JP3075139B2 - Coarse-grained case hardened steel, surface-hardened parts excellent in strength and toughness, and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a case hardened steel and a case hardened part, and a method for producing the case hardened part, and more particularly, to a coarse grained case hardened steel and a case hardened part excellent in strength and toughness. And its manufacturing method.

[0002]

2. Description of the Related Art Conventionally, various mechanical structural parts such as those for automobiles and industrial machines, especially surface hardened parts represented by gears,
Using case hardened steel as a material, it is formed into a desired shape by hot forging, cold forging, or machining, and then carburizing or carbonitriding the surface of the part to improve wear resistance and fatigue strength. It is used after being subjected to a surface hardening treatment.

Conventionally, JIS standard steels (manganese steel for machine structure (SMn steel) and manganese chrome steel (SMnC steel)) have been used as case hardening steel for machine structure used as the material for surface hardened parts.
(Both JIS G 4106), chrome molybdenum steel (SCM
Steel) (JIS G 4105), chrome steel (SCr steel) (JIS G 41
04), Nickel chrome molybdenum steel (SNCM steel) (JI
SG 4103), Nickel chrome steel (SNC steel) (JIS G 41
02) etc.) have been used. However, such JIS standard steels have a problem in terms of cost because they contain a large amount of alloy components and require a long time for ordinary surface hardening treatment. That is, under recent economic circumstances, there has been a request from the industry for a reduction in the material cost of various surface-hardened components, but this request has not been sufficiently met.

[0004] For this reason, studies have been made to reduce various alloying components to be added to the base steel. In order to compensate for the decrease in hardenability due to the reduction in the alloying components, boron steel to which a small amount of B is added has attracted attention. However, in conventional boron steel, which has simply reduced the amount of alloy components, abnormal grain growth occurs when heated to about 930 ° C. during surface hardening treatment such as carburizing treatment or carbonitriding treatment, and distortion occurs during quenching and There is a problem that the material strength is reduced.

In response to the above demands from the industry, Japanese Patent Application Laid-Open No. 4-176816 proposes a method for producing a carburizing steel material in which surface hardening is performed at a high temperature to shorten the processing time. However, the method disclosed in this publication,
The purpose is simply to prevent the crystal grains from becoming coarse during high-temperature carburizing, thereby preventing the occurrence of heat treatment distortion and the reduction in strength. For this reason, in the current situation where the use environment of automobiles and industrial machines is severe, it is not always possible to obtain a product having a sufficient strength-toughness balance to withstand use.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a low-cost type surface-hardened component which has a sufficient strength-toughness balance and which can sufficiently withstand use in a harsh environment, and a material which is resistant to the hardening. To provide a method for producing coarse-grained case hardened steel and its surface-hardened parts, especially, high-strength and high-toughness surface-hardened parts with no abnormal grain growth during surface hardening treatment and little heat treatment distortion, and their materials. Another object of the present invention is to provide a method for producing a coarse-grained case hardened boron steel and a surface-hardened component thereof.

[0007]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors have conducted investigations and researches on the chemical composition of the steel material used as the material of the surface-hardened part, the structure of the surface-hardened part, and the heat treatment method. As a result, the following important matters became clear.

Heretofore, it has been known that a small amount of Nb may be added as a means for preventing the occurrence of abnormal grain growth in low alloy steel. This is due to the fine N deposited by the addition of Nb.
By utilizing the pinning effect of bC, abnormal growth of austenite grains during heating in surface hardening treatment such as carburizing treatment or carbonitriding treatment is prevented. On the other hand, in the case of boron steel, the added B (boron) is very easily linked to N in the steel and precipitates as BN, so that the amount of solute B effective for hardenability decreases. Therefore, Ti is added and Ti
N, which fixes N in steel with Ti and combines with B
A method of securing the amount of solid solution B by reducing the amount is generally performed. However, prevention of abnormal grain growth and solid solution B
In order to simultaneously achieve the securing of Nb and Ti, it is necessary to use niobium titanium carbonitride [NbTi
The present inventors have found that (CN)] is coarsely precipitated, and that the carbonitride does not readily dissolve during heating or heat treatment during subsequent lumping, rolling and forging.
Therefore, it is conceivable that the precipitation amount of fine NbC and TiC which act as a pinning function effective for preventing abnormal grain growth is reduced, and coarse grains are generated during the surface hardening treatment.

