JP5792108B2 - Heat treatment method - Google Patents

Description

  The present invention relates to a heat treatment method for steel materials, and more particularly to a heat treatment method for steel materials that achieves both strength and toughness of steel materials.

  In the metal quenching and tempering method generally called tempering, the steel is heated from about 850 ° C to about 900 ° C, austenitized, then rapidly cooled and transformed into martensite to increase the strength and hardness of the processed material. Can do. However, in this case, the hardness of the treatment material is increased and at the same time, the material becomes brittle and the impact resistance is deteriorated.

  On the other hand, there is austempering as a processing method for generating a bainite structure having high toughness although hardness is inferior to martensite. Austempering is a process of quenching from the austenite state to a temperature higher than the martensitic transformation temperature (hereinafter sometimes referred to as “Ms temperature”), and maintaining at that temperature to generate a bainite structure and toughness. It is a processing method which aims to improve. However, although bainite is excellent in toughness, its hardness and strength do not reach martensite. In general, since the austempering treatment is performed at a quenching / holding temperature as high as about 300 ° C. to 400 ° C., a salt (salt bath) is used as the cooling medium.

Therefore, in order to compensate for the disadvantages of the bainite and martensite, it is desirable to obtain a mixed structure of both (martensite / bainite mixed structure). Several methods have been proposed for obtaining a martensite / bainite mixed structure.
For example, a method of mixing martensite and bainite for dimensional stabilization has been proposed (see, for example, Patent Document 1). This method involves quenching to a temperature above the Ms temperature, partially transforming the bainite, and then cooling to obtain martensite.

Moreover, in the said general austemper process, the method of austempering using gas instead of salt is proposed (for example, refer patent document 2).
Furthermore, a method of austempering using mineral oil instead of salt has also been proposed (see, for example, Patent Document 3).

JP 2000-129361 A International Publication No. 2003/080876 Pamphlet JP 2000-234124 A

  However, the salt used as a cooling medium for general austempering treatment has toxic demerits, and when removed from the salt furnace, a large amount of salt adheres to the product surface. However, there are problems such as cost demerits due to an increase in salt consumption, complicated waste liquid treatment after washing, cost, and jig corrosion.

  Further, in the heat treatment method described in Patent Document 1, when martensite is generated by cooling after bainite transformation, martensite becomes brittle and, at the same time, retained austenite increases, resulting in dimensional change over time and deterioration of fatigue characteristics. . Furthermore, rapid cooling after partial transformation to bainite and subsequent instantaneous tempering are essential, and the process becomes complicated.

Further, in the heat treatment described in Patent Document 2, even when a gas is used, even if the low temperature can be maintained, the cooling rate from the high temperature is not sufficient, and in the heat treatment of a material with low hardenability, pearlite is not used. To produce toughness degradation.
Further, in the treatment method described in Patent Document 3, the treatment material is cast iron and the heating is induction hardening, so that the structure only near the surface becomes bainite. Further, this austemper treatment requires a separate heating furnace. Therefore, there is a problem that the processing becomes complicated.

  The present invention has been made under such circumstances, and without using the salt used in conventional austempering treatment, it is possible to generate a mixed structure of martensite and bainite that can achieve both strength and toughness. It is possible to provide a heat treatment method for a steel material that can obtain a mixed structure that is excellent in strength while having the same toughness of a bainite structure as compared with conventional austempering treatment, and has an extremely simple process. With the goal.

As a result of intensive studies to achieve the above object, the present inventors have found that the above-mentioned problems can be solved by heat treating a steel material under a predetermined quenching condition using a specific heat treatment oil. It was. The present invention has been completed based on such findings.
That is, the present invention
[1] C: 0.25 to 1.0, Si: 1.0 or less, Mn: 0.3 to 2.0, P: 0.03 or less, S: 0.01 or less, based on mass% , Sol. A step of heating a steel material having a chemical composition containing Al: 0.3 or less and N: 0.01 or less and containing the remainder Fe to an austenitizing temperature or more and austenitizing,
The heat treatment oil having a cooling rate faster than the upper critical cooling rate of the steel material was used, and the oil temperature was maintained in the range of the temperature Ms (° C.) to (Ms-80) (° C.) represented by the following formula (1). Introducing the austenitized steel into the heat treated oil, quenching the temperature of the steel to the oil temperature, and quenching;
A step of holding the quenched steel material for a predetermined time with the oil temperature, and a heat treatment method for the steel material,
Ms (° C.) = 550-361 × [C] −39 × [Mn] + 30 × [Al] (1)
(In the above formula, [C], [Mn] and [Al] represent C, Mn, and sol.Al content (mass%) in the steel material, respectively.)
[2] In the chemical composition, a part of the remaining Fe is, based on mass%, B: 0.005 or less, Ti: 0.1 or less, Cr: 3.0 or less, Nb: 0.1 or less, V : 0.1 or less, Ni: 1.0 or less, Cu: 1.0 or less, Mo: 1.0% or less and Co: 1.0 or less selected from the group consisting of 1.0 or less, and the temperature Ms The heat treatment method according to [1], wherein (° C.) is represented by the following formula (2):
Ms (° C.) = 550-361 × [C] −39 × [Mn] + 30 × [Al] −20 × [Cr] −35 × [V] −17 × [Ni] −10 × [Cu] −5 × [Mo] + 15 × [Co] (2)
(In the above formula, [C], [Mn], [Al], [Cr], [V], [Ni], [Cu], [Mo] and [Co] are respectively C, Mn, sol.Al, Cr, V, Ni, Cu, Mo, and Co content (% by mass))
[3] The heat treatment method according to [1] or [2], wherein the heat treatment oil has an oil temperature of 140 ° C. or more and 300 ° C. or less, and [4] the heat treatment oil has a 100 ° C. kinematic viscosity of 10 mm ² / s. The heat treatment method according to any one of [1] to [3], which is 50 mm ² / s or less,
Is to provide.

