WO2013018893A1

WO2013018893A1 - Untempered steel for hot casting, hot-casted untempered article and method for producing same

Info

Publication number: WO2013018893A1
Application number: PCT/JP2012/069861
Authority: WO
Inventors: 真也寺本; 啓督高田
Original assignee: 新日鐵住金株式会社
Priority date: 2011-08-03
Filing date: 2012-08-03
Publication date: 2013-02-07
Also published as: JPWO2013018893A1; CN103228809B; JP5206911B1; KR20140012209A; CN103228809A; KR101458348B1

Abstract

The purpose of the present invention is to provide: an untempered steel for hot casting which is available in induction hardening; a hot-casted untempered article, in which the structure therein is controlled in the process of cooling following the hot-casting so as to prevent lowering in machinability associated with high strength, thereby improving fatigue strength; and a method for producing the same. The steel according to the present invention is characterized by comprising, in terms of mass%: 0.45-0.60% of C; 0.02-0.15% of Si; 1.50-3.00% of Mn; 0.0002-0.150% of P; 0.001-0.200% of S; 0.02-1.00% of Cr; 0.001-0.300% of Al; 0.01-0.30% of V; 0.03-1.00% of Mo; 0.0020-0.0070% of N; and the balance being Fe and unavoidable impurities. The hot-casted untempered article, which can be induction-hardened, having the aforesaid steel composition, comprising a bainitic structure amounting to 95% or greater, in terms of area, of the steel structure, and containing Mo carbonitride dispersed in the steel, and the method for producing the same.

Description

Non-tempered steel for hot forging, non-tempered hot forged product, and method for producing the same

The present invention relates to a non-heat treated steel material for hot forging that is processed into machine parts such as automobiles and industrial machines without being subjected to a tempering treatment after hot forging, and a hot work using the same. The present invention relates to a forged non-tempered product and a method for producing the same, and is particularly hot forged, has a high strength and a high durability ratio without being tempered, and can be induction hardened.

Traditionally, many mechanical structural parts such as automobiles and industrial machinery have been given high strength and high toughness by hot forging from raw steel bar to part shape and then reheating and quenching and tempering treatment. It was. In recent years, the tempering process for quenching and tempering has been omitted from the viewpoint of reducing the manufacturing cost. For example, as seen in Patent Document 1, the hot forging remains as it is without tempering. Non-tempered steel that can impart high strength and high toughness has been proposed. Among them, for shaft parts such as crankshafts, after hot working, they are cooled to give a predetermined strength, further processed into a predetermined shape by machining, etc., and then subjected to induction quenching on the necessary parts. By forming the hardened layer, wear resistance and fatigue strength are improved.

Patent Document 1 has a strength and low temperature toughness that are higher than those of conventional tempered materials while still being hot forged, using a steel material to which Si is added in excess of 1.0% and a large amount of S, V, and N are added. It is described that hot forged non-tempered steel was obtained. However, there is no description regarding the fatigue strength and durability ratio of this non-heat treated steel.

There have been some reports on non-heat treated steel for induction hardening. For example, the invention described in Patent Document 2 prevents a decrease in surface layer hardness and surface hardened layer depth due to generation of residual ferrite after induction hardening by setting the structure before induction hardening to a bainite ratio of 75% or more. It is an invention. However, the non-heat treated steel for induction hardening described in Patent Document 2 has no description regarding fatigue strength and durability ratio, and these characteristics are not considered at all.

For example, the invention described in Patent Document 3 is an invention that reduces the amount of retained austenite after induction hardening. However, in the non-heat treated steel for induction hardening described in Patent Document 3, there is no description regarding fatigue strength and durability ratio, and these characteristics are not considered at all. Further, it is described that an appropriate amount of S, Pb, Bi, Te, Se, and Ca may be added to improve machinability. However, when the tensile strength is 1100 MPa or more, the machinability is improved. The effect is known to be small.

