JPH08295981A - Case hardening steel with grain coarcening resistance, surface-hardened parts excellent in strength and toughness, and their production - Google Patents

Abstract

PURPOSE: To produce surface-hardened parts excellent in strength and toughness by using an Mn type case hardening steel of specific composition and applying hot forging prior to surface-hardening treatment. CONSTITUTION: The case hardening steel has a composition which consists of, by weight ratio, 0.10-0.30% C, 0.01-0.50% Si, 0.60-2.00% Mn, <=2.0% Cr, <=1.0% Mo, 0.020-0.080% Nb, <=0.100% Ti, <=0.100% Al, 0.0010-0.0100% B, and the balance Fe with inevitable impurities and in which the contents of P, S, and N among the impurities are regulated to <=0.025%, <=0.040%, and <=0.0100%, respectively, and also the values of fn1 and fn2 in equations I, II are regulated, respectively, to <=0. Moreover, at the time of producing surface-hardened parts by using this case hardening steel, the steel is heated to 1150 deg.C and hot-forged prior to hardening treatment to obtain >=Hv300 core hardness and >=20J/cm<2> impact value after hardening treatment.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a case-hardening steel and a surface-hardened part, and a method for producing the surface-hardened part. More specifically, the present invention relates to a coarse-grained case-hardened steel and a surface-hardened part excellent in strength and toughness. And the manufacturing method thereof.

[0002]

2. Description of the Related Art Conventionally, various mechanical structural parts for automobiles and industrial machines, particularly surface-hardened parts represented by gears,
Case hardening steel is used as a material, which is then hot forged, cold forged, or machined into the desired shape, and then carburized or carbonitrided on the surface of the component for the purpose of improving wear resistance and fatigue strength. It is used after being subjected to surface hardening treatment such as.

Conventionally, JIS case steels (manganese steels for machine structures (SMn steels) and manganese chrome steels (SMnC steels) have been used as case hardening steels for machine structures used as materials for surface-hardened parts.
(All 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) and the like) have been used. However, such JIS standard steel has a problem in terms of cost because it contains a large amount of alloy components and it takes a long time for a normal surface hardening treatment. That is, under the recent economic circumstances, there is a demand from the industrial world to reduce the material cost of various surface-hardened parts, but this demand has not been sufficiently met.

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

In response to the above-mentioned demands from the industrial world, on the other hand, a method for producing a steel material for carburizing, which carries out a surface hardening treatment at a high temperature to shorten the treatment time, has been proposed in Japanese Patent Laid-Open No. 4-176816. However, the method disclosed in this publication is
The purpose is simply to prevent coarsening of crystal grains during high temperature carburization, thereby preventing heat treatment distortion and strength reduction. Therefore, under the present circumstances where the use environment of automobiles and industrial machines has become severe, it is not always possible to obtain a product having a sufficient strength-toughness balance to withstand use.

[0006]

The object of the present invention is to provide a low-cost surface-hardened component which has a sufficient strength-toughness balance and is sufficiently durable to be used in a harsh environment, and the resistance to be used as the material thereof. To provide a grain-hardened case-hardening steel and a method for manufacturing a surface-hardened part thereof, and in particular, a surface-hardened part having high strength and high toughness, which does not cause abnormal grain growth during surface hardening treatment and has little heat treatment distortion, and its material. Another object of the present invention is to provide a coarse-grained case-hardening boron steel and a method for producing a surface-hardened part thereof.

[0007]

In order to solve the above-mentioned problems, the present inventor has conducted an investigation and research on the chemical composition of the steel material as the material of the surface-hardened parts, the structure of the surface-hardened parts and the heat treatment method.

As a result, the following important matters have been revealed.

It is conventionally 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 the fine N precipitated by the addition of Nb.
By utilizing the pinning effect of bC, abnormal growth of austenite grains at the time of 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 likely to combine with N in the steel and precipitates as BN, so the amount of solid solution B effective for hardenability decreases. Therefore, add Ti by adding Ti
N in the steel, N in the steel is fixed by Ti and combined with B
A method of ensuring 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 achieve the same as the above, the steel containing Nb and Ti together is added to niobium titanium carbonitride [NbTi
(CN)] was coarsely precipitated, and it was revealed by the present inventor that the carbonitride does not easily dissolve during subsequent heating of slab, rolling and forging, and heat treatment.
Therefore, it is conceivable that the amount of fine NbC and TiC that have a pinning effect effective in preventing abnormal grain growth is reduced and coarse grains are generated during the surface hardening treatment.

