JP2011241424A - Steel material excellent in machinability and damping property - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel material excellent in productivity without performing heat treatment over a long time, that is, annealing over a long time for graphitization, and excellent in machinability and a damping property.SOLUTION: This steel material contains, by mass%, 0.3-1.2% of C, 2.0-3.0% of Si, >0.05% and ≤0.5% of Mn, 0.2-1.0% of Al, ≤0.02% of P, 0.007-0.2% of S, 0.0015-0.030% of N and ≤0.003% of O, wherein the content of Si and Mn is in a range satisfying the following expression: {5≤Si/Mn}; the residue has a component composition comprising Fe and unavoidable impurities; a steel structure includes a ferrite structure and graphite; and a graphitization degree is >80%.

Description

  The present invention relates to a steel material excellent in machinability and vibration damping properties, and more particularly to a steel material excellent in machinability and vibration damping properties used for parts such as automobiles and industrial machines.

  In recent years, there has been a demand for further improvements in machinability and vibration damping for steel materials used for parts such as automobiles and industrial machines, for example, machined parts such as undercarriage parts and gears of automobiles. Yes.

  Here, as a steel material used for the cutting parts as described above, steel whose machinability is improved by adding Pb (lead) is used as free-cutting steel. However, since Pb is a harmful substance, it has been demanded not to use it as much as possible in recent years. As free-cutting steel to which Pb is not added, sulfur-free-cutting steel to which S is added is used, but MnS present in the sulfur-free-cutting steel has a drawback that the toughness of the steel is deteriorated. Here, graphite steel is excellent in that there is no health damage to the human body because there is no need to add Pb, and the toughness of the steel material is not deteriorated unlike sulfur free-cutting steel. . Furthermore, graphite steel has a characteristic that it has vibration damping properties that cannot be obtained at all with lead free-cutting steel or sulfur free-cutting steel. Therefore, much research has been conducted on graphite steel.

Here, in patent document 1, it contains as a main element in the range of C: 0.1-1.2%, Si: 0.5-3.0%, Mn: 0.1-1.5% In addition, the graphite grains in the steel have an excellent vibration damping property (described as performance to reduce noise in Patent Document 1) by controlling the graphite grains in the steel to an average particle size of 0.1 to 10 μm and a maximum diameter of 20 μm or less. A steel part is disclosed.
Moreover, in patent document 2, it contains as a main element in the range of C: 0.1-2.0%, Si: 0.5-2.0%, Mn: 0.1-2.0%. Further, a graphite free-cutting steel that is softened by heat treatment in a short time by containing S: 0.1 to 0.7% and Mg: 0.0001 to 0.009% is disclosed.
In Patent Document 3, as main elements, C: more than 0.8% to 2.0%, Si: 0.5 to 2.0%, Mn: 0.1 to 2.0% Further, by containing S: 0.05 to 0.5% and Zr: 0.0005 to 0.005%, it is possible to achieve both excellent machinability, that is, tool life and cutting surface roughness. A graphite free-cutting steel that is effective is disclosed.

  On the other hand, there is no health damage to the human body because Pb is not added. Furthermore, as free-cutting steel that does not deteriorate the toughness of steel because a large amount of S is not added, Al is more than 0.1% and 0.3% or less. The added steel is disclosed in Patent Document 4. Here, generally, the graphite free-cutting steel as described in Patent Documents 1 to 3 and the like can be obtained only by heat-treating the steel containing carbon. The steel described in Document 4 has the advantage that it does not require heat treatment to obtain machinability.

JP 2009-228049 A JP 2003-34841 A JP 2003-239038 A JP 2008-13788 A

  However, as a result of intensive studies by the present inventors, it has become clear that the conventionally developed technology has the following problems.

  First, although the technique described in Patent Document 1 must be a graphite free-cutting steel with excellent vibration damping properties, it meets the needs from the industry including the automobile industry, and in particular machinability, especially tools. The service life and the steel surface roughness after cutting do not meet the requirements, and further improvement is necessary.

  Further, the technique described in Patent Document 2 is improved with respect to the disadvantage of graphite steel that machinability cannot be obtained unless heat treatment is performed for a long time. However, since the steel described in Patent Document 2 softens (graphitizes) in a short time, it requires addition of S and Mg. Actually, the examples of Patent Document 2 disclose a number of invention examples in which the amount of S is added in excess of 0.2%. As a result of intensive studies by the present inventors on this point, the addition of such a large amount of S causes a problem that the toughness of the steel is lowered and the application to parts of industrial machines and automobiles is limited.

