JP4773106B2 - Steel parts with excellent balance between strength and torsional characteristics, manufacturing method thereof, and steel materials for steel parts - Google Patents

Description

  The present invention relates to a steel part having an excellent balance between strength and torsional characteristics, a method for producing the steel part, and a steel material for the steel part.

  Among structural steel parts in the automotive field, mechanical field, construction field, and the like, and electrical steel parts in the electrical field, it may be required as a characteristic that it has high strength and exhibits excellent twist characteristics. For example, a torsion bar that is a steel part for automobiles is a small part that exhibits high strength (particularly strength that can be withstood in a collision) and excellent torsional characteristics such that it is not twisted even when twisted at high speed. Required.

As a method for obtaining steel parts with high strength, for example, at the manufacturing stage of steel parts, it is possible to increase the strength by work hardening in cold work. It is difficult to ensure twisting characteristics. In Patent Document 1, while increasing the amount of C to increase the strength and precipitating strengthening by precipitating an Nb compound in the ferrite structure, when the amount of C is increased in this way, a pearlite structure is generated and ductility is increased. Decreases, and it becomes difficult to improve the twist characteristics as well as the strength. Also, there is a method to ensure high strength by quenching and tempering after cold working, but under the manufacturing conditions proposed so far, it is possible to demonstrate excellent torsional characteristics even if strength can be ensured. was difficult.
From the above, realization of a steel part excellent in balance between strength and torsional characteristics is desired.
JP 2003-105495 A

  This invention is made | formed in view of the said situation, Comprising: The objective is to provide the steel material excellent in intensity | strength-twisting property balance, its manufacturing method, and the steel materials used for manufacture of this steel component. is there.

The steel material according to the present invention is
C: 0.03 to 0.12% (in the case of chemical components, the meaning of mass%, the same shall apply hereinafter),
Si: 0.5% or less (excluding 0%),
Mn: 0.25 to 2%,
P: 0.02% or less (excluding 0%),
S: 0.02% or less (excluding 0%),
Ni: 1.5% or less (excluding 0%),
Cr: 1% or less (excluding 0%),
Mo: 1% or less (excluding 0%),
Al: 0.01 to 0.06%,
N: 0.01% or less (excluding 0%),
Ti: 0.005 to 0.08%,
B: 0.0008 to 0.003%
Consists of the balance iron and inevitable impurities,
The critical diameter DI value represented by the following formula (1) is 20 to 40,
Ac3 point represented by the following formula (2) is 825 to 925 ° C., and when quenched and tempered under the following conditions, it has a characteristic in that it exhibits hardness: Hv110-350 and the following twist characteristics. ing. The steel material of the present invention may further contain Nb: 0.01 to 0.04% and / or V: 0.04 to 0.25%.
DI value = 15.07 x exp (-0.08 x γGS) x (0.7Si + 1) x (2.19Cr + 1) x (3.5Mn + 1)
× (2.9Mo + 1) × (0.9Ni + 1) × [1.5 (0.9−C) +1] × √C (1)
[In the formula (1), C, Si, Cr, Mn, Mo, Ni indicate the content (mass%) of each component.
And γGS = 9]
Ac3 point (℃) ＝ 908−2.237C × 100 + 0.4385P × 1000 + 0.3049Si × 100
−0.3443Mn × 100−0.23Ni × 100 (2)
[In formula (2), C, P, Si, Mn, and Ni indicate the content (% by mass) of each component]

<Quenching and tempering conditions>
Specimen shape: Diameter 10.9mm x Length 150mm
Quenching condition: Water cooling after holding at 875 ° C. for 15 minutes Tempering condition: Cooling after holding at 600 ° C. for 120 minutes <Torsion characteristics>
Twist test: Distance between gauge points = 50 mm
Twist speed = 1rpm
Measurement items: number of twists (times), breaking torque (N · m)
Twist breaking stress = (12 x breaking torque) / [π x (diameter) ³ ] = 360 N / mm ² or more Twist value (100D conversion) = number of twists x (diameter / distance between gauge points) x 100 = 130 times or more