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

Coarse alloy carbonitrides which precipitate during solidification in steel containing Nb and Ti in combination are NbC, TiC,
It is not a carbide, nitride or carbonitride made 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-mentioned composite carbonitride is stably present in steel.

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

(C) In the case of 1350 ° C. <T: The above composite carbonitride completely forms a solid solution (Ti also forms a solid solution).

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

Fn1 = 8 [% N] -2 [% Ti]-[% Nb] fn2 = 3.4 [% N]-[% Ti] where [% N] means% by weight of N component. , [% Ti]
The same applies to [% 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 higher before the surface hardening treatment, the pinning action of finely precipitated NbC and TiC causes an abnormality during the surface hardening treatment. It can prevent grain growth.

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

The present invention based on the above-mentioned findings provides the rough-grained case-hardening steel having the chemical composition shown in (1) below, the surface-hardened parts excellent in strength and toughness shown in (2), and (3) and (4). The gist is the method for producing a surface-hardened part having excellent strength and toughness.

(1) C: 0.10 to 0.30 by weight%
%, Si: 0.01 to 0.50%, Mn: 0.60
2.00%, Cr: 2.0% or less, Mo: 1.0% or less, Nb: 0.020 to 0.080%, Ti: 0.10.
0% or less, Al: 0.100% or less, B: 0.0010
.About.0.0100%, the balance consisting 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, and A coarse-grained case-hardening steel characterized in that the values of fn1 and fn2 are both 0 or less.

(2) The material is the steel described in (1) above, and after the surface hardening treatment, the core hardness of Hv300 or more and 2
A surface-hardened component excellent in strength and toughness, which has an impact value of 0 J / cm ² or more.

(3) A method for producing a surface-hardened component excellent in strength and toughness, characterized in that the steel described in (1) above is heated to 1150 ° C. or higher prior to the surface hardening treatment and then hot forged. .

(4) A strength characterized by subjecting the steel according to (1) above to at least one of steps of slabbing, rolling and heat treatment by heating to 1150 ° C. or higher, followed by forging and surface hardening. And a method of manufacturing a surface-hardened part having excellent toughness.

The core portion after the surface hardening treatment means a portion where the surface is not hardened.

[0026]

The present invention will be described in detail below along with its effects. "%" Means "% by weight".

(A) Chemical composition of material steel C: C is added to secure the static strength of the steel, but if the content is less than 0.10%, the effect of addition is poor, while 0.30% If it is contained in excess of 0.1%, the toughness of the steel will decrease, so the content was made 0.10 to 0.30%.

Si: Si is an element effective in 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 temperature deteriorates, and if it exceeds 0.50%, the toughness deteriorates. Therefore, the content is 0.01 to 0.50%
And

Mn: Mn has the effects of improving the hardenability and the hot ductility of steel. However, its content is 0.6
If it is less than 0%, sufficient hardenability cannot be obtained, and if it exceeds 2.00%, segregation occurs and conversely the hot ductility decreases. Therefore, the Mn content is 0.60
It was set 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 is combined with C to form a composite carbide, so that it has an effect of improving wear resistance. In order to reliably obtain this effect, the content of Cr is preferably 0.05% 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%.

Mo: Mo may not be added. If added, the hardenability of the steel is improved and the hardness of the core portion after the surface hardening treatment is increased. In order to surely obtain this effect, it is desirable that the content of Mo be 0.05% or more. However, if the content exceeds 1.00%, the machinability deteriorates significantly, so the Mo content is 1.0
It was set to 0% or less.

Nb: Nb is an element which is effective in improving the toughness by making the crystal grains of steel fine and preventing abnormal grain growth during heating for surface hardening treatment. However, if its content is less than 0.020%, the effect of addition is poor, while 0.0
Even if the content exceeds 80%, the effect of grain refinement is saturated and the economic efficiency is impaired, and the deformation resistance increases and the cold forgeability and hot forgeability also deteriorate. . Therefore, the content of Nb is set to 0.020 to 0.080%.

Ti: Ti may not be added essentially. However, since it is impossible to completely set the N content in the steel to 0, it is necessary to add some amount in the present invention from the condition of fn2 ≦ 0. If Ti is added, it has the effect of refining crystal grains and preventing abnormal grain growth during heating for surface hardening treatment. Furthermore, Ti
Also reacts with N to form TiN, reduces the solute N in the steel and suppresses the formation of BN, and thus also has the effect of securing an amount of solute B effective for improving the hardenability. In order to surely obtain such effects, the Ti content is preferably 0.005% or more. However, if the content exceeds 1.00%, not only the effect of refining the crystal grains is saturated and the economic efficiency is impaired, but also the toughness deteriorates. Therefore, the upper limit of the Ti content is set to 1.00%.

