WO2008084749A1 - Steel for machine structure excelling in machinability and strength property - Google Patents

Abstract

A steel for machine structure that in a wide range of cutting speed, exhibits appropriate machinability, and that simultaneously has high impact performance and high yield ratio. The steel for machine structure is one excelling in machinability and strength property, comprising, by mass, 0.1 to 0.85% C, 0.01 to 1.5% Si, 0.05 to 2.0% Mn, 0.005 to 0.2% P, 0.001 to 0.15% S, over 0.05 to 0.3% total Al, less than 0.0150% Sb (including 0%), 0.0035 to 0.020% total N wherein solid solution N is limited to 0.0020% or less, and the balance Fe and unavoidable impurities.

Description

Machine structural steel with excellent machinability and strength characteristics

Technical field

 The present invention relates to a machine structural steel to which cutting is performed, and in particular, from a cutting process at a relatively low speed range by a high-speed drill to a super steel coating.

Wide range of cutting speeds up to cutting at relatively high speeds such as longitudinal turning with tools

A book on machine structural steels with excellent machinability and strength characteristics applicable to the region

Background art

 In recent years, the strength of steel has been increasing, but on the other hand, there has been a problem that workability is reduced. For this reason, there is a growing need for steels that retain strength but do not reduce cutting efficiency. Conventionally, it is known that it is effective to add machinability improving elements such as S, Pb and Bi to improve the machinability of steel. However, Pb and B i are known to improve machinability and have relatively little effect on forging, but to reduce strength characteristics.

Recently, there is a tendency to avoid the use of Pb as an environmental load, and it is in the direction of reducing its use. In addition, S improves the machinability by forming inclusions that become soft under the cutting environment like MnS, but the size of MnS is larger than that of particles such as Pb and tends to be a source of stress concentration. In particular, when extending by forging and rolling, anisotropy occurs due to MnS, and the specific direction of steel becomes extremely weak. It is also necessary to consider such anisotropy when designing steel. Therefore, when S is added, a technique for reducing its 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 kinds selected from solute V, solute Nb, and solute A 1 are contained in a total of 0.005% by mass or more, and solute N is contained in an amount of 0.001% or more. Therefore, steel for machine structural use has been proposed in which a nitride generated by cutting heat during cutting adheres to the tool and functions as a tool protection film, thereby extending the life of the cutting tool (JP 2004-107787 A). No. publication). In addition, the contents of C, Si, Mn, S and Mg are defined, the ratio of Mg content to S content is defined, and the aspect ratio and number of sulfide inclusions in steel There has also been proposed a steel for machine structural use that optimizes chip disposal and mechanical properties (see Japanese Patent No. 3706560). In the steel for machine structural use described in this Patent No. 3706560, Mg is 0.02% or less (excluding 0%), and when A1 is contained, the content is made 0.1% or less. Regulated. Disclosure of the invention

However, the conventional techniques described above have the following problems. That is, the steel described in Japanese Patent Application Laid-Open No. 2004-107787 is presumed that the above-mentioned phenomenon does not occur unless the amount of heat generated by cutting exceeds a certain level. For this reason, there is a problem that the cutting speed at which the effect is exerted 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. In addition, the steel described in Japanese Patent No. 3706560 has no consideration for cutting tool life and yield ratio. There is a problem that a sufficient strength characteristic cannot be obtained.

 The present invention has been devised in view of the above-mentioned problems, and is 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 steel for machine structural use having excellent machinability and strength characteristics according to the present invention is provided by mass%, C: 0.1 to 0.85%, Si: 0.01 to 1.5%, Mn: 0.05 to 2.0%. , P: 0.005-0.2%, S: 0.001-0.15%, Total A1: Over 0.05% and 0.3% or less, Sb: Less than 0.0150% (including 0%), and Total N: Including 0.0035-0.020% At the same time, the solid solution N is limited to 0.0020% or less, and the balance is composed of Fe and inevitable impurities.

 The steel for machine structure may further contain Ca: 0.0003 to 0.0015% by mass.

 In addition, 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% in mass% May be contained.

 Furthermore, it may contain one or more elements selected from the group consisting of Mg: 0.0001 to 0.0040%, Zr: 0.0003 to 0.01%, and Rem: 0.0001 to 0.015% by mass%. .

