is preferably Nb: 0.15%. More preferably, Nb: 0.1% or less.
[0035]
Furthermore, in the steel of the present invention, the following Pb and / or Bi can be positively added.
[0036]
Pb: 0.3% or less (excluding 0%)
Bi: 0.3% or less (excluding 0%)
These elements are all elements that contribute to the improvement of machinability. In order to effectively exhibit such an action, it is preferable to add Pb: 0.01% or more and Bi: 0.01% or more. However, since the effect will be saturated even if it adds excessively, it is preferable to make the upper limit into Pb: 0.3% and Bi: 0.3%. More preferably, they are Pb: 0.25% and Bi: 0.25% or less. These elements may be used alone or in combination of two or more.
[0037]
The steel of the present invention contains the above-described elements as necessary or necessary. Others: The steel of the present invention is substantially composed of Fe, but contains other permissible components within a range not impairing the function of the present invention. It can also be contained.
[0038]
Hereinafter, the present invention will be described in more detail by way of examples. However, the following examples are not intended to limit the present invention, and any appropriate design change in accordance with the gist of the preceding and following description is within the technical scope of the present invention. Is included.
[0039]
【Example】
Steel Nos. 2 to 12 having the chemical composition shown in Table 1 were melted in a 50 kg experimental furnace, then forged into a plate having a thickness of 25 mm and a width of 70 mm by hot forging, and cut to a predetermined length. .
[0040]
A 1 mm ² field of view was observed with an optical microscope (1000 ×) in a longitudinal section of ¼ part width of the flat plate thus obtained, and the number of sulfide inclusions having a width of 1 μm or more, and the inclusions. The average aspect ratio of the objects was measured.
[0041]
Further, tensile test pieces and impact test pieces were taken in parallel to the forging direction of the flat plate, and each test was performed. At that time, the surface of a flat plate that was flattened and forged to a thickness of 15 mm was visually observed, and the presence or absence of forging cracks was also investigated at the same time. The brittle fracture surface ratio was visually observed according to the method described in JIS Z 2242, and the ratio of the area of the brittle fracture surface to the total area of the fracture surface was calculated by comparison with the standard fracture surface.
[0042]
Table 1 shows the number of sulfide inclusions having a width of 1 μm or more, the average aspect ratio of the inclusions, the presence or absence of cracks during forging, the tensile strength, the impact value, and the brittle fracture surface ratio. In the table, “−” in each item of “tensile strength”, “impact value” and “brittle fracture surface ratio” means that the measurement was stopped.
[0043]
[Table 1]

[0044]
From these results, it can be considered as follows.
First, Nos. 4 and 5 are examples that satisfy all of the requirements defined in the present invention. Forging cracks do not occur, the impact value is smaller than the standard 20 J / cm ² , and the brittle fracture surface ratio. Is 100%, which is an essential condition, and it can be seen that good fracture splitting properties are obtained.
[0045]
On the other hand, Nos. 8 to 12 that do not satisfy any of the requirements specified in the present invention have the following problems.
[0046]
No. 8 is an example in which the number of sulfide inclusions is large, and hot forging cracks are observed. On the other hand, No. 9 is an example in which the number of sulfide inclusions is small, and neither the impact value nor the brittle fracture surface ratio has reached the target level.
[0047]
In No. 10, the number of sulfide inclusions satisfies the requirements of the present invention, but the aspect ratio exceeds the upper limit, and neither the impact value nor the brittle fracture surface ratio has reached the target level.
[0048]
Further, No. 11 is an example in which both the number of sulfide inclusions and the aspect ratio do not satisfy the requirements of the present invention, and neither the impact value nor the brittle fracture surface ratio has reached the target level.
[0049]
No. 12 is an example in which the S content is high and the aspect ratio of the sulfide inclusions is large, but hot forging cracks were observed.
[0050]
【The invention's effect】
Since the present invention is configured as described above, it is possible to obtain a non-heat treated steel for hot forging that satisfies high strength, low toughness, and excellent fracture splitting property.

Claims (3)

Steel component is mass% (hereinafter the same)
C: 0.15-0.6%,
Si: 1.5% or less (excluding 0%),
P: 0.15% or less (excluding 0%),
Cr: 2% or less (excluding 0%),
Al: 0.06% or less (excluding 0%),
N: 0.03% or less (excluding 0%),
Mn: 2.0% or less (excluding 0%),
S: 0.038-0.061%,
V: 0.45% or less (excluding 0%)
Ca: 0.0018% or less (excluding 0%)
And Zr: 0.2% or less (excluding 0%)
Ti: 0.1% or less (excluding 0%)
Mg: 0.01% or less (excluding 0%)
REM: 0.3% or less (excluding 0%)
Containing at least one selected from the group consisting of :
(However, when Zr is contained, Zr is set to 0.08% or more.)
The balance consists of Fe and inevitable impurities ,
In longitudinal section of the 1/4 thickness position from the surface of the forging material, the sulfide inclusions in the above range 1μm is 100 to 4000 pieces / mm ² is present, the average aspect ratio of sulfides (length / width) Is a non-tempered steel for hot forging for fracture split type connecting rods, which is excellent in fracture splitting characteristics, characterized in that is 10 or less. Nb: 0.15% or less (excluding 0%)
The non-tempered steel for hot forging for fracture split type connecting rods according to claim 1. Furthermore, Pb: 0.3% or less (not including 0%) and / or Bi: 0.3% or less (not including 0%)
The non-heat treated steel for hot forging for fracture split type connecting rods according to claim 1 or 2.