JPS6299416A - Production of case hardening steel - Google Patents

Abstract

PURPOSE:To prevent the formation of coarse austenite crystal grains in the state of a high-temp. surface treatment of a case hardening steel in the stage of producing the case hardening steel by adding a specific ratio of Nb to said steel to decrease the content of N and adding elements having high affinity to N to said steel. CONSTITUTION:N is decreased to <0.009% and at least one kind among <0.05% SolAl, <0.04% Ti and <0.05% Zr having the affinity to N higher than the affinity of Nb are added to the case hardening steel contg. 0.10-0.45% C, <1.5% Si and 0.1-1.5% Mn and the content of Nb is decreased to 0.01-0.14% expressed by formula (1) according to the ratio thereof in order to accelerate the formation of Nb carbide for preventing the formation of the coarse austenite crystal grains in the stage of the high-temp. surface treatment by carburization and further to prevent the formation of the Nb nitride having the lower effect of preventing the formation of the coarse austenite crystal grains. Otherwise, >=1 kinds of Cr, Mo, and Ni or >=1 kinds of S, Pb, Ca, and Te are added alone or in combination to said steel and after the steel is worked at >=1,050 deg.C, the steel is immediately heated and held for >=10min at 400-880 deg.C to prevent the formation of the coarse austenite crystal grains by the formation of the Nb carbide.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for manufacturing case hardening steel. More specifically, the present invention relates to a method for manufacturing case hardening steel that can be subjected to surface hardening treatment such as carburization while suppressing coarsening of crystal grains.

(Prior art) Structural carbon case-hardened steel and alloy case-hardened steel are used for power transmission parts and sliding parts of industrial machinery, construction machinery, automobiles, etc. Since surface pressure strength is required, dephosphorization treatment, that is, surface hardening treatment is generally performed. There are various methods for surface hardening, but carburizing and quenching or carbonitriding and quenching are still the mainstream methods because they significantly improve the properties of steel. However, in carburizing or carbonitriding, heating is usually performed at 900 to 950°C for several hours, which tends to increase the austenite grain strength <m.

Therefore, as a countermeasure to this problem, a method has been considered in which 8Q and N are added in a well-balanced manner and the austenite crystal grains are prevented from becoming coarser by the effect of suppressing the growth of crystal grains (pinning effect) by the 8Q nitride.

However, in reality, these measures alone are not sufficient, especially when high-temperature carburizing is performed at 950°C or higher.
It becomes extremely difficult to stop the coarsening of austenite grains.

Therefore, various studies have been carried out in the past, and for example, 9In was developed as a steel that is difficult to increase in austenite grain size.
2-500 March, No. 54-1647, No. 57-156
Steels to which Nb is added have been proposed as described in Publication No. 57 and the like.

The basic idea of these proposals is to precipitate Nb carbonitride and utilize the pinning effect at austenite grain boundaries.

For example, Japanese Patent Publication No. 5'l-1565'7 discloses boron-treated case-hardened steel, but until then it had been sufficiently deoxidized with Al in order to prevent the nullification of B. The grain growth suppressing effect of TiN produced by adding Ti was utilized, but since the effect of TiN is not sufficient, the synergistic effect of the coexistence of Nb carbide produced by Nb addition and TiN causes finer grains. This means that the change will become more noticeable. However, it does not specifically disclose how to finely and uniformly precipitate Nb carbide. Note that Nb has conventionally been used as a nitride to suppress crystal grain growth.

In addition, in Japanese Patent Publication No. 54-1647, a large amount of Nb, 8Q compounds (nitrides, carbides) is decomposed into austenite as a solid solution, and after hot working, $n grains (particle size number 5 or less) are formed. It is disclosed that the material is cooled as it is. With this type of phase-grained silk weave, the grain growth force will be small even during carburization.
- It is said that the situation will be such that the grain growth suppressing effect due to the precipitation of Nb and Q compounds can be utilized to the maximum.

