JP4568362B2 - Machine structural steel with excellent machinability and strength characteristics - Google Patents

Description

  The present invention relates to a machine structural steel to which cutting is performed, and in particular, a wide cutting speed range from cutting in a relatively low speed range with a high-speed drill to cutting in a relatively high speed range such as longitudinal turning with a super steel coating tool. The present invention relates to a machine structural steel excellent in machinability and strength properties applicable to the steel.

In recent years, the strength of steel has been increased, but on the other hand, there is a problem that workability is lowered. For this reason, the need for steel that does not decrease cutting efficiency while maintaining strength is increasing. Conventionally, in order to improve the machinability of steel, it is known that it is effective to add machinability improving elements such as S, Pb and Bi. However, Pb and Bi improve machinability and have a relatively small influence on forging, but are known to reduce strength characteristics.
In addition, recently, there is a tendency to avoid using Pb as an environmental load, and it is in the direction of reducing its usage. Further, S forms inclusions that become soft under a cutting environment such as MnS to improve machinability, but the size of MnS is larger than that of particles such as Pb and is likely to become a stress concentration source. In particular, when the steel is stretched by forging and rolling, anisotropy occurs due to MnS, and the specific direction of the steel becomes extremely weak. Moreover, it is necessary to consider such anisotropy in designing the steel. Therefore, when S is added, a technique for reducing the anisotropy is required.
As described above, even if an element effective for improving the machinability is added, the strength characteristics are lowered, so that it is difficult to achieve both the strength characteristics and the machinability. For this reason, further technological innovation is required to achieve both the machinability and strength characteristics of steel.
Therefore, conventionally, for example, one or more selected from solid solution V, solid solution Nb, and solid solution Al is contained in a total amount of 0.005% by mass or more, and solid solution N is contained in an amount of 0.001% or more during cutting. There has been proposed a steel for machine structural use that allows nitride generated by cutting heat to adhere to a tool to function as a tool protection film and extend the life of the cutting tool (see Japanese Patent Application Laid-Open No. 2004-107787). In addition, the content of C, Si, Mn, S and Mg is specified, the ratio of Mg content to S content is specified, and the aspect ratio and number of sulfide inclusions in steel are optimized. As a result, a steel for machine structural use that has improved chip disposal and mechanical properties has also been proposed (see Japanese Patent No. 3706560). In the steel for machine structure described in Japanese Patent No. 3706560, Mg is set to 0.02% or less (not including 0%), and when Al is contained, its content is restricted to 0.1% or less.

However, the above-described conventional techniques have the following problems. That is, it is estimated that the steel described in Japanese Patent Application Laid-Open No. 2004-107787 does not cause the above-described phenomenon unless the amount of heat generated by cutting is more than a certain level. For this reason, there is a problem that the cutting speed at which the effect is exhibited is limited to high speed cutting to some extent, and the effect in the low speed region cannot be expected. In the steel described in Japanese Patent No. 3706560, no consideration is given to the strength characteristics. Furthermore, the steel described in Japanese Patent No. 3706560 has a problem that sufficient strength characteristics cannot be obtained because no consideration is given to the cutting tool life and the yield ratio.
The present invention has been devised in view of the above-mentioned problems, and provides a machine structural steel having good machinability in a wide cutting speed region and having both high impact characteristics and a high yield ratio. The purpose is to do.

The steel for machine structural use having excellent machinability and strength characteristics according to the present invention is mass%, C: 0.1 to 0.85%, Si: 0.01 to 1.5%, Mn: 0.05 to 2.0%, P: 0.005 to 0.2%. , S: 0.001 to 0.15%, total Al: more than 0.05% and not more than 0.3%, Sb: less than 0.0150% (including 0%) and total N: 0.0035 to 0.020%, and solid solution N: 0.0020% or less It is limited, and the balance is made of Fe and inevitable impurities.
This steel for machine structure may further contain Ca: 0.0003 to 0.0015% by mass.
Moreover, it contains one or more elements selected from the group consisting of Ti: 0.001 to 0.1%, Nb: 0.005 to 0.2%, W: 0.01 to 1.0%, and V: 0.01 to 1.0% by mass%. You may do it.
Furthermore, it may contain Mg: 0.0001 to 0.0040% by mass%.
Furthermore, one or more elements selected from the group consisting of Sn: 0.005 to 2.0%, Zn: 0.0005 to 0.5%, B: 0.0005 to 0.015%, and Te: 0.0003 to 0.2% in mass%. You may contain.
Furthermore, Cr: 0.01-2.0% may be contained by the mass%.
Furthermore, it may contain one or two elements selected from the group consisting of Ni: 0.05 to 2.0% and Cu: 0.01 to 2.0% by mass%.

