JP5181619B2 - Hardened steel with excellent machinability and hardenability - Google Patents

Description

  The present invention relates to an alloy steel that is subjected to cutting and heat treatment, and in particular, when the life of a cutting tool can be suppressed, the amount of expensive additive elements used can be reduced, and the amount of heat generated during cutting is small. However, it relates to a hardened steel material having excellent machinability and hardenability.

  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 are known to improve machinability and have relatively little influence on forging, but to reduce strength characteristics such as impact characteristics. Further, S forms inclusions that become soft under a cutting environment such as MnS to improve machinability. However, the size of MnS is larger than that of particles such as Pb and is likely to be a stress concentration source. In particular, when extending by forging and rolling, anisotropy occurs due to MnS, and a specific direction of steel such as impact characteristics becomes extremely weak. In addition, it is necessary to consider such anisotropy when designing an object using 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 impact characteristics are lowered, so that it is difficult to achieve both strength characteristics and machinability. Furthermore, recently, there is a tendency to avoid using Pb as an environmental load, and the amount of Pb used tends to be reduced. For this reason, in order to achieve both the machinability and strength characteristics of steel, further technological innovation is required.

  Therefore, conventionally, for example, one or more selected from solute V, solute Nb, and solute Al is contained in a total amount of 0.005% by mass or more, and solute N is contained in an amount of 0.001% or more. Thus, a steel for machine structural use has been proposed in which a nitride generated by cutting heat during cutting is attached to a tool to function as a tool protection film, and the cutting tool life can be extended (see, for example, Patent Document 1). ).

  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, for example, Patent Document 2). In the steel for machine structure described in Patent Document 2, 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. Yes.

JP 2004-107787 A Japanese Patent No. 3706560

  However, the conventional techniques described above have the following problems. That is, in the steel described in Patent Document 1, it is presumed that the above-described phenomenon does not occur unless the amount of heat generated by cutting exceeds a certain level. For this reason, in order to exhibit an effect, it is necessary to make cutting speed high to some extent, and an effect cannot be expected in a normal speed range. Further, in the steel described in Patent Document 2, since no consideration is given to the cutting tool life, sufficient characteristics cannot be obtained in terms of suppressing shortening of the cutting tool life.

  Moreover, in order to ensure the strength of alloy steel typified by case-hardened steel or tough steel, heat treatment for surface hardening treatment such as carburizing or quenching and tempering may be performed. In this case, it is necessary to ensure sufficient hardenability of the steel depending on the size of the parts, etc., but the quenching improving elements, such as Mo and Ni, are so expensive that the alloy costs are high. It is necessary to be able to substitute with a relatively cheap element.

  The present invention was devised in view of the above-described problems, and can reduce the life of a cutting tool, reduce the amount of expensive additive elements used, and generate a small amount of heat during cutting. However, an object is to provide a hardened steel material having good machinability and excellent hardenability.

  The present inventors have found that, in steels to which an appropriate amount of Al is added, if the abundance of coarse AlN is limited, it is possible to suppress the shortening of the cutting tool life and to obtain a steel material having good machinability. I found out. Furthermore, the present inventors have found that the hardenability of the steel material can be ensured by securing a sufficient amount of solute Al, so that Al can be used as a substitute element for so-called hardenability improving elements such as Mo and Ni. Completed the invention. The steel according to the present invention can be produced, for example, by adding an appropriate amount of Al as a chemical component of the steel, limiting the amount of N, and performing an appropriate heat treatment.

The hardened steel materials excellent in machinability and hardenability according to the present invention are as follows.
(1) As a chemical component in mass%,
C: 0.14 to 0.85%,
Si: 0.01 to 1.5%,
Mn: 0.05 to 2.5%
S: 0.005-0.35%,
Cr: 0.2-6.0%,
N: 0.020% or less,
Al: 0.055-1.0%
And the balance consists of Fe and inevitable impurities,
The total volume of AlN maximum diameter exceeds 200nm is 20% or less of the total volume of all AlN, Ri der solute Al is 0.062 mass% or more, manufactured from molded round-bar steel by hot rolling, machinability and hardenability excellent in hardening steel, wherein the steel der Rukoto the quenching process is applied.

