JP5231101B2 - Machine structural steel with excellent fatigue limit ratio and machinability - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a machine structural steel useful for producing parts such as automobiles, industrial machines, and electrical products, and more particularly to a steel for machine structural use having a high fatigue limit ratio and excellent machinability. Is.

  In recent years, the strength of steel has been increased, but usually the workability of steel is lowered due to the increase in strength of steel. Against this background, there is an increasing need for free-cutting steel for machine structures that does not reduce cutting efficiency. Until now, in order to improve machinability, a method of adding machinability improving elements such as S and Pb to machine structural steel is known.

  However, when S is added to the steel for machine structural use, inclusions of MnS are formed to improve the machinability of the steel for machine structural use. However, since the inclusions themselves are deformed during the production, they are manufactured from the steel for machine structural use. In some cases, anisotropy of material characteristics may occur in steel materials and parts. In particular, when MnS is stretched by forging or rolling, the above anisotropy appears remarkably and the strength in a specific direction becomes extremely weak.

  As for Pb, there is a movement to limit its use because of its large environmental load. Bi and Te also have an effect as a machinability improving element, but since the hot deformability deteriorates, the production efficiency is lowered. For this reason, the technique which does not rely on use of a machinability improvement element is needed.

  There are various properties related to machinability, such as chip disposal, tool life, and cutting surface roughness. The improvement of chip disposal can be dealt with by devising the shape of the tool. However, the improvement of the tool life requires a step of coating the surface of the tool, so that the production efficiency is lowered. With regard to the machined surface roughness, no significant improvement is seen in machine structural steel even when a machinability improving element is added.

As a cause of the deterioration of the tool life, hard particles of alumina (Al ₂ O ₃ ) can wear the tool surface. Although the amount of alumina can be suppressed by suppressing the amount of Al added, it is difficult to apply to applications that require high fatigue properties because the crystal grain size is large and the toughness is poor.

  Patent Document 1 describes that “in order to improve machinability, it is necessary to adjust to a coarse particle size of 7 or less in the particle size number of the austenite particle measurement method of JIS G0551” (paragraph 0038). . However, a steel material having a small particle size number has poor toughness, and there is a problem that good fatigue strength cannot be obtained.

  Generally, in a part obtained by cooling or air cooling after hot forging, the steel structure needs to be ferrite-pearlite in order to ensure fatigue strength. This is because fatigue strength decreases when bainite is generated. Further, sufficient fatigue strength cannot be obtained unless V is added at least 0.03% (meaning mass%, the same applies to the chemical components of steel) and Mn is added at least 0.5%. On the other hand, in Patent Document 1, the implementation No. 1 to 17 (Table 1) and 35 to 39 (Table 2) are of a ferrite-pearlite structure (non-heat treated steel), but in these examples, since V is not added, fatigue strength is insufficient. It is thought that there is.

  Implementation No. In 46-67, since the structure of steel is not indicated, the structure cannot be specified. However, the implementation No. in which V was added. 49, 63, 64 will be described in more detail. In 63 and 64, the maximum cutting speed (VL1000) is as low as 35 and 28, respectively, so that the structure is considered to be tempered martensite in view of the level of VL1000 shown in FIG. On the other hand, the implementation No. 49, it is also considered that VL1000 is 52 and that it is a ferrite-pearlite structure from FIG. However, no. In 49, the amount of oxygen is quite high at 0.0068%, so it is considered that a large amount of alumina is produced, which is not advantageous in terms of tool life, and the fatigue strength is insufficient because the amount of Mn is as small as 0.41%. it seems to do.

  Patent Document 2 states that “the microstructure is composed of ferrite and pearlite, the average grain size of the ferrite grain size and the pearlite grain size is less than 10 μm, and the total decarburized layer depth DM-T defined by JIS G0588 is 0.02 to 0.02”. `` Super high temperature hot forged non-tempered parts characterized by 0.06 mm, tensile strength 800-1300 MPa, yield ratio 0.7-0.95 '', heating before forging It is necessary to raise the temperature to 1250 ° C. or more, which is disadvantageous in terms of production efficiency. Although it is described that the ferrite particle size and the pearlite particle size are reduced, it is the coarsening of austenite that causes the deterioration of the toughness. By suppressing the ferrite particle size and the pearlite particle size, In addition, deterioration of toughness is inevitable. When the austenite crystal grain size increases, the grain interface area per unit volume decreases, embrittlement elements such as P tend to concentrate at the grain boundaries, and the austenite remains brittle after ferrite-pearlite transformation. Therefore, it becomes easy to break at the site that was originally an austenite grain boundary.