  ADVANTAGE OF THE INVENTION According to this invention, the steel for machine structures which has favorable machinability in a wide cutting speed area | region, and has a high impact characteristic and a high yield ratio can be provided.

Hereinafter, the best mode for carrying out the present invention will be described in detail. In order to solve the above-described problems, in the steel for machine structure excellent in machinability and strength characteristics according to the present invention (hereinafter also simply referred to as machine structure steel), as a component composition of the steel, Al and other nitrides Adjusting the amount of addition of product-generating elements and N, and applying appropriate heat treatment to keep solid solution N harmful to machinability and impact characteristics low, and improve machinability by high temperature embrittlement A wide range of cutting speeds from low to high speeds by securing an appropriate amount of solid solution Al and Sb having matrix embrittlement effect and securing an appropriate amount of AlN which improves machinability by high temperature embrittlement effect and crystal structure of cracking. With effective cutting performance, and by increasing the amount of Al added, the segregation at the slab stage is smaller than that of conventional Al-killed steel, and MnS with high uniform dispersibility (according to SIMS classification) (III type MnS) increases, high impact Is intended to machine structural steel for both sex, even by fine precipitation and solid solution of Al AlN, it is to obtain a high yield ratio.
That is, the steel for machine structural use according to the present invention is in mass%, C: 0.1 to 0.85%, Si: 0.01 to 1.5%, Mn: 0.05 to 2.0%, P: 0.005 to 0.2%, S: 0.001 to 0.15%, Total Al: more than 0.05% and less than 0.3%, Sb: less than 0.0150% (including 0%) and total N: 0.0035% to 0.020%, solid solution N: limited to 0.0020% or less, the balance being Fe and inevitable It has a composition consisting of mechanical impurities.

  First, each component element and its content in the steel for machine structure of the present invention will be described. In the following description, mass% in the composition is simply described as%.

C: 0.1-0.85%
C is an element that greatly affects the basic strength of steel. However, if the C content is less than 0.1%, sufficient strength cannot be obtained, and a larger amount of other alloy elements must be added. On the other hand, if the C content exceeds 0.85%, it becomes close to hypereutectoid and a large amount of hard carbide precipitates, so that machinability is remarkably lowered. Therefore, in the present invention, in order to obtain sufficient strength, the C content is set to 0.1 to 0.85%. A preferred lower limit is 0.2%.

Si: 0.01-1.5%
Although Si is generally added as a deoxidizing element, it also has the effect of imparting ferrite strengthening and temper softening resistance. However, when the Si content is less than 0.01%, a sufficient deoxidizing effect cannot be obtained. On the other hand, when the Si content exceeds 1.5%, material properties such as embrittlement are deteriorated, and machinability is also deteriorated. Therefore, the Si content is set to 0.01 to 1.5%. A preferred upper limit is 1.0%.

Mn: 0.05-2.0%
Mn is an element necessary to fix and disperse sulfur (S) in steel as MnS and to dissolve in a matrix to improve hardenability and ensure strength after quenching. However, if the Mn content is less than 0.05%, S in the steel combines with Fe to become FeS, and the steel becomes brittle. On the other hand, when the Mn content increases, specifically, when the Mn content exceeds 2.0%, the hardness of the substrate increases and cold workability deteriorates, and the effects on strength and hardenability are saturated. . Therefore, the Mn content is set to 0.05 to 2.0%.

P: 0.005-0.2%
P has an effect of improving machinability, but when the P content is less than 0.005%, the effect cannot be obtained. In addition, when the P content increases, specifically, when the P content exceeds 0.2%, the hardness of the substrate increases in the steel, and not only cold workability but also hot workability and casting characteristics. Also decreases. Therefore, the P content is set to 0.005 to 0.2%.