 Furthermore, by mass%, Sn: 0.005 to 2.0%, Zn: 0.0005 to 0.5%, B: 0.0005 to 0.015%, Te: 0.0003 to 0.2%, Bi: 0.005 to 0.5% and Pb: 0.005 to 0.5% One or two or more elements selected from the group may be contained.

Furthermore, it may contain one or two elements selected from the group consisting of Cr: 0.01 to 2.0% and Mo: 0.01 to 1.0% by 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%. Brief Description of Drawings

 Fig. 1 is a diagram showing a cut-out portion of a specimen for a Charpy impact test. BEST MODE FOR CARRYING OUT THE INVENTION

 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 steel for machine structure), the component composition of the steel is A1 and other By adjusting the addition amount of nitride-forming elements and N, and by applying appropriate heat treatment, solid solution N, which is harmful to machinability and impact properties, is kept low, and machinability is improved by high temperature embrittlement. Optimized solid solution Al and Matrix embrittlement effect Sb, and high temperature embrittlement effect and cracked crystal structure to improve machinability By securing an appropriate amount of A1N, from low speed to high speed With effective cutting performance over a wide range of cutting speeds, and by increasing the amount of A1 added, MnS (highly uniform dispersibility with less segregation at the flake stage than conventional A1-killed steel) (Type III MnS by SIMS classification) And many are those that the machine structural steel having both high impact properties, even by fine precipitation and solid solution A1 of A1N, is to obtain a high yield ratio.

 That is, the steel for machine structure of the present invention is in mass%, C: 0.1 to 0.85%, S i: 0.01 to 1.5%, Mn: 0.05 to 2.0%, P: 0.005 to 0.2%, S: 0.001 to 0. 15%, total A1: 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 less than 0.000020%, The balance is composed of Fe and inevitable 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 will be close to prayer and a large amount of hard carbide will be precipitated, so the machinability will be significantly reduced. Therefore, in the present invention, in order to obtain sufficient strength, the C content is set to 0.1 to 0.85%. The preferred lower limit is 0.2%.

Si: 0.01 to 5%

 Although Si is generally added as a deoxidizing element, it also has the effect of strengthening Ferai and imparting temper softening resistance. However, when the Si content is less than 0.01%, a sufficient deoxidation effect cannot be obtained. On the other hand, if the Si content exceeds 1.5%, material properties such as embrittlement deteriorate, and machinability also deteriorates. Therefore, the Si content is set to 0.01 to 1.5%. The 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 it in the 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 decreases, and the effects on strength and hardenability are saturated. To do. Therefore, the Mn content is 0.05-2.0%.

P: 0.005-0.2% P has the effect of improving machinability, but if 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 forging. The characteristics also deteriorate. Therefore, the P content is 0.005 to 0.2%.

S: 0.001 to 0.15%

 S exists as an MnS inclusion as a bond with Mn. MnS has the 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 should be 0.001 to 0.15%.

All A1: more than 0.05% and less than 0.3%

 In addition to forming oxides, A1 has the effect of precipitating A1N, which is effective for grain size adjustment and machinability, and further becoming solid solution A1 to improve machinability. In order to sufficiently produce solid solution A1 effective for this machinability, it is necessary to add an amount exceeding 0.05%. A1 also affects the MnS crystal * precipitation form. Addition of more than 0.05% of A1 reduces the prayer at the flake stage compared to conventional A1-killed steel, and MnS with high uniform dispersibility (type III MnS by SIMS classification) Therefore, a mechanical structural steel having high impact characteristics can be obtained, and furthermore, a high yield ratio can be obtained by fine precipitation and solid solution A1 of A1N. However, when the total A1 content exceeds 0.3%, the machinability starts to deteriorate. Therefore, the total A1 content is more than 0.05% and 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%)

It has the effect of improving machinability by moderately embrittlement of ferrite. This effect is particularly noticeable when the amount of solute A1 is large. u ι -u · Not allowed if 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, the macrosegregation of Sb becomes excessive, and the impact value greatly decreases. Therefore, the Sb content should be 0.0005% or more and less than 0.0150%. If high machinability is not required or if the total A1 exceeds 0.1%, no additive (0% ·) can be added.