Al: Al may not be added. If added, it has the effect of stabilizing the deoxidation of steel and homogenizing it.
In order to reliably obtain this effect, it is desirable that the content of Al be 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.
It was set to 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, and if it exceeds 0.0100%, the effect is saturated and not only the cost rises, but also the hardenability deteriorates. In some cases, the B content is set to 0.0010 to 0.0100%.

The content of the impurity elements P, S and N is limited as follows.

P: P deteriorates the toughness of steel and reduces cold and hot forgeability.
If it exceeds 025%, the toughness and the cold / hot forgeability are significantly deteriorated. Therefore, the upper limit of the content of P as an impurity element is set to 0.025%.

S: S not only deteriorates the toughness of the surface hardened layer, but also deteriorates the cold and hot forgeability. Especially, when the content exceeds 0.040%, the toughness deteriorates, the cold and hot forgeability. The deterioration of sex becomes remarkable. Therefore, the upper limit of the content of S as an impurity element is set to 0.040%.

N: N reacts with B to form BN and reduces the amount of solid solution B that is effective for improving the hardenability, so the content of N is made as small as possible in order to secure the proper amount of B. There is a need. However, it is actually impossible to set the N content to 0 during steelmaking. Therefore, it is important to fix the 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 dissolved N and the amount of added Ti in terms of both the cost and the action of Ti described above. When the content of N exceeds 0.0100%, N
The toughness is reduced by adding Ti in an amount corresponding to. Therefore, the upper limit of the content of N as an impurity element is 0.010.
It was set to 0%.

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

The raw material steel having the above chemical composition is, for example, hot-agglomerated into a steel slab, which is then hot-rolled and then hot- or cold-forged. It is machined into a predetermined surface-hardened component shape. Finally, the surface hardening treatment is performed.

(B) Hot forging, slabbing, rolling and heat treatment In the above-mentioned Japanese Patent Laid-Open No. 4-176816, Nb, T
Disclosed is a manufacturing method for preventing the coarsening of crystal grains during high temperature carburization by using case-hardening steel to which at least one of i and V is added. In the case of the case-hardening steel described in this publication, in general, if fine alloy carbonitrides are deposited, it is possible to suppress the crystal grain growth during the surface hardening treatment due to the pinning effect. When precipitating fine alloy carbonitrides during heating of so-called surface hardening treatments such as carburizing and carbonitriding, the coarse alloy carbonitrides precipitated during solidification after melting should be treated before surface hardening treatment. At the stage, it is necessary to form a solid solution in steel sufficiently to prepare a base material for fine alloy carbonitride precipitation. For this purpose, it is sufficient to once heat to a high temperature in the step before the surface hardening treatment. Conventionally, the high temperature heating temperature for suppressing the crystal grain growth has been set to 1200 ° C. from the solid solution temperature obtained from the solubility product of each alloy carbonitride.

However, as described above, the coarse alloy carbonitrides that precipitate during solidification in the steel to which Nb and Ti have been added are the complex carbonitrides 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.

(B) In the case of T <1150 ° C .: The above-mentioned composite carbonitride is stably present in steel.

(B) In the case of 1150 ° C. ≦ T ≦ 1350 ° C .: Only Nb in the above composite carbonitride is solid-dissolved and Ti is concentrated.

(C) When 1350 ° C. <T: The above composite carbonitride completely forms a solid solution (Ti also forms a solid solution).

Therefore, in the present invention, (a) from the viewpoint of improving the hardenability, N is fixed mainly by Ti in order to secure the amount of solid solution B, and (b) finely precipitated NbC and TiC.
In order to prevent the occurrence of abnormal grain growth due to the pinning effect of (1), the material is once heated to 1150 ° C. or higher in the step before the surface hardening treatment.

Therefore, when hot forging is included in the processing step for the surface-hardened part, at least the heating temperature in this hot forging should be set to 1150 ° C. or higher to form a solid solution of Nb (claim). Invention of 3).

Of the preceding steps of the surface hardening treatment already mentioned, those involving "heating" treatment other than hot forging are lump lumping, rolling and so-called "heat treatment". The heating temperature may be set to 1150 ° C. or higher in at least one step (the invention of claim 4).