Therefore, the present inventors have continued further detailed research, and as a result, have obtained the following new findings.

[0010] The coarse alloy carbonitride precipitated during solidification in steel to which Nb and Ti are added in combination is NbC, TiC,
It is not a carbide, nitride or carbonitride of a single alloy such as NbN, TiN, Nb (CN) and Ti (CN), but a composite carbonitride of Nb and Ti [NbTi (CN)].

The relationship between the solid solution of the composite carbonitride [NbTi (CN)] and the heating temperature (T) is as follows.

(A) When T <1150 ° C .: The above composite carbonitride exists stably in steel.

(B) When 1150 ° C. ≦ T ≦ 1350 ° C .: Only Nb in the above composite carbonitride forms a solid solution, and Ti is concentrated.

(C) When 1350 ° C. <T: The above composite carbonitride completely dissolves (Ti also dissolves).

When the following values of fn1 and fn2 are both 0 or less, an amount of solid solution B effective for improving hardenability can be secured.

Fn1 = 8 [% N] −2 [% Ti] − [% Nb] fn2 = 3.4 [% N] − [% Ti] where [% N] means weight% of N component. , [% Ti]
And [% Nb].

The above fn1 ≦ 0 and fn2 ≦ 0
And when the material steel and / or the surface-hardened parts are heated to a temperature range of 1150 ° C. or more before the surface hardening, abnormalities at the time of the surface hardening due to the pinning action of finely precipitated NbC and TiC. Grain growth can be prevented.

After the surface hardening treatment, if the core has a hardness of Hv 300 or more and an impact value of 20 J / cm ² or more, the surface hardened part has sufficient durability even in harsh environments where automobiles and industrial machines are used. Is shown.

The present invention based on the above findings provides a coarse-grained case hardening steel having the chemical composition shown in (1) below, a surface-hardened part excellent in strength and toughness shown in (2), and (3), (4) The method of manufacturing a surface-hardened part having excellent strength and toughness described in (1). (1) C: 0.10 to 0.30% by weight, Si:
0.01 to 0.50%, Mn: 0.60 to 2.00%,
Cr: 2.0% or less, Mo: 0.05 to 1.0% , N
b: 0.020-0.080%, Ti: 0.100% or less, Al: 0.100% or less, B: 0.0010-0.
0100%, the balance being Fe and unavoidable impurities, P in the impurities is 0.025% or less, S is 0.0
A coarse-grain-hardened case hardening steel characterized by being 40% or less and N being 0.0100% or less, and both the values of fn1 and fn2 being 0 or less. (2) Material is C: 0.10 to 0.30% by weight.
Si: 0.01 to 0.50%, Mn: 0.60 to 2.0
0%, Cr: 2.0% or less, Mo: 0.05 to 1.0
%, Nb: 0.020 to 0.080%, Ti: 0.10
0% or less, Al: 0.100% or less, B: 0.0010
~ 0.0100%, the balance being Fe and inevitable
It is composed of pure substance, P in impurities is 0.025% or less, and S is
0.040% or less and N is 0.0100% or less, and
In addition, the values of fn1 and fn2 are both 0 or less.
A granulating hardened steel, excellent surface curing component strength and toughness characterized by having a Hv300 or more core hardness and 20 J / cm ² or more impact value after the surface hardening treatment.