Total N: 0.0035 to 0.020%

 In addition to solid solution N, N also exists as nitrides such as Ti, A1, or V, and suppresses the growth of austenite grains. However, if the total N content is less than 0.0035%, no significant effect can be obtained. On the other hand, if the total N content 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 the steel. In particular, in cutting, dynamic strain aging hardens in the vicinity of the cutting edge, reducing the tool life, and in rolling, it causes rolling defects. 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 the increase in local hardness during cutting. Therefore, the amount of solute N is suppressed to 0.0020% or less. Thereby, tool friction can be improved. In addition, 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 controlled to 0.0020% or less, this matrix embrittlement can also be improved. The amount of solute N mentioned here is the value obtained by subtracting the amount of N contained in nitrides such as A1N, NbN, TiN and VN from the total amount of N. For example, by the inert gas melting and thermal conductivity method In addition to measuring the total N content, the NPE in the nitride was measured by the SPEED method, a potentiostatic electrolytic corrosion method using a non-aqueous solvent electrolyte, and the residue obtained by electrolytic extraction using a 0.1 μm filter. It can be calculated by the following formula (1). (Solution N content) = (Total N content) 1 (N content in nitride) ... (1) The solute N content can be kept low by the following method.

 1) Keep the total N amount low within the range specified in the present invention. The total N range is specified to be 0.020% or less, preferably 0.01% or less, and more preferably 0.006% or less.

 2) When the total amount of N is high, it is better to increase the amount of N compound by adding an appropriate amount of nitride generating element Α1 and other nitride generating elements.

 3) In order to reduce the solid solution 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 structural use. In order to reduce the amount of solute N by fine precipitation of nitrides, it is essential to maintain a high temperature at which it forms a complete solution depending on the amount of N and the amount of nitride-forming elements. After heat treatment for solution treatment at 1200 ° C or higher, more preferably 1250 ° C or higher, heat treatment such as normalization or carburization is performed for precipitation. In particular, in the case of A1N, it is possible to increase the amount of precipitation and reduce solute N by holding it 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. In addition, in the steel for machine structural use according to the present invention, in addition to the above components, Ca is added. You may contain.

Ca: 0.0003-0, 0015%

Ca is a deoxidizing element and generates oxides in steel. In the mechanical structural steel of the present invention having a total A1 content of more than 0.05% and less than 0.3%, calcium aluminate (CaOAl ₂ 0 ₃ ) is formed, but this CaOAl ₂ 0 ₃ is compared to A1 ₂ 0 _3. Since it is a low melting point oxide, it becomes a tool protection film during high-speed cutting, and has the effect of improving machinability. However, the Ca content is If it is less than 0.0003%, this machinability improvement effect cannot be obtained, and if the Ca content exceeds 0.0015%, CaS is generated in the steel, and on the contrary, the machinability decreases. 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%, 0.005 to 0.2%, W May contain one or more elements selected from the group consisting of: 0.01 to 0% and V: 0.01 to 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. Steel that requires high strength and steel that requires low strain is used to prevent coarse grains. It is 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 soot content is less than 0.001%, the effect is not recognized, and when the Ti content exceeds 0.1%, undissolved coarse carbonitride that causes hot cracking is precipitated. On the other hand, the mechanical properties are impaired. Therefore, if Ti is added, its content should be 0.001 to 0.1%.

Nb: 0.005-0.2%

Nb also forms carbonitrides and is an element that contributes to strengthening steel by secondary precipitation hardening and suppressing and strengthening the growth of austenite grains. Steel that requires high strength and steel that requires low strain Is used as a sizing element to prevent 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 exceeding 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 Nb is added, its content is made 0.005 to 0.2%. W: 0.01 to 1.0%

 w is also an element that forms carbonitrides and can strengthen steel by secondary precipitation hardening. However, if the W content is less than 0.01%, the effect of increasing the strength cannot be obtained, and if W is added in excess of 1.0%,

This causes precipitation of undissolved coarse carbonitrides that cause hot cracking, and 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 added as appropriate to steels that require high strength. However, if the V content is less than 0.01%, the effect of increasing the strength cannot be obtained, and if V is added in excess of 1.0%, undissolved coarse coal that causes hot cracking. Nitrides are deposited and the mechanical properties are impaired.