  The present invention also defines a steel part having an excellent balance between strength and torsional characteristics obtained by using such a steel material, and the steel part includes C, Si, Mn, P, S, Ni, Cr, Each component of Mo, Al, N, Ti, and B (further Nb and / or V if necessary) satisfies the above range, consists of the remaining iron and inevitable impurities, and has a critical diameter DI represented by the above formula (1) It is characterized in that the value is 20 to 40, the Ac3 point represented by the above formula (2) is 825 to 925 ° C., the metal structure is mainly tempered martensite, and the hardness is Hv 110 to 350. is doing.

In the steel part of the present invention, each component of C, Si, Mn, P, S, Ni, Cr, Mo, Al, N, Ti, and B (if necessary, Nb and / or V) is within the above range. The critical diameter DI value represented by the above formula (1) is 20 to 40, the Ac3 point represented by the above formula (2) is 825 to 925 ° C. Is Hv110-350, and also has the characteristic in the following torsional characteristics.
<Twisting characteristics>
Twist test: Distance between gauge points = 50 mm
Twist speed = 1rpm
Measurement items: number of twists (times), breaking torque (N · m)
Twist breaking stress = (12 x breaking torque) / [π x (diameter) ³ ] = 360 N / mm ² or more Twist value (100D conversion) = number of twists x (diameter / distance between gauge points) x 100 = 130 times or more

The present invention also prescribes a method for manufacturing the steel part. The manufacturing method includes drawing the steel material and then cold-working it into a part shape, followed by quenching under the following conditions ( No tempering) or is characterized by performing quenching and tempering.
Quenching temperature: Ac3 point to 950 ° C
Tempering conditions: held at a temperature of 200 to 690 ° C. for 30 to 120 minutes

  According to the present invention, a steel part having an excellent balance between strength and torsional characteristics can be realized, and particularly steel parts that are used in severe conditions such as torsion bars that are subjected to impact and require extremely high torsional characteristics. Can be provided.

The inventors of the present invention conducted intensive research to realize a steel part having an excellent balance between strength and torsional characteristics.
(A) C amount of steel material used for manufacturing steel parts,
-From the viewpoint of ensuring ductility after quenching and tempering, which has a large effect on torsional characteristics, and-From the viewpoint of ensuring the minimum amount of C to be quenched,
(B) adjusting the amount of chemical components other than C in the steel material used for the production of steel parts within a range in which sufficient strength can be secured by quenching and tempering and no adverse effect on torsional properties;
(C) In the manufacturing process of steel parts, it is essential to perform quenching (no tempering) or quenching and tempering after forming the part shape, and control the quenching conditions (and control the tempering conditions when tempering is performed). By melting all C during quenching and suppressing tempering by precipitating cementite instead of pearlite when tempering,
With the emphasis on these points, it has been found that the component design and manufacturing method should be specified.

  First, the reasons for defining the steel component and the composition of the steel material for obtaining the component will be described.

<C: 0.03 to 0.12%>
C is essential for sufficient quenching in the quenching treatment and needs to be at least 0.03%. Preferably it is 0.04% or more, more preferably 0.05% or more. On the other hand, when the amount of C is too large, cementite is excessively precipitated when tempered, which adversely affects torsional characteristics. Further, the strength of the steel material is increased more than necessary, and cold workability and the like are deteriorated, making it difficult to perform good processing. Therefore, the C content is suppressed to 0.12% or less. Preferably it is 0.08% or less, More preferably, it is 0.07% or less.

<Si: 0.5% or less (excluding 0%)>
When deoxidation is performed by adding Si, the amount of Si in the steel becomes 0.01% or more. Si is an element effective for improving hardenability, and also effective for improving temper softening resistance. Therefore, it may be added positively from these viewpoints, but when the amount of Si becomes excessive, Since the hardness of the ferrite is increased and defects such as the life of the mold used for cold working are shortened, the ferrite content is limited to 0.5% or less. Preferably it is 0.35% or less, More preferably, it is 0.20% or less.