(3) C: 0.10 to 0.30% by weight
%, Si: 0.01 to 0.50 %, Mn: 0.60 to
2.00%, Cr: 2.0% or less, Mo: 0.05-
1.0%, Nb: 0.020 to 0.080%, Ti:
0.100% or less, Al: 0.100% or less, B: 0.
0010-0.0100%, the balance being Fe and
Consisting of unavoidable impurities, P in the impurities is 0.025% or less
Below, S is 0.040% or less and N is 0.0100% or less.
Below, and both the values of fn1 and fn2 are 0 or less.
The lower coarse-grained case hardened steel was removed 1
A method for producing a surface-hardened part having excellent strength and toughness, wherein the part is heated to 150 ° C. or higher and then hot forged.

(4) By weight%, C: 0.10 to 0.30
%, Si: 0.01 to 0.50%, Mn: 0.60 to
2.00%, Cr: 2.0% or less, Mo: 0.05-
1.0%, Nb: 0.020 to 0.080%, Ti:
0.100% or less, Al: 0.100% or less, B: 0.
0010-0.0100%, the balance being Fe and
Consisting of unavoidable impurities, P in the impurities is 0.025% or less
Below, S is 0.040% or less and N is 0.0100% or less.
Below, and both the values of fn1 and fn2 are 0 or less.
Excellent in strength and toughness, characterized in that at least one of the steps of lumping , rolling and heat treatment is performed by heating the lower coarse-grained case hardening steel to 1150 ° C. or higher, followed by forging and surface hardening. Manufacturing method of hardened surface parts.

The core after the surface hardening treatment means a part that is not surface hardened.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail together with its operation and effect. “%” Means “% by weight”. (A) Chemical composition of base steel C: C is added to secure the static strength of the steel, but if its content is less than 0.10%, the effect of addition is poor.
If the content exceeds 0.30%, the toughness of the steel decreases, so the content was set to 0.10 to 0.30%.

Si: Si is an element effective for improving the hardenability of steel, improving the static strength, and preventing surface oxidation at high temperatures. However, if the content is less than 0.01%, the desired static strength cannot be ensured, and the oxidation resistance of the surface at high temperatures deteriorates,
If it exceeds 0.50%, the toughness will be degraded, so its content is made 0.01 to 0.50%.

Mn: Mn has the effect of improving the hardenability and hot ductility of steel. However, if the content is less than 0.60%, sufficient hardenability cannot be obtained, and if the content exceeds 2.00%, segregation occurs, and on the contrary, hot ductility decreases.
Therefore, the content of Mn is set to 0.60 to 2.00%.

Cr: Cr may not be added. If added, the hardenability of the steel is improved, and at the time of surface hardening treatment such as carburizing treatment, it combines with C to form a composite carbide, so that there is an effect that wear resistance is improved. To ensure this effect, Cr should be 0.0
Preferably, the content is 5% or more. However, if the content exceeds 2.00%, the toughness deteriorates. Therefore, the upper limit of the Cr content is set to 2.00%.

[0027] Mo: Mo, if added pressure as well as improving the hardenability of steel, has the effect of raising the core hardness after surface hardening treatment. To ensure this effect, it is desirable that the content of Mo be 0.05% or more. However, if the content exceeds 1.00%, the machinability will be greatly deteriorated.
The content was 0.05 to 1.00% .

Nb: Nb is an element effective for improving the toughness by making the crystal grains of steel fine and for preventing abnormal grain growth during heating for surface hardening treatment. However, if the content is less than 0.020%, the effect of addition is poor. On the other hand, if the content exceeds 0.080%, the effect of crystal grain refinement is saturated and economic efficiency is only lost, resulting in deformation. The resistance increases, and the cold forgeability and the hot forgeability deteriorate. Therefore, the content of Nb is set to 0.020 to 0.080%.

Ti: Ti may not be essentially added. But N in steel
Since it is impossible to completely reduce the content to 0, some amount must be contained in the present invention from the condition of fn2 ≦ 0. If Ti is added, it has the effect of making crystal grains finer and preventing abnormal grain growth during heating for surface hardening treatment. Further, Ti reacts with N to react with Ti.
By forming N and reducing the solute N in the steel to suppress the generation of BN, there is also an effect of securing the amount of solute B effective for improving hardenability. To ensure these effects, T
It is preferable that i has a content of 0.005% or more.
However, if the content exceeds 1.00%, the effect of crystal grain refinement is saturated and not only impairs economic efficiency but also deteriorates toughness. Therefore, the upper limit of the Ti content is set to 1.0
0%.