In Patent Document 3, C: 0.1 to 0.4%, Si: 0.02 to 1.5%, Mn: 0.3 to 0.3 for the purpose of preventing coarse austenite grains during carburization and improving fatigue characteristics. 1.8%, S: 0.001 to 0.15%, Al: 0.005 to 0.05%, Ti: 0.05 to 0.2%, N: limited to less than 0.0051% Furthermore, Cr: 0.4 to 2.0%, Mo: 1.5% or less, Ni: 3.5% or less, V: 0.5% or less, B: 0.005% or less Containing two or more, P: 0.025% or less, O: limited to 0.0025% or less, the balance consists of iron and inevitable impurities, the amount of precipitation of AlN after hot rolling is 0.01% The following is a description of the case-hardened steel having a grain size number of 8 to 11 as defined in JIS G0552 and a method for producing the same, as described below. It is listed. However, the method described in Patent Document 3 requires a carburizing process and has a disadvantage in terms of production efficiency. Patent Document 3 describes that “the ferrite grain size number is 8 to 11,” but does not describe the grain size number of the austenite crystal.
JP 2000-256785 A JP 2004-346415 A JP-A-2005-240175

As described above, the present situation is that a steel for machine structural use that satisfies both the fatigue limit ratio and the machinability and that can be easily manufactured has not been obtained.
In view of such circumstances, an object of the present invention is to provide a machine structural steel that satisfies both the fatigue limit ratio and the machinability in a balanced manner without depending on the use of a machinability improving element, and can be easily manufactured. And

The machine structural steel excellent in machinability of the present invention that can achieve the above-mentioned object,
C: 0.15-0.55% (meaning mass%, the same applies to the chemical components of steel),
Si: 0.01 to 1.5%,
Mn: 0.5-2%
P: 0.01-0.2%
S: 0.01-0.2%
Al: 0.015% or less (excluding 0%),
Cr: 0.1 to 1.5%
N: 0.002 to 0.02%,
O: 0.001 to 0.005%,
V: 0.03-0.5%,
A balance of non-refined steel consisting of iron and inevitable impurities, the total structural fraction of ferrite and pearlite is 95% or more in area ratio, and the former austenite grains defined in JIS G0551 Machine structural steel with a Gf grain size number of 5 or more.

In addition to the above chemical components, the steel for machine structural use according to the present invention may further include, if necessary,
Cu: 0.02 to 0.6%,
Ni: 0.02 to 0.6%,
Mo: 0.01 to 0.5%,
It is also effective to contain at least one selected from the group consisting of, and the properties of the steel are further improved depending on the type of component to be contained.

In addition to the above chemical components, the steel for machine structural use according to the present invention may further include, if necessary,
Ti: 0.01-0.2%
Zr: 0.01 to 0.2%,
Nb: 0.01 to 0.1%,
It is also effective to contain at least one selected from the group consisting of, and the properties of the steel are further improved depending on the type of component to be contained.

In addition to the above chemical components, the steel for machine structural use according to the present invention may further include, if necessary,
Pb: 0.01 to 0.35%,
Bi: 0.01-0.2%
Mg: 0.0001 to 0.005%,
Ca: 0.0001 to 0.005%,
Se: 0.01 to 0.5%,
Rare earth element (REM): 0.001 to 0.2%,
It is also effective to contain at least one selected from the group consisting of, and the properties of the steel are further improved depending on the type of component to be contained.

  According to the present invention, machinability is improved while manufacturing is easy by appropriately adjusting the component composition and structure of steel for machine structural use and by setting the Gf grain size number of the prior austenite crystal grains to 5 or more. Without relying on the use of elements, it is possible to provide a steel for machine structural use in which both the fatigue limit ratio and the machinability are improved in a well-balanced manner.

