JP2021195588A - Case-hardened steel material and method for producing the same - Google Patents

Abstract

To provide a case-hardened steel material that can suppress abnormal grain growth during high-temperature carburization.SOLUTION: The case-hardened steel material is made of a steel having a predetermined component composition. Regarding the number of precipitates containing AlN and/or NbC per 1 μm2 and setting pieces having a circle-equivalent diameter of 10 nm or less as S, pieces having a circle-equivalent diameter of more than 10 nm to 40 nm or less as M, and pieces having a circle-equivalent diameter of 100 nm or more as L, S<10, M>10, L<0.001, and, furthermore, 2S+2000L<M ...Equation (1) is satisfied.SELECTED DRAWING: None

Description

The present invention relates to a skin-baked steel material and a method for producing the same.

For example, in gear parts and the like used as power transmission parts of automobiles, carburized steels such as SCr420H and SCM420H, which are JIS steels, are carburized in order to secure surface strength and internal toughness. Since the carburizing treatment usually takes a long time, shortening the carburizing time is an issue from the viewpoint of cost. Although it is necessary to raise the carburizing temperature to shorten the carburizing time, the carburizing treatment at a high temperature causes coarsening of crystal grains, which may lead to a decrease in fatigue strength and static strength.

As a technique for preventing the coarsening of crystal grains, it is widely practiced to disperse and precipitate particles such as AlN in the manufacturing process before carburizing and pinning (pinning) the crystal grains (specifically, the crystal grain boundaries). (For example, see Patent Documents 1 to 3 below).

Japanese Unexamined Patent Publication No. 2005-163168 Japanese Unexamined Patent Publication No. 2015-140449 Japanese Unexamined Patent Publication No. 2012-207244

However, in the technique of pinning crystal grains by this kind of precipitate particles (pinning particles), there is a problem that it is difficult to sufficiently prevent the phenomenon of abnormal grain growth in which the crystal grains are locally abnormally coarsened. rice field.

Here, the abnormal grain growth means that the pinning force of the precipitate particles was larger than the driving force of the crystal grain growth at the initial stage of carburizing, but the force relationship was reversed during the carburizing, and the pinning force of the precipitate particles was increased. However, it is a phenomenon that occurs when the driving force for crystal grain growth becomes large, and such a reversal of the force relationship is caused by the solid solution of the precipitate particles during carburizing and the osteostatic growth of the precipitates, which causes the pinning force to become small. It occurs due to factors such as becoming.

Against the background of the above circumstances, it is an object of the present invention to provide a skin-baked steel material capable of suppressing abnormal grain growth during high-temperature carburizing and a method for producing the same.

As a result of diligent research focusing on the size and density of the precipitates that act as pinning particles in the skin-baked steel material, the present inventors have conducted diligent studies on fine precipitates having a circle-equivalent diameter of 10 nm or less and a circle-equivalent diameter of 100 nm or more. When the density of coarse precipitates is high, the pinning force, which causes abnormal grain growth during high-temperature carburization, is greatly reduced, and in order to suppress abnormal grain growth, the diameter equivalent to a circle is in the range of more than 10 nm to 40 nm or less. It has been found that increasing the density of precipitates is effective. The present invention has been made based on such findings, and ^{the number of precipitates containing AlN and / or NbC per 1 μm 2} is as follows: S pieces having a circle-equivalent diameter of 10 nm or less and a circle-equivalent diameter of more than 10 nm to 40 nm or less. When M pieces and circle equivalent diameter of 100 nm or more are L pieces, S <10, M> 10, L <0.001 and 2S + 2000L <M are further specified.

The steel material having such a precipitate density can be obtained by using a steel having a predetermined component composition and specifying the conditions for heating before forging and normalizing after forging.

Therefore, the gist of the present invention is as follows.

[1] By mass%, C: 0.10 to 0.30%, Si: 0.01 to 1.50%, Mn: 0.40 to 1.50%, P: 0.03% or less, S: 0.005 to 0.100%, Cu: 0.01 to 1.00%, Ni: 0.01 to 1.00%, Cr: 0.01 to 2.00%, Mo: 0.01 to 0. 50%, s-Al: 0.005 to 0.050%, N: 0.005 to 0.030%, and optionally Nb: 0.001 to 0.100%.
It has a composition in which the balance is Fe and unavoidable impurities.
Regarding the number of precipitates containing AlN and / or NbC ^{per 1 μm 2} , when S pieces have a circle-equivalent diameter of 10 nm or less, M pieces have a circle-equivalent diameter of more than 10 nm to 40 nm or less, and L pieces have a circle-equivalent diameter of 100 nm or more. S <10, M> 10, L <0.001 and
Furthermore, the following formula (1)
2S + 2000L <M ... Equation (1)
A skin-baked steel material characterized by satisfying.

