JP2006291237A - Steel superior in cold-forgeability and machinability for machine structural use - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel material which contains a comparatively large amount of carbon, is used in a machine structural field, and can show excellent cold-forgeability and machinability even when having skipped a spheroidizing step. <P>SOLUTION: The steel for a machine structural use superior in cold-forgeability and machinability satisfies a prescribed component composition; and has a metallurgical structure which is a two-phase structure consisting of ferrite and pearlite, and in which an average space between lamellars of pearlite is 220 to 500 nm, an average particle diameter of pearlite grains is 30 μm or smaller, and a difference between the ferritic grain size numbers in a region from a D/2 point (D: diameter in cross section of steel material) to a D/8 point in the cross section of the steel material and in the outmost surface is 1 or less. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  TECHNICAL FIELD The present invention relates to a machine structural steel excellent in cold forgeability and machinability, and in particular, to a steel for machine structure having a markedly improved cold forgeability and finished surface roughness during cutting. is there.

Conventionally, the steel material (rolled material) used for the manufacture of steel parts for machine structures is cold, which is a subsequent process, by providing a spheroidizing annealing process as shown in the manufacturing process (excluding pickling and lubrication film formation) below. The processability during cold forging (cold forgeability) and machinability were ensured. However, in recent years, attempts have been made to omit the spheroidizing annealing process which requires a long time in order to increase productivity.
<Conventional manufacturing process>
Rolling-Wire drawing-Spheroidizing annealing-Wire drawing-Cold forging-Cutting <Process without the above spheroidizing annealing>
Rolling − Wire drawing − Cold forging − Cutting

  However, if the spheroidizing annealing is omitted, the wire obtained by rolling is directly used for wire drawing, cold forging, and cutting. A conventional steel material with a small amount of C is excellent in cold forgeability and machinability even in hot rolling, but if the amount of C is increased in order to increase the strength of machine structural parts, the machinability is significantly reduced. Specifically, there is a problem that the cutting edge is generated and “peeling” occurs in the steel part, and the finished surface becomes rough.

  As a preventive measure for the above problem, it has been generally proposed to increase the cutting speed, but it is required to improve the machinability of the steel because it is restricted by the equipment.

  As a technique for improving the machinability, for example, in Patent Document 1, an amount of S of 0.03% or more is secured, and an area ratio of pearlite grains having a particle size exceeding 1 μm in the microstructure is set to 5% or less. It is shown. However, when the amount of S is increased, it becomes difficult to ensure the cold forgeability required for the machine structural steel. Moreover, it is difficult to sufficiently improve the finished surface roughness only by controlling the size of the pearlite grains.

Patent Document 2 discloses a technique for containing Pb, Bi, and Te and improving chip disposal as a cutting property. However, it is difficult to improve the finished surface roughness even when such a general machinability improving element is added.
JP 2004-169051 A Japanese Patent No. 1586383

  The present invention has been made in view of the above circumstances, and its object is a steel for machine structure having a relatively large amount of C, and has excellent cold forgeability and machinability without providing a spheroidizing annealing step. It is in providing the steel material which can exhibit.

The steel for machine structure excellent in cold forgeability and machinability according to the present invention is
C: 0.10 to 0.42% (meaning mass%, the same shall apply hereinafter)
Si: 0.01 to 0.5%,
Mn: 0.1 to 1.6%,
P: 0.030% or less (excluding 0%),
S: Less than 0.030% (excluding 0%),
Cr: 0.01 to 1.0%,
Sol.Al: 0.01 to 0.06%,
N: 0.0005 to 0.0070%,
(Cu + Ni + Mo) ≦ 0.2%,
(Nb + V) ≦ 0.1%
And consisting of the balance iron and inevitable impurities,
The metal structure is a two-phase structure of ferrite and pearlite,
The average lamellar interval of pearlite is 220 to 500 nm, and the average particle size of pearlite grains is 30 μm or less,
In steel cross section,
It is characterized in that the difference between the ferrite grain size number in the region from D / 2 (D: steel section diameter) to D / 8 and the outermost ferrite grain size number is 1 or less.