(2) The hardened steel material having excellent machinability and hardenability as described in (1) above, further containing P: 0.020% or less by mass% as a chemical component.
(3) Excellent in machinability and hardenability as described in (1) or (2) above, wherein the chemical component further contains Ca: 0.0003 to 0.0015% by mass%. Hardened steel.
(4) Further, as chemical components, by mass%, Ti: 0.001 to 0.1%, Nb: 0.005 to 0.2%, W: 0.01 to 1.0%, V: 0.00. One type or two or more types selected from the group consisting of 01% to 1.0% are contained, and excellent in machinability and hardenability according to any one of the above (1) to (3) Hardened steel.
(5) The chemical component is further 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% in terms of mass%. The hardened steel material excellent in machinability and hardenability according to any one of the above (1) to (4), wherein the hardened steel material contains one or two or more types.
(6) Further, as a chemical component, by mass%, Sb: 0.0005% or more and less than 0.0150%, Sn: 0.005 to 2.0%, Zn: 0.0005 to 0.5%, B: One selected from the group consisting of 0.0005 to 0.015%, Te: 0.0003 to 0.2%, Bi: 0.005 to 0.5%, Pb: 0.005 to 0.5% Or a hardened steel material excellent in machinability and hardenability according to any one of the above (1) to (5), characterized in that it contains two or more kinds.
(7) Excellent in machinability and hardenability as described in any one of (1) to (6) above, further containing Mo: 0.01 to 1.0% as a chemical component Hardened steel.
(8) As a chemical component, it further contains one or two selected from the group consisting of Ni: 0.05 to 2.0% and Cu: 0.01 to 2.0% by mass%. A hardened steel material excellent in machinability and hardenability according to any one of the above (1) to (7).

  According to the present invention, it is possible to suppress the cutting tool life from being shortened, to reduce the amount of expensive additive elements used, and to provide good machinability and excellent hardenability even when the amount of heat generated during cutting is small. A hardened steel material can be provided.

Hereinafter, the best mode for carrying out the present invention will be described in detail. A hardened steel material excellent in machinability and hardenability according to the present invention has a chemical composition of steel of Al: 0.055-1.0%, N: 0.020% or less, and a maximum diameter of AlN exceeding 200 nm. adjust the total volume of less than 20% of the total volume of all AlN, and all SANYO that the solute Al amount more than 0.062%, produced from molded round-bar steel by hot rolling, quenching Ru der which has been subjected to. By making the total volume of AlN whose maximum diameter exceeds 200 nm 20% or less of the total volume of all AlN, machinability is improved without deteriorating impact characteristics unlike conventional free-cutting elements S and Pb. An effect can be obtained. Furthermore, by making the solid solution Al amount 0.062 % or more, it becomes possible to improve machinability and improve hardenability, and other alloy elements to be added for improving hardenability. The amount can be reduced.

  First, the content of each chemical component in the quenched steel material of the present invention will be described. Hereinafter, the mass% is simply described as%.

(C: 0.05-0.85%)
C is an element that greatly affects the basic strength of steel. However, if the C content is less than 0.05%, 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 and machinability, the C content is set to 0.06 to 0.85%.
In this invention, since C content of an Example is all 0.14% or more, C content shall be 0.14-0.85%.

(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, in the present invention, the Si content is set to 0.01 to 1.5%.

(Mn: 0.05-2.5%)
Mn is an element necessary to fix and disperse S in steel as MnS and to dissolve in a matrix to improve hardenability and to 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 2.5%, the hardness of the substrate increases and the cold workability decreases, and also the influence on the strength and hardenability. Saturates. Therefore, in the present invention, the Mn content is set to 0.05 to 2.5%.

(S: 0.001-0.35%)
S combines with Mn and exists as MnS inclusions. 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 0.35%, the effect of improving the machinability is saturated, but the strength reduction is significantly promoted. Therefore, in the present invention, the S content is set to 0.001 to 0.35%.

(Cr: 0.2-2.0%)
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 0.2%, these effects cannot be obtained, and when a large amount of Cr (specifically, more than 2.0%) is added, Cr carbide is generated. Steel becomes brittle. Therefore, in the present invention, the content is set to 0.01 to 2.0%.

(N: 0.020% or less)
N (total N in steel) is combined with a nitride-forming element such as Al and is present as nitride or as solute N. However, if it exceeds 0.020%, nitrides are coarsened, and solid solution N is increased to deteriorate machinability, and problems such as wrinkles occur during rolling. For this reason, in the present invention, the upper limit of the content is 0.020%, more preferably 0.01%.

(Al: 0.055-1.0%)
Al (total Al in steel) is partly combined with N and precipitated as AlN, and the rest exists as solute Al. In order to sufficiently secure solid solution Al, Al needs to be 0.055% or more, but if it exceeds 1.0%, the transformation characteristics are greatly affected. For this reason, in this invention, the content was 0.55-1.0%.

  Further, in the quenched steel material of the present invention, P is an element that embrittles the grain boundaries after quenching and tempering and lowers the fatigue strength, and therefore is preferably 0.020% or less.

  The hardened steel material of the present invention may contain Ca in addition to the above components.