However, in all of these cases, there is no mention of how to precipitate Nb carbide finely and uniformly, and the relationship between the precipitation form of Nb carbide and its grain growth suppressing effect has not been clarified. It is tr to do
Moreover, regarding the effect of the Nb compound, the effect W varies greatly depending on the history of hot balling before carburizing or carbonitriding, and is extremely unstable.

(Problems to be Solved by the Invention) The purpose of the present invention is to solve the problems of the prior art as described above. It is an object of the present invention to provide a method for producing structural carbon steel and alloy steel that can perform dephosphorization treatment while reliably preventing corrosion.

(Means for Solving the Problems) That is, the present inventors have conducted extensive experiments and studies to provide a case hardening steel in which the austenite grains do not become coarse. At the stage of
It has been found that the pinning effect of Nb can be maximized by precipitating all the Nb contained in the steel finely and uniformly as Nb carbonitride so that no Nb remains. In addition, for this purpose, Nb carbonitrides precipitated by various heat treatments including carburizing or carbonitriding have little effect, and Nb carbonitrides must be heated to 1050°C or higher
It has been found that the fine Nb carbonitrides that precipitate when processed with most of the carbonitrides in solid solution and then immediately maintained at a temperature of 400 to 800°C are very effective. That is, after hot working, direct heat retention (so-called direct charge) is performed.

Furthermore, in order to precipitate all the Nb contained in steel as carbonitrides, which are effective in preventing coarsening of austenite grains, it is more effective to recrystallize austenite after hot working. For this purpose, it has been found that hot working with a working degree of 10% or more in a temperature range of 800° C. or more is desirable. In other words, the ferrite immediately after the Kaniostenite Mi weave or the Kaniostenite structure is transformed.
Innumerable dislocations exist in the pearlite structure, and these become the generation nuclei of Nb carbides that are finely and uniformly precipitated.

If this is cooled to -giant room temperature and then reheated, the strain will be released during reheating and dislocations will disappear. Therefore, as mentioned above, heat retention is performed immediately after hot working (or warm working).

Further, the present inventors conducted further studies and discovered the following. That is, Nb carbide has a higher solid solution temperature than Nb nitride, and does not dissolve until high temperatures. On the other hand, carbonitrides are
When processing strain is introduced, the solid solution temperature decreases. Therefore, when performing normal carburizing or high-temperature carburizing on processed cold-forged or warm-forged materials, Nb carbide is more effective at suppressing grain coarsening than Nb nitride. was found to be effective.

Therefore, in the present invention, the N content is regulated and 8(1, Ti) has a stronger bonding force with N than Nb.

Zr is added to keep N fixed. Al nitride, T
i nitride, Zr nitride) and Nb carbide.

Furthermore, as a result of our studies, the present inventors found that the total amount of solute Nb (that is, the amount of Nb after subtracting the chemical equivalent amount corresponding to the amount of solute N) that can form Nb carbides in the steel is at least It has been found that it is necessary to be present in an amount of 0.01% or more.

Therefore, in the present invention, Nb content, sol. AlC
It is specified that the total 1iNb'' of solid solution Nb defined by the content Ti content, Zr content and N content is greater than 0.01%. That is, the Nb content, sol.Al
The requirement was that the following relational expression be satisfied among the C content, Ti content, Zr content, and Nb content.

Therefore, the gist of the present invention is that, in weight percent, C: 0.10 to 0.45%, Si: 1.5% or less, Mn: 0.1 to 1.5%, Nb: 0. 01～
0.14%, N: 0.009% or less, and sol. Al: 0.05% or less, Ti: 0.
04% or less and Zr: 0.05% or less, and if necessary, Cr: 1.5% or less, Mo: 0.03% or less, and old: 2.0% or less. or more preferably S: 0.04 to 0.13%, Pb:
0.03-0.35%, Ca: 0.0(II-0.
01% and Te: 0.005 to 0.05% of one or more kinds, the balance consists of Fe and inevitable impurities, and contains Nb 1ji, sol, Q content,
A steel having a chemical composition in which Nb", which is determined by the following formula according to the Ti content, Zr content, and N content, is 0.%1 double-1- is heated to 1050°C or higher and then processed. This is a method for producing case-hardened steel in which crystal grains are less likely to become coarse, which is characterized in that the steel is heated in a temperature range of 400 to 800° C. for 10 minutes or more immediately thereafter.