S: 0.001 to 0.15%
S exists as an MnS inclusion as a bond with Mn. MnS has an effect of improving machinability, but in order to obtain the effect remarkably, it is necessary to add S 0.001% or more. On the other hand, when the S content exceeds 0.15%, the impact value of the steel is significantly reduced. Therefore, when improving machinability by adding S, the S content is set to 0.001 to 0.15%.

Total Al: more than 0.05% and less than 0.3%
In addition to forming an oxide, Al has the effect of precipitating AlN effective for grain size adjustment and machinability, and further becoming solid solution Al to improve machinability. In order to sufficiently generate solid solution Al effective for the machinability, it is necessary to add an amount exceeding 0.05%. Al also affects the MnS crystal / precipitation form. Addition of more than 0.05% Al reduces segregation at the slab stage compared to conventional Al-killed steel, and increases MnS (III-type MnS by SIMS classification) with high uniform dispersibility. Therefore, a steel for mechanical structure having high impact characteristics can be obtained, and furthermore, a high yield ratio can be obtained by fine precipitation of AlN and solute Al. However, when the total Al content exceeds 0.3%, the machinability starts to decrease. Therefore, the total Al content exceeds 0.05% and is 0.3% or less. A preferred lower limit is 0.08%, and a more preferred lower limit is more than 0.1%.

Sb: Less than 0.0150% (including 0%)
Sb has the effect of moderately embrittlement of ferrite and improving machinability. This effect is particularly remarkable when the amount of solid solution Al is large, but is not observed when the Sb content is less than 0.0005%. On the other hand, when the Sb content increases, specifically, when the Sb content is 0.0150% or more, macrosegregation of Sb becomes excessive, and the impact value greatly decreases. Therefore, the Sb content is 0.0005% or more and less than 0.0150%. When high machinability is not required or when the total Al exceeds 0.1%, it can be added without addition (0%).

Total N: 0.0035 to 0.020%
N is present as a nitride such as Ti, Al or V in addition to solute N, and suppresses the growth of austenite grains. However, if the total N amount is less than 0.0035%, a remarkable effect cannot be obtained. On the other hand, if the total N amount exceeds 0.020%, it will cause rolling defects in the rolling process. Therefore, the total N amount is 0.0035 to 0.020%.

Solid solution N: 0.0020% or less Solid solution N hardens steel. In particular, in cutting, it hardens in the vicinity of the cutting edge due to dynamic strain aging, thereby shortening the tool life, and in rolling, it becomes a cause of rolling wrinkles. When the amount of solute N is large, specifically, when the amount of solute N exceeds 0.0020%, tool wear is promoted due to an increase in cutting resistance accompanying an increase in local hardness during cutting. Therefore, the solid solution N amount is suppressed to 0.0020% or less. Thereby, tool friction can be improved. Further, if the amount of solute N is large, matrix embrittlement is caused and impact properties are deteriorated. However, if the amount of solute N is suppressed to 0.0020% or less, this matrix embrittlement can also be improved. The solid solution N amount here is a value obtained by subtracting the N amount contained in nitrides such as AlN, NbN, TiN and VN from the total N amount. For example, the total N amount is determined by the inert gas melting-thermal conductivity method. The amount of N in the nitride was measured by the SPEED method of the potentiostatic electrolytic corrosion method using a nonaqueous solvent electrolytic solution and the electrolytic extraction of the 0.1 μm filter by the indophenol absorbance method, and the following formula (1) Can be calculated.
(Solubility N amount) = (Total N amount) − (N amount in nitride) (1)
In addition, the amount of solid solution N can be restrained low by the method shown below.
In order to reduce the solute N by fine precipitation of nitrides, fine precipitation is preferable from the viewpoint of suppressing grain coarsening, considering that it is used as steel for machine structures. In order to reduce the amount of solute N due to fine precipitation of nitrides, it is essential to maintain a high temperature at which the solution is completely formed by N and the amount of nitride-forming elements, but considering this, 1100 ° C or higher, preferably 1200 ° C More preferably, after heat treatment for solution treatment at 1250 ° C. or higher, precipitation is performed by heat treatment such as normalization or carburization. In particular, in the case of AlN, it is possible to increase the amount of precipitation and reduce the solid solution N by holding at around 850 ° C. for a long time. The long time here means 0.8 hours or more, preferably 1 hour or more, more preferably 1.2 hours or more.
Moreover, in the steel for machine structure of this invention, in addition to said each component, you may contain Ca.