(Ca: 0.0003 to 0.0015%)
Ca is a deoxidizing element and generates an oxide. Calcium aluminate (CaOAI ₂ O ₃ ) is formed in a steel containing 0.05% or more as total Al (T-Al) as in the present invention steel, but this CaOAI ₂ O ₃ is AI ₂ O. _Since it is a lower melting point oxide than ₃ , it becomes a tool protective film during high-speed cutting and improves machinability. However, when the Ca content is less than 0.0003%, this machinability improvement effect cannot be obtained. When the Ca content exceeds 0.0015%, CaS is generated in the steel, and on the contrary, the machining is performed. Decrease the sex. Therefore, when adding Ca in this invention, the content shall be 0.0003 to 0.0015%.

  Furthermore, when the hardened steel material of the present invention forms carbonitrides and high strength is required, in addition to the above components, Ti: 0.001 to 0.1%, Nb: 0.005 You may contain 1 type, or 2 or more types of elements selected from the group which consists of 0.2%, W: 0.01-1.0%, 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, adjustment is required to prevent coarse grains. Used as a granulating 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 carbonitriding that causes hot cracking. A thing precipitates and a mechanical property is impaired on the contrary. Therefore, when adding Ti in the present invention, the content is made 0.001 to 0.1%.

(Nb: 0.005 to 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. 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 in excess of 0.2%, it is an undissolved coarse that causes hot cracking. New carbonitrides are deposited and the mechanical properties are impaired. Therefore, when adding Nb in this invention, the content shall be 0.005-0.2%.

(W: 0.01-1.0%)
W is also an element that forms carbonitride 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. If W is added in excess of 1.0%, it is an undissolved coarse that causes hot cracking. New carbonitrides are deposited and the mechanical properties are impaired. Therefore, when adding W in this invention, the content shall be 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 in excess of 1.0%, it is an undissolved coarse that causes hot cracking. New carbonitrides are deposited and the mechanical properties are impaired. Therefore, when adding V in this invention, the content shall be 0.01%-1.0%.

  Furthermore, in the quenched steel material of the present invention, when performing sulfide form control by adjusting deoxidation, in addition to the above components, Mg: 0.0001 to 0.0040%, Zr: 0.0003 to 0.01 % And Rem: one or more elements selected from the group consisting of 0.0001 to 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. In addition, 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. Further, Mg forms a composite sulfide with MnS and spheroidizes MnS. However, when Mg is added excessively (specifically, more than 0.0040%), the formation of single MgS is promoted. Deteriorates machinability. Therefore, when adding Mg in this invention, the content shall be 0.0001 to 0.0040%.

(Zr: 0.0003 to 0.01%)
Zr is a deoxidizing element and generates an oxide in steel. The oxide is considered to be ZrO ₂ , but since this ZrO ₂ becomes a precipitation nucleus of MnS, there is an effect of increasing the precipitation sites of MnS and uniformly dispersing MnS. Zr also has a function of forming a composite sulfide in MnS, reducing its deformability, and 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%, a remarkable effect cannot be obtained for these. On the other hand, even if Zr is added in excess of 0.01%, the yield is not only extremely deteriorated, but a large amount of hard compounds such as ZrO ₂ and ZrS are formed, and on the contrary, machinability, impact value and fatigue are increased. Mechanical properties such as characteristics are degraded. Therefore, when adding Zr in this invention, the content shall be 0.0003-0.01%.

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

  Furthermore, in the hardened steel material of the present invention, when further improving the machinability, in addition to the above components, Sb: 0.0005% or more and less than 0.0150%, 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%. One or more elements selected from the group consisting of 005 to 0.5% can be added.

(Sb: 0.0005% or more and less than 0.0150%)
Sb moderately embrittles ferrite and improves machinability. The effect is particularly remarkable when the amount of dissolved Al is large, and is not observed when the Sb content is less than 0.0005%. Further, when the Sb content increases (specifically, 0.0150% or more), Sb macrosegregation becomes excessive and the impact value is greatly reduced. Therefore, when adding Sb in this invention, the content shall be 0.0005% or more and less than 0.0150%.

(Sn: 0.005 to 2.0%)
Sn has an 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 over 2.0%, the effect is saturated. Therefore, when adding Sn in this invention, the content shall be 0.005-2.0%.

(Zn: 0.0005 to 0.5%)
Zn has the effect of making the ferrite 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 recognized, and even if Zn is added in excess of 0.5%, the effect is saturated. Therefore, when adding Zn in this invention, the content shall be 0.0005 to 0.5%.

(B: 0.0005 to 0.015%)
B is effective in grain boundary strengthening and hardenability when dissolved, and is precipitated as BN when precipitated, which is effective in improving machinability. These effects are not significant when the B content is less than 0.0005%. On the other hand, even if B is added over 0.015%, the effect is saturated and a large amount of BN is precipitated, so that the mechanical properties of the steel are impaired. Therefore, when adding B in this invention, the content shall be 0.0005 to 0.015%.