The machine structural steel of the present invention is
As other elements,
(A) B: 0.0010 to 0.0055%, and / or Ti: 0.004 to 0.05%,
(B) one or more selected from the group consisting of Pb, Bi, Mg and Ca,
0.1% or less in total (excluding 0%)
May be included.

  If the steel for machine structure of the present invention is used for the production of, for example, automobile parts obtained by performing cutting after cold forging, cold forging can be performed satisfactorily without providing a spheroidizing annealing process. A high-strength steel part with a beautiful finished surface can be produced efficiently.

  As described above, in order to obtain a steel for machine structural use that exhibits excellent cold forgeability and machinability without performing spheroidizing annealing on steel materials containing a relatively large amount of C to ensure strength, it is generally cut. Although attempts were made to add S, Pb, Bi, etc., which show an improvement effect on the machinability such as scrap disposal, it was difficult to ensure the excellent cold forgeability essential for steel for machine structures. It was also confirmed that the finished surface roughness could not be improved sufficiently even when free-cutting elements such as Pb and Bi were added.

Therefore, the present inventor conducted intensive research on the following means. That is, since the steel material of the present invention contains 0.10% or more of C as described later, the metal structure is a two-phase structure of pearlite and ferrite, and the pearlite is present in about 5 to 40%. Based on the premise, we studied the morphology of the tissue. As a result, the precipitation form of pearlite
It has been found that the average lamellar interval of pearlite is 220 to 500 nm, and the average particle size of pearlite particles is 30 μm or less. Hereinafter, the reason for this definition will be described in detail.

  First, in the present invention, the average lamellar spacing of pearlite is set to 220 nm or more, which is wider than that of conventional steel materials. By widening the pearlite lamellar spacing, the lamellar becomes relatively short and the pearlite becomes brittle, so that the pearlite can be well destroyed during cutting and the formation of the component edge can be sufficiently reduced, resulting in a finished surface roughness of Very small and beautiful steel parts can be obtained. The pearlite lamellar spacing is preferably 240 nm or more. However, if the lamellar spacing is too wide, the ductility is adversely affected. Preferably it is 450 nm or less, More preferably, it is 400 nm or less.

  When the pearlite area ratio is constant, finely dispersing pearlite grains having a relatively wide lamellar spacing as described above facilitates destruction of pearlite during cutting, and can effectively suppress formation of the constituent cutting edge. Accordingly, in the present invention, the size of the pearlite grains is set to 30 μm or less in terms of average particle diameter. The smaller the pearlite grains are, the more difficult it is to hit the cutting tool during cutting. From the viewpoint of improving machinability, there is no particular lower limit for the average particle size of the pearlite particles. In addition, since the said problem does not arise in the steel type with few pearlites, in this invention, what has the pearlite grain whose average particle diameter is 3 micrometers or more is objected.

  The average lamellar spacing of the pearlite and the average particle size of the pearlite grains are measured and defined by the method shown in the examples described later.

  In the present invention, the effect of controlling the pearlite structure can be reliably exhibited by controlling the ferrite crystal grain size together. This is because by making the size of the ferrite crystal grains uniform, variation in the size of the pearlite grains can be suppressed, and as a result, variation in cutting resistance can be reduced and cutting can be performed uniformly.

Therefore, in the present invention, when the diameter of the cross section of the steel material is D, the ferrite grain size number (specified by Fgc, JIS G 0552) in the region from D / 2 to D / 8 in the cross section of the steel material,
・ The outermost ferrite grain size number (specified by Fgc, JIS G 0552)
The difference was 1 or less. The difference is preferably 0.5 or less, and most preferably, the region from D / 2 to D / 8 and the outermost surface Fgc are the same. When manufactured by the recommended method described later, both the region from D / 2 to D / 8 (inside the steel material) and the outermost ferrite crystal grain size number in the steel sheet cross section are in the range of 6-11. .

  The ferrite grain size number is measured and defined by the method shown in the examples described later.

  In the pearlite, there is a case where devoted pearlite (carbonized carbide is agglomerated) is inevitably formed in the manufacturing process, but the devoted pearlite is not an object of particle size measurement.

  The present invention is particularly characterized in that the structure is controlled as described above. However, in order to easily form the structure and ensure excellent cold forgeability and machinability, it is necessary to satisfy the following component composition: There is.