Al: Al may not be added. If added, it has the effect of stabilizing and homogenizing steel deoxidation. To ensure this effect, the content of Al is desirably 0.005% or more. However, if the content exceeds 0.100%, the effect is saturated and the toughness deteriorates. Therefore, the Al content is set to 0.100% or less.

B: B is an element effective for improving the hardenability of steel. However, if the content is less than 0.0010%, the desired effect cannot be obtained. If the content exceeds 0.0100%, the effect is saturated and not only the cost is increased, but also the hardenability decreases. In some cases, the content of B was set to 0.0010 to 0.0100% because of the possibility of the occurrence.

The contents of the impurity elements P, S and N are limited as follows.

P: P degrades the toughness of the steel and lowers the cold and hot forgeability, especially when the content exceeds 0.025%, the toughness and the cold / hot forgeability deteriorate. It becomes remarkable. Therefore, the upper limit of the content of P as an impurity element is set to 0.02.
5%.

S: S not only deteriorates the toughness of the surface-hardened layer, but also lowers the cold and hot forgeability, especially when the content is 0.040.
%, The toughness deteriorates and the cold and hot forgeability deteriorates remarkably. Therefore, the upper limit of the content of S as an impurity element is set to 0.040%.

N: Since N reacts with B to form BN and reduces the amount of solid solution B effective for improving hardenability, the content of N is reduced as much as possible in order to secure the amount of intrinsic B. There is a need. However, it is actually impossible to reduce the N content to 0 during steelmaking. Therefore, it is important to fix solid solution N mainly in the form of TiN by adding Ti. In this case, it is necessary to consider the balance between the amount of solid solution N and the amount of added Ti from both the cost aspect and the above-described Ti action aspect. If the content of N exceeds 0.0100%, the amount of Ti corresponding to N
Addition decreases the toughness. Therefore, the upper limit of the content of N as an impurity element is set to 0.0100%.

Fn1.ltoreq.0 and fn2.ltoreq.0: When the values of fn1 and fn2 are both 0 or less, an effective amount of solid solution B for improving hardenability can be secured, and the above condition is satisfied. In addition, when the material steel and / or the surface-hardened part is heated to a temperature range of 1150 ° C. or more before the surface hardening, abnormal grains during the surface hardening due to the pinning action of finely precipitated NbC and TiC. Since the growth can be prevented, the above limitation is set in the present invention.

The raw steel having the above chemical composition is, for example, hot-lumped into a billet, then hot-rolled, and then hot- or cold-forged. It is machined and processed into a predetermined surface-hardened component shape. Then, a surface hardening treatment is finally performed. (B) Hot forging, lumping, rolling, and heat treatment Nb, T
A production method is disclosed that uses case hardening steel to which one or more of i and V are added to prevent coarsening of crystal grains during high-temperature carburization. In general, if a fine alloy carbonitride is precipitated, such as the case hardened steel described in this publication, it is possible to suppress the crystal grain growth during the surface hardening treatment by its pinning effect. In order to precipitate fine alloy carbonitrides during heating in so-called surface hardening treatment such as carburizing or carbonitriding, coarse alloy carbonitrides precipitated during solidification after melting must be removed before surface hardening treatment. It is necessary to sufficiently dissolve the steel in the steel at the stage, and to prepare a base material of fine alloy carbonitride precipitation. For this purpose, it is sufficient to temporarily heat to a high temperature in a step before the surface hardening treatment. Conventionally, the high-temperature heating temperature for suppressing crystal grain growth has been set to 1200 ° C. from the solid solution temperature determined from the solubility product of each alloy carbonitride.

However, as described above, the coarse alloy carbonitride precipitated during solidification in steel to which Nb and Ti are added in combination is a composite carbonitride of Nb and Ti [NbTi (C
N)]. The relationship between the solid solution of the composite carbonitride [NbTi (CN)] and the heating temperature (T) is as follows.

