JP2007211272A - Non-heat treated steel for hot forging - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide non-heat treated steel for hot forging in which, although the additive quantity of expensive V is reduced, sufficient strength can be obtained even if heat treatment, such as quenching and tempering, is omitted after hot forging and also excellent machinability can be obtained. <P>SOLUTION: The non-heat treated steel for hot forging has a composition consisting of, by mass, 0.30 to 0.60% C, 0.05 to 2.00% Si, 0.90 to 1.80% Mn, 0.10 to 1.00% Cr, 0.010 to 0.045% Al, 0.005 to 0.025% N, 0.01 to 0.20% S, 0.0005 to 0.02% Ca, 0.0005 to 0.01% O, either or both of 0.002 to 0.010% Ti and 0.002 to 0.025% Zr, V in an amount controlled to ≤0.010% and the balance Fe with inevitable impurity elements. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a non-heat treated steel for hot forging.

JP-A-11-350065

For the purpose of omitting tempering treatment such as quenching and tempering after hot working, V of about 0.1% by mass is added to medium carbon steel (C content: 0.3% to 0.5% by mass) Ferrite + pearlite type non-heat treated steel is widely used for machine structural parts, and the use of such non-heat treated steel is further expanded against the background of cost savings (for example, Patent Document 1). ). However, in recent years, the need for further cost saving has increased, and the development of an inexpensive ferrite + pearlite type non-heat treated steel that does not contain an expensive additive element such as V is desired.

  By the way, conventionally, in order to obtain the target strength without performing heat treatment such as quenching and tempering in non-heat treated steel, V of about 0.1% by mass has been added. However, V is a very expensive additive element, and although the heat treatment cost can be reduced by detempering, the material cost is greatly increased compared to the conventional carbon steel. For this reason, the conventional V-added ferrite + pearlite type non-heat treated steel could not sufficiently meet the cost saving needs. Moreover, the conventional non-tempered steel to which V is added has difficulty in machinability after hot forging, and easily leads to an increase in processing cost.

  The object of the present invention is to achieve hot forging with sufficient strength even if heat treatment such as quenching and tempering is omitted after hot forging while reducing the amount of expensive V added, and excellent in machinability. The purpose is to provide non-heat treated steel.

Means and actions / effects for solving the problems

In order to solve the above problems, the non-heat treated steel for hot forging of the present invention is
C: 0.30 mass% or more and 0.60 mass% or less;
Si: 0.05 mass% or more and 2.00 mass% or less;
Mn: 0.90 mass% or more and 1.80 mass% or less;
Cr: 0.10% by mass or more and 1.00% by mass or less;
Al: 0.010 mass% or more and 0.045 mass% or less;
N: 0.005 mass% or more and 0.025 mass% or less;
S: 0.01% by mass or more and 0.20% by mass or less;
Ca: 0.0005 mass% or more and 0.02 mass% or less;
O: 0.0005 mass% or more and 0.01 mass% or less;
Containing
Ti: 0.002% by mass or more and 0.010% by mass or less; and
Zr: 0.002% by mass or more and 0.025% by mass or less;
One or both of
And V content shall be 0.010 mass% or less, and it consists of remainder Fe and an unavoidable impurity element.

  According to the non-heat treated steel for hot forging according to the present invention, although the V content is reduced to 0.010% by mass or less, by adjusting the remaining components to the above composition, Even if heat treatment such as quenching and tempering is omitted after hot forging, a sufficient strength can be obtained, and excellent machinability is obtained even after hot forging, so that the processing cost can be reduced.

  The structure of the non-tempered steel having a C content of 0.60% by mass or less is a mixed phase structure of ferrite (predeposition) + pearlite (presenting a eutectoid lamellar structure of ferrite and cementite). In the ferrite + pearlite structure, the greater the lamellar spacing in the pearlite and the larger the area ratio of the pro-eutectoid ferrite, the easier the proof stress and fatigue limit decrease, and this tendency is particularly remarkable when the V content is reduced. Therefore, it is desirable to keep the lamellar spacing of pearlite of ferrite + pearlite structure at 0.8 μm or less, and it is desirable that the area ratio in the (deposited) ferrite structure is 30% or less.