<C: 0.10 to 0.42%>
C is an element essential for securing the strength of the steel material and requires at least 0.10%. Preferably it is 0.12% or more, more preferably 0.13% or more. On the other hand, if the amount of C is excessive, the area ratio of pearlite is increased and the deformability cannot be secured. Moreover, since it has the effect | action which stabilizes the component blade edge which arises at the time of cutting, it is difficult to improve finishing surface roughness. Therefore, the C amount is 0.42% or less. Preferably it is 0.40% or less, More preferably, it is 0.38% or less.

<Si: 0.01 to 0.5%>
When deoxidation is performed by adding Si, the Si content in the steel is 0.01% or more. However, if the amount of Si is excessive, deformation resistance increases due to solid solution strengthening, and not only the forging load in cold forging increases, but it also causes a decrease in deformability, so it is desirable that it be as low as possible. Therefore, in the present invention, the Si amount is 0.5% or less. Preferably it is 0.4% or less, More preferably, it is 0.3% or less.

<Mn: 0.1 to 1.6%>
Mn is not only necessary for deoxidation and desulfurization, but is also an element effective for improving the quenching and tempering softening resistance during heat treatment after cold working. In order to exert such an effect, the content is 0.1% or more. Preferably it is 0.15% or more, more preferably 0.2% or more. On the other hand, when the amount of Mn becomes excessive, the growth rate of ferrite and pearlite after hot rolling decreases, and pearlite with a narrow lamellar interval that reduces machinability is easily formed. Therefore, in the present invention, the amount of Mn is set to 1.6% or less. Preferably it is 1.4% or less, More preferably, it is 1.2% or less.

<P: 0.030% or less (excluding 0%)>
P segregates microscopically during solidification, segregates at grain boundaries during hot rolling, and easily forms a band structure, which causes the pearlite block to become coarse. Therefore, the P content is suppressed to 0.030% or less. Preferably it is 0.025% or less, more preferably 0.02% or less. The lower limit of P is about 0.001% from the viewpoint of cost and productivity.

<S: Less than 0.030% (excluding 0%)>
S forms sulfide inclusions such as MnS, segregates at the grain boundaries during hot rolling, embrittles the grain boundaries, and easily cracks during cold working. Therefore, in the present invention, S is preferably reduced as much as possible, and is suppressed to less than 0.030%. Preferably it is 0.025% or less, more preferably 0.020% or less. The lower limit of S is about 0.001% from the viewpoint of cost and productivity.

<Cr: 0.01 to 1.0%>
Cr is an element effective in promoting ferrite + pearlite transformation during hot rolling and precipitating carbides without increasing the strength more than necessary. In order to exhibit such effects, the Cr content is preferably 0.01% or more. Preferably it is 0.03% or more. However, when the amount of Cr is excessive, the tensile strength is increased more than necessary, and pearlite with a narrow lamellar spacing that tends to increase the hardenability and lower the machinability is liable to occur. Therefore, in the present invention, the Cr content is set to 1.0% or less. Preferably it is 0.9% or less.

<Sol.Al: 0.01-0.06%>
sol.Al not only has a deoxidizing effect, but also combines with N to form a nitride (AlN: aluminum nitride) and prevent austenite grain coarsening during quenching after cold forging. It is effective for. In order to exhibit such an effect, in the present invention, the amount of sol.Al is set to 0.01% or more. Preferably it is 0.02% or more. However, even if the sol. Preferably it is 0.05% or less.

<N: 0.0005 to 0.0070%>
As described above, N is an element useful for forming Al and AlN and preventing austenite grain coarsening during quenching after cold forging. Further, as described later, when B is added, BN is formed, and it is also an element useful for suppressing dynamic strain aging during cold working. Therefore, in the present invention, the N content is set to 0.0005% or more, preferably 0.0020% or more. However, when the amount of N becomes excessive, the solid solution N increases and the solid solution strengthening is promoted, the strength is increased more than necessary, and the cold forgeability is lowered. Therefore, the N content is suppressed to 0.0070% or less. Preferably it is 0.0060% or less, More preferably, it is 0.0050% or less.