(Te: 0.0003 to 0.2%)
Te is a machinability improving element. Moreover, it produces MnTe or coexists with MnS, thereby reducing the deformability of MnS and suppressing the extension of the MnS shape. Thus, Te is an element effective 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%, not only the effect is saturated but also the hot ductility is lowered. And easily cause wrinkles. Therefore, when adding Te in this invention, the content shall be 0.0003 to 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 improving effect but also increases the heat The ductility is reduced and it is easy to cause wrinkles. Therefore, when adding Bi in this invention, the content shall be 0.005%-0.5%.

(Pb: 0.005 to 0.5%)
Pb is a machinability improving element. However, when the Pb content is less than 0.005%, the effect is not recognized, and addition of Pb exceeding 0.5% not only saturates the machinability improving effect but also heat The ductility is reduced and it is easy to cause wrinkles. Therefore, when adding Pb in this invention, the content shall be 0.005-0.5%.

  Furthermore, in the hardened steel material of the present invention, when improving the hardenability and resistance to temper softening and imparting strength to the steel material, in addition to the above components, Mo: 0.05 to 1.0% It may be added.

(Mo: 0.01 to 1.0%)
Mo is an element that imparts resistance to temper softening and improves hardenability, and is added to steel that requires high strength. However, when the Mo content is less than 0.01%, these effects cannot be obtained, and even if Mo is added in excess of 1.0%, the effects are saturated. Therefore, when adding Mo in this invention, the content shall be 0.01 to 1.0%.

  Furthermore, in the hardened steel material of the present invention, when strengthening ferrite, in addition to the above components, Ni: 0.05 to 2.0% and / or Cu: 0.01 to 2.0% are added. can do.

(Ni: 0.05-2.0%)
Ni strengthens ferrite, improves ductility, and is an element effective for 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 the work is cut. Sexuality decreases. Therefore, when adding Ni in the present invention, 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 in this invention, the content shall be 0.01 to 2.0%. Note that Cu particularly decreases hot ductility and tends to cause defects during rolling. Therefore, when adding Cu, it is preferable to add Ni as well.

  Next, the reason why the total volume of AlN having a maximum diameter exceeding 200 nm is set to 20% or less of the total volume of all AlN will be described.

  When the total volume of AlN with a maximum diameter exceeding 200 nm exceeds 20% of the total volume of total AlN, mechanical wear of the cutting tool due to coarse AlN becomes remarkable, and machinability improvement by securing solid solution Al Since the effect is lowered, the total volume of AlN having a maximum diameter exceeding 200 nm is set to 20% or less of the total volume of all AlN. The maximum diameter is the longest diameter of each AlN.

The volume ratio of AlN can be calculated by, for example, the following method. First, AlN exposed on the cross section of the steel material is attached to the film by the extraction replica method. The membrane is then observed with a transmission electron microscope. At this time, 20 or more fields of 1000 μm ² were randomly observed, and the total volume (area) of AlN having a maximum diameter exceeding 200 nm and the total volume (total area) of all AlN were determined. The total volume of AlN exceeding / the total volume of all AlN) × 100] is calculated.

  In order to make the total volume of AlN having a maximum diameter exceeding 200 nm 20% or less of the total volume of all AlN, before hot rolling or hot forging, so that AlN is solutionized or the undissolved residue is sufficiently reduced. It is necessary to adjust the heating temperature.

Based on the solubility product of AlN and the experimental results, the present inventors have determined that the heating temperature before rolling or forging before the forging or forging necessary for setting the volume ratio of coarse AlN exceeding 200 nm to 20% or less of the total AlN ( The equation (1) for deriving (C) was found.

T1 = −6770 / [log {(wt% Al) (wt% N)} − 1.03] −423
... (1)

Here, wt% Al is the total Al content (mass%) of the steel material, and wt% N is the total N content (mass%) of the steel material. The same applies hereinafter.

  That is, before rolling or forging, the total volume of AlN having a maximum diameter exceeding 200 nm is reduced to 20% or less of the total volume of all AlN by heating to a temperature equal to or higher than the temperature T1 obtained by the formula (1). Can do.

Next, the reason why the solid solution Al amount is 0.05 mass% or more will be described. The solid solution Al amount here is defined by the following equation (2).

Solid Al content (mass%) = Total Al content in steel (mass%)-Al content present as AlN (mass%)
(2)

The amount of Al present as AlN can be determined, for example, by analyzing the residue obtained by electrolytic extraction with a SPEED method of a constant potential electrolytic corrosion method using a nonaqueous solvent electrolyte and a 0.1 μm filter with an ICP emission spectrometer. .

Solid solution Al has a machinability improving effect and a hardenability improving effect, but if less than 0.05%, these effects are not observed, so the amount of solid solution Al is set to 0.05% or more.
In the present invention, Invention Example No. Since the solute Al experimental value of 26 is 0.062%, the solute Al amount is set to 0.062% or more.