[2] The skin-baked steel material according to [1], which contains Si: 0.15 to 1.50% in mass%.

[3] It is characterized by further containing any one or more of Ti: 0.001 to 0.150% and Zr: 0.001 to 0.300% in mass% [1], [2]. The skin-baked steel material described in any of.

[4] The skin-baked steel material according to any one of [1] to [3], which further contains B: 0.0005 to 0.010% in mass%.

[5] Using a steel material having the chemical composition according to any one of [1] to [4],
When the pre-forging heating temperature is T1 and the pre-forging heating time is tm1, the forging process in which the steel material is heated under the conditions satisfying the following formulas (2) and (3) and then hot forging is performed.
After the forging step, when the normalizing heating temperature is T2 and the normalizing heating time is tm2, the normalizing heating temperature T2 is 900 ° C. or higher and 1000 ° C. or lower, and the following formula (4) is satisfied. The normalizing step of heat-treating the steel material under the conditions and
A method for producing a skin-baked steel material, which comprises.
T1 × log ₁₀ (tm1)> 400 × (8 + log ₁₀ [Al] [N])… Equation (2)
T1 × log ₁₀ (tm1)> 300 × (8 + log ₁₀ [Nb] [C])… Equation (3)
3000 <T2 x log ₁₀ (tm2) <4000 ... Equation (4)
Here, T1 and T2 indicate the temperature (° C.), tm1 and tm2 indicate the time (seconds), and [] in the formula indicates the content mass% of each element.

According to the present invention, it is possible to provide a skin-baked steel material capable of suppressing abnormal grain growth during high-temperature carburizing and a method for producing the same.

It is a figure which showed the example of the temperature pattern in the forging process and the normalizing process of the skin-baked steel material of this embodiment.

The skin-baked steel material according to this embodiment includes C, Si, Mn, P, S, Cu, Ni, Cr, Mo, s-Al, and N, and Nb can be optionally selected. It is contained and the balance consists of Fe and unavoidable impurities. Further, Ti, Zr and B may be further contained.

The reasons for limiting each chemical component in the skin-baked steel material of the present embodiment will be described in detail below. In the following description, "%" means "mass%" unless otherwise specified.

C: 0.10 to 0.30%
C is an element indispensable for ensuring hardness and strength. In order to secure the internal (core) hardness of the carburized parts, it is contained in an amount of 0.10% or more. However, since excessive addition causes deterioration of processability, the upper limit is set to 0.30%. The suitable range of C is 0.15 to 0.30%, and the internal hardness can be stably secured. Even more preferably, it is 0.15 to 0.25%, and the internal hardness can be more stably secured.

Si: 0.01 to 1.50%
Si is an element effective for improving hardenability. Further, Si is effective as a deoxidizing element. In order to obtain these effects, it is contained in an amount of 0.01% or more. However, since excessive addition causes deterioration of processability, the upper limit is set to 1.50%. A suitable Si range is 0.15 to 1.50%. More preferably, it is 0.15 to 1.30%.

Mn: 0.40 to 1.50%
Mn is an element effective for improving hardenability and increasing strength, and is contained in an amount of 0.40% or more in order to obtain this effect. However, since excessive addition causes deterioration of processability, the upper limit is set to 1.50%.

P: 0.030% or less Since P promotes grain boundary embrittlement, it is preferable that the amount is small. However, excessive suppression causes an increase in cost due to the extension of the process, so the upper limit is set to 0.03%.

S: 0.005 to 0.100%
S is contained in an amount of 0.005% or more in order to combine with Mn to increase the amount of MnS and improve machinability. However, since excessive addition reduces fatigue strength, the upper limit is set to 0.100%.

Cu: 0.01-1.00%
Cu is contained in an amount of 0.01% or more because it has an effect of enhancing hardenability. However, since excessive addition causes a decrease in hot forging property, the upper limit is set to 1.00%.

Ni: 0.01-1.00%
Ni is contained in an amount of 0.01% or more in order to improve hardenability and toughness. However, since excessive addition causes the formation of bainite, the upper limit is set to 1.00%. A suitable Ni range is
It is 0.05 to 1.00%. More preferably, it is 0.05 to 0.50%.