<(Cu + Ni + Mo) ≦ 0.2%>
<(Nb + V) ≦ 0.1%>
These elements (Cu, Ni, Mo, Nb, V) are included as inevitable impurities in the steel material of the present invention. In this invention, this element is suppressed and it suppresses that the intensity | strength of steel materials increases more than necessary, and the outstanding cold forgeability is ensured. In addition, it is necessary to suppress the pearlite having a narrow lamellar interval that easily increases the hardenability of the steel material due to these elements and lowers the machinability. Therefore, Cu, Ni, and Mo are suppressed to 0.2% or less in total. Preferably it is 0.1% or less, More preferably, it is 0.05% or less. Nb and V are also kept to 0.1% or less in total. Preferably it is 0.05% or less, More preferably, it is 0.02% or less.

  The contained elements specified in the present invention are as described above, and the remaining component is substantially Fe, but it is allowed to mix inevitable impurities brought into the steel depending on the conditions of raw materials, materials, manufacturing equipment, etc. Needless to say, it is also possible to positively contain other elements as described below as long as the effects of the present invention are not adversely affected.

<B: 0.0010 to 0.0055%>
B binds to N to form BN, thereby suppressing dynamic strain aging during cold working and also effective for suppressing solid solution strengthening due to solid solution N. Moreover, it has the effect | action which suppresses that an intensity | strength raises when AlN precipitates more than needed and a ferrite crystal grain refines | miniaturizes remarkably. Furthermore, when combined with Ti, which will be described later, N is fixed as TiN, B is effectively acted as a hardenable element in a solid solution state, and the strength is adjusted by quenching and tempering in the final process of the part. It is also a useful element.

  In order to exert these effects, the B content is preferably 0.0010% or more, more preferably 0.0015% or more. In addition, it is good to increase B amount according to N content, and since the N amount upper limit prescribed | regulated by this invention is 0.0070%, let the upper limit of B amount be 0.0055%.

<Ti: 0.004 to 0.05%>
Ti is an element that exhibits the same effect as BN by forming TiN. Moreover, if it adds simultaneously with B as above-mentioned, the hardenability improvement effect of B can fully be exhibited. In order to exert these effects, the Ti content is preferably 0.004% or more. More preferably, it is 0.010% or more. However, if the amount of Ti is too large, excess Ti forms TiC and the precipitation strengthening action increases, so it is better to keep it to 0.05% or less. More preferably, it is 0.04% or less.

<One or more selected from the group consisting of Pb, Bi, Mg and Ca:
<0.1% in total>
Pb, Bi, Mg, and Ca are effective elements for improving the machinability of the steel material, and may be contained alone or in combination of two or more if necessary. However, when the content of these elements is excessive, the ductility and toughness of the steel material are reduced and the cold forgeability is also hindered, so it is preferable to keep the total to 0.1% or less.

  Although this invention does not prescribe | regulate to manufacturing conditions, in order to obtain the said structure | tissue efficiently using the steel material of the said component composition, manufacturing on the following conditions is recommended.

  First, in order to make the ferrite crystal grains uniform as described above, in hot rolling, the temperature from the start of rolling to the start of adjustment cooling (steel surface temperature) is within the range of 800 to 1100 ° C., and these steps are performed. In this case, it is recommended that the temperature difference on the steel surface is kept as constant as possible within 250 ° C. Preferably, the temperature difference is within 100 ° C.

  In addition, regulated cooling is recommended to obtain pearlite having a relatively wide lamellar spacing and a smaller particle size as specified. Specifically, the controlled cooling starts slow cooling at a cooling rate of 0.1 to 0.5 ° C./s from a temperature range of 875 to 750 ° C. (adjusted cooling start temperature).

  The adjusted cooling start temperature is controlled by installing a cooling facility using water as a medium after intermediate rolling. When the above-described slow cooling is performed from a temperature range higher than 875 ° C., the structure after recrystallization becomes coarse, so that the pearlite grains cannot be reduced. In view of stable operation, it is preferable to perform slow cooling preferably from a temperature range of 850 ° C. or lower, more preferably 825 ° C. or lower. On the other hand, when controlled cooling is performed from a low temperature range below 750 ° C., the finish rolling temperature also decreases, so the crystal grains become fine, and the difference in crystal grains occurs between the steel material inside and the outermost surface due to the latent heat, and at the time of cutting Finished surface roughness tends to be rough. In terms of stable operation, the controlled cooling is preferably performed from a temperature range of preferably 775 ° C. or higher, more preferably 800 ° C. or higher. The controlled cooling can be performed using a slow cooling facility (for example, the facility described in JP-A-10-156417). The adjustment cooling (slow cooling) may be performed up to about 600 ° C.