Here, the upper limit of the heating temperature in the invention of claim 3 and the invention of claim 4 is preferably 1350 ° C. in order to reduce the surface oxidation during heating and to secure the solid solution B.

In order to deposit fine alloy carbonitrides during heating of so-called surface hardening treatment such as carburizing and carbonitriding, the cooling rate after heating is 0.2 ° C./s or more. It is desirable to do.

(C) Surface Hardening Treatment The surface hardening treatment is an essential treatment for hardening the surface of a predetermined surface-hardened component and ensuring the wear resistance and fatigue strength required for the product. However, this processing method is not particularly specified, and a normal method may be used.

(D) Core hardness and toughness of the surface-hardened component after the surface-hardening treatment In order for the surface-hardened component to exhibit sufficient durability even in a harsh environment where automobiles and industrial machines are used, After curing, hardness of the core is Hv300 or higher and 20 J / cm
It is necessary to have an impact value of ² or more. If it deviates from one and / or both of these, the durability of the surface-hardened component in the actual environment will be extremely deteriorated. Therefore, the hardness of the core of the surface-hardened component is Hv 300 or more, and the impact value is 20 J / cm ² or more.

(E) Tempering When tempering at a low temperature, the toughness can be improved without a large decrease in surface hardness.
The surface may be tempered after the surface hardening treatment if necessary. When tempering, it is desirable to set the temperature to 150 to 200 ° C. in order to secure the surface hardness.

[0055]

【Example】

[Example 1] Steels having the chemical compositions shown in Tables 1 to 3 were melted by a usual method using a 150 kg vacuum furnace. Tables 1-3
In the above, Steels A to H are steels of the present invention, and Steels I to X are comparative steels in which any of the components is out of the range of the content specified in the present invention. In comparison steels, steels V, W and X are JIS SMn420 steel, SCr420 steel and S, respectively.
It corresponds to CM420 steel.

Next, after heating these steels to 1140 ° C., they are made into steel pieces by a usual method, and further 1100 ° C.
Then, it was hot forged into a round bar having a diameter of 30 mm at a temperature of 1100 to 1000 ° C. An 8 mm diameter × 12 mm length coarse graining measurement test piece was cut out from the thus obtained round bar after hot forging, and a thermomechanical treatment test under the following four conditions was performed using this test piece, and abnormal grain growth occurred. The rates were investigated by light microscopy.

(Condition 1) The test piece was subjected to 1100 in vacuum.
15 at ℃, 1175 ℃ and 1250 ℃ respectively
After heating for a minute, a deformation amount of 30% was applied by compression processing, and the mixture was cooled to room temperature at a cooling rate of 1.0 ° C./s. After this, 930 ° C. × 6 hr (carbon potential: 0.8%)
After carrying out the carburizing treatment of No. 3, it was oil-quenched.

(Condition 2) The test piece was placed in a vacuum at 1100 ° C.
After heating for 15 minutes at 30 ° C., a deformation amount of 30% was applied by compression processing, and the material was once cooled to room temperature at a cooling rate of 2.0 ° C./s. After this, further 1100 ° C, 1175 ° C and 1
After heating for 15 minutes at a temperature of 250 ℃, 1.0 to room temperature
It cooled at the cooling rate of ° C / s. Then 930 ° C x 6h
After carrying out carburizing treatment of r (carbon potential: 0.8%), oil quenching was performed.

(Condition 3) A 30% deformation amount was given to the test piece by compression processing at room temperature in the air. Then, after heating in vacuum at a temperature of 1100 ° C., 1175 ° C. and 1250 ° C. for 15 minutes, respectively, it was cooled to room temperature at a cooling rate of 1.0 ° C./s. After this, carburizing treatment was performed at 930 ° C. × 6 hr (carbon potential: 0.8%), and then oil quenching was performed.

(Condition 4) 1100 the test piece in vacuum
15 at ℃, 1175 ℃ and 1250 ℃ respectively
After heating for a minute, it was once cooled to room temperature at a cooling rate of 1.0 ° C./s. Then, it was heated in vacuum at 1100 ° C. for 15 minutes, 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. After this, 9
After carburizing at 30 ° C. for 6 hours (carbon potential: 0.8%), oil quenching was performed.

The results of investigation of the occurrence rate of abnormal grain growth are shown in Tables 4-6. The occurrence rate of abnormal grain growth is 1 at 100 times magnification.
The area ratio when 0-field microscopy was performed was displayed.