  In addition, although the technique described in Patent Document 3 has improved tool life and improved steel surface roughness after cutting, this is largely due to the addition of S. Discloses a number of invention examples in which the S content exceeds 0.2%. For this reason, similarly to the case of Patent Document 2 described above, the addition of a large amount of S causes a problem that the toughness of the steel is lowered and application to industrial machinery and automobile parts is limited.

  In addition, the technique described in Patent Document 4 does not add Pb, so there is no health damage to the human body, and since a large amount of S is not added, excellent free-cutting steel that does not deteriorate mechanical properties such as toughness Although it can be said, when it compares with the machinability performance, for example, the tool life which will be described later, it is inferior to graphite steel. Furthermore, the free-cutting steel described in Patent Document 4 has a drawback that it is not different from ordinary steel materials in terms of vibration damping properties. Accordingly, there is a problem that only one of the excellent machinability and vibration damping properties, which is an object of the present invention, can be satisfied.

  The present invention has been made in view of the above problems, and further improves the machinability of graphite free-cutting steel having vibration damping properties, and without performing long-time heat treatment, that is, long-time annealing of graphitization. The object is to provide a steel material that is excellent in productivity and excellent in machinability and vibration control.

The inventors of the present invention conducted intensive research in order to produce a steel material having excellent machinability and vibration damping properties by a short heat treatment. As a result, in particular, the machinability is further improved by adding a large amount of Al to graphite steel, the content of silicon (Si) is increased, and the amount of manganese (Mn) is decreased, and Si and It has been found that a steel capable of shortening the time required for graphitization can be obtained by increasing the manganese composition ratio Si / Mn. Furthermore, the present inventors have found that excellent machinability and vibration damping properties can be obtained by making the ratio of the ferrite structure in the steel structure after graphitization annealing and the ratio of the graphite structure in the graphite structure appropriate. I let you.
That is, the gist of the present invention is as follows.

  [1] By mass%, C: 0.3 to 1.2%, Si: 2.0 to 3.0%, Mn: more than 0.05% and 0.5% or less, Al: 0.2 to 1. 0%, P: 0.02% or less, S: 0.007 to 0.2%, N: 0.0015 to 0.030%, O: 0.003% or less, and of Si and Mn The content is in a range satisfying the following formula {5 ≦ Si / Mn}, the balance has a component composition consisting of Fe and inevitable impurities, the steel structure includes a ferrite structure and graphite, and the graphitization rate is 80 A steel material with excellent machinability and vibration control characteristics, characterized by being over%.

[2] Further, it is characterized by containing one or more of Cr: 0.3% or less, Mo: 0.2% or less, W: 0.2% or less in mass%. The steel material excellent in machinability and vibration damping properties according to the above [1].
[3] Further, it is characterized by containing one or more of V: 0.1% or less, Nb: 0.05% or less, Ti: 0.05% or less in mass%. A steel material excellent in machinability and vibration damping properties according to [1] or [2].
[4] The machinability and vibration suppression according to any one of the above [1] to [3], further comprising Ni: 0.1 to 3.0% by mass. Excellent steel material.
[5] The machinability and vibration suppression according to any one of the above [1] to [4], further comprising B: 0.0003 to 0.01% by mass%. Excellent steel material.
[6] The machinability and vibration suppression according to any one of [1] to [5] above, further containing Mg: 0.0001 to 0.01% by mass%. Excellent steel material.
[7] Furthermore, it is excellent in machinability and damping properties according to any one of the above [1] to [6], characterized by containing, in mass ppm, Zr: 5 to 80 ppm or less. Steel material.

According to the steel material excellent in machinability and vibration damping property of the present invention, as described above, the component composition of the base material is within an appropriate range, and the proportion of graphite in the steel structure is appropriately controlled. It is possible to realize a steel material having excellent machinability and also having vibration damping properties.
Therefore, by applying the present invention to steel materials used for parts such as automobiles and industrial machines, in addition to reducing noise generated from parts, there are advantages such as improved productivity and longer tool life. The social contribution is immeasurable.