  In order to explain the outline of the present invention, the Al content in the steel, the O (oxygen) content, and the grain size number of the prior austenite crystal grains (the grain size number of the prior austenite crystal grains defined in JIS G0551, Explained the results of investigating the relationship between the boundary appearance method according to Annex 1 “a) Gradual Cooling Method” (hereinafter referred to as “Gf grain size number”) and the fatigue limit ratio and machinability of steel for machine structural use To do.

  In order to improve the machinability of the steel for machine structural use, the inventors first studied to suppress the addition amount of Al. As a result, as shown in FIG. 1, it was confirmed that the drill life was improved when the Al content was suppressed. On the other hand, when the Al content is suppressed, it has been found that the prior austenite crystal grains of the mechanical structural steel tend to be coarsened (for example, when the Al content is suppressed from 0.020% to 0.002%). Gf particle size number has decreased from 8.0 to 4.0).

  As a result of further experiments for investigating the effects caused by the decrease in the Gf grain size number, the fatigue limit ratio of the steel for machine structures tended to decrease as shown in FIG. 2 (Al content was 0). Fixed at .002%).

  Therefore, in order to maintain the fatigue limit ratio of steel for machine structural use, as a result of examining a method for preventing coarsening of prior austenite crystal grains (method of increasing Gf grain size number), the O content in the steel as shown in FIG. (Al content fixed at 0.002%), it was found that coarsening of prior austenite crystal grains can be prevented. When the O content is increased, the fatigue limit ratio is high because Al forms oxides to reduce free Al, and Al nitride decreases, resulting in an increase in free N. It is presumed that V carbonitrides are easily formed and the strength of the steel is increased.

  The present invention has been made on the basis of the above findings, the gist of which is (1) that chemical components of machine structural steel are appropriately adjusted, (2) non-tempered steel, (3) The total structure fraction of ferrite and pearlite is 95% or more, and (4) the Gf particle size number is increased.

1. Steel for machine structure (1) Chemical composition (C: 0.15-0.55%)
C is an important element for securing the strength of steel for machine structural use. However, if the amount of C is excessive, the toughness of the steel is lowered, and it becomes too hard and the machinability (particularly the tool life) is lowered. Therefore, the C content is 0.15% or more (preferably 0.20% or more, more preferably 0.25% or more), 0.55% or less (preferably 0.45% or less, more preferably 0.40%). The following.

(Si: 0.01-1.5%)
Si is effective as a deoxidizing element at the time of melting, and has the effect of improving the strength of mechanical structural parts by solid solution strengthening. However, if the Si content is excessive, the machinability is adversely affected. Therefore, the Si content is 0.01% or more (preferably 0.05% or more, more preferably 0.1% or more, further preferably 0.2% or more), 1.5% or less (preferably 1.2%). Or less, more preferably 0.9% or less, and still more preferably 0.6% or less.

(Mn: 0.5-2%)
Mn is an important element that improves the machinability by forming sulfide inclusions. However, if the Mn content is excessive, the machinability is rather lowered. Therefore, the amount of Mn is 0.5% or more (preferably 0.7% or more, more preferably 0.9% or more), 2% or less (preferably 1.8% or less, more preferably 1.6% or less). It was.

(P: 0.01-0.2%)
P is added for improving machinability, particularly for improving the finished surface properties of steel for machine structural use. If the P content is less than 0.01%, the effect cannot be obtained. Conversely, if the P content exceeds 0.2%, the toughness of the steel for machine structure deteriorates. Therefore, the P content is 0.01% or more (preferably 0.015% or more, more preferably 0.02% or more), 0.2% or less (preferably 0.1% or less, more preferably 0.05%). The following:

(S: 0.01-0.2%)
S is an element effective for forming a sulfide inclusion such as MnS and improving the machinability of the steel for machine structure. If the S content is less than 0.01%, the effect cannot be obtained. Conversely, if the S content exceeds 0.2%, the toughness and ductility of the steel for machine structural use deteriorate. Therefore, the S amount is 0.01% or more (preferably 0.02% or more, more preferably 0.03% or more), 0.2% or less (preferably 0.15% or less, more preferably 0.1% The following:

(Al: 0.015% or less (excluding 0%))
Al is an element necessary for deoxidation of steel. However, since Al ₂ O ₃ is formed in the steel, the machinability of the steel for machine structural use is deteriorated. Therefore, in the present invention, the Al amount is set to Al: 0.015% or less. More preferably, it is 0.01% or less, More preferably, it is 0.005% or less.