<Mn: 0.25 to 2%>
Mn is an element effective for improving hardenability, and is particularly an element effective for improving strength while maintaining toughness. As described above, when the amount of C is suppressed to a relatively low amount, the austenitizing temperature (Ac3 point) increases. However, when the temperature is raised to 900 ° C. or higher so as to dissolve all C during quenching, the crystal grains become coarse when tempered. Is likely to occur and the twisting characteristics are reduced. Mn is an effective element that lowers the Ac3 point of such a steel material with a reduced amount of C and enables quenching at a relatively low temperature. Furthermore, Mn also has a role of capturing S (sulfur) as MnS and rendering it harmless. In the present invention, Mn is contained in an amount of 0.25% or more in order to sufficiently exhibit these effects. Preferably it is 0.35% or more, more preferably 0.45% or more.

  On the other hand, if the amount of Mn becomes excessive, Mn promotes the formation of grain boundary oxides and segregation of P (phosphorus), and delayed fracture tends to occur, so it is suppressed to 2% or less. Preferably it is 1.85% or less, More preferably, it is 1.7% or less.

<Ni: 1.5% or less (excluding 0%)
Cr: 1% or less (excluding 0%)
Mo: 1% or less (excluding 0%)>
These elements are also useful elements for improving the hardenability, and the hardenability may be adjusted by combining the above Mn and these elements. In order to develop hardenability, it is preferable to contain 0.01% or more in the case of Ni, 0.01% or more in the case of Cr, and 0.01% or more in the case of Mo.

  On the other hand, if these elements are contained excessively, cold workability is impaired, so Ni is 1.5% or less (preferably 1.25% or less, more preferably 1% or less), and Cr is 1%. Or less (preferably 0.6% or less, more preferably 0.3% or less), and Mo is suppressed to 1% or less (preferably 0.8% or less, more preferably 0.6% or less).

<P: 0.02% or less (excluding 0%)>
When P is excessively contained, grain boundary segregation occurs and the toughness is deteriorated. Therefore, the P content is suppressed to 0.02% or less. Preferably it is 0.012% or less.

<S: 0.02% or less (excluding 0%)>
Since S causes MnS to be formed and deteriorates the deformability during cold working, it is suppressed to 0.02% or less. Preferably it is 0.012% or less.

<Al: 0.01 to 0.06%>
Al is an element effective for deoxidation, and contributes to refinement of crystal grains by forming N (nitrogen) and AlN, and is effective in improving toughness. In order to obtain such an effect, it is necessary to contain Al 0.01% or more (preferably 0.02% or more). On the other hand, if Al is contained excessively, AlN becomes coarse and adversely affects toughness. Therefore, the Al content is limited to 0.06% or less (preferably 0.05% or less).

<N: 0.01% or less (excluding 0%)>
N is an element that is inevitably included. If excessively contained, nitride formation with B is promoted, and the effect of improving hardenability by solid solution B cannot be sufficiently achieved. Therefore, the N content is suppressed to 0.01% or less. Preferably it is 0.005% or less.

<Ti: 0.005-0.08%>
Ti is an element effective for forming N and TiN, suppressing the formation of nitrides of N and B, and enhancing the effect of improving the hardenability by B. Moreover, it has the effect of suppressing the coarsening of crystal grains by the TiN. In order to sufficiently exhibit such an effect, the Ti amount is set to 0.005% or more (preferably 0.01% or more, more preferably 0.015% or more). On the other hand, when the amount of Ti is excessive, the precipitation strengthening action by TiC is increased and the cold workability is deteriorated, which is not preferable. Therefore, the Ti content is suppressed to 0.08% or less (preferably 0.07% or less, more preferably 0.06% or less).