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram explaining an example of the steel material excellent in the machinability and vibration suppression nature which concerns on this invention, and is a schematic diagram which shows the cutting method at the time of evaluating machinability by the plunge cutting method in an Example. BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram explaining an example of the steel material excellent in the machinability and vibration damping nature which concerns on this invention, and is a graph which shows the influence of Si / Mn exerted on the drill life VL1000 at the time of evaluating machinability in an Example. It is. BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram explaining an example of the steel material excellent in the machinability and vibration damping nature which concerns on this invention, and it has Si / Mn which affects the cutting surface roughness at the time of evaluating machinability by a plunge cutting method in an Example. It is a graph which shows the influence of.

  Hereinafter, an embodiment of a steel material excellent in machinability and vibration damping properties of the present invention will be described. In addition, since this embodiment is described in detail for better understanding of the gist of the invention, the present invention is not limited unless otherwise specified.

"Steel component (component composition of base material)"
First, the reason for limiting the component range of the essential elements defined in carrying out the present invention will be described. In the following description, the amount of each element added is all represented by “mass%”.

“C: Carbon” 0.3-1.2%
C is an element having a strong solid solution hardening ability and a strong precipitation hardening ability, and is therefore the most basic element that brings strength to steel. Further, when steel is quenched and tempered, it is an element that increases the strength of the martensite structure. C is an element essential for precipitation as graphite in the steel structure and improving machinability and vibration damping.
However, if the C content is less than 0.3%, the strength of the steel material decreases, and the amount of graphite deposited becomes insufficient, so that the effect of improving machinability and vibration damping properties cannot be obtained. Moreover, since the workability at the time of hot rolling will fall when content of C exceeds 1.2%, the content was made into the range of 0.3-1.2%. The C content is more preferably in the range of 0.4 to 1.2%.

"Si: silicon" 2.0-3.0%
Si has a small bonding force with C in the base material, and is one of influential elements for promoting graphitization treatment in the steel structure, and has an effect of improving strength. In the present invention, in order to deposit sufficient graphite in the steel structure by a short annealing treatment, it is necessary to control the Si content within an appropriate range, so the lower limit is 2.0. %. However, when the Si content is increased, the ferrite phase is solid-solution hardened and the life of the cutting tool is shortened. Therefore, the upper limit is set to 3.0%.

  Note that when the amount of Mn to be described later increases, the graphitization treatment is remarkably inhibited. Therefore, in order to prevent this, Si is made larger than Mn, specifically, Si / Mn is added. It should be 5 or more.

"Al: Aluminum" 0.2-1.0%
Al is added as a deoxidizing element that combines with O to form an oxide, and has the effect of promoting graphitization. Also, by sufficiently securing solid solution Al, machinability is improved. be able to. Furthermore, Al combines with N to form a nitride (AlN), and the effect of this AlN has the effect of reducing crystal grains and improving the toughness of the steel.
However, if the Al content is less than 0.2%, a sufficient effect of promoting the graphitization treatment as described above cannot be obtained. Further, when the Al content is less than 0.2%, the effect of improving the machinability in the case of graphite steel is small. On the other hand, when Al exceeds 1.0%, a steel material crack occurs during hot rolling. In addition, when a quenching process is performed, a sufficient amount of martensite structure cannot be obtained, and the strength decreases, so the Al content is set to a range of 0.2 to 1.0%.

“Mn: Manganese” Mn: more than 0.05% and 0.5% or less Mn combines with to form MnS and has an effect of preventing hot brittleness due to S. However, when Mn is added in an amount of 0.05% or less, the effect of preventing hot brittleness as described above cannot be obtained. Therefore, the lower limit of Mn is set to 0.05%. On the other hand, when the amount of Mn increases, graphitization is significantly inhibited, so the upper limit of Mn is set to 0.5%.

  Moreover, in this invention, in order to suppress the inhibitory effect of the graphitization process by addition of Mn, Si is added more than Mn. Specifically, by prescribing the content of Si and Mn to the relationship represented by the following formula {5 ≦ Si / Mn}, sufficient graphite is precipitated in the steel structure by a short annealing treatment. Is possible. Thereby, the machinability and vibration control property of steel materials can be improved.