Ca: 0.0003 to 0.0015%
Ca is a deoxidizing element and generates an oxide in steel. Calcium aluminate (CaOAl ₂ O ₃ ) is formed in the mechanical structural steel of the present invention having a total Al content of more than 0.05% and not more than 0.3%. This CaOAl ₂ O ₃ is compared to Al ₂ O ₃ Since it is a low melting point oxide, it becomes a tool protective film during high-speed cutting, and has the effect of improving machinability. However, when the Ca content is less than 0.0003%, this machinability improvement effect cannot be obtained, and when the Ca content exceeds 0.0015%, CaS is generated in the steel, and the machinability is reduced. . Therefore, when adding Ca, the content is made 0.0003 to 0.0015%.

  Furthermore, in the steel for machine structure of the present invention, when carbonitride is formed and high strength is required, in addition to the above components, Ti: 0.001 to 0.1%, Nb: 0.005 to 0.2%, You may contain 1 type, or 2 or more types of elements selected from the group which consists of W: 0.01-1.0% and V: 0.01-1.0%.

Ti: 0.001 to 0.1%
Ti is an element that forms carbonitrides and contributes to the suppression and strengthening of austenite grain growth. For steels that require high strength and steels that require low strain, Used as a sizing element. Ti is also a deoxidizing element and has the effect of improving machinability by forming a soft oxide. However, when the Ti content is less than 0.001%, the effect is not recognized, and when the Ti content exceeds 0.1%, undissolved coarse carbonitrides that cause hot cracking are precipitated. On the other hand, the mechanical properties are impaired. Therefore, when adding Ti, the content is made 0.001 to 0.1%.

Nb: 0.005-0.2%
Nb is also an element that forms carbonitrides and contributes to the strengthening of steel by secondary precipitation hardening and the suppression and strengthening of austenite grain growth.For steels that require high strength and steels that require low strain, It is used as a sizing element for preventing coarse grains. However, when the Nb content is less than 0.005%, the effect of increasing the strength cannot be obtained, and when Nb is added over 0.2%, undissolved coarse carbonitride that causes time cracking is not obtained. It precipitates and on the other hand mechanical properties are impaired. Therefore, when adding Nb, the content is made 0.005 to 0.2%.

W: 0.01-1.0%
W is also an element that forms carbonitride and can strengthen steel by secondary precipitation hardening. However, when the W content is less than 0.01%, the effect of increasing the strength cannot be obtained, and when W is added in excess of 1.0%, undissolved coarse carbonitride that causes hot cracking. On the contrary, the mechanical properties are impaired. Therefore, when adding W, the content is made 0.01 to 1.0%.

V: 0.01 to 1.0%
V is also an element that forms carbonitride and can strengthen the steel by secondary precipitation hardening, and is appropriately added to steel that requires high strength. However, when the V content is less than 0.01%, the effect of increasing the strength cannot be obtained, and when V is added exceeding 1.0%, undissolved coarse carbonitride that causes hot cracking. On the contrary, the mechanical properties are impaired. Therefore, when adding V, the content is made 0.05 to 1.0%.
Furthermore, in the steel for machine structure of the present invention, when the sulfide form control is performed by adjusting the deoxidation, in addition to the above components, Mg: 0.0001 to 0.0040%, Zr: 0.0003 to 0.01%, and Rem: 0.0001 One or more elements selected from the group consisting of ˜0.015% can also be added.

Mg: 0.0001-0.0040%
Mg is a deoxidizing element and generates an oxide in steel. In the case of Al deoxidation, Al ₂ O ₃ harmful to machinability is modified into MgO or Al ₂ O ₃ .MgO that is relatively soft and finely dispersed. Further, the oxide tends to be a nucleus of MnS, and has an effect of finely dispersing MnS. However, when the Mg content is less than 0.0001%, these effects are not recognized. In addition, Mg forms a composite sulfide with MnS and spheroidizes MnS, but when Mg is added excessively, specifically, when the Mg content exceeds 0.0040%, single MgS generation is generated. Promotes and degrades machinability. Therefore, when adding Mg, the content shall be 0.0001 to 0.0040%.

Sn: 0.005-2.0%
Sn has the effect of embrittlement of ferrite to extend the tool life and improve the surface roughness. However, when the Sn content is less than 0.005%, the effect is not recognized, and even if Sn is added in excess of 2.0%, the effect is saturated. Therefore, when adding Sn, let the content be 0.005-2.0%.