  According to the present invention, it is possible to generate a martensite and bainite mixed structure capable of achieving both strength and toughness without using the salt used in the conventional austemper treatment. Compared to the tempering treatment, a mixed structure having excellent strength while having the same toughness of the bainite structure can be obtained, and a heat treatment method for a steel material having a very simple process can be provided.

It is a quenching diagram of time and temperature which shows the heat treatment method of the present invention typically.

Hereinafter, the present invention will be described with reference to embodiments.
The heat treatment method of the present embodiment is based on mass%, C: 0.25 to 1.0, Si: 1.0 or less, Mn: 0.3 to 2.0, P: 0.03 or less, S : 0.01 or less, sol. From the step of heating the steel material having a chemical composition containing Al: 0.3 or less and N: 0.01 or less and including the remainder Fe to the austenitizing temperature or more and austenitizing, and the upper critical cooling rate of the steel material The austenite is used in the heat-treated oil that has a high cooling rate and is maintained at an oil temperature in the range of Ms (° C.) to (Ms-80) (° C.) represented by the following formula (1). And a step of rapidly quenching and quenching the temperature of the steel material to the oil temperature, and a step of holding the rapidly cooled steel material for a predetermined time at the oil temperature. .

When carbon steel is transformed into martensite after bainite transformation by conventional austempering treatment, for example, C concentrates in austenite during bainite transformation, stabilizes retained austenite, and finally increases the amount of retained austenite. In addition, the martensite to be produced becomes hard. On the other hand, in the present embodiment, martensite is first transformed below the Ms temperature to produce martensite equivalent to the material composition, and bainite is produced by transformation of retained austenite during the subsequent holding at a constant temperature. As a result, it becomes a martensite (martensite / bainite mixed structure) which is soft and has excellent toughness mixed with bainite.
In addition, the remaining austenite undergoes bainite transformation during the holding, and this bainite is produced at a lower temperature than the bainite produced by the usual conventional austemper treatment, and therefore has excellent toughness. For this reason, the obtained martensite / bainite mixed structure is excellent in dimensional change and fatigue characteristics, and can achieve both strength and toughness.

(Steel)
First, the steel material used for the heat treatment method of the present embodiment will be described.
Steel materials used in the present embodiment are based on mass%, C: 0.25 to 1.0, Si: 1.0 or less, Mn: 0.3 to 2.0, P: 0.03 or less, S: 0.01 or less, sol. It has a chemical composition containing Al: 0.3 or less and N: 0.01 or less and the balance including Fe.

-C (carbon)-
C is an important element that mainly determines the strength of the steel material after the heat treatment. If the C content is 0.25% by mass or less, sufficient strength cannot be obtained after the heat treatment. Therefore, the C content is 0.25% by mass or more. On the other hand, if the C content exceeds 1.0% by mass, the toughness of the steel after heat treatment is significantly deteriorated. Moreover, since the carbide | carbonized_material in the steel materials before heat processing increases remarkably, an intensity | strength rises and a moldability deterioration becomes remarkable. Therefore, the C content is 1.0% by mass or less.
The C content is preferably from 0.3 to 0.9, more preferably from 0.4 to 0.8, based on mass%.

-Si (silicon)-
Although Si is generally contained as a deoxidizer, it has an effect of enhancing the hardenability of the steel material, and therefore may be positively contained in the steel material used in the present embodiment. However, when the Si content exceeds 1.0% by mass, the increase in Ac ₃ point (temperature at which ferrite and pearlite complete transformation to austenite during heating) becomes significant, and the solid solution of carbide is delayed and hardenability is increased. Cause a decline. It also induces surface flaws during hot rolling. Therefore, the Si content is 1.0% by mass or less. Preferably it is 0.5 mass% or less, More preferably, it is 0.35 mass% or less.
In addition, the minimum of content is 0.01 mass% from a viewpoint of expression of the effect which improves hardenability.