  Moreover, in the non-heat treated steel for hot forging of the present invention, inclusions mainly composed of oxide are cored by containing S, Mn, Ca, O, Ti and / or Zr in the above composition range. And a first sulfide-based inclusion composed of a double structure inclusion surrounded by an inclusion mainly composed of sulfide, and an average particle size smaller than that of the first sulfide-based inclusion. The second sulfide inclusions made of MnS are dispersed in the structure, and the machinability after hot forging is remarkably improved without deteriorating the strength of the steel.

In the first sulfide-based inclusion, the core portion is an oxide of Ca, Mg, Si or Al, and the periphery thereof is formed as a double structure particle surrounded by MnS containing CaS. This is desirable from the viewpoint of improving machinability. Specifically, the oxide phase having a CaO content of 0.2% by mass or more and 62% by mass or less has a double structure surrounded by a sulfide-based layer, and the overall Ca content is 1 The mass is not less than 45% by mass. The first sulfide-based inclusion plays a role of providing an appropriate lubricating effect with the tool when cutting the material and preventing thermal diffusion wear of the tool. In order to this effect shall notably, the area occupied by the first sulfide inclusions in the tissue, in the observation field area 3.5 mm ² per, 2.0 × ¹⁰ -4 mm ² or more It is desirable to be 5.0 × 10 ⁻³ mm ² or less. If the occupied area is less than 2.0 × 10 ⁻⁴ mm ² , the lubricating effect is insufficient, and if it exceeds 5.0 × 10 ⁻³ mm ² , the strength of the steel after hot forging is impaired.

  In the present invention, when the Ti and / or Zr is contained in the above composition range, the second sulfide-based inclusion made of MnS having an average particle size smaller than that of the first sulfide-based inclusion. The product can be dispersed and formed in the structure with an average particle size smaller than that of the first sulfide inclusion. Ti and / or Zr combine with oxygen to form fine Ti or Zr oxide particles dispersed in the structure, and MnS precipitates with this as a nucleus. By the formation of such second sulfide inclusions, the friability of chips generated by cutting is remarkably improved, and the problem that long continuous chips are entangled around the tool is less likely to occur.

  Moreover, in order to apply the non-heat treated steel for hot forging of the present invention to general mechanical structural parts, it is necessary to ensure the strength after the hot forging to a certain level or more. The strength in this case can be simply evaluated by the Rockwell hardness after hot forging. For example, to use as a crankshaft or a connecting rod for automobiles, the hardness is ensured to be HRB96 or higher. There is a need to. On the other hand, since it is necessary to perform cutting into a necessary shape after hot forging, if the hardness is excessively high, there is a problem that the machinability of the material is lowered. However, in the present invention, as described above, the machinability is remarkably enhanced as compared with the conventional non-heat treated steel for hot forging due to the formation of two types of sulfide inclusions. The upper limit of the Rockwell C scale hardness after hot forging so that the hindering is not hindered can be higher than HRC30 and conventional non-tempered steel. As a result, both machinability and strength improvement can be achieved.

Hereinafter, the reasons for limiting the composition of each element will be described.
V: 0.010% by mass or less In the present invention, from the viewpoint of reducing material costs, it is a technical premise that V is not contained as much as possible. However, the inclusion of V at the impurity level is not excluded. V may be contained as long as it does not exceed 0.010% by mass.

C: 0.30% by mass or more and 0.60% by mass or less C is an element effective for securing the strength, and in order to obtain such an effect, the strength is not secured at less than 0.30% by mass. On the other hand, if it exceeds 0.60% by mass, the hardness becomes excessive and the machinability is deteriorated. The C content is more preferably 0.35% by mass or more and 0.55% by mass or less.

Si: 0.05% by mass or more and 2.00% by mass or less Si is contained as a deoxidizing agent during steel melting, and is solid-dissolved in ferrite to effectively improve the strength. Thereby, as an alternative to V, there is a function of improving the proof stress and fatigue limit. In order to acquire such an effect, it is necessary to make it contain 0.05 mass% or more. However, if the content is too large, the machinability is deteriorated or significant decarburization occurs on the surface due to heating during hot forging, so it is necessary to set the content to 2.00% by mass or less.
The Si content is more preferably 0.20% by mass or more and 1.50% by mass or less.