. Therefore, when V is added, the content is set to 0.05 to 1.0%. Furthermore, in the steel for machine structural use according to the present invention, when sulfide form control is performed by deoxidation adjustment, in addition to the above components, One element or two or more elements selected from the group consisting of Mg: 0.0001 to 0.0040%, Zr: 0.0003 to 0.01% and Rem: 0.0001 to 0.015% can also be added. Mg: 0.0001-0.0040%

Mg is a deoxidizing element and produces oxides in steel. In the case of A1 deoxidation, A 1 ₂ 0 ₃ harmful to machinability is modified to MgO or A 1 ₂ 0 ₃ · MgO that is relatively soft and finely dispersed. In addition, the oxide tends to become the core of MnS and has the effect of finely dispersing MnS. However, these effects are not observed when the Mg content is less than 0.0001%. In addition, Mg forms a composite sulfide with MnS and spheroidizes MnS. When Mg is added excessively, specifically, when the Mg content exceeds 0.0040%, Promotes single MgS formation and degrades machinability. Therefore, when adding Mg, the content is made 0.0001 to 0.0040%.

Zr: 0.0003-0.01%

Zr is a deoxidizing element and produces oxides in steel. The oxide is thought to be Zr 0 ₂ , but since this Zr0 ₂ becomes a precipitation nucleus of MnS, it has the effect of increasing the precipitation site of MnS and uniformly dispersing MnS. Zr also forms a composite sulfide in solid solution in MnS, lowers its deformability, and has the function of suppressing the elongation of the MnS shape during rolling and hot forging. Thus, Zr is an effective element for reducing anisotropy. However, when the Zr content is less than 0.0003%, no significant effect is obtained for these. On the other hand, even with the addition of Zr exceeding 0.01%, the yield is extremely well evil Kunar, Zr0 ₂ and hard compounds are mass produced, such as ZrS, conversely the machinability, impact value and fatigue characteristic or the like The mechanical properties of are reduced. Therefore, when adding Zr, the content is made 0.0003 to 0.01%.

 Rem: 0.0001 to 0.015%

 Rem (rare earth element) is a deoxidizing element, which generates a low melting point oxide, which not only suppresses nozzle clogging during fabrication, but also dissolves or bonds with MnS, lowering its deformability, reducing rolling and heat It also has the function of suppressing the elongation of the MnS shape during cold forging. Thus, Rem is an effective element for reducing anisotropy. However, when the total amount of Rem is less than 0.0001%, the effect is not significant, and when Rem is added in excess of 0.015%, a large amount of Rem sulfide is generated, and the machinability deteriorates. . Therefore, if Rem is added, its content should be 0.0001 to 0.015%.

Furthermore, in the steel for machine structure of the present invention, when improving machinability, in addition to the above components, Sn: 0.005 to 2.0%, Zn: 0.0005 to 0.5% B: 0.0005 to 0.015%, Te : 0.0003-0.2%, Bi: 0.005-0. One or more elements selected from the group consisting of 5% and Pb: 0.005 to 0.5% can be added.

Sn: 0.005 to 2.0%

 Sn has the effect of making the ferritic brittle and extending the tool life and improving 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, the content is made 0.005 to 2.0%.

Zn: 0.0005 to 0.5%

 Zn has the effect of making the ferritic brittle and extending the tool life and improving the surface roughness. However, when the Zn content is less than 0.0005%, the effect is not observed, 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 in 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 an amount exceeding 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 is made 0.0005 to 0.015%. Te: 0.0003-0.2%

Te is a machinability improving element. In addition, it produces MnTe and coexists with MnS, thereby 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 recognized, and when the Te content exceeds 0.2%, the effects are saturated. In addition, the hot ductility is reduced and it is easy to cause wrinkles. Therefore

When adding Te, the content should be 0.0003-0.2%.

Bi: 0.005-0.5%

 Bi is a machinability improving element. However, if the Bi content is less than 0.005%, the effect cannot be obtained, and adding more than 0.5% not only saturates the machinability improvement effect but also hot ductility. It tends to decrease and cause wrinkles. Therefore, when adding Bi, the content is made 0.005 to 0.5%.

Pb: 0.005-0.5%

 Pb is a machinability improving element. However, when the Pb content is less than 0.005%, the effect is not recognized.Addition of Pb exceeding 0.5% not only saturates the machinability improvement effect but also reduces the hot ductility. It tends to cause wrinkles. Therefore, when adding Pb, the content is made 0.005 to 0.5%.

 Furthermore, in the steel for machine structure of the present invention, when improving the hardenability and resistance to temper softening and imparting strength to the steel, Cr: 0.01 to 2.0% , Mo: 0.05 to 0% of 1 type or 2 types may be added.