-Mn (manganese)-
Mn has an effect of decreasing the Ac ₃ point and increasing the hardenability of the steel. If the Mn content is less than 0.3% by mass, it is difficult to obtain the effect by the above action. Therefore, the Mn content is 0.3% by mass or more. On the other hand, if the Mn content exceeds 2.0% by mass, the steel for heat treatment becomes hard and it becomes difficult to ensure excellent workability before the heat treatment. Moreover, it becomes easy to produce the band-like structure resulting from the segregation of Mn, and deteriorates toughness. Therefore, the Mn content is 2.0% by mass or less. The Mn content is preferably 1.5% or less.

-P (phosphorus)-
P is generally contained as an inevitable impurity, and has an action of degrading the formability of the heat-treating steel material and the toughness of the steel material after the heat treatment. When the P content is more than 0.03 mass%, the adverse effects due to the above action become significant. Therefore, the P content is 0.03% by mass or less. The P content is preferably 0.015% or less, more preferably 0.01% by mass or less.

-S (sulfur)-
S is generally contained as an inevitable impurity, and has the effect of degrading the formability of the steel for heat treatment and the toughness of the steel after heat treatment. If the S content exceeds 0.01% by mass, the adverse effects due to the above-described action become significant. Therefore, the S content is 0.01% by mass or less. The S content is preferably 0.005% by mass or less.

-Sol. Al (soluble Aluminum)-
Since Al has the effect | action which makes steel sound by deoxidation, you may make it contain in the steel materials used for this embodiment positively. However, sol. When the Al content exceeds 0.3% by mass, the Ac ₃ point is remarkably increased and the solid solution of the carbide is delayed, resulting in a decrease in hardenability. Therefore, sol. Al content shall be 0.3 mass% or less. sol. Al content becomes like this. Preferably it is 0.1 mass% or less, More preferably, it is 0.08 mass% or less.
In order to more reliably obtain the effect of the above action, sol. The Al content is preferably 0.005% by mass or more. In addition, soluble aluminum is shown as a value which subtracted the amount of Al couple | bonded with oxygen from the total amount of Al (total Aluminum) in steel materials.

-N (nitrogen)-
N is generally contained as an unavoidable impurity and has an effect of degrading the formability of the heat-treating steel material. In addition, when B is added to the steel, the amount of the solid solution B in the steel is reduced by combining with the solid solution B to form BN, thereby inhibiting the action of B described later. Have. When the N content exceeds 0.01% by mass, the adverse effects due to the above-described action become significant. Therefore, the N content is 0.01% by mass or less.

-Other optional elements-
In addition to the above essential elements, the steel material in the present embodiment preferably contains one or more selected from the element group shown below together with a suitable content.
・ B (boron)
B, which is an optional element, has an effect of improving hardenability during heat treatment and improving toughness after heat treatment by being present in a solid solution state in steel. However, if the B content exceeds 0.005% by mass, B may form a compound with Fe or the like, and the above-mentioned effects by B may be diminished. Therefore, the B content is preferably 0.005% by mass or less, and more preferably 0.003% by mass or less. In addition, in order to acquire the effect by the said action more reliably, it is preferable that B content shall be 0.0001 mass% or more.

・ Ti (titanium)
Ti, which is an optional element, has the effect of improving hardenability and forms TiN by combining with solid solution N in the steel, thereby reducing the amount of solid solution N in the steel and forming a steel material for heat treatment. Has the effect of improving the properties. In addition, Ti preferentially bonds with solute N in steel as compared with B, so that the decrease in the amount of solute B due to the formation of BN is suppressed, and the above-described action of B is more reliably exhibited. Have Therefore, it is preferable to contain Ti when B is added. However, if the Ti content exceeds 0.1% by mass, it may combine with C in the steel to form a large amount of TiC. Here, C that contributes to an increase in strength of the steel material by the heat treatment is C that exists in a solid solution state in the heating step of the heat treatment. Accordingly, if a large amount of TiC is formed in the steel, the amount of C that contributes to an increase in the strength of the steel material is reduced by the heat treatment, and the target strength may not be obtained in the steel material after the heat treatment. Therefore, the Ti content is preferably 0.1% by mass or less, and more preferably 0.05% by mass or less. In addition, in order to acquire the effect by the said action | operation more reliably, it is preferable to make Ti content 0.005 mass% or more.

Cr (chromium), Nb (niobium), V (vanadium), Ni (nickel), Cu (copper), Mo (molybdenum) and Co (cobalt)
The optional elements Cr, Nb, V, Ni, Cu, Mo, and Co all have the effect of enhancing the hardenability of the steel. Moreover, Nb also has the effect | action which improves the toughness of the steel materials after heat processing. Therefore, the steel material in the present embodiment preferably contains one or more of Cr, Nb, V, Ni, Cu, Mo, and Co. However, if any element is excessively contained, the formability of the steel material before heat treatment may be significantly lowered. In addition, Cr and Mo may be concentrated in carbides in the steel, delaying the solid solution of carbides in the heating step of heat treatment, and reducing the hardenability. Furthermore, the addition of Cu may cause red heat embrittlement during hot rolling, and Ni is preferably added simultaneously in order to avoid red heat embrittlement.