Mn: 0.90% by mass or more and 1.80% by mass or less Cr: 0.10% by mass or more and 1.00% by mass or less Mn and Cr refine the lamellar spacing of pearlite and effectively improve the yield strength and fatigue limit. Further, Mn is an essential element for forming sulfide inclusions that contribute to improvement of machinability. If both Mn and Cr are less than the lower limit, the effect of refining the lamellar spacing is not significant. In addition, with respect to Mn, the amount of sulfide inclusions formed becomes insufficient, leading to a decrease in machinability. On the other hand, when the contents of Mn and Cr exceed the above upper limit values, bainite is generated even in air cooling, the material is cured, and the machinability is significantly reduced.
The Mn content is more preferably 0.90% by mass or more and 1.30% by mass or less. The Cr content is more preferably 0.10% by mass or more and 0.40% by mass or less.

Al: 0.010% by mass or more and 0.045% by mass or less Al forms N and nitride in steel and is finely dispersed in the steel to suppress crystal grain growth during hot forging. However, if the content is less than 0.010% by mass, the effect is not significant, and if it exceeds 0.045% by mass, the effect is saturated.
The Al content is more preferably 0.015% by mass or more and 0.030% by mass or less.

N: 0.005% by mass or more and 0.025% by mass or less N forms a nitride with Al and contributes to strength improvement by suppressing crystal grain growth during hot forging by fine precipitation of this nitride. However, if the content is less than 0.005% by mass, the effect is not significant, and if it exceeds 0.025% by mass, the effect is saturated.
The N content is more preferably 0.010% by mass or more and 0.020% by mass or less.

S: 0.01% by mass or more and 0.20% by mass or less S, together with Mn, is an essential element for forming a sulfide inclusion that contributes to improvement of machinability. If it is less than 0.01% by mass, the amount of sulfide inclusions produced is insufficient and the machinability becomes insufficient. On the other hand, if it exceeds 0.20% by mass, the toughness and ductility of the steel are impaired, and CaS having a high melting point is produced together with Ca to be described later, which leads to the inhibition of hot metal flowability in the ingot casting process. The S content is more preferably 0.050 mass% or more and 0.120 mass% or less.

Ca: 0.0005 mass% or more and 0.02 mass% or less Ca is an element essential for forming the first sulfide inclusion, and if its content is less than 0.0005 mass%, Sulfide inclusions are not formed in a sufficient amount, leading to a decrease in machinability. On the other hand, if the content exceeds 0.02% by mass, the above-described high melting point CaS is generated, leading to an obstacle to the inhibition of the flow of molten metal in the ingot casting process. The Ca content is more preferably 0.0010% by mass or more and 0.0050% by mass or less.

O: 0.0005 mass% or more and 0.01 mass% or less O is an element required for the production | generation of an oxide. In excessively deoxidized steel, a large amount of high melting point CaS is formed, which hinders casting for the reasons described above, so 0.0005% by mass or more, preferably more than 0.0015% by mass of O is required. . On the other hand, O exceeding 0.01% by mass results in a large amount of hard oxide, and as a result, the machinability is impaired, and the sulfide containing Ca that constitutes the first sulfide-based inclusion is contained. Generation becomes difficult. When composite deoxidation is performed using Ca and Al, a CaO—Al ₂ O ₃ -based composite oxide is formed, which is an inclusion with a low melting point, which is preferable for machinability, There is no effect on. Therefore, it is better to minimize the formation of CaO—Al ₂ O ₃ -based composite oxides. Specifically, when steel is melted, it is first removed by adjusting the Al content to the above range. It is advisable to adopt a process in which the degree of acid is made appropriate and then Ca or the like is added. The O content is more preferably 0.0010% by mass or more and 0.0050% by mass or less.