<B: 0.0008 to 0.003%>
B is an extremely effective element for ensuring hardenability, and in the present invention, B is contained in an amount of 0.0008% or more. Preferably it is 0.0010% or more, More preferably, it is 0.0015% or more. On the other hand, if the amount of B becomes excessive, the toughness deteriorates, so the content is suppressed to 0.003% or less. Preferably it is 0.0025% or less, More preferably, it is 0.0020% or less.

  The contained elements defined in the present invention are as described above, and the remaining component is substantially Fe, but Cu, Sn, O as inevitable impurities brought into the steel depending on the situation of raw materials, materials, manufacturing equipment, etc. Needless to say, mixing of (oxygen) or the like is allowed, and other elements can be positively contained as described below within a range not adversely affecting the operation of the present invention.

<Nb: 0.01 to 0.04%>
Nb is effective in improving the toughness by forming carbides and nitrides to refine crystal grains. In order to sufficiently exhibit such an effect, it is preferable to contain 0.01% or more of Nb. More preferably, it is 0.015% or more. On the other hand, when Nb is contained excessively, the precipitation strengthening action is increased and the cold workability is deteriorated.

<V: 0.04 to 0.25%>
V is also effective in forming carbides and nitrides as in Nb, making the crystal grains finer, and improving toughness. It is also an element that contributes to the suppression of delayed fracture. In order to sufficiently exhibit such an effect, it is preferable to contain V by 0.04% or more. More preferably, it is 0.08% or more. On the other hand, if the amount of V is excessive, the precipitation strengthening effect becomes large and it becomes impossible to ensure excellent cold workability, so it is preferable to suppress it to 0.25% or less (more preferably 0.2% or less).

Further, in the present invention, in order to ensure the hardenability of the steel material, the component composition (C, Si, Mn, Cr, Mo, Ni, so that the critical diameter DI value represented by the following formula (1) is 20-40. ). If this DI value is 20 or more, even a steel material having a relatively small amount of carbon as in the present invention can be sufficiently quenched. Preferably it is 24 or more. On the other hand, if the DI value becomes too high, the strength of the steel material becomes higher than necessary and adversely affects cold workability, so it is suppressed to 40 or less. Preferably it is 36 or less.
DI value = 15.07 x exp (-0.08 x γGS) x (0.7Si + 1) x (2.19Cr + 1) x (3.5Mn + 1)
× (2.9Mo + 1) × (0.9Ni + 1) × [1.5 (0.9−C) +1] × √C (1)
[In the formula (1), C, Si, Cr, Mn, Mo, Ni indicate the content (mass%) of each component.
And γGS = 9]

Moreover, in this invention, component composition (C, P, Si, Mn, Ni) is adjusted so that Ac3 point calculated | required from following formula (2) may be 825-925 degreeC. If the Ac3 point is too high, it is necessary to increase the heating temperature during quenching, resulting in coarsening of crystal grains. Therefore, the Ac3 point is set to 925 ° C. or lower, preferably 900 ° C. or lower. On the other hand, if the Ac3 point is too low, undissolved carbides remain, so the component composition is adjusted so that the Ac3 point is 825 ° C. or higher (preferably 850 ° C. or higher).
Ac3 point (℃) ＝ 908−2.237C × 100 + 0.4385P × 1000 + 0.3049Si × 100
−0.3443Mn × 100−0.23Ni × 100 (2)
[In formula (2), C, P, Si, Mn, and Ni indicate the content (% by mass) of each component]

A steel material that satisfies the above requirements exhibits hardness: Hv110-350 and the following torsional characteristics when quenched and tempered under the following conditions.
<Quenching and tempering conditions>
Specimen shape: Diameter 10.9mm x Length 150mm
Quenching condition: Water cooling after holding at 875 ° C. for 15 minutes Tempering condition: Cooling after holding at 600 ° C. for 120 minutes <Torsion characteristics>
Twist test: Distance between gauge points = 50 mm
Twist speed = 1rpm
Measurement items: number of twists (times), breaking torque (N · m)
Twist breaking stress = (12 x breaking torque) / [π x (diameter) ³ ] = 360 N / mm ² or more Twist value (100D conversion) = number of twists x (diameter / distance between gauge points) x 100 = 130 times or more