Cr: 0.01-2.0%

 Cr is an element that improves hardenability and imparts temper softening resistance, and is added to steels that require 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%.

Mo: 0.05-1.0%

Mo imparts temper softening resistance and improves hardenability It is an element and is added to steels that require high strength. However,

When the Mo content is less than 0.05%, these effects cannot be obtained, and even if Mo is added in excess of 1.0%, the effects are saturated. Therefore, when adding Mo, the content is made 0.05 to 1.0%.

 Furthermore, in the steel for machine structural use of the present invention, when strengthening ferrite, in addition to the above components, one or two of Ni: 0.05 to 2.0% and Cu: 0.01 to 2.0% are added. can do.

Ni: 0.05-2.0%

 Ni is an element that strengthens ferrite and improves ductility, and is effective in 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 in excess of 2.0%, the effect is saturated in terms of mechanical properties and machinability is reduced. . Therefore, if Ni is added, its content should be 0.05-2.0%.

Cu: 0.01 to 2.0%

 Cu is an element that strengthens ferrite and is effective in improving hardenability and improving 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, if Cu is added, its content should be 0.01-2.0%. Cu is particularly preferable to be added at the same time as Ni because it lowers 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 A1 content and Sb content, appropriate amounts of solid solution Al, Sb, and A1N, which have an effect of improving machinability, are secured, so a wide range of cutting speeds from low speed to high speed can be achieved. Against Effective cutting performance. Furthermore, a high yield ratio can be obtained by this fine precipitation and solid solution A1N. Furthermore, since the content of elements that affect the precipitation of MnS is optimized and the amount of MnS with high uniform dispersibility is increased, the impact characteristics are also excellent.

 Machine structural steels with excellent machinability and strength properties according to the present invention are hot-forged at 1200 ° C or higher for a billet having the above-mentioned steel composition, then forged into a cylindrical shape and then solutionized at 1100 or higher. It can be manufactured by heat treatment and then heat treatment such as normalization and carburization. In particular, for steels containing A1N nitride, the solution is maintained for a long time at 1100 ° C or higher after solution heat treatment at 0.8 hours or longer, preferably 1 hour or longer, more preferably 1.2 hours or longer. A machine structural steel in which N is significantly reduced can be obtained. Example 1

Next, the effects of the present invention will be specifically described with reference to examples and comparative examples. In this example, 15 Okg of steel with the composition shown in Table 1 and Table 2 was melted in a vacuum melting furnace, then hot forged under a temperature condition of 1250 ° C and forged into a columnar shape with a diameter of 65. did. 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. The underline in Table 2 indicates that it is outside the scope of the present invention.

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Machinability test

 In the machinability test, first, each of the steel materials of Examples and Comparative Examples heated to 1250 ° C and warm forged for 1 hour under the temperature condition of 850 ° C, Comparative Example N 0. For Nos. 49 and 50, after 0.5 hours of normalization, heat treatment was performed by air cooling. After that, a test piece for machinability evaluation was cut out from each steel material after heat treatment, 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 in the example was evaluated. At that time, the maximum cutting speed VL 1000 capable of cutting to a cumulative hole depth of 1000 mm was used for the drilling test, and the maximum flank wear width VB_max after 10 minutes was used for the longitudinal turning test.

Table 3

シャルピー衝撃試験

Charpy impact test

Fig. 1 is a diagram showing the cut-out part of a specimen for Charpy impact test. In the Charbi impact test, first, as shown in FIG. 1, from each steel material 1 heat-treated under the same method and conditions as the above-described machinability test, Cylinder 2 having a diameter of 25 min was cut out so that the shaft was perpendicular to the forging direction of steel 1. Next, each columnar material 2 was held for 1 hour under a temperature condition of 850 ° C, and for Comparative Examples No. 49 and No. 50, 0.5 hours were held, and then oil quenching was performed to cool to 60 ° C. Further, tempering was carried out by holding for 30 minutes at a temperature of 550 ° C. and then cooling with water. After that, each cylindrical member 2 was machined to produce a Charbi test piece 3 specified in JIS Z 2202, and a Charpy impact test at room temperature was carried out by the method specified in JIS Z 2242. . At that time, as the evaluation index, it was adopted absorbed energy per unit area (J / CD1 ^2).