Ti: 0.002% by mass or more and 0.010% by mass or less; and
One or both of Zr: 0.002% by mass or more and 0.025% by mass or less; Ti combines with O in the steel to form a fine oxide. This serves as a nucleus for the precipitation of the first sulfide inclusion (MnS), and is useful for finely dispersing the Mn sulfide as described later. Two types of Ti and Zr are used in combination (for example, Ti: 0.005% by mass + Zr: 0.015% by mass) in order to further enhance the effect of refining MnS. In order to produce an appropriate amount of Ti oxide or Zr oxide without interfering with the formation of oxides necessary for the core of the first sulfide inclusions and other oxides, the amounts of Ti and Zr Must be adjusted to the ranges of 0.002% by mass to 0.010% by mass and 0.002% by mass to 0.025% by mass. In order to reliably produce the first sulfide inclusion, it is desirable to add Ti and / or Zr after performing the adjusted deoxidation as described above. However, since the effect of improving machinability is saturated by adding excessively, the upper limit value is set as described above in consideration of the suppression of the increase in material cost. Note that Ti is more desirably adjusted in a range of 0.005 mass% to 0.010 mass%, and Zr is desirably adjusted in a range of 0.005 mass% to 0.020 mass%.

In addition, the following components can be appropriately added to the non-heat treated steel for hot forging of the present invention.
Nb: 0.30% by mass or less Nb is useful for preventing coarsening of crystal grains at a high temperature. The effect saturates as the amount increases, so it is advisable to add it in the range of 0.30% by mass or less.

Pb: 0.30 mass% or less Bi: 0.30 mass% or less Both Pb and Bi can be optionally added for the purpose of further improving the machinability of the steel. Pb exists alone or in the form of adhering to the outer periphery of the sulfide, and itself improves machinability. The upper limit of 0.30% by mass was set because it does not dissolve in the steel even if Pb more than this is added, but agglomerates and precipitates, resulting in defects in the steel. The same applies to Bi.

  In the present invention, the “inevitable impurities” is not limited in the type of elements contained therein unless the effect of the above-described non-heat treated steel for hot forging of the present invention is prevented. In a narrow sense, it refers to impurities derived from factors inevitably occurring in the steel manufacturing process, but in the present invention, elements added intentionally in the form of replacing part of Fe are also present. If the above effect of the non-heat treated steel for hot forging of the invention is not hindered, the element is also considered to belong to the concept of “inevitable impurities”.

Hereinafter, experimental results performed to confirm the effects of the present invention will be described.
First, raw materials were blended so that the composition shown in Table 1 was obtained, and a 5-ton steel ingot was melted in an electric furnace. This steel ingot was made into a forging material having a square cross section with a side of 157 mm by hot forging, then heated to 1250 ° C. to hot forged into a round bar with a diameter of 22 mm, and then cooled by air cooling. Moreover, the Rockwell C scale hardness of the center part of each forged product was measured by the method prescribed | regulated to JIS: Z2245. Moreover, the test piece prescribed | regulated to JIS: Z2210 was cut out from the forged product, this was subjected to the tensile test with the Amsler universal testing machine, and 0.2% yield strength was measured from the obtained stress-strain curve. The fatigue limit was measured by a rotating bending fatigue test. A rotating bending fatigue test was also conducted. In this test, the maximum load load was changed in various ways, and the maximum load load that did not cause breakage at 10 million rotations was determined as the fatigue limit.

  Next, the hot forged product is cut in a cross section perpendicular to the central axis, mirror-polished, and observed with a scanning electron microscope at a magnification of 3000 to observe 10 fields of view, and a pearlite phase lamellar interval (ILS: Inter Lamellar). Spacing)) was measured. Specifically, a plurality of measurement lines crossing the lamellar are drawn on each visual field, the value with the smallest interval between the lamellars across the measurement line is obtained, and the average value of the values in 10 visual fields is obtained as the final ILS. It was. Also, the pro-eutectoid ferrite grains and pearlite grains are identified on the image (the pearlite grains can be easily distinguished from the ferrite grains because of the lamellar stripe pattern described above), and the area ratio of the pro-eutectoid ferrite grains is analyzed It was measured by.

On the other hand, for sulfide inclusions, the first inclusion (having a double structure) and the first inclusion (double structure) on the optical microscope observation image (magnification 200 times) of the cross section. And the formation area of the first inclusion per observation field area of 3.5 mm ² was determined. Further, the number of second inclusions was measured to determine the number formation density per unit visual field area (1 mm ² ).