“P: Phosphorus” P: 0.02% or less P is a grain boundary segregation and center segregation in steel, and causes a deterioration in toughness. In order to improve the roughness of the cutting surface, a small amount is added in the case of steel materials that require surface roughness. However, when the P content exceeds 0.02%, the above problem becomes remarkable, so the upper limit is defined as 0.02% or less.

“S: sulfur” S: 0.007 to 0.2%
S reacts with alloy elements such as Mg in addition to Mn and exists as a sulfide. These sulfides function as graphite nucleation sites and improve lubricity to improve the cutting surface roughness, so that excellent machinability is obtained. However, if the content of S is less than 0.007%, a sufficient amount of sulfide cannot be secured, and the above effect is difficult to obtain. Further, if the content of S is too large, not only the toughness of the steel deteriorates but also the cold and hot ductility deteriorates, so that cracking occurs during cold working, hot rolling and forging. Since cracks sometimes occur, the upper limit was made 0.2%.

“N: Nitrogen” 0.0015 to 0.030%
N combines with Al and Ti to form nitrides such as AlN and TiN, and is effective in making crystal grains fine and has the effect of improving the toughness of steel. If the N content is less than 0.0015%, the above effect is difficult to obtain, and even if added over 0.030%, the effect is not only saturated, but also reacts with Al to form a solid solution Al amount. By lowering, the action of improving graphitization and machinability is inhibited. Therefore, the N content is in the range of 0.0015 to 0.030%.

“O: Oxygen” 0.003% or less O is bound to other elements to form an oxide, which may reduce workability, so the content needs to be suppressed. When the content of O exceeds 0.003%, the above tendency becomes remarkable. Therefore, the upper limit of the content of O is defined as this value.

In the present invention, in addition to the above essential elements, elements as described below can be selectively added. Specifically, the component composition further contains one or more of Cr: 0.3% or less, Mo: 0.2% or less, W: 0.2% or less in mass% or mass ppm. It can be. Moreover, it can be set as the component composition which further contains 1 type or 2 types or more in V: 0.1% or less, Nb: 0.05% or less, and Ti: 0.05% or less.
Moreover, it can be set as the component composition which further contains Ni: 0.1-3.0%. Moreover, it can be set as the component composition which further contains B: 0.0003-0.01%. Moreover, it can be set as the component composition which further contains Mg: 0.0001-0.01%. Moreover, it can be set as the component composition which further contains Zr: 5-80 ppm or less.
Hereinafter, the reason for limiting the addition range of the selected component element in the present invention will be described.

“Ni: nickel” 0.1 to 3.0%,
Ni is an element effective for destabilizing cementite and promoting graphitization treatment, enhancing hardenability, and ensuring strength. If the Ni content is less than 0.1%, the above effect is insufficient, and even if Ni is added in excess of 3%, the effect is saturated and extremely disadvantageous economically. Therefore, the Ni content is specified in the range of 0.1 to 3.0%.

“B: Boron” 0.0003 to 0.01%
B has the effect of improving the hardenability and increasing the strength of the steel material. Further, B combines with N to generate BN, which is a nucleation site of graphite, and promotes the graphitization treatment and refines the precipitated graphite. In order to obtain such an effect, it is necessary to add B at 0.0003% or more. However, if B is added in excess of 0.01%, the graphite structure becomes too fine, so the machinability of the steel may be reduced, and the B compound is precipitated at the grain boundaries. Reduce toughness. Therefore, the B content is preferably in the range of 0.0003 to 0.01%.

“Mg: Magnesium” 0.0001 to 0.01%
Mg combines with O to form oxides such as MgO and MgAl ₂ O ₄ , which generate single inclusions or complex inclusions with sulfides. Such Mg-based inclusions are effective as pinning particles, and serve as nucleation sites for graphite or BN, and have an effect on the uniform dispersion of graphite. When the Mg content is less than 0.0001%, the effect is small, and when the Mg content is 0.01% or more, the steelmaking cost increases.

  Further, the steel material according to the present invention may be made into a part by induction hardening after cutting steel into a part shape. The addition of Mg is effective in making the graphite grains finer, and even if the graphitization rate is the same, the finely dispersed one is superior in performance such as induction hardening. That is, in order to form a uniform surface hardened layer in a hardening treatment by short-time heating, such as induction hardening, graphite must be dissolved and diffused in the steel in a short time. Therefore, it is very effective to finely disperse graphite so that C can be uniformly diffused over the entire surface with a short diffusion distance. In this respect, Mg is a very effective element. However, as described above, if the Mg content is too small, the effect of refining the graphite cannot be obtained, and if the content is too large, the graphite becomes too fine and the machinability of the steel may be reduced. Therefore, the content was specified in the range of 0.0001 to 0.01%.