  In the manufacture of conventional steel parts, after wire was drawn, the parts were formed by, for example, cold forging, but this method could not obtain a high-strength part. However, in the present invention, the steel material whose component composition is adjusted as described above is drawn, then cold worked to form a part shape, and then quenched (no tempering) or quenched and tempered under the following conditions. Thus, high strength can be achieved while maintaining excellent twist characteristics.

<Quenching temperature: Ac3 point to 950 ° C.>
In the quenching process, the temperature is first increased to the austenitizing temperature. This is because if the temperature is too low, sufficient baking will not occur during cooling. In order to fully quench, it is preferable to raise it to 870 degreeC or more. On the other hand, if the temperature is too high, the crystal grains are likely to be coarsened, so the temperature is suppressed to 950 ° C. or lower. Preferably it is 930 degrees C or less.

<Tempering conditions: held at a temperature of 200 to 690 ° C. for 30 to 120 minutes>
Tempering may be performed according to a desired strength level, but when tempering is performed, it is preferable to heat to at least 200 ° C. to increase ductility. The higher the temperature, the higher the ductility and the better torsional properties can be achieved, but the tempering temperature should not exceed the limit temperature at which transformation occurs: 690 ° C. Further, when the tempering temperature is increased, the strength decreases in inverse proportion to the improvement of the twist property, and therefore it is preferable to determine the tempering temperature according to the desired strength as described below. In addition, what is necessary is just to adjust intensity | strength between tempering time for 30 to 120 minutes. Moreover, what is necessary is just to cool by methods, such as standing to cool, after heating holding.
When obtaining steel parts with a tensile strength of 800 to 1000 N / mm ² No tempering or tempering at 200 to 250 ° C. When obtaining steel parts with a tensile strength of 550 to 800 N / mm ² No tempering or tempering at 400 to 620 ° C. When obtaining steel parts with a tensile strength of 360 to 550 N / mm ² No tempering or tempering at 620 to 690 ° C.

  The present invention is not particularly specified for manufacturing conditions other than those described above, and general manufacturing conditions can be adopted. A method for melting steel, a predetermined shape (plate, wire, rod, etc.) The hot rolling, the cold rolling, and the wire drawing for the above may be performed under conditions generally employed.

  Cold working is performed to obtain a part shape, and examples of the cold working include cold forging, cold forging, and cold rolling. In the forming process, after the cold working, cutting may be performed before quenching and tempering. From the viewpoint of easy processing, quenching and tempering are performed after the cold processing. Examples of the process (part) for producing the steel part of the present invention include the following process example 1 and process example 2.

<Process example 1>
Descaling (pickling or mechanical descaling) → Lubrication (lime or zinc phosphate coating, etc.) → Wire drawing → Cold working (cold forging, Tempering) → quenching (no tempering) or quenching and tempering under the above conditions <Example 2>
Descaling (pickling or mechanical descaling) → Softening heat treatment → Descaling (pickling or mechanical descaling) → Lubrication treatment (lime or zinc phosphate coating, etc.) → Wire drawing → Cold working (cold forging, cold forging) → Quenching (no tempering) or quenching and tempering under the above conditions

In the present invention, as a steel part obtained by wire drawing using a steel material satisfying the above component composition, then cold-working into a part shape, and then quenching (no tempering) or quenching and tempering under the above conditions , Hardness: Hv110-350 and the following twist characteristics are obtained.
<Twisting characteristics>
Twist test: Distance between gauge points = 50 mm
Twist speed = 1rpm
Measurement items: number of twists (times), breaking torque (N · m)
Twist breaking stress = (12 x breaking torque) / [π x (diameter) ³ ] = 360 N / mm ² or more Twist value (100D conversion) = number of twists x (diameter / distance between gauge points) x 100 = 130 times or more