Therefore, it is preferable that the upper limit of the content of each element is set to the following.
Cr: Preferably it is 3.0 mass% or less, More preferably, it is 1.0 mass% or less.
Nb: Preferably it is 0.1 mass% or less, More preferably, it is 0.05 mass% or less.
V: Preferably it is 0.1 mass% or less, More preferably, it is 0.05 mass% or less.
Ni: Preferably it is 1.0 mass% or less, More preferably, it is 0.5 mass% or less.
Cu: Preferably it is 1.0 mass% or less, More preferably, it is 0.5 mass% or less.
Mo: Preferably it is 1.0 mass% or less, More preferably, it is 0.5 mass% or less.
Co: preferably 1.0% by mass or less, more preferably 0.5% by mass or less.
Moreover, in order to obtain the effect by the above action more reliably, Cr: 0.02 mass% or more, Nb: 0.005 mass% or more, V: 0.005 mass% or more, Ni: 0.02 mass% or more, It is preferable to satisfy any of Cu: 0.02 mass% or more, Mo: 0.02 mass% or more, and Co: 0.02 mass% or more.
In addition, the measurement of element content contained in the steel materials described above is performed according to JIS G1253-2002.

In the present embodiment, the chemical composition of the steel material includes remaining Fe (remainder Fe) other than the above elements. Moreover, since the said arbitrary element occupies a part of remainder Fe, the amount of remainder Fe will reduce, so that content of this arbitrary element increases. Furthermore, in addition to the inevitable impurities of P, S, and N, the steel material contains, for example, impurity elements such as Ca, Mg, Bi, and rare earth elements (REM), oxides, nitrides, and the like. May be.
In addition, the remainder Fe in this embodiment does not mean the part of Fe alone other than the Fe part in which the specific element is contained in the steel material, but the specific element uniformly contained in the entire steel material Needless to say, it means Fe elements other than.

Next, the heat treatment method of the present embodiment will be described with reference to the drawings.
FIG. 1 is a time and temperature quenching diagram schematically showing the heat treatment method of the present embodiment. In the figure, a thick solid line (arrow) is a cooling curve showing an example of the heat treatment of this embodiment, and a thin solid line is a continuous cooling transformation curve (abbreviated as “C.C.T.” in the figure). The thick dotted line indicates the Ms temperature line, and the thin dotted line indicates the austenitizing temperature line.

-Austenitization process-
In the heat treatment method of the present embodiment, first, a steel material having the above-described chemical composition is heated to an austenitizing temperature or higher on the Y axis in the figure to austenitize the steel material (austenitizing step). Here, although the austenitizing temperature (Ac ₃ point) varies slightly depending on the composition of the steel material, it is preferably 800 ° C. or higher and 950 ° C. or lower, and more preferably 850 ° C. or higher and 950 ° C. or lower.

-Quenching process-
And after completion | finish of the said austenitization process, in the said heat processing oil which hold | maintained the steel material of an austenitic state to the oil temperature of the range of the temperature Ms (degreeC) represented by following formula (1) to (Ms-80) (degreeC). Then, the steel material is rapidly cooled to the oil temperature (quenching process, curve (a) in FIG. 1).
Ms (° C.) = 550-361 × [C] −39 × [Mn] + 30 × [Al] (1)
(In the above formula, [C], [Mn] and [Al] represent C, Mn, and sol.Al content (mass%) in the steel material, respectively.)
Moreover, temperature Ms (degreeC) is represented by following formula (2), when steel materials contain the said other arbitrary elements.
Ms (° C.) = 550-361 × [C] −39 × [Mn] + 30 × [Al] −20 × [Cr] −35 × [V] −17 × [Ni] −10 × [Cu] −5 × [Mo] + 15 × [Co] (2)
(In the above formula, [C], [Mn], [Al], [Cr], [V], [Ni], [Cu], [Mo] and [Co] are respectively C, Mn, sol.Al, Cr, V, Ni, Cu, Mo, and Co content (% by mass))

As described above, in the present embodiment, in the quenching treatment, it is necessary to perform martensitic transformation before bainite transformation. In order to cause martensitic transformation during cooling prior to bainite transformation, the heat treatment oil temperature (that is, the temperature achieved by rapid cooling from the austenite state) needs to be equal to or lower than the Ms temperature. Moreover, since the mixed structure effect of a martensite and a bainite will not be exhibited if all make martensitic transformation, the minimum of heat processing oil temperature shall be (Ms-80) degreeC.
The heat treatment oil temperature is more preferably in the range of (Ms-20) ° C to (Ms-80) ° C.