The amount of solute Al can be adjusted by the total amount of Al and / or the total amount of N in the steel, depending on the heating temperature during carburizing and quenching. The present inventors have found the following formula (3) that estimates the amount of solute Al after carburizing and quenching based on the heating temperature (T2) at the time of carburizing and quenching, and the total Al amount and the total N amount in the steel. .

Solid solution Al amount (mass%) = (27/28) (((wt% N- (14/27) (wt% Al)) ² + (56/27) × 10 ^{(1.03-6770 / (T2 +273))} ) ^1/2 -((wt% N)-(14/27) (wt% Al))) ... (3)

Usually, the heating temperature at the time of quenching (T2), when the heating temperature during quenching is set to more than a temperature obtained by adding 40 to 60 ° C. to determined A ₃ transformation temperature by chemical components is large and also the heating temperature at the time of carburizing (T2) is often set to 900 to 1000 ° C. That is, the heating temperature (T2) at the time of carburizing and quenching is usually in the temperature range of 800 ° C. to 1000 ° C., but the amount of solid solution Al calculated by the above formula (3) is determined according to the heating temperature (T2). What is necessary is just to adjust the total amount of Al in steel (mass%) and / or the total amount of N in steel (mass%) within the specified range so that it becomes 0.05 mass% or more.

  As described above, in the alloy steel of the present invention, since the formation of coarse AlN is suppressed, the machinability and hardenability can be improved even if the calorific value at the time of cutting is small, and the cutting tool It is possible to prevent the life from becoming shorter. In addition, since the amount of solid solution Al effective for improving the hardenability and machinability is increased, the concentration of these elements is unavoidable at an inevitable impurity level without using expensive additive elements (for example, Mo and Ni). Even in some cases, the hardenability can be improved. Furthermore, even when it is necessary to use the above-described additive element, the amount of use can be reduced.

  In addition, unlike typical hardenability improving elements such as Mo, in the case of Al, the effect of improving hardness is small at a slow cooling speed (for example, air cooling) in which quenching does not occur. In addition to machining, other cold working (cold forging, cold drawing, etc.) can also suppress the improvement in hardness, so the workability is reduced compared to the case where hardenability is increased with Cr and Mo. Can be suppressed.

  Next, the effects of the present invention will be specifically described with reference to examples and comparative examples. The alloy steel of the present invention has improved machinability regardless of heat treatment after heat treatment after hot rolling, and has improved hardenability that is important when heat treatment such as carburizing and quenching is performed. . Then, the effect at the time of applying this invention is demonstrated concretely in three steel types from which a basic component type | system | group or heat processing differs. However, since the machinability is greatly affected when the basic strength or heat treatment structure is different, and the hardenability is greatly affected when the heat treatment temperature or basic components are different, the examples are also divided into three. I will explain.

Example 1
Example 1 shows a test example when the present invention is applied to an alloy steel of about 0.15% C (for example, C is in the range of 0.12 to 0.18%). Other chemical components are Si: 0.1-0.3%, Mn: 0.5-1.0%, P: 0.020% or less, S: 0.01% or more and 0.1% or less, Cr: 0.5% to 1.5% or less, Al: 0.055 to 0.3%. In Example 1, after the normalization, a cutting test and a Jomini test at a heating temperature assuming carburization were performed. Specifically, after 150Kg of steel having the composition shown in Table 1-1 was melted in a vacuum melting furnace, it was hot forged at the heating temperature shown in Table 1-3 to forge into two types of cylinders with diameters of 65mm and 35mm. Stretched. And about the steel material of this Example, the machinability test, observation of AlN, and measurement of the amount of solute Al were performed with the test piece cut out from the 65 mm forged material by the method shown below. Moreover, the Jomini test was implemented with the Jomini test piece cut out from 35 mm forged material, and the characteristic was evaluated.

[Machinability test]
First, the steel material after forging was held at a temperature of 930 ° C. for 1 hour, then air-cooled, and subjected to heat treatment for normalization, and the hardness of each steel material was adjusted to a range of 135 to 145 at Hv10. Then, the test piece for machinability evaluation was cut out from each steel material after heat processing, the drill drilling test was performed on the cutting conditions shown to following Table 1-2, and the machinability of each steel material of an Example and a comparative example was evaluated. At that time, as an evaluation index, a maximum cutting speed VL1000 capable of cutting to a cumulative hole depth of 1000 mm was adopted in the drill drilling test.

なお、NACHI通常ドリルとは株式会社不二越製の型番SD3.0のドリルを示す。以後、本文中でNACHI通常ドリルと表示した場合にも株式会社不二越製の型番SD3.0のドリルを示す。

The NACHI normal drill is a model SD3.0 drill manufactured by Fujikoshi Co., Ltd. Hereafter, when the NACHI normal drill is indicated in the text, the model number SD3.0 drill made by Fujikoshi Co., Ltd. is shown.