In the application of these high-strength non-heat treated steels to steel parts for machine structures, what actually becomes an obstacle is to achieve both the high fatigue strength and the contradictory properties of machinability. In general, the fatigue strength depends on the tensile strength, and the fatigue strength increases as the tensile strength is increased. On the other hand, an increase in tensile strength decreases machinability. Many steel parts for machine structures require cutting after hot forging, leading to a significant increase in cutting costs. In general, when the tensile strength is increased in order to increase the fatigue strength of steel parts for machine structural use, it has been found that if the tensile strength exceeds 1300 MPa, the machinability is significantly reduced. Since the cutting manufacturing cost is greatly increased, it is practically difficult to increase the strength exceeding the tensile strength of 1300 MPa. Therefore, in these machine structural parts, an increase in cutting cost due to a decrease in machinability is a bottleneck in achieving high fatigue strength, and a technique for achieving both high fatigue strength and machinability is required.

For example, Patent Document 4 describes an invention in which the improvement in strength after hot forging is suppressed to ensure machinability, and the fatigue strength of the entire component is improved by increasing the depth of the surface hardened layer by induction hardening. Has been.

Also, as a means of achieving both high fatigue strength and machinability, it is effective to improve the ratio of fatigue strength to tensile strength, that is, the durability ratio (= fatigue strength / tensile strength). For example, Patent Document 5 proposes that it is effective to reduce the high-carbon island-like martensite and retained austenite in the bainite-based metal structure. However, the durability ratio is at most 0.56 or less, and there is a limit to increasing the strength without reducing the machinability, and these fatigue strengths are both low.

Patent Document 6 describes a crankshaft that can achieve high wear resistance and fatigue strength, and that achieves both high machinability and a method for manufacturing the crankshaft. In this method, the microstructure of the hot forged product before soft nitriding is made to be a bainite-based (70% or more) microstructure, and this hot forged product is soft nitrided under a temperature condition of 550 to 650 ° C. The mechanical properties such as fatigue strength of the crankshaft are improved. In order to increase the internal hardness after nitrocarburizing moderately and to obtain high fatigue strength, the steel material components are obtained using the parameters Hg representing the amounts of C, Si, Mn, Cr, Mo and V of the steel material with specific relational expressions. Is stipulated. However, nitrocarburizing treatment usually requires exposure to an atmosphere containing nitrogen and heating in a temperature range below the austenitizing temperature. Compared with surface hardening treatment by induction hardening, the equipment and cost are reduced. It takes. In addition, because the steel material intended for soft nitriding over a certain time has a large amount of Si, in the induction hardening by instantaneous induction heating only on the surface, residual austenite remains in the internal structure, and high fatigue strength is I can't get it.

Japanese Patent Laid-Open No. 1-198450 Japanese Patent Laid-Open No. 63-1000015 JP-A-11-286744 JP 2005-68518 A Japanese Patent Laid-Open No. 4-176842 JP 2010-189697 A

Therefore, the present invention advantageously solves the above-described problems and controls the microstructure in the part by non-heat treated steel for hot forging that enables induction hardening and cooling after hot forging. An object of the present invention is to provide a hot forged non-heat treated product that suppresses a decrease in machinability associated with strength and improves fatigue strength, and a manufacturing method thereof.

The present invention uses a steel in which a large amount of Mo is added to high carbon steel that can be induction hardened to obtain a high surface hardness, and in the cooling process after hot forging, a large amount of Mo carbonitride is precipitated, It has been found that it has a high durability ratio by reducing the defect density such as dislocation, and suppresses the deterioration of machinability associated with high strength, and obtains a hot forged non-heat treated product with improved fatigue strength, The present invention has been completed. Here, “carbonitride” used in the present specification means carbonitride and carbide.

The gist of the present invention is as follows.

(1) By mass%, C: 0.45 to 0.60%, Si: 0.02 to 0.15%, Mn: 1.50 to 3.00%, P: 0.0002 to 0.150% , S: 0.001 to 0.200%, Cr: 0.02 to 1.00%, Al: 0.001 to 0.300%, V: 0.01 to 0.30%, Mo: 0.03 A non-tempered steel for hot forging that can be induction-hardened and contains 1.00%, N: 0.0020-0.0070%, the balance being Fe and inevitable impurities.

(2) Further, by mass%, one or more of Ca: 0.0002 to 0.0100%, Te: 0.0002 to 0.1000%, Zr: 0.0002 to 0.2000% The non-tempered steel for hot forging capable of induction hardening as described in (1) above.

(3) The steel component according to the above (1) or (2) is included, the steel structure is an area ratio of 95% or more is a bainite structure, and the average size of Mo carbonitride dispersed in the steel is 4 nm. As described above, a hot-forged non-tempered product capable of induction hardening characterized by being 11 nm or less.