Zn: 0.0005-0.5%
Zn has the effect of embrittlement of ferrite to extend the tool life and improve the surface roughness. However, when the Zn content is less than 0.0005%, the effect is not recognized, and even if Zn is added in excess of 0.5%, the effect is saturated. Therefore, when adding Zn, the content is made 0.0005 to 0.5%.

B: 0.0005 to 0.015%
B is effective in grain boundary strengthening and hardenability when dissolved, and is effective as machinability because it precipitates as BN when precipitated. These effects are not significant when the B content is less than 0.0005%. On the other hand, even if B is added in excess of 0.015%, the effect is saturated and too much BN is precipitated, so that the mechanical properties of the steel are impaired. Therefore, when adding B, the content shall be 0.0005 to 0.015%.

Te: 0.0003-0.2%
Te is a machinability improving element. In addition, by producing MnTe or coexisting with MnS, it has a function of reducing the deformability of MnS and suppressing the extension of the MnS shape. Thus, Te is an effective element for reducing anisotropy. However, when the Te content is less than 0.0003%, these effects are not observed, and when the Te content exceeds 0.2%, not only the effect is saturated, but the hot ductility is reduced and Prone to cause. Therefore, when adding Te, the content is made 0.0003 to 0.2%.

Cr: 0.01-2.0%
Cr is an element that improves hardenability and imparts temper softening resistance, and is added to steel that requires high strength. However, when the Cr content is less than 0.01%, these effects cannot be obtained, and when a large amount of Cr is added, specifically, when the Cr content exceeds 2.0%, Cr carbide is generated. Steel becomes brittle. Therefore, when adding Cr, the content is made 0.01 to 2.0%.

Ni: 0.05-2.0%
Ni is an element effective for strengthening ferrite and improving ductility, as well as improving hardenability and corrosion resistance. However, when the Ni content is less than 0.05%, the effect is not recognized, and even if Ni is added over 2.0%, the effect is saturated in terms of mechanical properties and the machinability is lowered. . Therefore, when adding Ni, the content is made 0.05 to 2.0%.

Cu: 0.01-2.0%
Cu is an element effective for strengthening ferrite and improving hardenability and corrosion resistance. However, when the Cu content is less than 0.01%, the effect is not recognized, and even if Cu is added over 2.0%, the effect is saturated in terms of mechanical properties. Therefore, when adding Cu, the content is made 0.01 to 2.0%. Cu is particularly preferable to be added simultaneously with Ni because it lowers the hot ductility and tends to cause defects during rolling.

  As described above, in the machine structural steel of the present invention, since the amount of solute N is reduced, the machinability and impact characteristics can be improved as compared with the conventional machine structural steel. In addition, by optimizing the total Al content and Sb content, the proper amount of solid solution Al, Sb, and AlN that have the effect of improving machinability is secured, so it can be used for a wide range of cutting speeds from low speed to high speed. Effective cutting performance. Furthermore, a high yield ratio is obtained by the fine precipitation of AlN and solute Al. Furthermore, since the content of elements that affect the precipitation of MnS is optimized and the amount of MnS having high uniform dispersibility is increased, the impact characteristics are also excellent.

Next, the effects of the present invention will be specifically described with reference to examples and comparative examples. In this example, 150 kg of steel having the composition shown in Tables 1 and 2 was melted in a vacuum melting furnace and then hot forged under a temperature condition of 1250 ° C. to forge into a cylindrical shape having a diameter of 65 mm. And about each steel material of this Example and the comparative example, the machinability test, the Charpy impact test, and the tension test were done by the method shown below, and the characteristic was evaluated. In addition, the underline in Table 2 shows that it is outside the scope of the present invention.

Machinability test In the machinability test, first, each steel material of Examples and Comparative Examples heated to 1250 ° C. and forged in warm condition was subjected to a comparative example No. 1 at 850 ° C. for 1 hour. For No. 49 and No. 50, after normalizing for 0.5 hour, heat treatment was applied to air cooling. Thereafter, a test piece for machinability evaluation was cut out from each steel material after the heat treatment, and a drill drilling test was conducted under the cutting conditions shown in Table 3 below, and a longitudinal turning test was conducted under the conditions shown in Table 4 below. The machinability of each steel material of the example was evaluated. At that time, as an evaluation index, in the drill drilling test, the maximum cutting speed VL1000 capable of cutting to a cumulative hole depth of 1000 mm is used, and the flank maximum wear width VB after 10 minutes in the longitudinal turning test. Max was adopted respectively.