[Jomini test]
The Jomini test was performed on each steel material after forging at a heating temperature of 930 ° C. assuming a carburizing heating temperature by a one-time quenching method by a method based on JIS G0561. Thereafter, the resulting Jomini - from the curve and C content, estimates the Jomini distance corresponding to the hardness of 50% M, and converting the Jomini distance hardenability D _I.

[Observation of AlN]
For the observation of AlN, an observation sample was prepared by a replica method using a sample cut out from the Q part of a steel material produced in the same manner as the test piece for machinability test evaluation, and this observation sample was transmitted using a transmission electron microscope. Was observed. Observation was carried out by randomly viewing 20 fields of 1000 μm ² . Then, the case where the total volume of AlN having a maximum diameter exceeding 200 nm was 20% or less of the total volume of all AlN was judged as ◯, and the case where it was 20% or more was judged as x.

[Measurement of solid solution Al content]
First, the amount of Al present as AlN (% by mass) is measured by analyzing the residue obtained by electrolytic extraction with the SPEED method, a potentiostatic electrolytic corrosion method using a non-aqueous solvent electrolyte, and a 0.1 μm filter, using an ICP emission spectrometer. By substituting into the above equation (2), the amount of solid solution Al (% by mass) was determined.

The results of the above tests are summarized in Table 1-3.

No. shown in Table 1-1 and Table 1-3 above. 1-7 are invention examples, No.1. 8 to 13 are comparative examples. As shown in Table 1-3 above, No. 1 as an example of the present invention. In 1-7 steel, which is an evaluation index VL1000, although the effect of improving hardenability D _I is observed, which is a comparative example No. In the steel materials of 8 to 13, at least one of these characteristics was inferior to the steel materials of the examples (see FIGS. 1 and 2).

  Specifically, no. Nos. 8, 10, and 12 are out of the range defined by the present invention, and as a result, the amount of solute Al is less than that defined by the present invention. It was inferior to the inventive steel having a content. Moreover, the effect of improving hardenability by solute Al was also small.

  No. 9 and 11, the total Al amount, the total N amount, and the required heating temperature T1 (° C.) calculated from the above formula (1), the heating temperature at the time of actual forging was low, so coarse AlN was generated, VL1000, which is an index of machinability, was inferior to the inventive steel having the same S content.

  No. In No. 13, the total amount of N and the total amount of Al are within the specified ranges, and the heating temperature during forging also satisfies the required heating temperature calculated from (1) above, but with respect to the total amount of N (mass%). Since the total Al amount (% by mass) is relatively small, the solute Al value is outside the range specified in the present invention at the carburizing heating temperature carried out, and the VL1000, which is an index of machinability, has the same S content. It was inferior to the invention steel having Moreover, the effect of improving hardenability by solute Al was also small.

(Example 2)
Example 2 shows a test example when the present invention is applied to an alloy steel of about 0.2% C (for example, C is in the range of 0.18 to 0.23%). Other chemical components are Si: 0.1-0.3%, Mn: 0.5-1.0%, P: 0.020% or less, S: 0.01% or more and 0.1% or less, Cr: 0.5% to 1.5% or less, Al: 0.055 to 0.3%. In Example 2, a Jomini test at a heating temperature assuming a cutting test and carburizing was performed after normalization. Specifically, after 150Kg of steel having the composition shown in Table 2-1 was melted in a vacuum melting furnace, it was hot forged at the heating temperature shown in Table 2-3 and forged into two types of cylinders with diameters of 65mm and 35mm. Stretched. And about the steel material of this Example, the machinability test, observation of AlN, and measurement of the amount of solute Al were performed with the test piece cut out from the 65 mm forged material by the method shown below. Moreover, the Jomini test was implemented with the Jomini test piece cut out from 35 mm forged material, and the characteristic was evaluated.

[Machinability test]
First, each steel material after forging was held for 1 hour under a temperature condition of 930 ° C. and then air-cooled and subjected to heat treatment for normalization, and the hardness of each steel material was adjusted to a range of 140 to 150 at Hv10. Then, the test piece for machinability evaluation was cut out from each steel material after heat processing, the drilling test was performed on the cutting conditions shown in the following Table 2-2, and the machinability of each steel material of an Example and a comparative example was evaluated. At that time, as an evaluation index, a maximum cutting speed VL1000 capable of cutting to a cumulative hole depth of 1000 mm was adopted in the drill drilling test.