(4) The steel material comprising the component composition described in (1) or (2) above is heated to 1000 ° C. or more and 1250 ° C. or less for hot forging, and after the hot forging, the average cooling rate up to 200 ° C. Is heated at 0.05 ° C./second or more and 0.80 ° C./second or less, and induction hardening is performed on a portion where strength is required.

The steel of the present invention is optimal as a raw material for hot forged non-heat treated steel parts capable of induction hardening with high fatigue strength while suppressing an increase in cutting cost. Moreover, the manufacturing method of this invention can manufacture the hot forging non-tempered goods which can be induction-hardened which has a high durability ratio and high fatigue strength. Furthermore, the hot forged non-heat treated product of the present invention can be induction hardened when used as a part for automobiles or industrial machines, so that the parts can be further strengthened, and the weight of the vehicle can be reduced. Contributes to reducing fuel consumption and cost.

The present inventors diligently studied the steel component range, the structure morphology, and the heat treatment conditions for the above-mentioned objects, and found the following findings. That is,
(A) It has a higher durability ratio (= fatigue strength / tensile strength) than conventional non-tempered steel by dispersing fine Mo carbonitrides in a bainite structure of 95% or more in area ratio. The influence of the cooling rate after hot forging is large, and the durability ratio is improved as the cooling rate is decreased. This is because the tensile strength and fatigue strength increase as the cooling rate decreases and the amount of precipitation increases as the time in which the Mo carbonitride precipitates increases, but the smaller the cooling rate, the more This is because the carbonitride is coarsened and the tensile strength is remarkably reduced, while the fatigue strength is increased or maintained without decreasing. In general, precipitation of carbonitride such as Mo used for precipitation strengthening increases not only fatigue strength but also tensile strength and remarkably reduces machinability, so that high fatigue strength and good machinability are not compatible. It was found that fatigue strength can be increased without increasing the tensile strength affecting machinability by coarsening the precipitate and controlling the size of the Mo carbonitride to 4 nm or more and 11 nm or less. However, it is necessary that the main structure is a bainite structure and the other pro-eutectoid ferrite and retained austenite structure is less than 5%.
(B) V, like Mo, forms carbonitrides and contributes to improving the durability ratio. However, high N forms V nitrides that are stable at higher temperatures, and proeutectoid ferrite in the cooling process after hot forging. It leads to a decrease in strength and durability ratio. Low N is a necessary condition for fully utilizing the effect of improving the durability ratio by V carbonitride.

The present invention has been made for the first time after further studies based on these findings.

Hereinafter, the present invention will be described in detail. First, the reason for limiting the steel component range described above will be described.

C: 0.45 to 0.60%
C is an important element that determines the strength of steel. Compared to other alloy elements, the alloy cost is low. If a large amount of C can be added, the alloy cost of the steel material can be reduced. Further, the surface hardness after induction hardening is determined by the amount of C in the steel, and in order to obtain the required strength, the lower limit is made 0.45%. However, when a large amount of C is added, residual austenite or island martensite in which C is concentrated at the boundary of the lath during bainite transformation is formed and the durability ratio is lowered, so the upper limit is made 0.60%. In addition, the steel which can be induction-hardened of the present invention is steel that can have a surface hardness after the induction-hardening treatment that is higher than the required strength. Therefore, in the present invention, it is steel having a C content of 0.45% or more. In order to obtain higher strength, an amount of C exceeding 0.5% is preferable.

Si: 0.02 to 0.15%
Si is an element that increases the amount of retained austenite in steel by bainite transformation in the cooling process after hot forging. When an induction hardening process for heating only the surface layer is performed, residual austenite remains in the non-heated part, and when the Si content exceeds 0.15%, the fatigue strength and the durability ratio are significantly reduced. Therefore, the amount is limited to 0.15% or less. However, if the content is suppressed to less than 0.02%, the manufacturing cost becomes great, so the lower limit is made 0.02%.

Mn: 1.50 to 3.00%
Mn is an element that promotes bainite transformation, and is an important element for making the structure bainite in the cooling process after hot forging. Furthermore, it combines with S to form sulfides and has the effect of improving machinability. In order to exert these effects, the lower limit is made 1.50%. On the other hand, if an amount of Mn exceeding 3.00 is added, the hardness of the substrate increases and becomes brittle, so that the machinability is significantly lowered. The upper limit is 3.00. In particular, an amount of Mn exceeding 2.0% is preferable because a bainite structure with an area ratio of 95% is obtained even at a low cooling rate.