(A) In the case of T <1150 ° C .: The above composite carbonitride exists stably in steel.

(B) When 1150 ° C. ≦ T ≦ 1350 ° C .: Only Nb in the above composite carbonitride forms a solid solution and Ti is concentrated.

(C) When 1350 ° C. <T: The above composite carbonitride completely dissolves (Ti also dissolves).

Therefore, in the present invention, (a) N is mainly fixed by Ti in order to secure the amount of solid solution B from the viewpoint of improving hardenability, and (b) NbC and TiC
In order to prevent the occurrence of abnormal grain growth due to the pinning effect of the above, the material is once heated to 1150 ° C. or more in a step before the surface hardening treatment. Therefore, when hot forging is included in the process of forming a surface-hardened part, it is sufficient that at least the heating temperature in this hot forging is set to 1150 ° C. or more to form a solid solution of Nb. ).

Alternatively, among the pre-processes of the surface hardening treatment described above, those involving "heating" treatment other than hot forging are lumping, rolling and so-called "heat treatment". In at least one step, the heating temperature may be set to 1150 ° C. or higher (the invention of claim 4).

The upper limit of the heating temperature in the third and fourth aspects of the present invention is preferably set to 1350 ° C. in order to reduce surface oxidation during heating and to secure solid solution B.

In order to precipitate fine alloy carbonitrides during heating in so-called surface hardening treatment such as carburizing or carbonitriding, the cooling rate after the above-mentioned heating is 0.2 ° C./s or more. It is desirable to do. (C) Surface Hardening Treatment The surface hardening treatment is an indispensable treatment for hardening the surface of a predetermined surface hardened component and securing the required wear resistance and fatigue strength as a product. However, this processing method is not particularly defined, and may be performed by a normal method. (D) Core hardness and toughness of surface-hardened parts after surface-hardening treatment In order for the surface-hardened parts to exhibit sufficient durability even in the harsh environment in which automobiles and industrial machines are used, it is necessary to use the surface-hardened parts after surface hardening. , Hv300 or higher core hardness and 20 J / cm
It is necessary to have an impact value of ² or more. If it is out of one and / or both, the durability of the surface-hardened component in a real environment is extremely deteriorated. Therefore, the core hardness of the surface-hardened component was set to Hv300 or more, and the impact value was set to 20 J / cm ² or more. (E) Tempering Tempering at a low temperature can improve toughness without a significant decrease in surface hardness.
After the surface hardening treatment, tempering may be performed if necessary. In the case of tempering, the temperature is preferably set to 150 to 200 ° C. in order to secure the surface hardness.

[0046]

EXAMPLES (Example 1) A steel having a chemical composition shown in Tables 1 and 2 was prepared by a usual method.
It was melted using a 0 kg vacuum furnace. In Tables 1 and 2 , steel
B, F and H are steels of the present invention, and steels I to X are comparative steels in which one of the components is out of the range of the content specified in the present invention. In the comparative steels, steels V, W and X are JIS SMn420 steel, SCr420 steel and SCM, respectively.
It is equivalent to 420 steel.

Next, these steels were heated to 1140 ° C., and then made into billets by a usual method.
, And hot forged into a round bar having a diameter of 30 mm at a temperature of 1100 to 1000 ° C. Cut out roughening measurement specimens 8mm diameter × 12 mm length from a portion of the round bar after hot forging thus obtained, subjected to thermomechanical treatment test of 4 under the following conditions using the test specimen, abnormal grain The incidence of growth was investigated by light microscopy.

(Condition 1) A test piece was heated at 1100 ° C., 1175 ° C. and 12
After heating at a temperature of 50 ° C. for 15 minutes each, a deformation amount of 30% was given by compression processing, and cooling was performed at a cooling rate of 1.0 ° C./s to room temperature. Thereafter, carburizing treatment was performed at 930 ° C. × 6 hr (carbon potential: 0.8%), followed by oil quenching.