“Cr: Chromium” 0.3% or less “Mo: Molybdenum” 0.2% or less “W: Tungsten” 0.2% or less Cr is an element that secures the hardenability of the steel material and contributes to strength improvement. On the other hand, if the Cr content is too high, the graphitization may be remarkably inhibited, so the upper limit is defined as 0.3%.

  Mo, like Cr, is an element that ensures the hardenability of the steel material and contributes to strength improvement. On the other hand, if the Mo content is too large, the hardness of the ferrite structure increases, the cold workability is impaired and the graphitization treatment is hindered, so the upper limit was defined as 0.2%.

  W, like Cr and Mo, is an element that secures the hardenability of the steel material and contributes to the improvement of strength. On the other hand, when added in excess, the transformation end time becomes longer and the productivity is lowered, and graphitization is achieved. Since the treatment is inhibited, the upper limit value is defined as 0.2%.

“V: Vanadium” 0.1% or less “Nb: Niobium” 0.05% or less “Ti: Titanium” 0.05% or less V, Nb, and Ti combine with C or N to form carbides and nitrides. In addition, it functions as pinning particles, thereby suppressing the growth of austenite grains and improving the fracture toughness value, and further acts as a nucleation site for graphite. However, if the content of V, Nb, Ti is too large, the recrystallization of the ferrite is greatly delayed when performing continuous annealing, etc., so that unrecrystallized ferrite tends to remain after annealing, resulting in a significant decrease in ductility. In addition, the machinability and toughness are also lowered, so the upper limit was defined as V: 0.1% or less, Nb: 0.05% or less, Ti: 0.05% or less.

“Zr: Zirconium” 5 to 80 ppm or less Zr has the effect of finely dispersing Zr oxide, which is a nucleation site of graphite, and sulfides such as MnS. These oxides and sulfides function effectively as graphite precipitation sites, and are effective for fine dispersion of graphite and short-time graphitization. However, the above effect is not observed when the amount of Zr added is less than 5 ppm, and when it exceeds 80 ppm, coarse and hard (Zr, X) S and (X is a sulfide such as Mn. Zr (CN) is formed, and not only the oxide refinement effect by Zr is reduced, but also the fracture characteristics are deteriorated. Therefore, also from the viewpoint of machinability, the content of Zr is specified in the range of 5 to 80 ppm or less.

"Steel structure"
As described above, the steel material according to the present invention is configured such that the steel structure includes a ferrite structure and graphite, and the graphitization rate exceeds 80%.

Generally, when a steel containing carbon is graphitized, the cementite structure in the steel structure is graphitized by performing a heat treatment. In addition, as an element that promotes such graphitization treatment, it is effectively graphitized by adding Al, Si, etc. to the base material, and since the treatment time is shortened, the heat treatment time such as annealing can be shortened, Productivity is improved. Such a graphitized steel material is excellent in machinability when it is cut to produce various parts. In addition, a part using such a steel material exhibits excellent vibration damping properties.
In addition, as described above, when Al is added to the base material, the machinability when processing the steel material is further improved, so that the product quality and the productivity when manufacturing the steel parts are further improved. An effect is obtained.

  In order to obtain the effect of improving machinability and vibration damping as described above, the ratio of graphite in the steel structure, that is, the graphitization rate, needs to be more than 80%. If the ratio of graphite in the steel structure exceeds, for example, 80% in terms of area ratio, the above-described effects of improving machinability and vibration damping can be stably obtained. If the graphitization rate in the steel structure is 80% or less, the above effects cannot be obtained stably.

  As described above, in the present invention, particularly, the steel structure can be appropriately controlled by strictly controlling the contents of Si, Al, and Mn in the composition of the base material. That is, it is possible to improve the machinability and vibration damping of the steel material by controlling the steel structure so as to include a ferrite structure and graphite and the graphitization rate exceeds 80%.