More specifically, the heat treatment oil temperature is preferably 140 ° C. or higher and 300 ° C. or lower. By setting the heat treatment oil temperature within this range, even when carbon steel having a high Ms temperature is used as a steel material, the martensite / bainite mixed structure can be obtained efficiently.
The heat treatment oil temperature is more preferably 200 ° C. or higher and 250 ° C. or lower.

In the quenching step, it is necessary to avoid transformation that generates a structure such as pearlite from austenite, and the cooling curve is a continuous cooling transformation curve (C.C.T.) as shown in FIG. Quench to avoid the nose. In this case, the ratio of the transformation from austenite to martensite is determined according to the temperature drop ratio of the temperature T ₁ (° C.) at which the steel material reaches by rapid cooling. The temperatures T ₁ steel reached (℃) is the starting temperature of bainitic transformation in the next step.

  In this embodiment, heat-treated oil is used for the heat treatment. When carbon steel such as S55C having a high Ms temperature and low hardenability is used, the cooling rate is low with gas and heat treatment is difficult. Moreover, since salt has an environmental problem, it is possible to realize heat treatment that is environmentally friendly and excellent in cooling ability by using heat treatment oil instead. At that time, it is important to use heat-treated oil corresponding to a higher temperature than before.

As the heat treatment oil, heat treatment oil having a cooling rate faster than the upper critical cooling rate of the steel material is used. Here, the critical cooling rate refers to a minimum cooling rate in which all the structure is only martensite, and by setting the cooling rate in the quenching process to be higher than this, the cooling curve becomes a continuous cooling transformation curve (CC). (T.) can be quenched to avoid the nose (FIG. 1).
As the steel material used in the present embodiment, it is preferable to use a steel having an upper critical cooling rate of 10 ° C./second or more.

  In the present embodiment, the upper critical cooling rate is obtained by the following test method. That is, a steel material having a predetermined component is machined to produce a plurality of test pieces having a diameter of 3 mm and a length of 10 mm. The test piece is heated to a predetermined temperature at a heating rate of 10 ° C./min in a high-frequency induction heating furnace and then controlled to be cooled at a constant cooling rate in the range of 0.1 to 10 ° C./min. The expansion / contraction of the test piece during the cooling is detected using an operating transformer, and the phase transformation temperature is obtained from the temperature change and length change (expansion / contraction) of the test piece with respect to time. This operation is repeated by sequentially changing the cooling rate, and the phase transformation temperature at each cooling rate is obtained. By plotting this, C.I. C. T. T. et al. A (continuous cooling transformation curve) diagram was created, and the upper critical cooling rate was determined from this diagram.

As heat-treatment oil in this embodiment, what mix | blended various components with the base oil as needed is used.
The base oil is not particularly limited, and is obtained by subjecting paraffin base crude oil, intermediate base crude oil, or naphthene base crude oil to atmospheric distillation residue or atmospheric distillation residue oil under reduced pressure. Examples thereof include distillate oils obtained, and refined oils obtained by refining them according to conventional methods, such as solvent refined oils, hydrogenated refined oils, dewaxed oils, and clay-treated oils.
Synthetic oils include, for example, α-olefin oligomers (obtained by oligomerizing α-olefins having 6 to 16 carbon atoms and those obtained by hydrogenation thereof), and (co) polymers of olefins having 2 to 16 carbon atoms. , Alkylbenzene, alkylnaphthalene, polyphenyl hydrocarbon, various esters, for example, fatty acid esters of polyhydric alcohols such as neopentyl glycol, trimethylolpropane, pentaerythritol, polyoxyalkylene glycol derivatives, and the like.
Among these, solvent refined oil, hydrogenated refined oil and the like are particularly preferably used.

As the base oil, those having a kinematic viscosity at 100 ° C. of 1 mm ² / s to 50 mm ² / s are preferably used. If the kinematic viscosity is lower than 1 mm ² / s, it may be flammable and dangerous, and if it exceeds 50 mm ² / s, it may be too viscous to handle. From this point, the kinematic viscosity is more preferably the following 1.5 mm ² / s or more 45 mm ² / s, more preferably those following 2 mm ² / s or more ^{40mm 2 / s, 10mm 2 /} s or more 25mm Particularly preferred is ² / s or less.
The base oil preferably has a flash point of 150 ° C or higher, more preferably 170 ° C or higher, and further preferably 230 ° C or higher and 320 ° C or lower. If it is 150 degreeC or more, dangers, such as ignition, are low, and generation | occurrence | production of the oil smoke at the time of heat processing can be suppressed simultaneously. Further, the base oil preferably has a viscosity index of 85 or more, more preferably 95 or more. Further, the aromatic content (% C _A ) is preferably 10 or less, more preferably 7 or less, further 3 or less, and particularly preferably 1 or less. When the viscosity index is 85 or more and the aromatic content (% C _A ) is 10 or less, the oxidation stability of the heat-treated oil composition can be maintained well.
Although said mineral oil and synthetic oil can also be used individually by 1 type, 2 or more types can also be mixed and used for them in arbitrary ratios.