  For other manufacturing conditions in the melting and hot rolling processes for obtaining the steel material of the present invention, general conditions may be adopted.

  The present invention is not limited to the shape or the like of the steel material, and can be obtained as a wire, a steel bar, or the like. Moreover, it does not limit about the use of steel materials, for example, as steel parts for machine structures, bolts, screws, nuts, sockets, ball joints, torsion bars, clutch cases, cages, housings, hubs, covers, cases, washers, Tappet, saddle, balg, inner case, clutch, sleeve, outer race, sprocket, core, stator, anvil, spider, rocker arm, body, flange, drum, fitting, connector, pulley, bracket, yoke, base, valve lifter, In addition to automobile parts such as spark plugs, bracket nuts, bracket bolts, and universal joints, the present invention can be applied to the manufacture of machine parts, electrical parts, and the like.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.

  Test steel No. 1 composed of the components shown in Table 1 Using A to L, hot rolling was performed under the conditions shown in Table 2, and then adjustment cooling was performed to obtain a wire (rolled material) having a diameter of 15.5 mm. In addition, the adjustment cooling after hot rolling performed the slow cooling for about 10 minutes using the slow cooling apparatus described in Unexamined-Japanese-Patent No. 10-156417, and it cooled by air after that. In addition, spheroidizing annealing is not performed except the conventional example mentioned later.

The metal structure of the obtained rolled material was observed as follows. First, the average particle size of pearlite grains is a comparison method according to ASTM standard, which is a microphotograph of D / 4 (D: steel section diameter) part of the rolled material cross section at a magnification of 400 times and compared with the standard particle size diagram. was applied to obtain the grain size number (G) of pearlite, the following formula (1) [ "Ferrum" vol.2 (1997) No.10 p.29~34] from perlite particle grain size (d _n ) The same measurement was performed on a total of three samples, and the average was taken as the average particle size of the pearlite particles.
d _n = 0.254 / 2 ^{(G-1) / 2} (1)
Wherein (1), d _n represents the grain size, G denotes a grain size number]

Moreover, the average lamellar interval of pearlite was calculated | required as follows. That is, a sample was taken so that a portion of D / 4 (D: steel cross section diameter) in the cross section of the rolled material could be observed, and after corroding with Picral, it was observed and photographed with an electron microscope at a magnification of 6000 times, as shown in FIG. In addition, the number of lamellar particles existing within a certain length in the pearlite grains was measured to obtain the lamellar spacing. Then, this measurement is performed with a total of 6 points in 3 fields of view, and an intercept (minimum lamellar) is obtained from a graph in which the relationship between the cumulative frequency (horizontal axis) and the lamellar interval (vertical axis) arranged from a small value of the lamellar interval is arranged. The interval was determined, and the average lamellar interval was determined from the following formula (2).
Average lamellar interval = intercept × 1.65 (2)

In the rolled material cross section,
The ferrite grain size number in the region from D / 2 (D: steel material cross-sectional diameter) to D / 8,
・ The outermost ferrite grain size number
Each was determined by the method prescribed in JIS G0552. And the same measurement was performed about a total of 3 samples, and the average value was calculated | required.

  In addition, after the above rolling, pickling was performed to remove the scale, a lubricating film was formed, and the wire was drawn to a diameter of 14.8 mm for cold forging tests (drawing strain: 0.1), and cutting For testing, a wire drawn to a diameter of 9.3 mm (drawing strain: 1.0) was obtained, and the following cold forging test and cutting test were performed.