(Cr: 0.1-1.5%)
Cr is an element that contributes to increasing the strength of steel for machine structural use by solid solution strengthening. However, if the amount of Cr is excessive, it is not only disadvantageous in cost but also machinability is lowered. Therefore, the Cr content is 0.1% or more (preferably 0.11% or more, more preferably 0.12% or more), 1.5% or less (preferably 1% or less, more preferably 0.5% or less). It was.

(O: 0.001 to 0.005%)
By adjusting the amount of O within an appropriate range, it is possible to improve both the machinability and the fatigue limit ratio of the steel for machine structure in a well-balanced manner. If the amount of O in the steel is excessive, inclusions increase, so the machinability (particularly the tool life) of the machine structural steel deteriorates. However, as described above, it has been found that securing a certain amount or more of oxygen can prevent coarsening of the prior austenite crystal grains of the steel for machine structural use and increase the fatigue limit ratio of the steel for machine structural use. Therefore, in the present invention, the amount of O is 0.005% or less (preferably 0.004% or less, more preferably 0.0035% or less), and 0.001% or more (preferably 0.0015% or more, more Preferably it was 0.002% or more. In order to increase the amount of O, a slag having a low basicity [(CaO + MgO) / SiO ₂ ] may be used when melting the structural structural steel. The basicity of the slag is, for example, in the range of 0.5 to 4, preferably 0.7 to 2.

(N: 0.002 to 0.02%)
N forms a nitride with Al or the like to refine the prior austenite crystal grains, and as a result, has an effect of improving toughness and fatigue strength. However, if the amount of N is excessive, the toughness is lowered and the casting process, which is a normal manufacturing process for steel for machine structures, is adversely affected. Therefore, the N amount is 0.002% or more (preferably 0.0025% or more, more preferably 0.003% or more), 0.02% or less (preferably 0.015% or less, more preferably 0.01%). The following.

(V: 0.03-0.5%)
V combines with C and N to produce carbonitrides and has the effect of refining the crystal grains of mechanical structural steel. In order to effectively exhibit this effect, the V amount is set to 0.03% or more (preferably 0.09% or more, more preferably 0.15% or more). On the other hand, even if V is contained in excess of 0.5%, the precipitates are enlarged and the toughness and fatigue strength are reduced. Therefore, the V amount is set to 0.5% or less (preferably 0.4% or less, more preferably 0.3% or less).

  The basic component composition of the steel for machine structure used in the present invention is as described above, and the balance is iron and inevitable impurities. For example, it is naturally allowed that inevitable impurities brought into the steel for machine structural use depending on the situation of raw materials, materials, manufacturing facilities, and the like are included in the steel for machine structure.

(2) Non-tempered steel The machine structural steel of the present invention is non-tempered steel. Machine structural steel is usually two-stage heat treatment ((1) quenching after heating to an austenite region of about 850 ° C. or higher and quenching to make the structure a martensitic phase, and (2) 550 ° C. to 650 ° C. thereafter. And a tempering process in which the mechanical properties are adjusted by heating. Tempered steel is obtained by adjusting the mechanical properties of steel by this quenching and tempering treatment. Non-tempered steel has mechanical properties comparable to those of tempered steel even when such tempered heat treatment is omitted, and forged and air-cooled or rolled and air-cooled. Non-tempered steel is obtained by adding vanadium (V) to carbon steel. The tempered steel has a tempered martensite phase, while the non-tempered steel has fine vanadium carbide (VC) precipitated in the pearlite-ferrite mixed structure and is strengthened by the precipitate. It is a thing.