Charpy Impact Test FIG. 1 is a diagram showing a cut-out site of a Charpy impact test specimen. In the Charpy impact test, first, as shown in FIG. 1, the center axis is perpendicular to the forging direction of the steel material 1 from each steel material 1 heat-treated by the same method and conditions as the above-described machinability test. Then, a cylindrical member 2 having a diameter of 25 mm was cut out. Next, each columnar material 2 was held for 1 hour under the temperature condition of 850 ° C., and Comparative Examples No. 49 and No. 50 were held for 0.5 hour, followed by oil quenching to cool to 60 ° C., Tempering was performed by maintaining the temperature at 550 ° C. for 30 minutes and then water cooling. Thereafter, each cylindrical member 2 was machined to produce a Charpy test piece 3 defined in JIS Z 2202, and a Charpy impact test at room temperature was performed by the method defined in JIS Z 2242. At that time, absorbed energy per unit area (J / cm ² ) was adopted as an evaluation index.

表１、表２及び表５に示すNo.１〜No.42の鋼材は本発明の実施例であり、表２及び表６に示すNo.43〜No.51の鋼材は本発明の比較例である。表５及び表６に示すように、実施例No.１〜No.42の鋼材では、評価指標であるVL1000、VB max、Impact Value（吸収エネルギー）及びYP／TS（降伏比）の全てにおいて良好な値を示していたが、比較例の鋼材では、これらのうちの少なくとも１つ以上の特性が、実施例の鋼材に比べて劣っていた。具体的には、比較例No.43〜No.46の鋼材は全Al含有量が本発明の範囲を下回っているため、被削性の評価指標であるVL1000及び降伏比（YP／TS）が実施例の鋼材よりも劣っている。また、比較例No.47の鋼材は、全Al含有量が本発明の範囲を極端に下回っているため、固溶Ｎ量が本発明の範囲を上回り、実施例の鋼材よりも被削性（VL1000、VB max）、衝撃値（Impact Value）及び降伏比（YS／TS）が劣っていた。
比較例No.48の鋼材は、全Al含有量が本発明の範囲を上回っているため、硬さが増加し、被削性（VL1000、VB max）が劣っていた。比較例No.49、No.50の鋼材は、実施例の鋼材に比べてAlNが析出しやすい850℃での温度保持時間が短いため、固溶Ｎ量が本発明の範囲を上回り、実施例の鋼材よりも被削性（VL1000、VB max）及び衝撃値（Impact Value）が劣っていた。比較例No.51〜No.54の鋼材は、Sb含有量が本発明の範囲を上回っているため、実施例の鋼材よりも衝撃値（Impact Value）が劣っていた。 Tensile test After cylindrical quenching and tempering were performed in the same manner and under the same conditions as the Charpy impact test described above, the diameter of the parallel part was 8 mm and the length of the parallel part was measured. A 30 mm tensile test piece was processed, and a tensile test was performed at room temperature based on the method specified in JIS Z 2241. At that time, the yield ratio (= (0.2% yield strength YP) / (tensile strength TS)) was adopted as an evaluation index.
The results of the above tests are shown in Tables 5 and 6.