In particular, when manufactured by the method of Process Example 1 or Process Example 2, a steel part that satisfies the following characteristics is obtained.
-Austenite grain size number: 7-12-If the part shape is axial,
Tensile strength (TS): 360 to 1000 N / mm ²
Aperture (RA): 70-95%
Twist value: 130 times or more (100D conversion)
Twisted rupture stress: 360-660 N / mm ²

The metal structure of the steel part after quenching and tempering is mainly tempered martensite, and ferrite, bainite, pearlite, and cementite are not actively included unless they are inevitably formed in the manufacturing process. The main tempered martensite referred to in the present invention is
-When tempered martensite is a single phase and ferrite or the like unavoidably formed in the manufacturing process as the balance is 3% by volume or less (including 0% by volume), or-Tempered martensite and the second phase are 5 volumes % Or less of retained austenite and the ferrite or the like inevitably formed in the manufacturing process as the balance is 3% by volume or less (including 0% by volume).

  The present invention is not limited to the shape, use, etc. of the steel parts. For example, as steel parts for machine structures, screws, bolts, nuts, sockets, ball joints, inner tubes, torsion bars, clutch cases, cages, housings, Hub, cover, case, washer, tappet, saddle, bulg, inner case, clutch, sleeve, outer race, sprocket, core, stator, anvil, spider, rocker arm, body, flange, drum, fitting, connector, pulley, In addition to metal fittings, yokes, caps, valve lifters, and spark plugs, it can be applied to machine parts, electrical parts, and the like.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.

  Test steels having the composition shown in Table 1 were obtained by melting. The steel type number G shown in Table 1 is JIS G 3507 SWRCH45K of the existing steel, and the steel type number H is JIS G 3508 SWRCHB220.

A rolled material was obtained by hot rolling to a diameter of about 14.0 mm using the test steel. Hardness was measured using a sample obtained by quenching and tempering the rolled material under the following condition (A). Further, the following twist test (B) was performed, and the following twist fracture stress and twist value (100D conversion) were obtained.
<Quenching and tempering conditions (A)>
Specimen shape: Diameter 10.9mm x Length 150mm
Quenching condition: Water cooling after holding at 875 ° C. for 15 minutes Tempering condition: Cooling after holding at 600 ° C. for 120 minutes <Torsion characteristics>
Torsion test (B): Distance between gauge points = 50 mm
Twist speed = 1rpm
Measurement items: number of twists (times), breaking torque (N · m)
Twist breaking stress = (12 x breaking torque) / [π x (diameter) ³ ]
Twist value (100D conversion) = Twist times x (Diameter / Distance between gauge points) x 100
Then, the drawn material obtained by drawing the rolled material to a diameter of about 13.0 mm was subjected to quenching and tempering under the conditions of FIG. 1 and Table 2.

In addition, the deformability shown in Table 2 was determined by performing a compression test as shown in FIG. 2 using a cylindrical sample having a diameter of 12 mm and a length of 18 mm cut out from the rolled material. (1-H / H ₀ ) × 100 (%), which is an index for evaluating the cold forgeability. In addition, about the deformability, since the rolling material and the wire drawing material show substantially the same numerical value, in this example, the deformability was measured using the rolled material.

Using the above tempered and tempered materials, hardness (Hv) measurement, tensile test and twist test were performed, and the properties after quenching and tempering were evaluated. In the hardness (Hv) measurement, five Vickers hardnesses were measured from (sample cross-section diameter / 4) to the central portion, and the average value was obtained. In the tensile test, tensile strength (TS) and drawing (RA) were measured using a JIS No. 9 test piece. In the twist test, a JIS No. 9 test piece was sampled and tested under the conditions of a distance between gauge points: 50 mm and a twist speed: 1 rpm, and the number of twists (times) and breaking torque (N · m) until breaking. Measurement was performed, and the number of twists (twist value in terms of 100D) when the distance between the gauge points was [diameter (D) × 100] was calculated based on the following formula. Further, the twist rupture stress was obtained from the following formula. These results are shown in Table 2.
Twist value (100D conversion) = Twist times x (Diameter / Distance between gauge points) x 100
Twist breaking stress = (12 x breaking torque) / [π x (diameter) ³ ]