Tensile test

 The cylindrical material 2 taken in parallel with the forging direction was subjected to oil quenching and tempering using the same method and conditions as the Charpy impact test described above, and then the parallel part had a diameter of 8 mm and the parallel part had a length of 30 mm. Based on the method specified in JIS Z 2241, a tensile test was performed at room temperature. At that time, the yield ratio (= (0.2% resistance to YP) / (tensile strength TS)) was adopted as an evaluation index.

The results of the above tests are shown in Tables 5 and 6.

Table 5

表 6

Table 6

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 一） 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〜N 0.46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 A1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 （Im pact Value) 及び降伏比 （YS/TS) が劣っていた。

The steel materials No. 1 to No. 42 shown in Table 1, Table 2 and Table 5 are examples of the present invention, and the steel materials No. 43 to No. 51 shown in Table 2 and Table 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, in all of the evaluation indices VL1000, VB_max, Impact Value (absorbed energy 1) and YP / TS (yield ratio). Although it showed good values, the steel material of the comparative example was inferior to the steel material of the example in at least one of these characteristics. Specifically, the steels of Comparative Examples No. 43 to N 0.46 have a total Al content that falls below the scope of the present invention, so the VL1000 and yield ratio (YP / TS), which are evaluation indices for machinability, are implemented. It is inferior to the example steel. In addition, since the total A1 content of the steel material of Comparative Example No. 47 is extremely lower than the range of the present invention, the amount of solute N exceeds the range of the present invention, and it is cut more than the steel material of the example. The properties (VL1000, VB_max), impact value (Im pact value), and yield ratio (YS / TS) were poor.

Since the steel material of Comparative Example No. 48 had a total A1 content exceeding the range of the present invention, the hardness increased and the machinability (VL1000, VB max. 1) was inferior. Comparative Examples No. 49 and No. 50 are more susceptible to precipitation of A1N than the steels of the examples, and the temperature holding time at 850 ° C. is short, so the amount of solute N is within the scope of the present invention. The machinability (VL1000, VB_max) and impact value (Impact Value) were inferior to the steel materials of the examples. Since the steel materials of Comparative Examples No. 5 l to No. 54 have Sb contents exceeding the range of the present invention, the impact value (Impact Value) was inferior to the steel materials of the examples.

Example 2

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 columnar shape with 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 indicates that it is outside the scope of the present invention.

 L 拏

OS £ S.0 / .OOidf / X3d 6t ^ 1 "80 / 800Z OAV n

 8 ¾

OSC≤.0 / Z.OOZdT / X3d 6t ^ 80/800 OAV Machinability test

 In the machinability test, first, each of the steel materials of Examples and Comparative Examples heated to 1250 ° C and warm forged for 1 hour under the temperature condition of 850 ° C, Comparative Example N 0. 48, No. 49, No. 97 to No. 101 were subjected to heat treatment by air cooling after normalizing for 0.5 hours. After that, a test piece for machinability evaluation was cut out from each steel material after heat treatment, 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. Examples and Comparative Examples The machinability of each steel material was evaluated. At that time, the maximum drilling speed VL 1000 capable of cutting up to a cumulative hole depth of 1000 was used as the evaluation index, and the maximum flank wear width VB_niax after 10 minutes was used in the longitudinal turning test.

Table 9

シャルピー衝撃試験

Charpy impact test

FIG. 1 is a view showing a cut-out portion of a specimen for a Charpy impact test. In the Charpy impact test, first, as shown in FIG. A cylindrical material 2 having a diameter of 25 mm was cut from each steel material 1 heat-treated in the same manner and under the same machinability test, with the central axis perpendicular to the forging direction of steel material 1. Next, each cylindrical member 2 was held for 1 hour under a temperature condition of 850 ° C., and Comparative Examples No. 48, No. 49, No. 97 to No. 101 were held for 0.5 hour. Oil quenching was performed to cool to 0 ° C, and tempering was further performed by holding for 30 minutes at a temperature of 550 ° C and then water cooling. After that, each cylindrical member 2 was machined to produce a Charpy test piece 3 specified in JIS Z Π02, and a Charpy impact test at room temperature was performed by the method specified in JIS Z 2242. . At that time, the absorbed energy per unit area (J Zcm ² ) was adopted as an evaluation index. Tensile test

 Each cylindrical material 2 that was oil-quenched and tempered under the same method and conditions as the Charpy impact test described above was processed into a tensile test piece with a parallel part diameter of 8 mm and a length of 30 mm, and JIS Z 2241 Based on the method stipulated in, a tensile test was performed at room temperature. At that time, the yield ratio (= (0.2% resistance to YP) / (tensile strength TS)) was adopted as an evaluation index.