The steel materials No. 1 to No. 42 shown in Tables 1, 2 and 5 are examples of the present invention, and the steel materials No. 43 to No. 51 shown in Tables 2 and 6 are comparative examples of the present invention. It is. As shown in Table 5 and Table 6, in the steel materials of Examples No. 1 to No. 42, evaluation indexes VL1000 and VB The values of max, impact value (absorbed energy) and YP / TS (yield ratio) all showed good values. However, in the steel material of the comparative example, at least one of these characteristics is the steel material of the example. It was inferior to. Specifically, since the total Al content of the steel materials of Comparative Examples No. 43 to No. 46 is below the range of the present invention, the VL1000 and the yield ratio (YP / TS), which are evaluation indexes of machinability, It is inferior to the steel material of an Example. Further, the steel material of Comparative Example No. 47 has a total Al content that is extremely lower than the range of the present invention, so the amount of solute N exceeds the range of the present invention, and the machinability ( VL1000, VB max), impact value (Ympact Value) and yield ratio (YS / TS) were inferior.
In the steel material of Comparative Example No. 48, the total Al content exceeds the range of the present invention, so the hardness increases and machinability (VL1000, VB max) was inferior. The steel materials of Comparative Examples No. 49 and No. 50 have a shorter temperature holding time at 850 ° C. where AlN is more likely to precipitate than the steel materials of the examples, so the amount of solute N exceeds the range of the present invention. Machinability (VL1000, VB) max) and Impact Value were inferior. The steel materials of Comparative Examples No. 51 to No. 54 were inferior in impact value (Impact Value) to the steel materials of the examples because the Sb content exceeded the range of the present invention.

In this example, 150 kg of steel having the composition shown in Table 7 and Table 8 was melted in a vacuum melting furnace, and then hot forged under a temperature condition of 1250 ° C. to forge into a cylindrical shape having a diameter of 65 mm. And about the steel material of this Example and the comparative example, the machinability test, the Charpy impact test, and the tension test were done by the method shown below, and the characteristic was evaluated. In addition, the underline in Table 7 and Table 8 shows that it is outside the scope of the present invention.

Machinability test In the machinability test, first, each steel material of Examples and Comparative Examples heated to 1250 ° C. and forged in warm condition was subjected to a comparative example No. 1 at 850 ° C. for 1 hour. No. 48, No. 49, No. 97 to No. 101 were subjected to heat treatment for air cooling after normalizing for 0.5 hour. Thereafter, a test piece for machinability evaluation was cut out from each steel material after the heat treatment, and a drill drilling test was conducted under the cutting conditions shown in Table 9, and a longitudinal turning test was conducted under the conditions shown in Table 10, and Examples and Comparative Examples The machinability of each steel material was evaluated. At that time, as an evaluation index, in the drill drilling test, the maximum cutting speed VL1000 capable of cutting to a cumulative hole depth of 1000 mm is used, and the flank maximum wear width VB after 10 minutes in the longitudinal turning test. Each adopted max.

Charpy Impact Test FIG. 1 is a diagram showing a cut-out site of a Charpy impact test specimen. In the Charpy impact test, first, as shown in FIG. 1, the center axis is perpendicular to the forging direction of the steel material 1 from each steel material 1 heat-treated by the same method and conditions as the above-described machinability test. Then, a cylindrical member 2 having a diameter of 25 mm was cut out. Next, each cylindrical member 2 was held for 1 hour under the temperature condition of 850 ° C., and Comparative Examples No. 48, No. 49, No. 97 to No. 101 were held for 0.5 hour, and then cooled to 60 ° C. Further, tempering was performed, followed by water-cooling after holding for 30 minutes at a temperature of 550 ° C. Thereafter, each cylindrical member 2 was machined to produce a Charpy test piece 3 defined in JIS Z 2202, and a Charpy impact test at room temperature was performed by the method defined in JIS Z 2242. At that time, absorbed energy per unit area (J / cm ² ) was adopted as an evaluation index.

なお、表７及び表11に示すNo.１の鋼材は請求項１の実施例であり、No.２〜No.42の鋼材は請求項２の実施例である。また表８及び表12に示すNo.52〜No.93は請求項１の実施例である。更に、比較例No.43〜No.49の鋼材は、Ｓ含有量及びCa含有量については請求項２の規定を満足しており、比較例No.94〜No.101の鋼材は、Ｓ含有量及びCa含有量については請求項１の規定を満足しているものである。
表11及び表12に示すように、実施例No.１〜No.42及びNo.52〜No.93の鋼材では、評価指標であるVL1000、VB max、Impact Value（吸収エネルギー）及びYP／TS（降伏比）の全てにおいて良好な値を示していたが、比較例の鋼材では、これらのうちの少なくとも１つ以上の特性が、実施例の鋼材に比べて劣っていた。具体的には、比較例No.43〜No.46の鋼材は全Al含有量が本発明の範囲を下回っているため、被削性（VL1000）及び降伏比（YP／TS）が実施例の鋼材よりも劣っていた。また、比較例No.47の鋼材は、全Al含有量が本発明の範囲を極端に下回っているため、固溶Ｎ量が本発明の範囲を上回り、実施例の鋼材よりも被削性（VL1000、VB max）、衝撃値（Impact Value）及び降伏比（YS／TS）が劣っていた。 Tensile test Each cylindrical material 2 that has been oil-quenched and tempered under the same method and conditions as the Charpy impact test described above is processed into a tensile test piece with a parallel part diameter of 8 mm and a length of 30 mm. JIS Z 2241 Based on the method prescribed in the above, a tensile test at room temperature was performed. At that time, the yield ratio (= (0.2% yield strength YP) / (tensile strength TS)) was adopted as an evaluation index.
The results of the above tests are summarized in Tables 11 and 12.