A vapor film breaking agent can be further blended in the heat-treated oil in the present embodiment. By adding this vapor film breaker, the vapor film stage can be shortened. Examples of the vapor film breaker include polymer polymers, specifically ethylene-α-olefin copolymers, polyolefins, polymethacrylates, etc., high molecular weight organic compounds such as asphalt, and oil-dispersed inorganic substances. Can be mentioned. These vapor film breakers may be used individually by 1 type, and may be used in combination of 2 or more type.
Further, the content in the heat-treated oil is preferably 1% by mass or more and 10% by mass or less, and more preferably 3% by mass or more and 6% by mass or less. When the content is 1% by mass or more, the effect of adding a vapor film breaker is sufficiently exerted. On the other hand, when the content is 10% by mass or less, the viscosity of the heat-treated oil composition does not become too high and is moderate. The performance as a heat-treated oil composition does not deteriorate.

If necessary, the heat-treated oil of the present embodiment can be blended with additives conventionally used in heat-treated oils, such as surfactants, deteriorated acid neutralizers, antioxidants, and glitter improvers. .
Surfactants include alkaline earth metals or alkali metal salicylates, sulfonates, sulfurized finates and the like. As the alkaline earth metal, calcium, barium and magnesium are preferred. Moreover, as an alkali metal, potassium and sodium are preferable. The content of the surfactant is preferably 0.1% by mass or more and 10% by mass or less, and more preferably 0.2% by mass or more and 7% by mass or less, based on the total amount of the heat-treated oil.
Examples of the degrading acid neutralizer include alkaline earth metal salicylates, sulfurized finates, sulfonates, and the like. As the alkaline earth metal, calcium, barium and magnesium are preferred. Examples of the antioxidant include conventionally known amine-based antioxidants and hindered phenol-based antioxidants. Further, examples of the glitter improver include conventionally known oils and fats, fatty oils and fatty acids, alkenyl succinimides, and substituted hydroxy aromatic carboxylic acid ester derivatives.

The heat-treated oil in the present embodiment preferably has a kinematic viscosity at 100 ° C. of 10 mm ² / s to 50 mm ² / s. When the kinematic viscosity at 100 ° C. is 10 mm ² / s or more, the smoke at a high oil temperature does not become intense, and the vapor film stage does not become long, so that the cooling performance is not lowered. On the other hand, when the kinematic viscosity at 100 ° C. is 50 mm ² / s or less, the cooling performance is not deteriorated and the workability is good.
From the above points, the kinematic viscosity at 100 ° C. is more preferably 12 mm ² / s to 30 mm ² / s.

In addition, the heat-treated oil in the present embodiment is not particularly limited in terms of its properties, but preferably has a flash point of 180 ° C. or higher, and more preferably 190 ° C. or higher. Further, those having higher flash points such as 200 ° C. or higher, 220 ° C. or higher, 240 ° C. or higher, 260 ° C. or higher are more preferable. Thus, if the flash point is high, it can be suitably used as a hot oil or semi-hot oil used for high oil temperature heat treatment.
In addition, if the flash point of heat-treated oil is 200 ° C or higher, it falls under the category of dangerous oils under the Fire Service Act, and the restriction by the restriction of the specified quantity can be eased. If it is 250 degreeC or more, it will be handled as a designated combustible material (non-dangerous material), and there exists an effect which can further ease the restrictions of the Fire Service Act.

Furthermore, the heat-treated oil in this embodiment preferably has excellent cooling performance. The cooling performance here is specifically the cooling time (seconds) from 800 ° C. to 400 ° C. of the cooling curve of the silver specimen in JIS K 2242 (hereinafter sometimes referred to as “400 ° C. seconds”). This indicates that the shorter the cooling time, the better the cooling performance, and it is possible to obtain a quenched product having sufficient hardness by quenching.
As described above, the heat-treated oil in the present embodiment needs to have a cooling rate faster than the upper critical cooling rate of the steel material. For this reason, the heat-treated oil has a 400 ° C. second of 4.5 seconds or less. Is preferred. When the number of seconds at 400 ° C. is in this range, the steel can be rapidly cooled to the bainite transformation start temperature T ₁ (° C.) without causing other transformations while maintaining sufficient hardness. More desirably, the number of seconds at 400 ° C. is more preferably 3.5 seconds or less.

-Holding process-
Next, as shown in FIG. 1B, the rapidly cooled steel material is held for a predetermined time with the oil temperature T ₁ (° C.), and the transformation of the remaining austenite to bainite is started, and martensite bainite is started. Create mixed tissue. The final mixed structure is a hard and tough structure in which martensite and bainite are mixed. Further, when the steel material is maintained at a temperature equal to or lower than the Ms temperature, the surface and the core are at the same temperature, so that the internal stress due to heat is reduced and deformation can be prevented.