(Function) The reason why the composition range and manufacturing conditions of the steel are limited as described above in the method of the present invention will be described below. In addition, 1%
-11J This is a phantom "weight %" without particular mention.

C: C is a basic component to ensure strength, and as a case hardening steel, the center hardness after carburizing or carbonitriding and tempering is required to be at least Hac20.
It is necessary that the total content be at least 0.05%. Also,
Fatigue strength, which is a necessary property for structural members, is greatly influenced by compressive residual stress introduced into the surface layer, but this depends on the difference in hardness between the surface and the center after surface hardening. The larger the compressive residual stress,
Fatigue strength is also improved. Therefore, if the center hardness is increased too much, the hardness difference with the surface becomes small, which is not only disadvantageous in terms of fatigue strength, but also significantly deteriorates the impact resistance.

This limit is generally about uic45, and for this purpose, the amount of C needs to be suppressed to 0.40% or less.

However, in the present invention, considering that the amount of solid solute C decreases due to the formation of Nb carbides, it is necessary to increase both the lower limit and the -I- limit of the amount of C. Therefore, the amount of C required to generate Nb carbide was considered to be 0.05%, and the lower limit of the C content was set to 0.10% and the upper limit to 0.45%. Preferably it is 0.15 to 0.40%.

Si: Si is usually added as a deoxidizing agent, but in some cases Mn. Addition of Al alone can sufficiently deoxidize. Si is an element that dissolves in ferrite and reduces the ductility, cold workability, and machinability of steel, so it is preferable to have less Si.
The upper limit of the content was set at 1.5%. Preferably 0.
05-1.0%.

Mn: Mn is not only essential as a deoxidizing agent, but also effective in improving the strength and toughness of steel. It is also the most effective element for improving hardenability, which is an important property for case hardening steel, and it is preferable to add at least 0.1% in consideration of synergistic effects with other hardenability improving elements. . On the other hand, 1
．． When added in excess of 5%, hot workability deteriorates and machinability begins to drop rapidly. Therefore, Mn
The lower limit of the content was set at 0.1%, and the lower limit was set at 1.5%.

Nb: Nb combines with C during heat retention at 400 to 800°C to form fine and uniform Nb carbide. At least 0.01% is required for this effect to be exhibited.

However, if it is added in an amount exceeding 0.14%, the solid solution C will be significantly reduced, which may lead to a decrease in strength. Therefore, Nb
The lower limit of gold content was 0.01%, and the second limit was 0.14%. Preferably it is 0.01 to 0.07%.

sol, heq; heql; are usually added as deoxidizing agents, but in some cases, the addition of SiXMn alone can be sufficient to deoxidize. In the present invention, as mentioned above, the purpose is to suppress the coarsening of austenite grains by the pinning effect of Nb carbides, and therefore, it is necessary to fix all N, which has a stronger bonding force with Nb than C. be. For this purpose, it is effective to add M, which has a stronger bonding force with Nb than Nb, but as the amount added increases, alumina-based inclusions increase, which increases the number of defects on the surface of the steel. However, the Q nitrides aggregated and coarsened, and the fine dispersion of the Nb carbides was not adversely affected. Therefore, sol
．． The smaller the amount of Al added, the more preferable it is, but up to 0.05% (1), since its adverse effects are slight.

1. The upper limit of the Al content was set to 0.05%.

Ti1T) Similar to the above-mentioned Al, TiN for Nb carbide generation has a large shape and deteriorates machinability, so T
The fewer i-added stars, the better. Therefore, the Ti content range was set at 0.04% or less, which is the range in which the deterioration of machinability is small.