In addition, the No. 1 steel materials shown in Table 7 and Table 11 are Examples of Claim 1, and the No. 2 to No. 42 steel materials are Examples of Claim 2. No. 52 to No. 93 shown in Tables 8 and 12 are examples of claim 1. Furthermore, the steel materials of Comparative Examples No. 43 to No. 49 satisfy the provisions of claim 2 regarding the S content and the Ca content, and the steel materials of Comparative Examples No. 94 to No. 101 contain S. Regarding the amount and Ca content, the provisions of claim 1 are satisfied.
As shown in Table 11 and Table 12, in the steel materials of Examples No. 1 to No. 42 and No. 52 to No. 93, VL1000 and VB which are evaluation indexes The values of max, impact value (absorbed energy) and YP / TS (yield ratio) all showed good values. However, in the steel material of the comparative example, at least one of these characteristics is the steel material of the example. It was inferior to. Specifically, since the steel materials of Comparative Examples No. 43 to No. 46 have a total Al content lower than the range of the present invention, the machinability (VL1000) and the yield ratio (YP / TS) are those of the examples. It was inferior to steel. Further, the steel material of Comparative Example No. 47 has a total Al content that is extremely lower than the range of the present invention, so the amount of solute N exceeds the range of the present invention, and the machinability ( VL1000, VB max), impact value (Ympact Value) and yield ratio (YS / TS) were inferior.

The steel materials of Comparative Examples No. 48 and No. 49 have a shorter temperature holding time at 850 ° C. where AlN is more likely to precipitate than the steel materials of the examples, so the amount of solute N exceeds the range of the present invention. Machinability (VL1000, VB) max) and Impact Value were inferior. Further, the steel materials of Comparative Examples No. 94 to No. 96 have machinability (VL1000, VB) because the total Al content is below the range of the present invention. max) and yield ratio (YP / TS) were inferior to the steel materials of the examples. Furthermore, the steel materials of Comparative Examples No. 97 to No. 101 have a shorter temperature holding time at 850 ° C. at which AlN is more likely to precipitate than the steel materials of the Examples, so the amount of solute N exceeds the range of the present invention. Machinability (VL1000, VB) max) and Impact Value were inferior.

  ADVANTAGE OF THE INVENTION According to this invention, the steel for machine structures which has favorable machinability in a wide cutting speed area | region, and has a high impact characteristic and a high yield ratio can be provided.

FIG. 1 is a diagram showing a cutout portion of a test piece for a Charpy impact test.

Claims (2)

% By mass
C: 0.1 to 0.85%
Si: 0.01 to 1.5%
Mn: 0.05-2.0%,
P: 0.005-0.2%
S: 0.001 to 0.15%,
Total Al: more than 0.05% and 0.3% or less,
Sb: less than 0.0150% (including 0%) and total N: 0.0035 to 0.020%,
Solid solution N: limited to 0.0020% or less,
A machine structural steel excellent in machinability and strength characteristics, characterized in that the balance consists of Fe and inevitable impurities.   Furthermore, in mass%, Ca: 0.0003 to 0.0015%, Ti: 0.001 to 0.1%, Nb: 0.005 to 0.2%, W: 0.01 to 1.0%, V: 0.01 to 1.0%, Mg: 0.0001 to 0.0040%, Sn: One of 0.005-2.0%, Zn: 0.0005-0.5%, B: 0.0005-0.015%, Te: 0.0003-0.2%, Cr: 0.01-2.0%, Ni: 0.05-2.0% and Cu: 0.01-2.0% The steel for machine structure excellent in machinability and strength characteristics according to claim 1, comprising two or more kinds.

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