[Jomini test]
The Jomini test was performed on each steel material after forging at a heating temperature of 930 ° C. assuming a carburizing heating temperature by a one-time quenching method by a method based on JIS G0561. Thereafter, a Jomini distance corresponding to a hardness of 50% M was estimated from the obtained Jomini-Curve and C amount, and this Jomini distance was converted to hardenability (D _I : ideal critical diameter).

[Observation of AlN]
For the observation of AlN, an observation sample was prepared by a replica method using a sample cut out from the Q part of a steel material produced in the same manner as the test piece for machinability test evaluation, and this observation sample was transmitted using a transmission electron microscope. Was observed. Observation was carried out with 20 fields of 1000 μm ² randomly. Then, the case where the total volume of AlN having a maximum diameter exceeding 200 nm was 20% or less of the total volume of all AlN was judged as ◯, and the case where it was 20% or more was judged as x.

[Measurement of solid solution Al content]
First, the amount of Al present as AlN (% by mass) is measured by analyzing the residue obtained by electrolytic extraction with the SPEED method, a potentiostatic electrolytic corrosion method using a non-aqueous solvent electrolyte, and a 0.1 μm filter, using an ICP emission spectrometer. By substituting into the above equation (2), the amount of solid solution Al (% by mass) was determined.

The results of the above tests are summarized in Table 2-3.

No. shown in Table 2-1 and Table 2-3 above. Nos. 14 to 32 are invention examples. 33 to 41 are comparative examples. As shown in Table 2-3 above, No. 1 as an example of the present invention. In steel 14 to 32, an evaluation index VL1000, although the effect of improving hardenability D _I is observed, which is a comparative example No. In the steel materials of 33-41, at least 1 or more of these characteristics were inferior compared with the steel material of an Example. (See Figs. 3 and 4)

  Specifically, no. Nos. 33, 36, and 39 are out of the range defined by the present invention, and as a result, the amount of solute Al is less than that defined by the present invention. It was inferior to the inventive steel having a content, and the effect of improving hardenability by solute Al was also small.

  No. 34, 35, 37, 40 and 41 are coarse AlN because the heating temperature at the actual forging was lower than the required heating temperature T1 (° C.) calculated from the Al amount, N amount, and the above formula (1). VL1000, which is an index of machinability, was inferior to the inventive steel having the same S content.

  No. In No. 38, the total amount of N and the total amount of Al are within the specified ranges, and the heating temperature during forging also satisfies the required heating temperature calculated from (1) above. Therefore, the solute Al value is outside the range specified in the present invention at the carburizing heating temperature carried out, and VL1000, which is an index of machinability, is inferior to the invention steel having the same S content. It was. Moreover, the effect of improving hardenability by solute Al was also small.

(Example 3)
Example 3 shows a test example when the present invention is applied to an alloy steel of about 0.4% C (for example, C is in the range of 0.35 to 0.45%). Other chemical components are Si: 0.1-0.3%, Mn: 0.5-1.0%, P: 0.020% or less, S: 0.01% or more and 0.1% or less, Cr: 0.5% to 1.5% or less, Al: 0.055 to 0.3%. In Example 3, a jomini test was performed at a heating temperature assuming a cutting test and quenching after normalization. Specifically, after 150Kg of steel having the composition shown in Table 3-1 was melted in a vacuum melting furnace, it was hot forged at the heating temperature shown in Table 3-3 and forged into two types of cylinders with diameters of 65 mm and 35 mm. Stretched. And about the steel material of this Example, the machinability test, observation of AlN, and measurement of the amount of solute Al were performed with the test piece cut out from the 65 mm forged material by the method shown below. Moreover, the Jomini test was implemented with the Jomini test piece cut out from 35 mm forged material, and the characteristic was evaluated.

[Machinability test]
First, the steel material after forging was kept at 850 ° C. for 1 hour, then air-cooled, and subjected to heat treatment for normalization, and the hardness of each steel material was adjusted to a range of 225 to 235 at Hv10. Then, the test piece for machinability evaluation was cut out from each steel material after heat processing, the drilling test was performed on the cutting conditions shown to the following Table 3-2, and the machinability of each steel material of an Example and a comparative example was evaluated. At that time, as an evaluation index, a maximum cutting speed VL1000 capable of cutting to a cumulative hole depth of 1000 mm was adopted in the drill drilling test.

[Jomini test]
The Jomini test was carried out on each steel material after forging at a heating temperature of 850 ° C. assuming a quenching heating temperature by a one-time quenching method by a method based on JIS G0561. Thereafter, the resulting Jomini - from the curve and C content, estimates the Jomini distance corresponding to the hardness of 50% M, and converting the Jomini distance hardenability D _I.