(3) Total structural fraction The mechanical structural steel of the present invention comprises a parent layer of the total structural fraction of ferrite and pearlite (t / 4 position cross section of mechanical structural steel (t: thickness of mechanical structural steel)). It is necessary to set the area ratio to 95% or more. This is because if the structure other than ferrite and pearlite, for example, bainite exceeds 5%, the fatigue limit ratio of the mechanical structural steel is lowered. The total structural fraction of ferrite and pearlite is more preferably 96% or more, and still more preferably 97% or more.

(4) Gf grain size number As mentioned above, the Gf grain size number is the grain size number of the former austenite crystal grain defined in JIS G0551, and the grain boundary appearance method is according to Appendix 1 “a) Slow Cooling Method”. It is. Further, as described above, in order to prevent a decrease in the fatigue limit ratio of steel for machine structural use, it is necessary to prevent coarsening of prior austenite crystal grains, that is, to increase the Gf grain size number. Then, it was set to 5 or more. Preferably it is 6 or more, More preferably, it is 7 or more, More preferably, it is 7.5 or more.

2. Temperature Control Details of the method for producing the steel for machine structural use according to the present invention will be described in the examples described later, but the present inventors have generally grasped the tendency shown in FIGS. The steel for machine structural use of the present invention can be manufactured by, for example, melting, casting, heating, hot working (rolling or forging), and air cooling a steel material having a predetermined chemical component. It has been confirmed that the relationship between the previous heating temperature and the Gf grain size number of the machine structural steel to be produced is as shown in FIG. As shown in FIG. 4, the lower the heating temperature, the larger the Gf particle size number. Therefore, it is recommended that the heating temperature be 1250 ° C. or lower (preferably 1200 ° C. or lower).

  Further, it has been confirmed that the relationship between the end of hot working, that is, the final working temperature and the fatigue limit ratio of the manufactured machine structural steel is as shown in FIG. As shown in FIG. 5, the lower the final processing temperature, the higher the fatigue limit ratio. Although not shown in FIG. 5, the present inventors confirmed that the fatigue limit ratio becomes low because the effect of strengthening the steel by vanadium cannot be obtained when the final processing temperature falls below 900 ° C. doing. Therefore, it is recommended that the final processing temperature is 900 ° C. or higher (preferably 950 ° C. or higher).

  Further, it has been confirmed that the relationship between the cooling rate after hot working and the Gf grain size number of the machine structural steel to be manufactured is as shown in FIG. As shown in FIG. 6, when the cooling rate is less than 0.5 ° C./s (second), the Gf particle size number rapidly decreases, so the cooling rate is 0.5 ° C./s or more (preferably 1.0 ° C. / S) is recommended.

  The mechanical structural steel used in the present invention may contain at least one selected from the group consisting of Cu, Ni, and Mo in order to improve the strength of the mechanical structural steel. The preferable addition amount of each element is as follows.

(Cu: 0.02-0.6%)
Cu is an element effective for improving the strength of steel for machine structural use by solid solution strengthening. In order to effectively exhibit such an effect, for example, 0.02% or more (preferably 0.05% or more, more preferably 0.1% or more) is contained. On the other hand, if the amount of Cu exceeds 0.6%, hot cracking occurs when solidifying the steel for machine structural use. Therefore, it is 0.6% or less (preferably 0.5% or less, more preferably 0.4% or less. ).

(Ni: 0.02-0.6%)
Ni, as well as Cu, is an element effective for improving the strength of steel for machine structural use. In order to effectively exhibit such an effect, for example, 0.02% or more (preferably 0.05% or more, more preferably 0.1% or more) is contained. On the other hand, if the amount of Ni exceeds 0.6%, not only is it disadvantageous in terms of cost, but also deteriorates the machinability of steel for machine structural use, so 0.6% or less (preferably 0.5% or less, more Preferably it is 0.4% or less).

(Mo: 0.01-0.5%)
Mo is also an element effective for improving the strength of steel for machine structural use. In order to effectively exhibit such an effect, for example, 0.01% or more (preferably 0.05% or more, more preferably 0.1% or more) is contained. On the other hand, if the amount of Mo exceeds 0.5%, not only is it disadvantageous in terms of cost, but also deteriorates the machinability of the steel for machine structural use, so 0.5% or less (preferably 0.4% or less, more Preferably it is 0.3% or less.