  Further, as described above, in the present invention, the amount of Mn added is increased to prevent the graphitization treatment from being significantly inhibited and the graphitization rate is increased. Defines the content of Si and Mn in the relationship represented by the following formula {5 ≦ Si / Mn}. As a result, sufficient graphite can be precipitated in the steel structure by a short annealing treatment, so that the machinability and vibration damping of the steel material can be improved and the productivity can be remarkably improved. .

Here, the machinability described in the present invention indicates the ease of cutting when cutting a steel material, for example, cutting resistance, tool life, degree of cutting finish surface, etc. And is represented by characteristics such as difficulty of processing.
In addition, the vibration damping property described in the present invention represents a characteristic that attenuates the magnitude of vibration by substituting vibration energy with thermal energy. It is an index different from the anti-vibration property. By improving such vibration damping properties, it is possible to suppress vibrations as much as possible and maintain a good environment in which various parts manufactured using the steel material according to the present invention are used.

  The steel structure of the steel material of the present invention can be examined, for example, by cutting and polishing the steel material, and then observing the polished surface with an optical microscope or the like, thereby examining the area ratio or size of the structure.

"Production method"
Below, an example of the method of manufacturing the steel material excellent in the machinability and vibration damping nature concerning the above-mentioned this invention is explained. In this embodiment, when manufacturing the steel material having the above-described configuration, it is possible to manufacture the steel material according to the following procedure and conditions. However, the method for manufacturing the steel material of the present invention is limited to the method described below. Instead, any method can be adopted as long as the configuration of the present invention can be realized.

  In manufacturing the steel material having the above-described structure and excellent machinability and vibration damping properties, first, a steel ingot having the above-described component composition is cast. Here, the manufacturing method of the steel ingot used for hot rolling is not particularly limited, and those manufactured by continuous casting or the like can be used as appropriate. The production method of the present invention can also be adapted to a continuous casting-direct rolling (CC-DR) process in which hot rolling is performed immediately after casting.

  Next, the rolled steel is cooled by online water cooling and air cooling. At this time, a structure mainly composed of martensite and bainite or a mixed structure thereof is precipitated in online water cooling, and a structure mainly composed of ferrite / pearlite or pearlite is precipitated in air cooling.

  Next, for example, C in the steel structure is graphitized by subjecting the steel material having the above component composition to re-heating to a temperature of 500 to 740 ° C. and performing graphitization annealing. At this time, by appropriately adjusting the heat treatment conditions (graphitization annealing conditions), the graphitization rate is controlled to be more than 80% while the steel structure is a ferrite structure and a structure containing graphite. At this time, in order to make the graphitization rate exceed 80%, it is necessary to set the reheating temperature in the range of 500 to 740 ° C. When this reheating temperature is less than 500 ° C. or exceeds 740 ° C., the efficiency of the graphitization treatment is lowered, the treatment time is lengthened, and the productivity is lowered. Moreover, it is preferable to set it as the range of 5-10 hr as holding time at the time of performing reheating. If the holding time during reheating is less than 5 hr, a sufficient amount of graphite may not be precipitated in the structure. If the holding time exceeds 10 hr, productivity is lowered.

  According to the steel material having excellent machinability and vibration damping properties according to the present invention as described above, the composition ratio of the base material is within an appropriate range as described above, and the proportion of graphite in the steel structure is By appropriately controlling, it is possible to realize a steel material having excellent machinability and also having vibration damping properties. Therefore, by applying the present invention to steel materials used for parts such as automobiles and industrial machines, in addition to improving product quality such as vibration damping of parts, there are merits such as improved productivity and longer tool life. The social contribution is immeasurable.

  Hereinafter, examples of the steel material excellent in machinability and vibration damping according to the present invention will be given and the present invention will be described more specifically, but the present invention is not limited to the following examples from the beginning, The present invention can be implemented with appropriate modifications within a range that can be adapted to the gist of the following, and these are all included in the technical scope of the present invention.

[Example]
In the Examples, first, steel ingots having the component compositions shown in Table 1 below were obtained by controlling the deoxidation / desulfurization of the molten steel and chemical components in the steelmaking process. Thereafter, the obtained steel ingot was hot-rolled to prepare a steel material having a diameter of 50 mm.