Moreover, the machinability of the material was evaluated as follows.
First, using a cemented carbide tool (JIS: B4503, P10), each test product was turned under the following conditions.
・ Turning speed: 200 m / min ・ Tool feed rate per rotation: 0.2 mm / rotation ・ Cutting depth: 2.0 mm
・ Cutting oil: Water-soluble.
Then, the tool life of sulfur free-cutting steel having an S content in the range of 0.01% by mass or more and 0.2% by mass or less was ranked as a standard as follows. Further, the tool life was evaluated by the processing time until the side flank average wear width of the tool reached 0.2 mm.
Good machinability when the tool life is 5 times that (○ mark)
Machinability possible when 2 times or more and less than 5 times (△ mark)
Less than 2 times machinability failure (x mark)

Furthermore, the chip crushability at the time of cutting was evaluated as follows. That is, a cutting test is performed under the following conditions using an NC lathe using a cemented carbide (JIS: B4503, K10) chip as a cutting tool:
Cutting speed: three conditions of 80 m / min, 100 m / min and 120 m / min;
-Cut amount per rotation: two conditions of 0.3 mm and 1.0 mm;
-Feed amount per rotation: three conditions of 0.025 mm, 0.050 mm, 0.100 mm;
・ Cutting oil: Water-soluble.
Then, the chips when the round bar test piece was turned in the longitudinal direction under a total of 18 conditions of the above cutting speed 3 conditions × cutting amount 2 conditions × feed amount 3 conditions were scored based on the criteria shown in Table 1. The total score was used as an index for evaluating chip crushability.

A higher score means better chip crushability. This was evaluated as follows according to the result of comparison with the chip crushability index of sulfur free-cutting steel having the same sulfur content.
Good when higher (○ mark)
If it is the same or low, it is bad (×)
These results are shown in Tables 2-9. (If the composition column is blank or "-", it means that no positive addition has been performed.)

  From the above results, it can be seen that the steels belonging to the examples of the present invention can obtain sufficient strength even when heat treatment such as quenching and tempering is omitted after hot forging and have excellent machinability.

Claims (5)

C: 0.30 mass% or more and 0.60 mass% or less;
Si: 0.05 mass% or more and 2.00 mass% or less;
Mn: 0.90 mass% or more and 1.80 mass% or less;
Cr: 0.10% by mass or more and 1.00% by mass or less;
Al: 0.010 mass% or more and 0.045 mass% or less;
N: 0.005 mass% or more and 0.025 mass% or less;
S: 0.01% by mass or more and 0.20% by mass or less;
Ca: 0.0005 mass% or more and 0.02 mass% or less;
O: 0.0005 mass% or more and 0.01 mass% or less;
Containing
Ti: 0.002% by mass or more and 0.010% by mass or less; and
Zr: 0.002% by mass or more and 0.025% by mass or less;
One or both of
And non-refining steel for hot forging characterized by V content being 0.010 mass% or less and consisting of the remainder Fe and inevitable impurity elements. The non-heat treated steel for hot forging according to claim 1, wherein the structure after hot forging is ferrite + pearlite, the pearlite lamellar spacing is 0.80 µm or less, and the proeutectoid ferrite area ratio is 30% or less. The oxide phase having a CaO content of 0.2% by mass or more and 62% by mass or less has a double structure surrounded by a sulfide-based layer, and the overall Ca content is 1% by mass or more and 45% by mass. in the first area occupied by the sulfide inclusion is, the observation field area 3.5 mm ² per% or less, 2.0 × and at ¹⁰ -4 mm ² or more, a second sulfide inclusions consisting MnS The non-heat treated steel for hot forging according to claim 1 or 2, wherein the product is dispersed and formed in the structure with an average particle size smaller than that of the first sulfide inclusion. The non-heat treated steel for hot forging according to any one of claims 1 to 3, wherein the Rockwell hardness after hot forging is HRB96 or more and HRC30 or less. In the form of replacing a part of the Fe,
Te: 0.01% by mass or more and 0.30% by mass or less;
Pb: 0.01% by mass to 0.30% by mass;
Bi: 0.01% by mass or more and 0.30% by mass or less;
The non-heat treated steel for hot forging according to any one of claims 1 to 4, comprising one or more selected from one or more.