  From the above results, it can be considered as follows. First, No. 2 in Table 2. Nos. 1 to 13 (shown in Experiment No. 2 in Table 2. The same applies hereinafter) are steel materials that satisfy the component composition defined in the present invention and are tempered under specified conditions. Is excellent in strength-twist characteristic balance. Further, it can be seen that the deformability is excellent and the workability can be secured.

  On the other hand, No. using conventional steel materials (steel materials G and H). Nos. 14 and 15 are both high in strength but small in twist value and inferior in strength-twist characteristic balance.

  For reference, experiment no. FIG. 3 shows a microstructure photograph (magnification: 400 times) of the cross section of the steel part obtained in 7 (Example of the present invention) observed with an optical microscope. FIG. 3 shows that the steel material of the present invention has a structure mainly composed of tempered martensite and has a small crystal grain size of prior austenite.

It is a figure which shows the quenching and tempering pattern employ | adopted in the Example. It is the schematic diagram which showed the mode of the compression test in an Example. Experiment No. It is a microscope picture of the steel part obtained by 7 (invention example).

Claims (6)

C: 0.03 to 0.12% (in the case of chemical components, the meaning of mass%, the same shall apply hereinafter),
Si: 0.5% or less (excluding 0%),
Mn: 0.25 to 2%,
P: 0.02% or less (excluding 0%),
S: 0.02% or less (excluding 0%),
Ni: 0.01 to 1.5%,
Cr: 0.01-1%,
Mo: 0.01 to 1%,
Al: 0.01 to 0.06%,
N: 0.01% or less (excluding 0%),
Ti: 0.005 to 0.08%,
B: 0.0008 to 0.003%
Consists of the balance iron and inevitable impurities,
The critical diameter DI value represented by the following formula (1) is 20 to 40,
Ac3 point represented by following formula (2) is 825-925 degreeC, and when hardened and tempered on the following conditions, while showing hardness: Hv110-350, the steel material characterized by showing the following twist property .
DI value = 15.07 x exp (-0.08 x γGS) x (0.7Si + 1) x (2.19Cr + 1) x (3.5Mn + 1)
× (2.9Mo + 1) × (0.9Ni + 1) × [1.5 (0.9−C) +1] × √C (1)
[In the formula (1), C, Si, Cr, Mn, Mo, Ni indicate the content (mass%) of each component.
And γGS = 9]
Ac3 point (℃) ＝ 908−2.237C × 100 + 0.4385P × 1000 + 0.3049Si × 100
−0.3443Mn × 100−0.23Ni × 100 (2)
[In formula (2), C, P, Si, Mn, and Ni indicate the content (% by mass) of each component]
<Quenching and tempering conditions>
Specimen shape: Diameter 10.9mm x Length 150mm
Quenching condition: Water cooling after holding at 875 ° C. for 15 minutes Tempering condition: Cooling after holding at 600 ° C. for 120 minutes <Torsion characteristics>
Twist test: Distance between gauge points = 50 mm
Twist speed = 1rpm
Measurement items: number of twists (times), breaking torque (N · m)
Twist breaking stress = (12 x breaking torque) / [π x (diameter) ³ ] = 360 N / mm ² or more Twist value (100D conversion) = number of twists x (diameter / distance between gauge points) x 100 = 130 times or more Furthermore,
Nb: 0.01-0.04% and / or V: 0.04-0.25%
The steel material of Claim 1 containing. C: 0.03-0.12%,
Si: 0.5% or less (excluding 0%),
Mn: 0.25 to 2%,