P: 0.0002 to 0.150%
Since P usually contains 0.0002% or more as an inevitable impurity in steel, the lower limit is made 0.0002% or more. When P is added in a large amount, P segregates at the grain boundaries of prior austenite and promotes cracking after induction hardening, so the upper limit is made 0.150%. Preferably it is 0.100% or less, More preferably, it is 0.050% or less.

S: 0.001 to 0.200%
S forms sulfides with Mn and has an effect of improving machinability, and in order to exert the effect, the lower limit is made 0.001%. S forms sulfides with Mn and has an effect of improving machinability, and also has an effect of suppressing the growth of austenite grains and maintaining high toughness. In order to exert these effects, the lower limit is made 0.001%. However, although depending on the amount of Mn, since anisotropy increases in mechanical properties when added in a large amount, the upper limit is made 0.200%.

Cr: 0.02 to 1.00%
Cr, like Mn, is an element effective for promoting the bainite transformation, and in order to exert its effect, the lower limit is made 0.02%. However, if a large amount of Cr is added, the Fe-based carbide is stabilized and the surface hardness when induction hardening is reduced, so the upper limit is made 1.00%.

Al: 0.001 to 0.300%
Al precipitates and disperses in the steel as a nitride, thereby preventing the austenite structure from coarsening during forging reheating and preventing the subsequent bainite structure from becoming coarse. Furthermore, Al combines with oxygen during machining, adheres to the tool surface, and is effective in preventing tool wear. In order to exert these effects, the lower limit is made 0.001%. Preferably, it is 0.050% or more, more preferably 0.100%. On the other hand, if it exceeds 0.300%, a large amount of hard inclusions are formed, and both the durability ratio and the machinability are lowered. Therefore, the upper limit is set to 0.300%.

V: 0.01 to 0.30%
V is an element effective for promoting the bainite transformation, and is an element effective for forming a carbonitride, strengthening the precipitation of the bainite structure, and increasing the strength and durability ratio. In order to exhibit this effect, a content of 0.01% or more is necessary. On the other hand, if it exceeds 0.30%, the effect is saturated, so the upper limit is made 0.30%.

Mo: 0.03-1.00%
Mo is not only an element effective for promoting bainite transformation, but also has the highest solid solubility in austenite compared to alloy elements such as V, Ti, Nb, etc., which can provide precipitation strengthening due to alloy carbide, and in the cooling process A large amount of Mo carbonitride is obtained. Generally, precipitation of carbonitride such as Mo used for precipitation strengthening is not preferable because not only fatigue strength but also tensile strength is increased and machinability is remarkably lowered. However, when the size of Mo carbonitride is controlled to 4 nm or more and 11 nm or less, only the fatigue strength can be increased without increasing the tensile strength affecting the machinability, that is, the fatigue strength and the durability ratio are increased. I understood it. In order to exhibit this effect, a content of 0.03% or more is necessary. On the other hand, if it exceeds 1.00%, the effect is saturated, so the upper limit is made 1.00%.

N: 0.0020 to 0.0070%
N is generally used to form a nitride with V to prevent coarsening of the austenite structure during hot forging, but V nitride serves as the nucleus of pro-eutectoid ferrite. The transformation is promoted to reduce the strength and durability ratio. In order to suppress the formation of V nitride, the upper limit of the N amount is set to 0.0070%. Moreover, since 0.0020% or more is usually contained as an inevitable impurity in steel, the lower limit is made 0.0020%.

Ca, Te, Zr containing one or more of Ca: 0.0002 to 0.0100%, Te: 0.0002 to 0.1000%, Zr: 0.0002 to 0.2000% are: All of them have an effect of forming oxides, becoming crystallization nuclei of Mn sulfide, and uniformly and finely dispersing Mn sulfide. In addition, any element dissolves in Mn sulfide, reduces its deformability, suppresses elongation of Mn sulfide shape after rolling or hot forging, and reduces the anisotropy of mechanical properties There is. In order to exert these effects, the lower limits of Ca, Te, and Zr are each 0.0002%. On the other hand, when Ca exceeds 0.0100%, Te exceeds 0.1000%, and Zr exceeds 0.2000%, a large amount of hard inclusions such as oxides and sulfides are generated, and the durability ratio and machinability are increased. Will decline. Therefore, the upper limit of Ca is 0.0100%, the upper limit of Te is 0.1000%, and the upper limit of Zr is 0.2000%.