Cr: 0.01-2.00%
Cr is contained in an amount of 0.01% or more because it has an effect of enhancing hardenability. However, since excessive addition deteriorates machinability, the upper limit is set to 2.00%. A suitable Cr range is 0.10 to 2.00%. More preferably, it is 0.10 to 1.50%.

Mo: 0.01-0.50%
Mo has the effect of improving hardenability and wear resistance. However, Mo is expensive, and when added in large amounts, a bainite structure appears. Therefore, the Mo content is set to 0.01 to 0.50%.

s-Al: 0.005-0.050%
Al is contained in steel by use as a deoxidizing agent. s-Al is an indispensable element in the present invention and binds to N in the normalizing step to pinn the crystal grains as AlN and suppress the coarsening of the crystal grains at the time of carburizing. In order to obtain this effect, it is contained in an amount of 0.005% or more. However, excessive addition leaves AlN in the middle of the process before normalizing, attracting the surrounding Al and N to make it huge, and rather hinders the formation of AlN in the normalizing process, thus suppressing the coarsening of crystal grains. The effect is reduced. Further, since excessive addition reduces the bending fatigue strength, the upper limit is set to 0.050%. A suitable range of s-Al is 0.015 to 0.050%. More preferably, it is 0.015 to 0.040%.

N: 0.005-0.030%
N is an important element in the present invention that binds to Al to form AlN, and contains 0.005% or more. However, since excessive addition leads to an excessive increase in the amount of AlN, the upper limit is set to 0.030%.

Nb: 0.001 to 0.100%
Nb may be selectively added in order to form fine NbC in the normalizing step and to assist the effect of suppressing grain grain coarsening of AlN. However, excessive addition increases the cost of the steel material and lowers the machinability, so the range should be within the range of 0.001 to 0.100%.

Ti: 0.001 to 0.150%, Zr: 0.001 to 0.300%
Ti and Zr may also form carbides and may be selectively added in order to assist the effect of suppressing grain grain coarsening of AlN. However, excessive addition increases the cost of the steel material and reduces the toughness. Therefore, the upper limit of Ti is 0.150% and the upper limit of Zr is 0.300%. It should be noted that Ti and Zr may contain only one of them, or may contain both of them.

B: 0.0005 to 0.010%
B is an element effective for improving hardenability, and has an effect of segregating at grain boundaries to strengthen grain boundaries and improve strength. However, excessive addition reacts with N in the steel to form BN and reduces toughness. Therefore, the content of B is preferably in the range of 0.0005 to 0.010%.

In the skin-baked steel material according to the present embodiment, ^{regarding the number of precipitates containing AlN and / or NbC per 1 μm 2} , S pieces have a circle-equivalent diameter of 10 nm or less, M pieces have a circle-equivalent diameter of more than 10 nm to 40 nm or less, and are equivalent to a circle. When L pieces have a diameter of 100 nm or more, S <10, M> 10, L <0.001 are specified.

Here, as the precipitate containing AlN and / or NbC, a precipitate containing only AlN, a precipitate containing only NbC, a composite precipitate containing AlN and NbC, and a carbide such as Ti and Zr in these precipitates. A composite precipitate further containing a nitride / carbonitride forming element is also included.

When the precipitate grows Ostwald during carburizing, the large particles grow and the small particles dissolve and disappear. According to the investigation by the present inventors, fine precipitates having a circle-equivalent diameter of 10 nm or less maintain fine crystal grains due to their pinning effect while they are present, but they easily disappear with Ostwald ripening and crystallize. ^{Since it causes local growth of grains, the number S per 1 μm 2} of fine precipitates having a diameter equivalent to a circle of 10 nm or less is set to less than 10 (that is, S <10).

On the other hand, a coarse precipitate having a diameter equivalent to a circle of 100 nm or more has a small pinning effect by itself, and the presence of the coarse precipitate promotes Ostwald ripening and eliminates fine precipitates having a diameter equivalent to a circle of 10 nm or less. ^{Therefore, here, the number L per 1 μm 2} of coarse precipitates having a diameter equivalent to a circle of 100 nm or more is set to less than 0.001 (that is, L <0.001).

On the other hand, precipitates having a diameter equivalent to a circle of more than 10 nm to 40 nm or less are not easily affected by Ostwald ripening, exist stably, and can exert a pinning effect. The number M of the precipitates ^{per 1 μm 2} is more than 10 (that is, M> 10).