Zr: Like sol, 8Q, and Ti, Zr is a very effective element for N fixation. However, when added to Tao,
In addition to causing deterioration of machinability, 7. The r-nitrides do not aggregate and become coarse, which does not adversely affect the fine dispersion of the Nb carbides. The limit for Zr content that does not have such a negative effect is 0.05
%, the lower limit was set at 0.05%.

In this way, at least one of sol, 8Q, Ti, and Zr is added as an N-fixing element.

N: In the present invention, the previous i! ! ; As mentioned above, the formation of Nb nitride is undesirable, so sol is used to fix N.

Although at least one of Al, TiXZr is required to be added, it is preferable that the amount of these elements added be as small as possible.

Therefore, it is preferable that the content of N to be fixed is also small. Therefore, by limiting the N content to 0.009% or less, SOl. Al
, Ti, Zr have a completely negligible effect. Therefore, the N content of -LI was set at 0.009%.

Cr: Cr is an optionally added element that improves the hardenability of steel, improves carburizability, and is effective in improving wear resistance even after carburizing. However, if it is added in excess of 1.5%, there is a tendency for excessive carburization to occur, and a network of cementite is formed on the surface layer, which significantly deteriorates fatigue strength and pitting resistance, so the upper limit of the Cr content is 1.
It was set at 5%.

Mo: Mo4) Like Cr, it is an optionally added element that improves hardenability and carburizability. Also, 51. , Mns Cr
Unlike Fe, since it has a lower oxidation potential than Fe, it is less likely to be oxidized during carburizing, and therefore has the effect of suppressing the formation of a so-called abnormal carburized layer (incompletely quenched layer) that forms on the surface layer. Furthermore, MO also has the effect of preventing quenching and tempering embrittlement. Therefore, it is preferable to add a larger amount, but since it is an expensive element, if you consider adding it in combination with other hardenability improving elements such as Mn and Cr, it is possible to make case hardening steel.
It is not a good idea to add more than 3%. Therefore, the MO gold-containing -1-II original concentration was set at 0.3%.

Ni: Ni, which is an optionally added element, is also an element that affects hardenability.
It also has the effect of significantly improving toughness.

Further, like Mo, it suppresses the formation of an abnormal carburized layer, but it is an expensive element and also has the effect of impairing carburizability, so adding a large amount of V is not preferable. Therefore, the upper limit of Ni content was set at 2.0%.

Note that C, Si, Mn, Cr, and MoXNi do not affect the coarsening behavior of austenite grains.

S, Pb, Ca, Te: These elements are optionally added h11 elements and are effective in improving machinability when cutting is performed before carburizing or carbonitriding. It is preferable to add one or more of these elements depending on the cutting method, cutting speed, depth of cut, etc. Note that these elements have no effect on the austenite grain coarsening tendency and carburizability.

The minimum addition amounts necessary to improve the machinability of structural steel are: S: 0.04%, pH: , o, oa%, Ca: 0.04%.
001%, Te: 0, 005%. Moreover, if S exceeds 0.13% and Pb exceeds 0.35%, the strength and toughness will deteriorate significantly, and it is difficult to add more than 0.01% of Ca, O, and 0.01% of Te. If it exceeds 0.05%, the hot workability decreases rapidly, so the lower limits for S (direct: 0.04%, IR value: 0.13%, P
For b, the lower limit is 0.03%, and the second limit is 0.35.
%, Ca, the lower limit is 0.001% and the upper limit is 0.
．． For Te, the lower limit was 0.005% and the upper limit was 0.05%.

Relational expression between Nb, 5olJQ, Ti, and N: In the present invention, as described above, it is necessary to precipitate Nb carbide, and for this purpose, the N content that can be combined with Nb is regulated, and the Sol. Al, Ti, and Zr are added to fix N.

Therefore, in order to coarsen the austenite grains, the amount of Nb that can bond with C, that is, Nb"', must be at least 0.
0.01% or more is required, so the lower limit of Nb” is set to 0.01% or more.
%.