  The steel for mechanical structure used in the present invention may contain at least one selected from the group consisting of Ti, Zr, and Nb for refinement of prior austenite crystal grains (Gf grain size number increases, Fatigue strength ratio is maintained). The preferable addition amount of each element is as follows.

(Ti: 0.01-0.2%)
Ti combines with N in the steel to form TiN and has a function of helping refine the crystal grains. In order to effectively exhibit such an effect, for example, 0.01% or more (preferably 0.02% or more, more preferably 0.03% or more) is contained. On the other hand, if the Ti amount exceeds 0.2%, TiN becomes excessive, and cracks occur in the steel during hot working. Therefore, the Ti content is 0.2% or less (preferably 0.15% or less, more preferably 0.1% or less).

(Zr: 0.01 to 0.2%)
Zr also has a function to help refine the crystal grain size. In order to effectively exhibit such an effect, for example, 0.01% or more (preferably 0.02% or more, more preferably 0.03% or more) is contained. On the other hand, the effect is saturated even if Zr is added over 0.2%. Therefore, the Zr content is 0.2% or less (preferably 0.15% or less, more preferably 0.1% or less).

(Nb: 0.01 to 0.1%)
Nb also has a function to help refine the crystal grain size. In order to effectively exhibit such an effect, for example, 0.01% or more (preferably 0.02% or more, more preferably 0.03% or more) is contained. On the other hand, even if Nb exceeds 0.1%, the effect is saturated. Therefore, the Nb content is 0.1% or less (preferably 0.08% or less, more preferably 0.06% or less).

  The steel for mechanical structure used in the present invention may contain at least one selected from the group consisting of Pb, Bi, Mg, Ca, Se, and rare earth elements (REM) for improving machinability. The preferable addition amount of each element is as follows.

(Pb: 0.01 to 0.35%)
Pb exists alone or in a form that adheres to the outer peripheral portion of sulfide, and is an element that improves the machinability of steel for machine structure. In order to effectively exhibit such an effect, for example, 0.01% or more (preferably 0.02% or more, more preferably 0.03% or more) is contained. On the other hand, if the content exceeds 0.35%, it tends to agglomerate and precipitate to form defects in the steel, so it is 0.35% or less (preferably 0.30% or less, more preferably 0.25%). The following.

(Bi: 0.01-0.2%)
Bi is an element having properties similar to Pb and improving the machinability of steel for machine structural use. In order to effectively exhibit such an effect, for example, 0.01% or more (preferably 0.02% or more, more preferably 0.03% or more) is contained. The upper limit of the content is also set to 0.2% or less (preferably 0.15% or less, more preferably 0.1% or less) for the same reason as in the case of Pb.

(Mg: 0.0001 to 0.005%)
Mg contributes to the improvement of machinability and anisotropy of mechanical structural steel because MnS is finely dispersed by forming fine oxide MgO. In order to effectively exhibit such an effect of Mg, for example, 0.0001% or more (preferably 0.0002% or more, more preferably 0.0003% or more) is contained. On the other hand, even if added over 0.005%, the effect is saturated, so 0.005% or less (preferably 0.004% or less, more preferably 0.003% or less).

(Ca: 0.0001 to 0.005%)
Ca is an element that dissolves in MnS and has an action of promoting spheroidization of MnS, and as a result, improves machinability (particularly tool life). In order to effectively exhibit such effects, for example, 0.0001% or more (preferably 0.0002% or more, more preferably 0.0003% or more) is contained. On the other hand, if added over 0.005%, coarse CaS is formed and causes casting defects, so 0.005% or less (preferably 0.004% or less, more preferably 0.003% or less). To do.

(Se: 0.01-0.5%)
Se is also a well-known machinability improving element. In order to effectively exhibit such effects of Se, for example, 0.01% or more (preferably 0.015% or more, more preferably 0.02% or more) is contained. On the other hand, if the content exceeds 0.5%, the hot workability of the steel for machine structural use is deteriorated and cracking tends to occur. Therefore, it is 0.5% or less (preferably 0.4% or less). , More preferably 0.3% or less).