Further, in the skin-baked steel material of the present embodiment, the relationship between the precipitates having different sizes is defined by the following formula (1).
2S + 2000L <M ... Equation (1)
The left side of this equation (1), that is, 2S + 2000L, indicates the magnitude of the decrease in pinning force accompanying Ostwald ripening, and according to the investigation by the present inventors, abnormal grain growth occurs when 2S + 2000L becomes M or more. Since it is likely to occur, the above formula (1) is provided.

Next, a method for manufacturing the skin-baked steel material of the present embodiment will be described.
The skin-baked steel material of the present embodiment can be produced through the forging step and the normalizing step described below. As the material to be forged (material for forging), a billet obtained by slab-rolling an ingot whose chemical composition is adjusted in the above range, a billet obtained by slab-rolling a continuous cast material, or a steel bar obtained by hot-rolling these billets. Etc. can be applied.

FIG. 1 shows an example of a temperature pattern in the forging step and the normalizing step of the skin-baked steel material of the present embodiment.
In the forging step, the forging material is heated in order to sufficiently dissolve the precipitate containing AlN and / or NbC precipitated in the previous steps in the austenite structure. Specifically, when the pre-forging heating temperature (the surface temperature of the forging material) is T1 and the pre-forging heating time is tm1, the pre-forging heating is performed under the conditions satisfying the following equations (2) and (3). ..
T1 × log ₁₀ (tm1)> 400 × (8 + log ₁₀ [Al] [N])… Equation (2)
T1 × log ₁₀ (tm1)> 300 × (8 + log ₁₀ [Nb] [C])… Equation (3)
Here, the unit of T1 is temperature (° C.), the unit of tm1 is time (seconds), and [] in the formula indicates the content mass% of each element.
Such pre-forging heating can be performed, for example, by high frequency induction heating. In the case of steel materials to which Nb is not added, pre-forging heating may be performed under the condition that only the formula (2) is satisfied.

Then, the forging material is forged, and then cooled to obtain a predetermined forged product by ferrite + pearlite transformation.
Here, in order to suppress abnormal grain growth during carburizing, it is also effective to reduce the ferrite average particle size number after forging (before carburizing) (larger ferrite particle size). In this example, as shown in FIG. 1, after forging, heat is kept for 10 minutes in a temperature range between 1000 and 800 ° C., or slowly cooled from 1000 to 800 ° C. at 10 ° C./min or less, followed by 750 to 750. Insulation is carried out in a temperature range between 650 ° C. for 30 minutes, or 750 to 650 ° C. is slowly cooled at 2 ° C./min or less, and then cooled to approximately room temperature. In this case, precipitates such as AlN that act as pinning particles are not precipitated during cooling, or the density of these precipitates is suppressed to a low level, and the austenite crystal grains can be coarsened, resulting in the ferrite + pearlite transformation. The ferrite grain size can be increased.

In the next normalizing step, the forged steel material is charged into the heat treatment furnace, and the precipitate containing AlN and / or NbC is dispersed and precipitated at a density according to a predetermined size. Heat treatment is applied under the conditions.
That is, when the normalizing heating temperature is T2 and the normalizing heating time is tm2, the normalizing heating temperature (internal atmosphere temperature) T2 is 900 ° C. or higher and 1000 ° C. or lower, and the following formula (4) is satisfied. The heat treatment is performed under the conditions to be used.
3000 <T2 x log ₁₀ (tm2) <4000 ... Equation (4)
Here, T2 indicates a temperature (° C.) and tm2 indicates a time (seconds).

After the above heat treatment, as in the case of the forging step, heat is kept for 30 minutes in a temperature range between 750 to 650 ° C., or slowly cooled from 750 to 650 ° C. at 2 ° C./min or less, and then to about room temperature. Cool down.

According to the above-mentioned production method, it is possible to obtain a skin-baked steel material in which the density of each precipitate size is controlled so that abnormal grain growth at the time of carburizing can be suppressed. The skin-baked steel material thus produced is processed into a desired shape by machining, and then is suitable for manufacturing mechanical structural parts through the steps of carburizing and quenching → tempering → finishing machining.

Next, examples of the present invention will be described below. Here, a test material was prepared based on the chemical composition and heat treatment conditions of Examples and Comparative Examples shown in Table 1 below, and the precipitate density and the coarsening temperature were evaluated. In each comparative example shown in Table 1, the chemical composition and / or heat treatment conditions are outside the scope of the present invention.