The results of the above tests are summarized in Tables 11 and 12.

11

12

12

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 VL 1000、 VB_max、 Impact Value (吸 収エネルギー） 及び YPZTS (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っ以 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 M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 （YPZTS) 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 A1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 （VL 1000、 VB— max) 、 衝撃値 （Impa ct Value) 及び降伏比 （YSZTS) が劣っていた。

The No. 1 steel materials shown in Tables 7 and 11 are the examples of claim 1, and the No. 2 to No. 42 steel materials are the examples of claim 2. Nos. 52 to 93 shown in Table 8 and Table 12 are the embodiments of claim 1. Furthermore, the steel materials of Comparative Examples No. 43 to No. 49 satisfy the provisions of claim 2 with respect to the S content and Ca content, and the steel materials of Comparative Examples No. 94 to No. 101 contain S. As shown in Table 11 and Table 12, the steel materials of Examples No. 1 to No. 42 and No. 52 to No. 93 are satisfied with respect to the amount and the Ca content. The evaluation indices VL 1000, VB_max, Impact Value (absorption energy) and YPZTS (yield ratio) all showed good values, but the comparative steel material had at least one of these values. The above characteristics were inferior to the steel materials of the examples. Specifically, the steel materials of Comparative Examples No. 43 to No. 46 have a total M content that falls below the scope of the present invention, so the machinability (VL1000) and the yield ratio (YPZTS) are higher than those of the steel materials of the examples. Was also inferior. In addition, since the total A1 content of the steel material of Comparative Example No. 47 is extremely lower than the range of the present invention, the amount of solute N exceeds the range of the present invention, and it is more difficult to cut than the steel materials of the examples. The properties (VL 1000, VB—max), impact value (Ympact value) and yield ratio (YSZTS) were poor.

Comparative Example No. 48 and No. 49 steel materials are easier to deposit A1N than the steel materials of the examples. Since the temperature holding time at 850 ° C is short, the amount of solid solution N exceeds the range of the present invention. The machinability (VL1000, VB-max) and impact value (Impact Value) were inferior to the steel materials of the examples. Further, since the total A1 content of the steel materials of Comparative Examples No. 94 to No. 96 was below the range of the present invention, the machinability (VL1G00, VB-max) and the yield ratio (YPZTS) It was inferior to steel. Furthermore, in the steel materials of Comparative Examples No. 97 to No. 101, A1N is more likely to precipitate than the steel materials of the examples, and the temperature holding time at 850 is short, so the amount of dissolved N exceeds the range of the present invention, Machinability (VL10) 00, VBjax) and Impact Value were inferior Industrial applicability

 According to the present invention, it is possible to provide a steel for machine structures having good machinability in a wide range of cutting speeds and having both high impact characteristics and a high yield ratio.

Claims

1. By mass%  C: 0.1-0.85%,  Si: 0.01 to 1.5%,  Mn: 0.05-2.0%  Contract  P: 0.005 to 0.1%,  S: 0.001 to 0.15%,  All A1: Over 0.05% and below 0.3%,  Sb: less than 0.0150% (including 0%) and  Total N: Enclosed with 0.0035 to 0.020%,  Solid solution N: limited to 0.0020% or less,  Machine structural steel with excellent machinability and strength characteristics, characterized in that the balance is Fe and inevitable impurities.  2. Further, in mass%, Ca: 0.0003 to 0.0015%, T i: 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%, Zr: 0.0003 to 0.01%, Rem: 0.0001 to 0.015%, Sn : 0.005 to 2.0%, Zn: 0.0005 to 0.5%, B: 0.0005 to 0.015%, Te: 0.0003 to 0.2%, Bi: 0.005 to 0.5% and Pb: 0.005 to 0.5%, Cr: 0.0 1 to 2.0%, Mo : Excellent in machinability and strength characteristics according to claim 1, characterized by containing one or more of 0.01 to L 0%, Ni: 0.05 to 2.0% and Cu: 0.01 to 2.0% Steel for machine structure.