  In the examples, as the cooling method after rolling, two types of on-line water cooling and air cooling are used, which are shown in Table 1 below. Here, in the on-line water cooling, the structure mainly composed of martensite and bainite or a mixed structure thereof is the structure before annealing, and the structure mainly composed of ferrite / pearlite or pearlite is the structure before annealing.

  Next, the steel material that has been subjected to hot rolling and cooling after rolling is reheated to a temperature of 680 ° C., and is retained for the time shown in Table 2 (graphitizing annealing time) to perform annealing, thereby graphitizing the steel structure. By performing the treatment, test pieces of the present invention and comparative examples were produced.

Next, the following evaluation test was done about the test piece manufactured by the said procedure and conditions.
First, the graphitized and annealed steel material was cut and polished, and the range of the total visual field of 0.4 mm ² was observed with an optical microscope to measure the average graphite particle size and the number of graphite particles. And from the average graphite particle diameter obtained by this measurement, the number of graphite particles, and the density of graphite, the graphite content in the steel was calculated, and the graphitization rate defined by the following formula (1) was calculated.
Graphitization rate (%) = (content of graphite in steel / carbon content of steel) × 100 (1)

  Further, the hardness after annealing, which is an index of cold forgeability, is an average obtained by measuring three points with a Vickers hardness meter at the center of the same sample used for obtaining the graphitization rate under a load of 10 kg. The value was evaluated.

In addition, for machinability evaluation, a machinability evaluation sample is prepared from a steel material subjected to graphitization annealing, and this sample is used to evaluate the drill life (VL1000) and the cutting surface roughness (Rz). did.
Here, the index VL1000 indicating the drill life is a maximum drill peripheral speed capable of drilling up to a cumulative hole depth of 1000 mm, and the larger this value is, the faster the cutting can be performed. It means that it has excellent properties. In this example, a φ5 mm straight drill was used as the drill, and at a feed of 0.25 mm / rev, the drill peripheral speed was changed using a water-soluble cutting oil, and the accumulated hole depth until drill breakage was measured. VL1000 was calculated based on this value, and the results are shown in Table 2. Note that a commercially available water-soluble oil was used as the cutting oil at this time.

  Further, the cutting surface roughness was measured by using a stylus roughness meter to measure the cutting surface when the machinability evaluation sample was cut by the plunge cutting method as shown in the schematic diagram of FIG. 1, and JIS B0601. The ten-point average roughness Rz based on The cutting conditions at this time were as follows. Using a cutting tool 1 as shown in FIG. 1, the cutting speed was 80 m / min, the tool feed was 0.05 mm / rev and the cutting was performed for 2.5 seconds, and then the tool was pulled out for 6 seconds. The operation of idling with time was defined as one cycle. And under this condition, the cutting surface roughness of the cutting surface 2 at the groove bottom of the 100th cycle was measured for the grooves sequentially created on the steel material surface by cutting. As the cutting surface roughness, the surface roughness Rz was evaluated using a high speed tool for plunge cutting: SKH57 at a cutting speed of 80 m / min and a feed of 0.05 m / rev. The results are shown in Table 2.

  Further, the vibration damping property was evaluated by the logarithmic attenuation rate of vibration. Specifically, first, a strain gauge was adhered to the central portion of one surface of a test piece that was processed into a plate shape having a width of 30 mm, a length of 300 mm, and a thickness of 5 mm by machining. Furthermore, the two test pieces are suspended with two threads so that the two points 67 mm away from both ends in the longitudinal direction of the test piece are fixed with a thread, the test piece is horizontal, and the strain gauge is on the lower side. Held in the same state. Then, in that state, a steel ball having a diameter of 10 mm was dropped from the upper side of the test piece 50 mm onto the center of the test piece using a guide. Then, the vibration of the test piece after dropping was detected with a strain gauge adhered to the lower surface of the test piece, and the logarithmic decay rate was measured.

Furthermore, in Example 1, in order to evaluate the difference in graphitization performance of each sample as a remarkable state, the required annealing time when the graphitization rate reached 80% and the average graphite particle size at this time were measured. . The required annealing time was defined as the holding time at which the graphitization rate reached 80% at a temperature of 680 ° C. The average graphite particle size was determined from an optical microscope having a total visual field of 0.4 mm ² .