(Condition 2) The test piece was heated in vacuum at 1100 ° C. for 15 minutes, and then subjected to a compression process to give a deformation amount of 30%, and once cooled to room temperature at a cooling rate of 2.0 ° C./s. . After this, one more
After heating at 100 ° C., 1175 ° C., and 1250 ° C. for 15 minutes, it was cooled to room temperature at a cooling rate of 1.0 ° C./s. Then, 930 ° C. × 6 hr (carbon potential:
(0.8%), followed by oil quenching.

(Condition 3) In the atmosphere, a test piece was subjected to compression at room temperature to give a deformation amount of 30%. Then in vacuum at 1100 ° C., 1175
After heating for 15 minutes at a temperature of 1250 ° C. and 1250 ° C., respectively, it was cooled to room temperature at a cooling rate of 1.0 ° C./s. Thereafter, 930 ° C. × 6 hr (carbon potential: 0.8%)
And then oil quenched.

(Condition 4) In a vacuum, the test piece was heated at 1100 ° C., 1175 ° C. and 12 ° C.
After heating at a temperature of 50 ° C. for 15 minutes each, it was once cooled to room temperature at a cooling rate of 1.0 ° C./s. Next, the mixture was heated at 1100 ° C. for 15 minutes in a vacuum, further subjected to compression processing to give a deformation amount of 30%, and cooled to room temperature at a cooling rate of 2.0 ° C./s. Thereafter, carburizing treatment was performed at 930 ° C. × 6 hr (carbon potential: 0.8%), followed by oil quenching.

Table 3 shows the results of the investigation on the incidence of abnormal grain growth. The occurrence rate of abnormal grain growth was 10 times at 100 times magnification.
It was indicated by the area ratio when the visual field was examined.

Table 3 shows that the steels B, F and H of the steels of the present invention were obtained.
It is clear that, among the comparative steels, the steels R and T do not undergo abnormal grain growth when subjected to heat treatment under the conditions specified in the present invention.

(Example 2) The steel slabs B, F and H to X prepared in Example 1 were heated to 1190 ° C, and then heated at a temperature of 1190 to 1000 ° C into a 30 mm diameter round bar. Forged. A JIS No. 3 Charpy impact test piece was cut out from the center of the thus obtained hot-forged round bar and subjected to 930 as a surface hardening treatment.
After carburizing at 6 ° C. × 6 hours (carbon potential: 0.8%), oil quenching was performed, and further tempering was performed at 160 ° C. Next, the hardness of the center of the test piece, that is, the core was measured together with the impact test.

Table 4 shows the test results. From Table 4 , steels B, F and H of the present invention have a core hardness of Hv 300 or more and an impact value of 20 J / cm ² or more. It can be seen that sufficient durability can be exhibited even in the harsh environment in which it is used. On the other hand, in the comparative steel, at least one of the core hardness and the impact value is low, and the durability of the surface-hardened component in a real environment is extremely deteriorated.

Example 3 The steel slabs of the steels B, F, H, R and T produced in Example 1 were heat-treated in a vacuum at 1180 ° C., and temporarily cooled to room temperature at a rate of 0.25 ° C./s. Cool at the cooling rate. Then, 11
After heating to 00 ° C., hot forging into a 30 mm diameter round bar at a temperature of 1100 to 1000 ° C., and a JIS No. 3 Charpy impact test specimen from the center of the hot forged round bar thus obtained. Cut out, 930 ° C as surface hardening treatment
After performing carburizing treatment of × 6 hr (carbon potential: 0.8%), oil quenching was performed, and further tempering was performed at 170 ° C. Next, the center hardness of the test piece, that is, the core hardness was measured together with the impact test.

Table 5 shows the test results. From Table 5 , steels B, F and H, which are the steels of the present invention, have a core hardness of Hv 300 or more and an impact value of 20 J / cm ² or more. It can be seen that sufficient durability can be exhibited even in the harsh environment in which it is used. On the other hand, R and T of the comparative steel, which did not undergo abnormal grain growth when subjected to the heat treatment under the conditions specified in the present invention in Example 1 above, had lower core hardness and / or impact value, indicating that the actual environment Endurance is extremely deteriorated.