The longer the time maintained at this temperature, the greater the amount (occupancy) of the bainite structure. In that case, the amount of bainite generated is not proportional to the retention time in the transformation range. By changing the holding time, the occupation ratio (bainite, martensite, retained austenite) in the structure changes, and thereby the product characteristics change.
The holding time until the bainite transformation is completed varies depending on each material component and the heat treatment oil temperature, but it can be determined from the time at which the expansion saturates during holding by measuring the thermal expansion in the heat pattern simulating the heat treatment conditions. The holding time is not particularly limited, but is preferably 10 minutes or longer and 2 hours or shorter.

  Thereafter, as shown in FIG. 1C, the steel material is cooled to room temperature. Cooling may be performed by air cooling, or other cooling means may be used. Further, the tempering process may be performed by another heat treatment means for adjusting the hardness without waiting for the completion of the isothermal transformation.

  If the steel material is heat-treated by the heat treatment method of the present embodiment described in detail above, a mixed structure of martensite and bainite capable of achieving both strength and toughness can be generated without using salt. In particular, even when carbon steel having a higher Ms temperature than cast iron or the like is used, a structure excellent in dimensional change and fatigue characteristics can be obtained. Further, after quenching from the austenitizing temperature to the heat-treated oil, the target structure can be obtained only by holding for a certain period of time, so that the processing method is extremely simple. Furthermore, since quenching is performed using a specific heat treatment oil, heat treatment excellent in mass productivity and cooling ability is possible as compared with the case where cooling is performed with gas.

  EXAMPLES Next, although an Example demonstrates this invention further in detail, this invention is not limited at all by these examples. In addition, the measurement values, physical property values, etc., steel materials used, heat-treated oil, etc. in the following examples are as follows.

<Analysis, physical property measurement method>
(1) Element content in steel material The element content contained in the steel material was measured by emission spectroscopic analysis according to JIS G1253-2002.
(2) Vickers hardness Using a Vickers hardness tester, measurement was performed at a half position of the radius of the object to be measured having a diameter of 3.75 mm.
(3) Charpy impact value Using a U-notch test piece (notch depth: 2 mm) with a width of 2.5 mm, a height of 10 mm, and a length of 55 mm, the Charpy impact value was measured according to JIS Z 2202. It was performed at room temperature (20 ° C.).
(4) Cooling performance of heat-treated oil In accordance with the cooling performance test stipulated in JIS K 2242, a specified silver specimen heated to 810 ° C is put into a sample (heat-treated oil), and a cooling curve of the silver specimen is obtained. It was measured. The cooling time (400 ° C. seconds) required for cooling from 800 ° C. to 400 ° C. of the cooling curve was measured as the cooling performance. The smaller the number of seconds, the higher the cooling performance.
(5) Kinematic viscosity of heat-treated oil at 100 ° C. Measured according to JIS K 2283.
(6) Flash point of heat-treated oil Measured according to JIS K 2265 (COC method).

<Used steel, heat-treated oil>
(1) Steel material S55C and S60C having the chemical composition shown in Table 1 below, which is carbon steel, were used.
Moreover, Ms temperature which has such a composition is 315 degreeC by S55C, and is 300 degreeC by S60C. These upper critical cooling rates are 30 ° C./second for S55C and 30 ° C./second for S60C.

なお、上記第１表に示すように、Ｓ５５Ｃ及びＳ６０Ｃには検出限度内でＣｒ以外のその他の任意元素は含まれない。

As shown in Table 1, S55C and S60C do not include any other element other than Cr within the detection limit.

(2) Heat-treated oil As heat-treated oil, high oil temperature quenching hot oil “Daphney High Temp Oil AS” (manufactured by Idemitsu Kosan Co., Ltd., base oil: hydrogenated refined oil, additive: alkaline earth as antioxidant Metal salicylate and sulfonate) were used. The characteristics of the heat treated oil are as follows.
Flash point: 276 ° C
- the kinematic viscosity ^{(40 ℃: 182.8mm 2 /s,100℃:17.93mm 2} / s)
Acid value: 0.48 mm KOH / g
・ Base number: 5.4
・ 400 ° C seconds: 3.2 seconds

<Example 1>
(1) Heat treatment Heat treatment was performed under the following experimental conditions.
-Material to be treated (test piece): S55C, width 2.5 mm x height 10 mm x length 55 mm
・ Heating condition: 840 ° C. × 50 minutes ・ Quenching condition: oil temperature 250 ° C., strong stirring ・ Retention time: 30 minutes (including cooling time)
(2) Evaluation About the quenching processed material, the Vickers hardness and the Charpy impact value were measured according to the said conditions. The results are shown in Table 2.