  As a conventional example, a sample (conventional material) was produced in a process including spheroidizing annealing. In this case, after hot rolling, spheroidizing annealing was performed with the heat pattern shown in FIG. 2, and then pickling / lubricating film was formed, and then wire drawing was performed. And the sample for cold forging test and the sample for cutting test similar to the above were produced, respectively, and the following cold forging test and cutting test were performed. As a result, the occurrence of cracks in the cold forging test was “none”, and in the cutting test, no peeling occurred, and Rmax was 9.0 μm.

<Cold forging test>
A cylindrical sample having a diameter of 14.8 mm and a height (H ₀ ) of diameter × 1.5 times was prepared, and as shown in FIG. 3, the compression ratio [(1-H / H ₀ ) × 100 (%) ] Was compressed 80% (ie, compressed until H = 0.2 × H ₀ ), and the presence or absence of cracks was confirmed.

<Cutting test>
Cutting was performed under the following conditions, the surface state (presence / absence of peeling) was confirmed from the appearance photograph, and the surface roughness (Rmax) was measured.
[Cutting conditions]
・ Cutting style: Forming (oil-based wet cutting)
・ Tool used: Carbide P10
・ Cutting speed: 80 m / min
・ Incision amount: 1.0mm
・ Feeding speed: 0.03mm / rev

  These results are also shown in Table 2.

  From Tables 1 and 2, it can be considered as follows (note that the following No. indicates the experiment No. in Table 2). That is, no. Since 1-6, 9-12, 14-16 satisfy the requirements specified in the present invention, they are excellent in cold forgeability, and the finished surface roughness is sufficiently improved as compared with conventional materials as machinability. Obtained.

  In contrast, no. No. 7 uses a steel material whose component composition satisfies the regulation, but in the manufacturing process, since the temperature difference from the start of the rolling temperature during hot rolling to the start of controlled cooling is large, and the cooling rate is also fast, A difference in the ferrite crystal grain size occurred inside, and the pearlite grains were fine and the lamellar spacing was narrow. As a result, the finished surface roughness became rough in the cutting test. No. In Nos. 8 and 13, since the cooling rate was high, fine pearlite grains and narrow lamellar spacing were obtained. As a result, the finished surface roughness was rough in the cutting test.

  For reference, an electron microscope observation photograph of the test piece obtained in this example is shown. FIG. 6 is an electron microscopic observation photograph (magnification: 6000 times), and FIG. 8 is an electron microscopic observation photograph (magnification: 6000 times). When FIG. 4 and FIG. 5 are compared, the pearlite in the metal structure of the example of the present invention has a wider lamellar spacing and shorter lamellar than the comparative example. Recognize.

It is explanatory drawing which shows the measuring method of the average lamellar space | interval of a pearlite. It is the schematic which shows the heat treatment process of spheroidization annealing performed conventionally. It is the schematic diagram which showed the mode of the cold forging test in an Example. No. which is an example of the present invention. 6 is an electron microscopic observation photograph of No. 6; No. which is a comparative example. 8 is an electron microscope observation photograph of No. 8;

Claims (3)

C: 0.10 to 0.42% (meaning mass%, the same shall apply hereinafter)
Si: 0.01 to 0.5%,
Mn: 0.1 to 1.6%,
P: 0.030% or less (excluding 0%),
S: Less than 0.030% (excluding 0%),
Cr: 0.01 to 1.0%,
Sol.Al: 0.01 to 0.06%,
N: 0.0005 to 0.0070%,
(Cu + Ni + Mo) ≦ 0.2%,
(Nb + V) ≦ 0.1%
And consisting of the balance iron and inevitable impurities,
The metal structure is a two-phase structure of ferrite and pearlite,
The average lamellar interval of pearlite is 220 to 500 nm, and the average particle size of pearlite grains is 30 μm or less,
Cold forgeability characterized in that the difference between the ferrite grain size number in the region from D / 2 (D: steel section diameter) to D / 8 in the steel section and the ferrite grain size number on the outermost surface is 1 or less And machine structural steel with excellent machinability. As other elements,
B: 0.0010 to 0.0055% and / or Ti: 0.004 to 0.05%
The steel for machine structure of Claim 1 containing. As other elements,
The steel for machine structural use according to claim 1 or 2 containing one or more selected from the group consisting of Pb, Bi, Mg and Ca in total of 0.1% or less (excluding 0%).

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