1. 1. Preparation of Test Material 50 kg of steel ingots with chemical components shown in Table 1 above were melted in a vacuum high frequency induction melting furnace to prepare an ingot having a diameter of 100 mm. Then, after heating the ingot at 1300 ° C. for 2 hours, it was forged to φ30 mm and processed into a rod shape.
After that, in order to simulate the actual process, a cylindrical material with a diameter of 8 mm and a length of 12 mm L was prepared, a thermocouple was joined to the side surface of the cylinder and the center in the length direction with the machining formaster, and Table 1 was used with the machining formaster. After simulating each pre-forging heating temperature and holding time shown in (1), the mixture was cooled with He gas. Next, the material was normalized under the heating conditions shown in Table 1 to prepare a test material.
Regarding the evaluation of the coarsening temperature, the test material was heat-treated at the carburizing simulated temperature for 2 hours in an atmospheric furnace as a simulation of carburizing, and then water-cooled. Here, the carburizing simulated temperature was set at intervals of 20 ° C. from 880 to 1100 ° C., and the presence or absence of coarse particles was investigated at each carburizing simulated temperature.

2. 2. Evaluation 2-1. Precipitate density A sample for an electron microscope was prepared from the center of the tempered test material by an extraction replica, and an Al-based precipitate was prepared from element mapping by STEM-EDS at an acceleration voltage of 300 kV at a magnification of 100,000 times with an electron microscope. Objects and Nb-based precipitates were detected, and the size (circle-equivalent diameter) and the number of precipitates containing AlN and / or NbC were measured from 10 visual fields in a bright-field image. The major axis and the minor axis of the precipitate were measured, and the geometric mean thereof was taken as the diameter equivalent to the circle of the precipitate.
Measurement results show that circle the number S of 1 [mu] m ² per equivalent diameter 10nm following precipitates, circle the number M of 1 [mu] m ² per equivalent diameter 10nm ultra ~40nm following precipitates, 1 [mu] m in circle equivalent diameter 100nm or more precipitates ² The number L per hit was obtained, and it was determined whether the obtained values of S, M, and L satisfied the formula (1) specified in the present invention, and these results are shown in Table 2 below.

2-2. Heating conditions For each Example and Comparative Example, it is determined whether or not the conditions for pre-forging heating and the conditions for normalizing satisfy the formulas (2), (3) and (4) specified in the present invention. The results are shown in Table 2 below.

2-3. Coagulation temperature Using a sample obtained by cutting the test material in the length direction after heat treatment simulating carburizing, the cut surface is embedded with resin, polished, and then with picric acid (picric acid 10 g, water 500 ml). Old austenite crystal grains were revealed. Then, the central part of the sample was observed in a cross section of 5 fields with an optical microscope (100 times), and the particle size was measured according to JISG0551. Then, the carburizing simulated temperature of the test material confirmed in any field of view for the coarse particles having a particle size number of 5 or less specified by JISG0551 was set as the coarsening temperature, and the results are shown in Table 2 below. In Table 2, those having a coarsening temperature of 1000 ° C. or higher were evaluated as acceptable “◯” (excellent in the property of suppressing abnormal grain growth).

From the evaluation results in Table 2, the following can be seen.
Comparative Example 1 is an example in which the temperature T1 (or the heating time tm1) in the pre-forging heating is low and the heating condition of the formula (2) is not satisfied. In Comparative Example 1, the density (number) of coarse precipitates of 100 nm or more exceeds the upper limit of the present invention, and the formula (1) that defines the relationship between precipitates of different sizes is not satisfied, resulting in coarseness. The temperature is as low as 960 ° C.

Similar to Comparative Example 1, Comparative Example 2 is also an example in which the temperature T1 (or the heating time tm1) in the pre-forging heating is low and the heating conditions of the formulas (2) and (3) are not satisfied. Similar to Comparative Example 1, the density (number) of coarse precipitates having a diameter of 100 nm or more exceeds the upper limit of the present invention, and the formula (1) that defines the relationship between precipitates of different sizes is not satisfied, so that the precipitates are coarse. The conversion temperature is as low as 940 ° C.

Comparative Example 3 is an example in which the amount of Nb is added in excess of the upper limit of the present invention. As a result, the density of the fine precipitates of 10 nm or less and the density of the coarse precipitates of 100 nm or more each exceed the upper limit of the present invention, and the coarsening temperature is as low as 980 ° C.
According to Comparative Example 3, it can be seen that simply increasing the total amount of precipitates as pinning particles does not contribute to the improvement of the coarsening temperature.