<Example 2 and Comparative Examples 1-5>
As shown in Table 2, each steel material was heat-treated in the same manner as in Example 1 except that the steel material type, oil temperature, etc. were changed, and the same evaluation was performed on the obtained treated materials. For Comparative Examples 4 and 5, the coolant was salt, and for Comparative Examples 2 and 3, tempering was performed. The results are summarized in Table 2.

Regarding the results in Table 2, first, when S55C is used as the steel material, the Ms temperature of the material is 315 ° C., so quenching to an oil temperature of 315 ° C. to 235 ° C. ((Ms−80) ° C.). desirable. In Example 1 where the oil temperature was 250 ° C., both the hardness and the impact resistance value were high, and a good martensite / bainite mixed structure was obtained.
On the other hand, in Comparative Examples 1 and 2 in which the oil temperatures were 210 ° C. and 120 ° C., sufficient hardness was obtained, but the impact resistance value was low. This is because the structure is mainly martensite and bainite is hardly generated. Further, tempering treatment is effective for increasing the impact resistance value, but in Comparative Example 2, the test piece having an oil temperature of 120 ° C. was subjected to tempering treatment at 230 ° C. × 2 hours. It was low.

Moreover, when S60C is used as the steel material, the Ms temperature of the material is 300 ° C, so it is desirable to quench the oil to an oil temperature of 300 ° C to 220 ° C ((Ms-80) ° C). In Example 2 in which the oil temperature was 250 ° C., both the hardness and the impact resistance value were high, and a good martensite / bainite mixed structure was obtained.
On the other hand, in Comparative Example 3 in which the oil temperature is 70 ° C., sufficient hardness is obtained, but the impact resistance value is low. This is because the structure is mainly martensite and bainite is hardly generated. Further, tempering is effective for increasing the impact resistance value. In Comparative Example 3, the test piece having an oil temperature of 70 ° C. was tempered at 230 ° C. for 2 hours. It was low.

  Further, in Comparative Examples 4 and 5 in which salt was used as the coolant and the salt temperatures were 330 ° C. and 340 ° C., respectively, so-called austempering treatment was performed, so that all became bainite structure and the Charpy impact value was high. The result was inferior.

  In the heat treatment method of the present invention, it is possible to generate a mixed structure of martensite and bainite that can achieve both strength and toughness without using the salt used in the conventional austemper treatment. Compared to the austempering treatment, it is possible to obtain a mixed structure having excellent strength while having the same toughness of the bainite structure. Moreover, the heat processing method of steel materials with a very simple process can be provided.

Claims (6)

C: 0.25 to 1.0, Si: 1.0 or less, Mn: 0.3 to 2.0, P: 0.03 or less, S: 0.01 or less, sol. A step of heating a steel material having a chemical composition containing Al: 0.3 or less and N: 0.01 or less and containing the remainder Fe to an austenitizing temperature or more and austenitizing,
The heat treatment oil having a cooling rate faster than the upper critical cooling rate of the steel material was used, and the oil temperature was maintained in the range of the temperature Ms (° C.) to (Ms-80) (° C.) represented by the following formula (1). Introducing the austenitized steel into the heat treated oil, quenching the temperature of the steel to the oil temperature, and quenching;
Holding the quenched steel material at the oil temperature for 10 minutes to 2 hours to transform the residual austenite into bainite .
Ms (° C.) = 550-361 × [C] −39 × [Mn] + 30 × [Al] (1)
(In the above formula, [C], [Mn] and [Al] represent C, Mn, and sol.Al content (mass%) in the steel material, respectively.) In the chemical composition, a part of the balance Fe is, based on mass%, B: 0.005 or less, Ti: 0.1 or less, Cr: 3.0 or less, Nb: 0.1 or less, V: 0.00. 1 or less, Ni: 1.0 or less, Cu: 1.0 or less, Mo: 1.0% or less, and Co: 1.0 or less selected from the group consisting of 1.0 or less, and the temperature Ms (° C.) The heat processing method of Claim 1 by which following is represented by following formula (2).
Ms (° C.) = 550-361 × [C] −39 × [Mn] + 30 × [Al] −20 × [Cr] −35 × [V] −17 × [Ni] −10 × [Cu] −5 × [Mo] + 15 × [Co] (2)
(In the above formula, [C], [Mn], [Al], [Cr], [V], [Ni], [Cu], [Mo] and [Co] are respectively C, Mn, sol.Al, Cr, V, Ni, Cu, Mo, and Co content (% by mass))   The heat treatment method according to claim 1 or 2, wherein an oil temperature of the heat treatment oil is 140 ° C or higher and 300 ° C or lower. The heat treatment method according to claim 1, wherein the heat-treated oil has a 100 ° C. kinematic viscosity of 10 mm ² / s to 50 mm ² / s. The heat treatment method according to claim 1, wherein the upper critical cooling rate is 10 ° C./second or more. The heat treatment method according to any one of claims 1 to 5, wherein the heat-treated oil has a 400 ° C seconds of 4.5 seconds or less. "

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