Next, the reason for limiting the manufacturing conditions will be explained.

Support 191: The present invention aims to prevent coarsening of austenite grains by uniformly dispersed fine Nb carbides. For this purpose, Nb carbide precipitated by a simple heat treatment, that is, a treatment in which the temperature is raised from room temperature to a warm or hot temperature range and then held and cooled (including carburizing or carbonitriding), is ineffective and insufficient. However, Nb carbide precipitated during heat retention after warm working or hot working is extremely effective for preventing coarsening of austenite grains.

However, if a large amount of Nb carbonitrides are present during heating before processing, the Nb carbides that precipitate during heat retention will agglomerate and coarsen using the remaining carbonitrides as nuclei, and therefore will not be effective in preventing the growth of austenite grains. Significantly decreased. Therefore, it is necessary to dissolve most of the Nb carbonitride in advance before processing and heat retention, and for this purpose, it is necessary to raise the heating temperature to 1050° C. or higher. In addition, in order to precipitate all of the solid INb as carbides that are effective in preventing coarsening of austenite grains, it is particularly effective to recrystallize austenite grains after processing, so after heating to 1050°C or higher, It is desirable that hot working be performed at 800° C. or higher and at a rate of 10% or higher.

Heat Retention Conditions: As described above, the present invention makes maximum use of the pinning effect of Nb carbide precipitated during heat retention immediately after processing. At a temperature lower than 400°C, Nb carbide may precipitate even if the heat is kept for a long time, and if the heat is kept at a temperature higher than 800°C, the Nb carbide aggregates and coarsens, and the pinning effect is not sufficiently exerted. Therefore, the heat retention temperature needs to be 400 to 800°C. In other words, after processing, before the temperature of the steel becomes lower than 800°C, it is charged into a heat retention furnace and kept at a temperature of 400 to 800°C.

In addition, when maintaining the temperature within this range, if the heat is held for less than 10 minutes, fine Nb carbides will not precipitate sufficiently and the austenite grain growth inhibiting effect will be reduced, so the heat holding time should be 1
The duration was 0 minutes or more. A heat retention time of up to 40 minutes is sufficient. It should be noted that the hot water temperature during this period does not need to be kept constant as long as it is within the above temperature range.

(Embodiment) Next, embodiments of the present invention will be described below.

In the production of steel bars, steel pipes, and thick steel plates, billets or blooms are heated in a heating furnace, as schematically shown in Figure 1.
heated to 1050°C or higher, then rolled using a bar rolling mill, steel pipe rolling mill, or steel plate rolling mill to 400-800°C.
The present invention can be carried out by directly conveying the material to a heat retention furnace (2) maintained at a temperature of 1000 mL and holding it there for 10 minutes or more.

For wire rods, as shown in Fig. 2, the material billet is heated to 1050°C or higher in a heating furnace (■), then rolled into a wire rod using a wire rod rolling machine (■), and then rolled into a wire rod using a wire winding machine (■) for a
The present invention can be carried out by directly winding up the film in a heat retention furnace (1) kept at 0 to 800°C and holding it for 10 minutes or more.

Furthermore, as shown in Fig. 3, for M steel plates, the raw material slab is heated to 1050°C or higher in a heating furnace (1), then rolled using a copper plate rolling mill (2), and then immediately heated to 400 to 800°C.
The present invention can be carried out by winding up the material in a heat retention furnace maintained at 0 and holding it for 10 minutes.

The present invention will be explained below with reference to Examples, but these Examples are merely illustrative of the present invention and do not limit the technical scope of the present invention in any way.

EXAMPLES Thirty-nine types of steel shown in Table 1 were melted in a high-frequency induction melting furnace and forged to a diameter of 50 mm to prepare sample materials.