It is clear from Tables 4 to 6 that among the steels A to H of the present invention and the comparative steels, only steels R and T do not undergo abnormal grain growth when heat-treated under the conditions specified in the present invention.

Example 2 The steel pieces of the steels A to X produced in the above-mentioned Example 1 were heated to 1190 ° C. and then 1190 to 1900.
Hot forging was performed on a round bar having a diameter of 30 mm at a temperature of 1000 ° C. From the center of the round bar after hot forging thus obtained, J
The IS3 Charpy impact test piece was cut out and subjected to a surface hardening treatment at 930 ° C. for 6 hours (carbon potential: 0.8
%) Carburizing treatment followed by oil quenching, then 160 ° C
It tempered in. Next, along with the impact test, the hardness of the center portion of the test piece, that is, the core portion was measured.

The test results are shown in Table 7. From Table 7, steels A to H, which are steels of the present invention, have a core hardness of Hv300 or more and 20 J /.
It can be seen that the surface-hardened parts having an impact value of cm ² or more and using these steels as raw materials can exhibit sufficient durability even in a severe environment where automobiles and industrial machines are used. On the other hand, R and T of the comparative steels in which abnormal grain growth did not occur when the heat treatment was performed under the conditions specified in the present invention in Example 1 above, the core hardness and the impact value were low, and the actual environment of the surface hardened parts was low. The durability of the product will be extremely deteriorated.

[Example 3] The steel pieces of the steels A to H, R and T produced in Example 1 were heat-treated in vacuum at 1180 ° C and once cooled to room temperature at a cooling rate of 0.25 ° C / s. Cooled in. Then, heat to 1100 ° C, then 1100
Hot forging is performed on a round bar having a diameter of 30 mm at a temperature of up to 1000 ° C., and a JIS No. 3 Charpy impact test piece is cut out from the center of the round bar after the hot forging thus obtained, and 930 ° C. as a surface hardening treatment. 6 hr (carbon potential:
0.8%) carburizing treatment followed by oil quenching, then 1
Tempering was performed at 70 ° C. Next, along with the impact test, the hardness of the center of the test piece, that is, the hardness of the core was measured.

The test results are shown in Table 8. From Table 8, steels A to H, which are steels of the present invention, have a core hardness of Hv300 or more and 20 J /
It can be seen that the surface-hardened parts having an impact value of cm ² or more and using these steels as raw materials can exhibit sufficient durability even in a severe environment where automobiles and industrial machines are used. On the other hand, R and T of the comparative steels in which abnormal grain growth did not occur when heat treatment was performed under the conditions specified in the present invention in Example 1 above, the core hardness and the impact value were low, and the actual environment of the surface-hardened parts was low. The durability of the product will be extremely deteriorated.

[0067]

[Table 1]

[0068]

[Table 2]

[0069]

[Table 3]

[0070]

[Table 4]

[0071]

[Table 5]

[0072]

[Table 6]

[0073]

[Table 7]

[0074]

[Table 8]

[0075]

As described above, the low-cost surface-hardened component according to the present invention has excellent strength and toughness and does not cause abnormal grain growth. Therefore, various mechanical structural components for automobiles, industrial machinery, etc., especially It can be used as a surface-hardened component represented by a gear. This surface-hardened component can be relatively easily manufactured 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.

Claims (4)

[Claims] 1. 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: 1.0% or less, Nb:
0.020 to 0.080%, Ti: 0.100% or less,
Al: 0.100% or less, B: 0.0010 to 0.01
00%, the balance consisting of Fe and unavoidable impurities, P in the impurities is 0.025% or less, and S is 0.040.
% Or less and N is 0.0100% or less and the following f
A coarse-grained case-hardening steel characterized in that the values of n1 and fn2 are both 0 or less. fn1 = 8 [% N] -2 [% Ti]-[% Nb] fn2 = 3.4 [% N]-[% Ti] where [% X] is the weight% of the element X. 2. A steel material according to claim 1, wherein the material is a core hardness of Hv300 or more and 20 J / c after surface hardening treatment.
A surface-hardened component excellent in strength and toughness, which has an impact value of m ² or more. 3. A method for producing a surface-hardened component having excellent strength and toughness, which comprises heating the steel according to claim 1 to 1150 ° C. or higher prior to the surface hardening treatment and then hot forging. 4. The strength and toughness, characterized in that the steel according to claim 1 is subjected to at least one step of slabbing, rolling and heat treatment by heating at 1150 ° C. or higher, and then forged and surface-hardened. A method of manufacturing a surface-hardened part that is excellent in heat resistance.