P: 0.02% or less (excluding 0%),
S: 0.02% or less (excluding 0%),
Ni: 0.01 to 1.5 %,
Cr: 0.01 to 1 %,
Mo: 0.01 to 1 %,
Al: 0.01 to 0.06%,
N: 0.01% or less (excluding 0%),
Ti: 0.005 to 0.08%,
B: 0.0008 to 0.003%
Consists of the balance iron and inevitable impurities,
The critical diameter DI value represented by the following formula (1) is 20 to 40,
The Ac3 point represented by the following formula (2) is 825 to 925 ° C., the metal structure is martensite or tempered martensite, and ferrite, bainite, pearlite, and cementite are 3% by volume or less in total (0 Steel parts having an excellent balance between strength and torsional characteristics, characterized in that the hardness is Hv110 to 350.
DI value = 15.07 x exp (-0.08 x γGS) x (0.7Si + 1) x (2.19Cr + 1) x (3.5Mn + 1)
× (2.9Mo + 1) × (0.9Ni + 1) × [1.5 (0.9−C) +1] × √C (1)
[In the formula (1), C, Si, Cr, Mn, Mo, Ni indicate the content (mass%) of each component.
And γGS = 9]
Ac3 point (℃) ＝ 908−2.237C × 100 + 0.4385P × 1000 + 0.3049Si × 100
−0.3443Mn × 100−0.23Ni × 100 (2)
[In formula (2), C, P, Si, Mn, and Ni indicate the content (% by mass) of each component] C: 0.03-0.12%,
Si: 0.5% or less (excluding 0%),
Mn: 0.25 to 2%,
P: 0.02% or less (excluding 0%),
S: 0.02% or less (excluding 0%),
Ni: 0.01 to 1.5 %,
Cr: 0.01 to 1 %,
Mo: 0.01 to 1 %,
Al: 0.01 to 0.06%,
N: 0.01% or less (excluding 0%),
Ti: 0.005 to 0.08%,
B: 0.0008 to 0.003%
Consists of the balance iron and inevitable impurities,
The critical diameter DI value represented by the following formula (1) is 20 to 40,
The Ac3 point represented by the following formula (2) is 825 to 925 ° C., the metal structure is martensite or tempered martensite, and ferrite, bainite, pearlite, and cementite are 3% by volume or less in total (0 A steel part having an excellent strength-twist characteristic balance, characterized in that the hardness is Hv110-350 and the following twist characteristics are exhibited.
DI value = 15.07 x exp (-0.08 x γGS) x (0.7Si + 1) x (2.19Cr + 1) x (3.5Mn + 1)
× (2.9Mo + 1) × (0.9Ni + 1) × [1.5 (0.9−C) +1] × √C (1)
[In the formula (1), C, Si, Cr, Mn, Mo, Ni indicate the content (mass%) of each component.
And γGS = 9]
Ac3 point (℃) ＝ 908−2.237C × 100 + 0.4385P × 1000 + 0.3049Si × 100
−0.3443Mn × 100−0.23Ni × 100 (2)
[In formula (2), C, P, Si, Mn, and Ni indicate the content (% by mass) of each component]
<Twisting characteristics>
Twist test: Distance between gauge points = 50 mm
Twist speed = 1rpm
Measurement items: number of twists (times), breaking torque (N · m)
Twist breaking stress = (12 x breaking torque) / [π x (diameter) ³ ] = 360 N / mm ² or more Twist value (100D conversion) = number of twists x (diameter / distance between gauge points) x 100 = 130 times or more Furthermore,
Nb: 0.01-0.04% and / or V: 0.04-0.25%
The steel part according to claim 3 or 4 containing. A method of manufacturing a steel part according to any one of claims 3 to 5, wherein the steel material according to claim 1 or 2 is drawn and then cold worked to obtain a part shape. And then quenching (no tempering) or quenching and tempering under the following conditions.
Quenching temperature: Ac3 point to 950 ° C
Tempering conditions: held at a temperature of 200 to 690 ° C. for 30 to 120 minutes

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