[0058]

[Table 1]

[Table 2]

[Table 3]

[Table 4]

[Table 5]

As described above, the low-cost surface-hardened parts according to the present invention are excellent in strength and toughness, and do not cause abnormal grain growth, so that various mechanical structural parts such as those for automobiles and industrial machines, in particular, It can be used as a surface hardened part represented by a gear. This surface hardened part can be manufactured relatively easily by using the coarse-grained case hardening steel of the present invention as a raw material and applying the method of the present invention thereto.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 8/00

Claims (4)

(57) [Claims] C. 0.10 to 0.30% by weight, S
i: 0.01 to 0.50%, Mn: 0.60 to 2.00
%, Cr: 2.0% or less, Mo: 0.05 to 1.0% ,
Nb: 0.020 to 0.080%, Ti: 0.100%
Hereinafter, Al: 0.100% or less, B: 0.0010
0.0100%, and the balance consists of Fe and unavoidable impurities. P in the impurities is 0.025% or less, S is 0.040% or less, and N is 0.0100% or less. A coarse-grained case hardened steel, wherein both values of fn2 are 0 or less. fn1 = 8 [% N] -2 [% Ti]-[% Nb] fn2 = 3.4 [% N]-[% Ti] where [% X] is the weight% of the element X. 2. The material is, by weight%, C: 0.10 to 0.3.
0%, Si: 0.01 to 0.50%, Mn: 0.60
2.00%, Cr: 2.0% or less, Mo: 0.05-
1.0%, Nb: 0.020 to 0.080%, Ti:
0.100% or less, Al: 0.100% or less, B: 0.
0010-0.0100%, the balance being Fe and
Consisting of unavoidable impurities, P in the impurities is 0.025% or less
Below, S is 0.040% or less and N is 0.0100% or less.
Below, and the values of fn1 and fn2 below are both 0 or less
Is a coarse-grained case hardening steel having an Hv of 30 after surface hardening treatment.
A surface-hardened part having excellent strength and toughness, having a core hardness of 0 or more and an impact value of 20 J / cm ² or more. fn1 = 8 [% N] -2 [% Ti]-[% Nb] fn2 = 3.4 [% N]-[% Ti] where [% X] is the weight% of the element X. 3. C: 0.10 to 0.30% by weight, S
i: 0.01 to 0.50%, Mn: 0.60 to 2.00
%, Cr: 2.0% or less, Mo: 0.05 to 1.0%,
Nb: 0.020 to 0.080%, Ti: 0.100%
Hereinafter, Al: 0.100% or less, B: 0.0010
0.0100%, the balance being Fe and inevitable impurities
P in impurities is 0.025% or less, and S is
0.040% or less and N is 0.0100% or less, and
In addition, the values of fn1 and fn2 below are both 0 or less.
A method for producing a surface-hardened part having excellent strength and toughness, comprising heating a coarse-grained case hardened steel to 1150 ° C. or more prior to surface hardening and then hot forging. fn1 = 8 [% N] -2 [% Ti]-[% Nb] fn2 = 3.4 [% N]-[% Ti] where [% X] is the weight% of the element X. 4. C .: 0.10 to 0.30% by weight, S
i: 0.01 to 0.50%, Mn: 0.60 to 2.00
%, Cr: 2.0% or less, Mo: 0.05 to 1.0%,
Nb: 0.020 to 0.080%, Ti: 0.100%
Hereinafter, Al: 0.100% or less, B: 0.0010
0.0100%, the balance being Fe and inevitable impurities
P in impurities is 0.025% or less, and S is
0.040% or less and N is 0.0100% or less, and
In addition, the values of fn1 and fn2 below are both 0 or less.
A surface-hardened part excellent in strength and toughness, characterized in that at least one of the steps of lumping, rolling and heat treatment is performed by heating the coarse-grained case hardened steel to 1150 ° C. or higher, and then forging and surface hardening. Manufacturing method.