[Evaluation results]
The graphs in Tables 1 and 2 and FIGS. 2 and 3 show a list of manufacturing conditions and evaluation results in this example. Here, FIG. 2 is a graph showing the influence of Si / Mn on the drill life VL1000, and FIG. 3 shows the influence of Si / Mn on the cutting surface roughness when the machinability is evaluated by the plunge cutting method. It is a graph which shows.

  As shown in Tables 1 and 2, the present invention examples (test Nos. 1 to 26) having the component composition defined in the present invention and the steel structure defined in the present invention are all steel structures. Was a steel material containing a ferrite structure and more than 80% graphite. As shown in the graphs of FIGS. 2 and 3, in the example of the present invention, the drill life (VL1000), which is an index indicating machinability, is 150 m / min or more, and the cutting surface roughness (Rz) is 25.6 μm. It was the following. Furthermore, in the example of the present invention, the logarithmic damping factor of vibration, which is an index indicating damping performance, was 0.010 or more. From these evaluation results, it was confirmed that the steel sheet of the example of the present invention had excellent machinability and also had vibration damping properties.

  In addition, as for the performance of the graphitization treatment, in any of the examples of the present invention, the annealing time was 10 hours or less and showed excellent characteristics. Among them, the samples that were water-cooled after rolling were further graphitized, and most of the samples reached a graphitization rate of 80% after annealing within 3 hours.

On the other hand, since the steel materials of the comparative examples (test Nos. 30 to 42) do not satisfy any of the component composition and steel structure defined in the present invention, machinability, vibration damping property, and graphitization rate 80 % Of the required annealing time until reaching% was inferior.
In the comparative example, the test No. No. 27 is an example in which Mn is below the specified range of the present invention, resulting in hot brittleness and cracking of the steel material during rolling. In addition, Test No. No. 28 is an example in which the steel material was cracked during hot rolling because Al exceeded the specified range of the present invention. In addition, Test No. No. 29 is an example in which hot brittleness occurs because S exceeds the specified range of the present invention, and the steel material cracks during rolling.

Here, as shown in the graph of Table 2 and FIG. 2, when Si / Mn was 5 or less, the value of VL1000 showed a low value. On the other hand, even when Si / Mn was 5 or more, the value of VL1000 was smaller than that of the invention steel when the Al content or C content was less than the specified value.
Moreover, as shown in the graph of Table 2 and FIG. 3, the characteristic of the cutting surface roughness (Rz) at the time of plunge cutting sharply decreased when Si / Mn was 5 or less, similar to VL1000.

  From the results of the examples described above, it is clear that the steel material of the present invention has excellent machinability and vibration damping properties.

1 ... cutting tool, 2 ... cutting surface

Claims (7)

% By mass
C: 0.3-1.2%
Si: 2.0 to 3.0%,
Mn: more than 0.05% and 0.5% or less,
Al: 0.2 to 1.0%,
P: 0.02% or less,
S: 0.007 to 0.2%,
N: 0.0015 to 0.030%,
O: 0.003% or less, and the content of Si and Mn is a range satisfying the following formula {5 ≦ Si / Mn}, and the balance has a component composition consisting of Fe and inevitable impurities,
A steel material excellent in machinability and vibration damping, wherein the steel structure contains a ferrite structure and graphite, and the graphitization rate is more than 80%. Furthermore, in mass%,
Cr: 0.3% or less,
Mo: 0.2% or less,
The steel material having excellent machinability and vibration damping properties according to claim 1, characterized by containing one or more of W: 0.2% or less. Furthermore, in mass%,
V: 0.1% or less,
Nb: 0.05% or less,
Ti: 0.05% or less,
The steel material excellent in machinability and vibration damping property according to claim 1 or 2, characterized by containing one or more of the above. Furthermore, in mass%,
Ni: 0.1 to 3.0%
The steel material excellent in machinability and vibration damping properties according to any one of claims 1 to 3, characterized in that Furthermore, in mass%,
B: 0.0003 to 0.01%
The steel material excellent in machinability and vibration damping property according to any one of claims 1 to 4, characterized in that Furthermore, in mass%,
Mg: 0.0001 to 0.01%
The steel material excellent in machinability and vibration-damping property according to any one of claims 1 to 5, characterized by comprising: Furthermore, in mass ppm,
Zr: 5-80 ppm or less, The steel material excellent in the machinability and vibration damping property of any one of Claims 1-6 characterized by the above-mentioned.

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