(REM: 0.001-0.2%)
REM contains 15 elements of Sc, Y and lanthanoids. Industrially, misch metal is used as REM. It has the effect of finely dispersing MnS, and improves the machinability of steel for mechanical structures. In order to effectively exhibit such an effect of REM, for example, 0.001% or more (preferably 0.005% or more, more preferably 0.01% or more) is contained. On the other hand, even if added over 0.2%, the effect is saturated, so the content is made 0.2% or less (preferably 0.15% or less, more preferably 0.1% or less).

  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, and appropriate modifications are made within a range that can be adapted to the above-described purpose. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

1. Production of Test Steel First, steel materials having various chemical components shown in Table 1 and Table 2 below were vacuum-melted and hot-rolled to obtain a columnar steel material having a diameter of 50 mmφ × 100 mm in length. And these steel materials were heated to the temperature shown in Table 3 and Table 4, the temperature of steel materials was measured, and this was made into the heating temperature before hot processing. It is recommended that the heating temperature be 1250 ° C. or lower. This is because if the heating temperature is too high, the austenite crystal grains become coarse.

  Next, a plate-shaped steel material having a thickness of 20 mm was obtained by hot forging a plurality of times in the circular diameter direction of the cylindrical steel material using a press machine. At this time, the temperature of the steel material was measured again, and this was taken as the final processing temperature after hot working. The final processing temperature is recommended to be 900 ° C. or higher in order to maintain the V strength improvement effect. The final processing temperature was adjusted by allowing the steel material to cool for an appropriate time before forging as required. Further, the time required for cooling from the final processing temperature to 600 ° C. was measured, and the average cooling rate was calculated. The cooling rate was adjusted by the strength of air cooling to the forged product. The steel materials obtained by such a process are the test steels A1 to A7, B1 to B5, C1 to C5, and D1 to D55 of Table 3 and Table 4, respectively.

2. Observation of manufactured test steel (1) Microstructure of test steel An optical microscope was used at a magnification of 100 to observe the microstructure of the test steel. The results are shown in Tables 3 and 4 below. (F + P) was described in which the total structural fraction of ferrite and pearlite was 95% or more in terms of area ratio. Among them, in addition to ferrite-pearlite, ferrite-pearlite-bainite structure (F + P + B) in which the bainite structure was observed in excess of 5% was also present as in test steel C5.

(2) Gf particle size number The Gf particle size number is based on JIS G0551 as already stated. Appearance of the former austenite grain boundary was in accordance with “a) slow cooling method” in Appendix 1 of JIS G0551. Since pro-eutectoid ferrite was observed in the structure, the Gf particle size was measured on the assumption that it was on the prior austenite grain boundary. The results are shown in Tables 3 and 4 above.

3. Evaluation of test steel (1) Fatigue limit ratio A 14A test piece described in JIS Z2201 was prepared from the test steel, and a tensile test was performed to measure the tensile strength. The fatigue limit was measured by a rotating bending test according to JIS Z2274. As a test piece, No. 1 test piece described in JIS Z2274 was produced in the same direction as the tensile test. The fatigue limit ratio is a value obtained by dividing the fatigue limit by the tensile strength. The results are shown in Tables 3 and 4 above.

(2) Machinability In order to evaluate the machinability of the obtained machine structural steel, a drill test was conducted. In the drill test, a 10 mmφ straight drill (SKH51, no surface coating) was used, cutting speed: 20 m / min, feed: 0.2 mm / rev, hole depth: 20 mm (through the above-mentioned test steel in a disk shape), Cutting oil: None (dry type). In this way, drill through holes in the test steel one after another, and describe the total length of the drill hole until the drill is seized and cannot be drilled (hereinafter simply referred to as “drill hole length”. The unit is cm), and this drill hole length was used as an index of tool life. The results are shown in Tables 3 and 4 above.