These specimens were heated to 1100°C for 1 hour in the heating furnace (■) shown in Fig. 1, and then rolled to 30φmm using a steel bar rolling machine (■).
(Wave surface ratio 43.8%), and then immediately rolled to 700%
It was charged into a heat retention furnace (2) maintained at ℃ and held for 30 minutes. In addition, for specimen floor 5, the heat retention temperature was set at 220°C and 1360°C.
420℃, 500℃, 620℃, 780℃, 850℃,
Hold at 930°C for 30 minutes, and
3 minutes, 8 minutes, 12 minutes, 20 minutes, 40 minutes at 00℃ hot water temperature
I also did a test where I held it for a minute. In addition, after completion of heat retention, all samples were allowed to cool in the atmosphere.

The obtained specimens were each cut to a thickness of 10 mm, and
After being held at 0° C. for 4 hours and cooling with water, prior austenite grain boundaries were exposed using a corrosive solution prepared by adding a surfactant to a saturated aqueous solution of picric acid, and the austenite grain size was determined.

Table 1 shows Nb-1, Nb”, which can form Nb carbide.
The calculated value and the austenite grain size of the sample when heated at 700°C for 30 minutes are also shown.

Specimen floors 1 to 27 are steel according to the present invention, and specimen floors 28 to 27 are steel according to the present invention.
30 is the sample size 31 in terms of Nb content or Nb amount.
, floor 32 is 133 in specimen 11 and floor 34 in terms of N content.
is s.

1. A (! In terms of content, specimens are 35 to 374; l:
In terms of T I content, specimens 38 and 39 are comparative steels that are outside the scope of the present invention, respectively, in terms of Zr-containing acid.

As can be seen from the results shown in Table 1, all of the steels of the present invention have fine austenite grain sizes of 8.5 or higher, whereas the comparative steel specimens have grades 28 to 34, 38 and 38. Room 3
No. 9 has coarse grains of No. 5 or below.

In addition, Figure 4 shows specimens 3.11, 17, 8, 35, 36, MB, which have substantially the same components except for the T+ content.
2 with a cutting tool made of cemented carbide P20 at a cutting depth of 0.8 mm, feed rate of 0.25 mm/rev, and no lubrication, the time required for flank wear of the tool to reach 0.2 mm depending on Ti content. It is organized. As shown in FIG. 4, when the Ti content exceeds 0.05%, the tool life rapidly decreases.

Next, FIG. 5 shows the relationship between the austenite grain size and the heat retention temperature of the sample l1kL5, which is outside the scope of the present invention.
It can be seen that the austenite grains suddenly become coarser.

Furthermore, FIG. 6 shows the relationship between the austenite grain size and heat retention time for specimen N115, which shows that once the range of the present invention is exceeded, the grains begin to coarsen rapidly. (Effects) As explained below, in order to maximize the pinning effect of Nb precipitates, the present invention heats the Nb precipitates to a temperature of 1050°C or higher and processes them in a state where Nb is completely dissolved. Then, by holding the temperature at 400 to 800°C for 10 minutes or more to precipitate fine and uniformly dispersed Nb carbide, and further adjusting the chemical composition of the steel, Nb is converted into Nb carbide with a high solid solution temperature. It is characterized by the fact that it is precipitated as Therefore, the total amount of Nb that can become Nb carbide in the Nb content is reduced to 0.0%.
It stipulates that:

In this way, the steel treated by the method of the present invention does not coarsen the austenite grains even when subjected to high-temperature surface hardening heat treatment, has small heat treatment strain after quenching, and has excellent base material toughness. It can be used as a case-hardened steel with excellent wear resistance, fatigue strength, and surface pressure strength.

[Brief explanation of drawings]

Figures 1 to 3 are schematic diagrams showing examples of the arrangement of various rolling and heat retention devices for carrying out the present invention; Figure 4 is a diagram showing the Ti content when a cutting test was conducted with a cemented carbide tool bit. FIG. 5 is a graph showing the relationship between austenite grain size and heat retention temperature; and FIG. 6 is a graph showing the relationship between austenite grain size and heat retention time.