Comparative Example 4 is an example in which the coarse precipitate of 100 nm or more exceeds the upper limit of the present invention as a result of the normalizing heat treatment for an excessively long time. In this case as well, the equation (1) that defines the relationship between the precipitates having different sizes is not satisfied, and it is presumed that the pinning force is greatly reduced due to the Ostwald ripening during carburizing, and the coarsening temperature is as low as 940 ° C.

Comparative Example 5 is an example in which, contrary to Comparative Example 4, the normalizing heat treatment was performed for an excessively short time. As a result, it does not satisfy the formula (1) that defines the relationship between precipitates of different sizes. Also in this example, the coarsening temperature is as low as 980 ° C.

Comparative Example 6 is an example in which the amount of Al is added in excess of the upper limit of the present invention. It does not satisfy the formula (1) that defines the relationship between precipitates of different sizes, and the coarsening temperature is as low as 980 ° C.

Comparative Example 7 is an example in which the amount of Al is below the lower limit of the present invention. In Comparative Example 7, the absolute amount of AlN, which is a pinning particle, is insufficient, and the number M of precipitates having a diameter equivalent to a circle of more than 10 nm to 40 nm or less, which is effective for suppressing abnormal grain growth, does not reach the specification of the present invention. In this case as well, the coarsening temperature is as low as 880 ° C.

As described above, in all the comparative examples, coarse grains were observed at a carburizing simulated temperature of less than 1000 ° C., and the occurrence of abnormal grain growth was confirmed.

On the other hand, in Examples 1 to 10 in which the chemical composition of the steel is within the range of the present invention and the density of each precipitate size is within the appropriate range, the coarsening temperature is 1000 ° C. or higher. It can be seen that the example skin-baked steel material is effective in suppressing abnormal grain growth during high-temperature carburizing.

Although the present invention has been described in detail above, the present invention is not limited to the above embodiments and examples, and various modifications can be made without departing from the spirit of the present invention.

Claims (5)

By mass%,
C: 0.10 to 0.30%,
Si: 0.01-1.50%,
Mn: 0.40-1.50%,
P: 0.030% or less,
S: 0.005 to 0.100%,
Cu: 0.01-1.00%,
Ni: 0.01-1.00%,
Cr: 0.01-2.00%,
Mo: 0.01-0.50%,
s-Al: 0.005 to 0.050%,
N: Containing 0.005 to 0.030% and
Further, Nb: 0.001 to 0.100% is optionally contained, and Nb: 0.001 to 0.100% is contained.
It has a composition in which the balance is Fe and unavoidable impurities.
Regarding the number of precipitates containing AlN and / or NbC ^{per 1 μm 2} , when S pieces have a circle-equivalent diameter of 10 nm or less, M pieces have a circle-equivalent diameter of more than 10 nm to 40 nm or less, and L pieces have a circle-equivalent diameter of 100 nm or more. S <10, M> 10, L <0.001 and
Furthermore, the following formula (1)
2S + 2000L <M ... Equation (1)
A skin-baked steel material characterized by satisfying. In claim 1, by mass%,
Si: 0.15 to 1.50%
A skin-baked steel material characterized by containing. In any of claims 1 and 2, in mass%,
Ti: 0.001 to 0.150%
Zr: 0.001 to 0.300%
A skin-baked steel material characterized by further containing any one or more of the above. In any one of claims 1 to 3, in mass%,
B: 0.0005 to 0.010%
A skin-baked steel material characterized by further containing. Using a steel material having the chemical composition according to any one of claims 1 to 4,
When the pre-forging heating temperature is T1 and the pre-forging heating time is tm1, the forging process in which the steel material is heated under the conditions satisfying the following formulas (2) and (3) and then hot forging is performed.
After the forging step, when the normalizing heating temperature is T2 and the normalizing heating time is tm2, the normalizing heating temperature T2 is 900 ° C. or higher and 1000 ° C. or lower, and the following formula (4) is satisfied. The normalizing step of heat-treating the steel material under the conditions and
A method for producing a skin-baked steel material, which comprises.
T1 × log ₁₀ (tm1)> 400 × (8 + log ₁₀ [Al] [N])… Equation (2)
T1 × log ₁₀ (tm1)> 300 × (8 + log ₁₀ [Nb] [C])… Equation (3)
3000 <T2 x log ₁₀ (tm2) <4000 ... Equation (4)
Here, T1 and T2 indicate the temperature (° C.), tm1 and tm2 indicate the time (seconds), and [] in the formula indicates the content mass% of each element.

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