[Observation of AlN]
For the observation of AlN, an observation sample was prepared by a replica method using a sample cut out from the Q part of a steel material produced in the same manner as the test piece for machinability test evaluation, and this observation sample was transmitted using a transmission electron microscope. Was observed. Observation was carried out with 20 fields of 1000 μm ² randomly. Then, the case where the total volume of AlN having a maximum diameter exceeding 200 nm was 20% or less of the total volume of all AlN was judged as ◯, and the case where it was 20% or more was judged as x.

[Measurement of solid solution Al content]
First, the amount of Al present as AlN (% by mass) is measured by analyzing the residue obtained by electrolytic extraction with the SPEED method, a potentiostatic electrolytic corrosion method using a non-aqueous solvent electrolyte, and a 0.1 μm filter, using an ICP emission spectrometer. By substituting into the above equation (2), the amount of solid solution Al (% by mass) was determined.

The results of the above tests are summarized in Table 3-3.

No. shown in Table 3-1 and Table 3-3 above. Nos. 42 to 48 are invention examples. 49 to 54 are comparative examples. As shown in Table 3-3 above, No. 1 is an example of the present invention. In steel 42 to 48, an evaluation index VL1000, although the effect of improving hardenability D _I is observed, which is a comparative example No. In the steel materials of 49-54, at least 1 or more of these characteristics were inferior compared with the steel material of an Example. (See FIGS. 5 and 6)

  Specifically, no. Nos. 49, 51, and 53 are out of the range defined by the present invention. As a result, the amount of solute Al is less than the present invention. It was inferior to the inventive steel having a content. The effect of improving hardenability by solute Al was also small.

  No. 50 and 52 are coarse AlN produced because the heating temperature during actual forging was lower than the required heating temperature T1 (° C.) calculated from the above formula (1) from the Al amount and N amount, and machinability. VL1000, which is an index of, was inferior to the inventive steel having the same S content.

  No. 54, the total N amount and the total Al amount are within the specified ranges, and the heating temperature during forging also satisfies the required heating temperature calculated from (1) above, but the total Al amount with respect to the total N amount Therefore, the solute Al value is out of the range specified in the present invention at the quenching heating temperature performed, and VL1000, which is an index of machinability, is inferior to the invention steel having the same S content. The effect of improving the hardenability by solute Al was also small.

It is a figure which shows the relationship between S amount and machinability in Example 1. FIG. It is a diagram showing a relationship between solute Al amount and hardenability in Example 1 (D _I value). It is a figure which shows the relationship between S amount and machinability in Example 2. FIG. It is a diagram showing a relationship between solute Al amount and hardenability in Example 2 (D _I value). It is a figure which shows the relationship between S amount and machinability in Example 3. FIG. It is a diagram showing a relationship between solute Al amount and hardenability in Example 3 (D _I value).

Claims (8)

As a chemical component,
C: 0.14 to 0.85%
Si: 0.01 to 1.5%,
Mn: 0.05 to 2.5%
S: 0.005-0.35%,
Cr: 0.2-6.0%,
N: 0.020% or less,
Al: 0.055-1.0%
And the balance consists of Fe and inevitable impurities,
The total volume of AlN maximum diameter exceeds 200nm is 20% or less of the total volume of all AlN, Ri der solute Al is 0.062 mass% or more, manufactured from molded round-bar steel by hot rolling, machinability and hardenability excellent in hardening steel, wherein the steel der Rukoto the quenching process is applied. The hardened steel material excellent in machinability and hardenability according to claim 1, further comprising P: 0.020% or less by mass% as a chemical component. The hardened steel material excellent in machinability and hardenability according to claim 1 or 2, further comprising Ca: 0.0003 to 0.0015% by mass% as a chemical component. Further, as chemical components, by mass%, Ti: 0.001 to 0.1%, Nb: 0.005 to 0.2%, W: 0.01 to 1.0%, V: 0.01% to The hardened steel material excellent in machinability and hardenability according to any one of claims 1 to 3, comprising one or more selected from the group consisting of 1.0%. The chemical component is 1% 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% in terms of mass%. The hardened steel material excellent in machinability and hardenability according to any one of claims 1 to 4, comprising seeds or two or more kinds. As a chemical component, Sb: 0.0005% to less than 0.0150%, Sn: 0.005-2.0%, Zn: 0.0005-0.5%, B : 0.0005 ˜0.015%, Te: 0.0003-0.2%, Bi: 0.005-0.5%, Pb: One or two selected from the group consisting of 0.005-0.5% The hardened steel material excellent in machinability and hardenability according to any one of claims 1 to 5, comprising the above. The chemical component further contains, in mass%, Mo : 0.01 to 1.0%, excellent in machinability and hardenability according to any one of claims 1 to 6. Hardened steel. Further, as a chemical component, the composition further comprises one or two selected from the group consisting of Ni: 0.05 to 2.0% and Cu: 0.01 to 2.0% by mass%. The hardened steel material excellent in machinability and hardenability according to any one of claims 1 to 7.

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