Next, the reason for limiting the steel structure of the above-mentioned hot forged non-tempered product will be described.

(Bainitic structure with an area ratio of 95% or more)
The structure is defined as a bainite structure with an area ratio of 95% or more. If the main structure is a bainite structure, it has a high durability ratio, but the remaining structure is ferrite, residual austenite, or island martensite. This is because the durability ratio is significantly reduced when the content is 5% or more. The fewer these remaining structures are, the higher the durability ratio is, and the area ratio is preferably 97% or more.

(The average size of Mo carbonitride dispersed in steel is 4 nm or more and 11 nm or less)
The reason why the average size of Mo carbonitrides in the bainite structure is specified to be 4 nm or more is that when the average size is less than 4 nm, the fatigue strength is high but the tensile strength is also high and the durability ratio is small. This is because it is impossible to achieve both fatigue strength and machinability. More preferably, the average size is 8 nm or more. Further, the upper limit value of the average size of Mo carbonitride is defined as 11 nm because when the average size exceeds 11 nm, not only the tensile strength but also the fatigue strength is remarkably lowered, so that high fatigue strength cannot be achieved. . In addition, the shape of Mo carbonitride is acicular and the size of Mo carbonitride used in this specification is the length in the longitudinal direction.

Next, the reason for the limitation of the manufacturing method of the above-mentioned hot forged non-heat treated product will be described.

(Steel is heated to 1000 ° C or lower and 1250 ° C or higher)
The reason why the steel material having the above-described component composition is heated to 1000 ° C. or lower and 1250 ° C. or higher is to sufficiently precipitate carbonitrides of Mo and V in the cooling process, before hot forging. This is because Mo and V are sufficiently dissolved in the steel by heating. If the heating temperature is less than 1000 ° C., Mo and V cannot be sufficiently dissolved in the steel, the precipitation strengthening amount in the subsequent cooling process is small, and the fatigue strength and durability ratio are small. On the other hand, raising the heating temperature more than necessary promotes the growth of austenite grains, and the structure transformed in the subsequent cooling process becomes coarse, and the durability ratio decreases. Therefore, the upper limit of the heating temperature is 1250 ° C.

(After hot forging, the average cooling rate is 0.05 ° C / second or more and 0.80 ° C / second or less to 200 ° C or less)
After hot forging, the average cooling rate up to 200 ° C or less is specified to be 0.05 ° C / second or more and 0.80 ° C / second or less because the time staying in the temperature range where Mo carbonitride precipitates is lengthened. This is because the precipitation amount is increased in the cooling process and the carbonitride size is controlled. When the average cooling rate is 0.80 ° C./second or more, the precipitation amount of Mo carbonized vagina is not sufficiently obtained, and the effect of improving the strength and the durability ratio is small. In particular, in order to coarsen Mo carbonitride and have a high durability ratio, an average cooling rate of 0.50 ° C./second or less is desirable. More preferably, it is 0.30 ° C./second or less. On the other hand, when the average cooling rate is less than 0.05 ° C./second, proeutectoid ferrite having an area ratio of 5% or more is generated at the bainite lath boundary, and the fatigue strength and the durability ratio are significantly reduced.

Although the present invention provides a hot forged non-tempered product capable of induction hardening with high fatigue strength, it is desirable that the tensile strength be 1300 MPa or less in order to ensure sufficient machinability. .

The present invention will be described in detail below by examples. These examples are for explaining the technical significance and effects of the present invention, and do not limit the scope of the present invention.

Steels having chemical compositions shown in Table 1 were melted in a 150 kg vacuum melting furnace. After rolling this into a steel bar having a diameter of 100 mm, a forging specimen was cut out and subjected to forging and heat treatment under the conditions shown in Table 2. After hot forging, the cooling method to 200 ° C. was performed by air cooling or furnace cooling, and the cooling rate was controlled by changing the diameter of the test piece. The average cooling rate was obtained by dividing the value obtained by subtracting 200 ° C. from the temperature of the test piece after hot forging by the time required for cooling to 200 ° C. after hot forging. For comparison, the conventional steel S55C was melted and heat-treated so as to have the same tensile strength as that of the present invention. The test piece was heated to 1100 ° C., cooled to room temperature, and heat-treated again at 450 ° C. for 1 hour. The underlined portions in Tables 1 and 2 are conditions outside the scope of the present invention.