Claims (4)

[Claims] (1) In weight%, C: 0.10 to 0.45%, Si: 1.5% or less, Mn
:0.1~1.5%, Nb:0.01~0.14%, N
: 0.009% or less, sol. Al: 0.05% or less,
Ti: 0.04% or less and Zr: 0.05% or less 1
species or two or more species, the balance being Fe and unavoidable impurities, and Nb content, sol. Al content, Ti
A steel having a chemical composition of 0.01% or more of Nb^*, which is determined by the following formula according to the Zr content and N content, is processed after being heated to 1050°C or higher, and then immediately heated to 400°C. 1. A method for producing case-hardened steel in which crystal grains are less likely to become coarse, the method comprising retaining heat within a temperature range of ~800° C. for 10 minutes or more. ▲Contains mathematical formulas, chemical formulas, tables, etc.▼ (2) In weight%, C: 0.10 to 0.45%, Si: 1.5% or less, Mn
:0.1~1.5%, Nb:0.01~0.14%, N
: 0.009% or less, sol. Al: 0.05% or less,
Ti: 0.04% or less and Zr: 0.05% or less 1
species or two or more species, Cr: 1.5% or less, Mo: 0.3
% or less, Ni: 2.0% or less of one or more types, the balance consisting of Fe and unavoidable impurities, and Nb content, sol. Al content, Ti
A steel having a chemical composition of 0.01% or more of Nb^*, which is determined by the following formula according to the Zr content and N content, is processed after being heated to 1050°C or higher, and then immediately heated to 400°C. 1. A method for producing case-hardened steel in which crystal grains are less likely to become coarse, the method comprising retaining heat within a temperature range of ~800° C. for 10 minutes or more. Nb^* = Nb (%) - (93/14) [N (%) - (
14/27) sol. Al(%)-(14/48)Ti
(%) - (14/91) Zr (%)] (3) In weight%, C: 0.10 to 0.45%, Si: 1.5% or less, Mn
:0.1~1.5%, Nb:0.01~0.14%, N
: 0.009% or less, sol, Al: 0.05% or less,
Ti: 0.04% or less and Zr: 0.05% or less 1
species or two or more species, S: 0.04-0.13%, Pb:
0.03-0.35%, Ca: 0.001-0.01%
, Te: 0.005 to 0.05% of one or more kinds, the balance being Fe and inevitable impurities, and Nb content, sol. Al content, Ti
A steel having a chemical composition of 0.01% or more of Nb^*, which is determined by the following formula according to the Zr content and N content, is heated to 1050°C or higher, processed, and then processed immediately. A method for producing case hardened steel in which crystal grains are less likely to become coarse, the method comprising retaining heat within a temperature range of 400 to 800°C for 10 minutes or more. Nb^* = Nb (%) - (93/14) [N (%) - (
14/27) sol. Al(%)-(14/48)Ti
(%) - (14/91) Zr (%)] (4) In weight%, C: 0.10 to 0.45%, Si: 1.5% or less, Mn
:0.1~1.5%, Nb:0.01~0.14%, N
: 0.009% or less, sol. Al: 0.05% or less,
Ti: 0.04% or less and Zr: 0.05% or less 1
species or two or more species, Cr: 1.5% or less, Mo: 0.3
% or less, Ni: 1 type or 2 or more types of 2.0% or less, S
:0.04~0.13%, Pb:0.03~0.35%
, Ca: 0.001~0.01%, Te: 0.005~
0.05% of one or more kinds, the balance being Fe and unavoidable impurities, and Nb content, sol. Al content, Ti
A steel having a chemical composition of 0.01% or more of Nb^*, which is determined by the following formula according to the Zr content and N content, is processed after being heated to 1050°C or higher, and then immediately heated to 400°C. 1. A method for producing case-hardened steel in which crystal grains are less likely to become coarse, the method comprising retaining heat within a temperature range of ~800° C. for 10 minutes or more. Nb^* = Nb (%) - (93/14) [N (%) - (
14/27) sol. Al(%)-(14/48)Ti
(%) - (14/91) Zr (%)]