4). Discussion The test steels A1 to A6 have different amounts of Al and oxygen. The machinability of the machine structural steel was evaluated by the drill hole length (cm). When the amount of Al (aluminum) is small like the test steel A1, it is as high as 38 (cm), but the test steel A3 When the amount of Al is large like A7, the drill hole length is less than 20 cm. Except for the test steels A3 and A7, the test steel A6 having an oxygen content of less than 0.001% has a Gf particle size number of less than 5.0 and a fatigue limit of less than 0.40. On the other hand, in the test steel having an oxygen content of 0.001% or more, the Gf particle size number is 5.0 or more, and the fatigue limit ratio is also excellent at 0.40 or more.

  In the test steels of the test steels B1 to B5, the chemical composition of the steel satisfies the provisions of the present invention, but the Gf particle size number is changed by changing the heating temperature before forging. In the test steel B5 having a heating temperature exceeding 1250 ° C., the oxygen content satisfies the provisions of the present invention, but the Gf particle size number is less than 5 and the fatigue limit ratio is less than 0.40.

  In the test steels C1 to C5, the chemical composition of the steel also satisfies the provisions of the present invention, but the steel structure is changed by changing the cooling rate after finishing the final processing. is there. In the test steel C1 having a cooling rate of less than 0.5 ° C./sec, the Gf particle size number is less than 5, and the fatigue limit ratio is less than 0.40. On the other hand, when the cooling rate is too high as in the case of the test steel C5, bainite is generated in excess of 5%, and the fatigue limit ratio is still less than 0.40.

Test steels D1 to D28, D30 to D33, D35 to D55 satisfy all of the basic components, structure, and Gf particle size number of the steel, and in addition, the selective additive elements were variously changed. is there. Both have excellent fatigue limit ratios and good machinability. On the other hand, when the amount of V is small like the test steel D29, the fatigue limit ratio is less than 0.40. On the other hand, when the amount of V is large as in the test steel D34, the fatigue limit ratio is still less than 0.40.

It is a figure which shows the relationship between Al content of machine structural steel, and a drill hole length. It is a figure which shows the relationship between the Gf grain size number and fatigue limit ratio of machine structural steel. It is a figure which shows the relationship between O content of machine structural steel, and Gf particle size number. It is a figure which shows the relationship between the heating temperature in the manufacturing process of steel for machine structures, and a Gf particle size number. It is a figure which shows the relationship between the final processing temperature and the fatigue limit ratio in the manufacture process of machine structural steel. It is a figure which shows the relationship between the cooling rate in the manufacture process of steel for machine structures, and a Gf particle size number.

Claims (4)

C: 0.15-0.55% (meaning mass%, the same applies to the chemical components of steel),
Si: 0.01 to 1.5%,
Mn: 0.5-2%
P: 0.01-0.2%
S: 0.01-0.2%
Al: 0.015% or less (excluding 0%),
Cr: 0.1 to 1.00 %,
N: 0.002 to 0.02%,
O: 0.001 to 0.005%,
V: 0.03-0. 3 %,
A non-tempered steel consisting of iron and unavoidable impurities,
The total structural fraction of ferrite and pearlite is 95% or more in area ratio,
A machine structural steel excellent in fatigue limit ratio and machinability, characterized in that the Gf grain size number of the prior austenite grains defined in JIS G0551 is 5 or more. further,
Cu: 0.02 to 0.6%,
Ni: 0.02 to 0.6%,
Mo: 0.01 to 0.5%,
The steel for machine structure of Claim 1 containing at least 1 sort (s) chosen from the group which consists of. further,
Ti: 0.01-0.2%
Zr: 0.01 to 0.2%,
Nb: 0.01 to 0.1%,
The steel for machine structure of Claim 1 or 2 containing at least 1 sort (s) chosen from the group which consists of. further,
Pb: 0.01 to 0.35%,
Bi: 0.01-0.2%
Mg: 0.0001 to 0.005%,
Ca: 0.0001 to 0.005%,
Se: 0.01 to 0.5%,
Rare earth element (REM): 0.001 to 0.2%,
The steel for machine structure in any one of Claims 1-3 containing at least 1 sort (s) chosen from the group which consists of.