A JIS Z 2201 No. 14 tensile test piece and a JIS Z 2274 No. 1 bending bending fatigue test piece were sampled from the center of these forgings, and the tensile strength and fatigue strength were determined. Here, the fatigue strength was defined as the stress amplitude that did not break in 10 ⁷ rotations in the rotating bending fatigue test. Further, the ratio of the obtained fatigue strength and tensile strength was obtained as the durability ratio (fatigue strength / tensile strength).

The test piece for structure | tissue observation was extract | collected from the 1/4 thickness part of the L direction of the forging material. The area ratio of bainite is determined by polishing the specimen until it becomes a mirror surface, then performing repeller etching, and confirming the microstructure of proeutectoid ferrite, residual austenite, island martensite, and the like other than bainite. The microphotographs were taken by 10 fields of view and then calculated by image analysis.

The average size of the Mo carbonitride was observed in 10 views of 15,000-fold transmission electron micrographs taken with a transmission electron microscope after finishing the test piece into a thin film by electrolytic polishing. The length in the longitudinal direction of the Mo carbonitride was determined by image analysis, and the average value was determined.

No. Each of the present invention steels 1 to 18 has a bainite structure of 95% or more in area ratio, the average size of Mo carbonitride is 4.6 nm or more and 10.8 nm or less, and the durability ratio is 0.58 or more. Has a high durability ratio. In order to ensure machinability, the tensile strength is 1300 MPa or less, but as is apparent when compared with the comparable tensile strength, the conventional example No. Higher fatigue strength is achieved than tempered steel of 28 carbon steel.

In contrast, Comparative Example No. Nos. 23, 24 and 27 have a high content of either C, Si or N. 21 is within the specified steel composition range, but the average cooling rate is not specified, the amount of the remainder such as ferrite and residual austenite is large at the bainite lath boundary, and the average size of Mo carbonitride is not specified, Low strength and durability ratio. No. Nos. 19 and 22 have steel compositions or heat treatment conditions that are not specified, and sufficient precipitation strengthening cannot be obtained, resulting in a low durability ratio. No. Since the heating temperature of No. 20 was increased more than necessary, the bainite structure was coarsened, and the durability ratio was rather low. No. No. 25 has Mn added more than necessary, has high tensile strength, and is very difficult to cut. On the other hand, no. In No. 26, Al is added more than necessary, and the fatigue strength and durability ratio are lowered.

As is clear from the above, those satisfying all of the conditions defined in the present invention are superior in toughness and fatigue characteristics to the comparative example and the conventional example.

Claims

% By mass
C: 0.45 to 0.60%,
Si: 0.02 to 0.15%,
Mn: 1.50 to 3.00%,
P: 0.0002 to 0.150%,
S: 0.001 to 0.200%,
Cr: 0.02 to 1.00%,
Al: 0.001 to 0.300%,
V: 0.01 to 0.30%
Mo: 0.03-1.00%,
N: 0.0020 to 0.0070%,
Non-tempered steel for hot forging, which can be induction-hardened with the balance being Fe and inevitable impurities.
Furthermore, in mass%,
Ca: 0.0002 to 0.0100%,
Te: 0.0002 to 0.1000%,
Zr: 0.0002 to 0.2000%
The non-tempered steel for hot forging capable of induction hardening according to claim 1, comprising one or more of them.
The steel composition according to claim 1 or 2, having a steel structure having an area ratio of 95% or more is a bainite structure, and an average size of Mo carbonitride dispersed in the steel is 4 nm or more and 11 nm or less. This is a hot forged non-tempered product that can be induction hardened.
The steel material having the component composition according to claim 1 or 2 is heated to 1000 ° C. or higher and 1250 ° C. or lower and hot forged, and after the hot forging, an average cooling rate up to 200 ° C. is 0.05 ° C. / A method for producing a hot forged non-tempered product, characterized by cooling at a rate of not less than 2 seconds and not more than 0.80 ° C./second and subjecting a portion requiring strength to induction hardening.