JP2006265704A - Steel for case hardening having excellent crystal grain coarsening resistance and cold workability and capable of obviating softening and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel for case hardening exhibiting excellent crystal grain coarsening resistance, for performing case hardening treatment such as carburizing and carbonitriding in a shorter time, even in the case carburizing is performed at a temperature higher than that in the conventional example, and exhibiting excellent cold workability even without performing softening as the stock, e.g., for rod-shaped machine parts such as a pulley for a CVT (continuously variable transmission) requiring carburizing depth and excellent workability. <P>SOLUTION: The steel for case hardening having excellent crystal grain coarsening resistance and cold workability and capable of obviating softening comprises a rolled steel in which the contents of C, Si, Mn or the like are specified, and further, the contents of N, Al and Ti are specified. The average value of the Vickers hardness in the cross-section of the steel is ≤180, and also, the maximum value of the standard deviation in the variation of Vickers hardness is ≤5. Its useful production method is also provided. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  TECHNICAL FIELD The present invention relates to a case hardening steel used as a material for machine parts used by carrying out case hardening processing in transportation equipment such as automobiles, construction machinery and other industrial machines, and in particular, bearings, pulleys for CVT, and shafts. When it is used as a material for gears, gears with shafts, etc., it is excellent in crystal grain coarsening properties, has good cold workability and can omit soft annealing before processing. Steel and its manufacturing method.

  In machine parts used for automobiles, construction machinery, and other various industrial machines, surface hardening heat treatment (case hardening) such as carburizing, nitriding, and carbonitriding has been conventionally performed for parts that require particularly high strength. ing. For these applications, the case-hardening steels such as SCr, SCM, SNCM, etc. are usually used, and after forming into the desired part shape by machining such as forging and cutting, carburizing and carburizing. A surface hardening heat treatment such as nitriding is performed, and then a finishing process such as polishing is performed.

  In recent years, it has been desired to reduce the manufacturing cost and the lead time for the mechanical parts as described above, and the heat treatment time has been shortened by increasing the case baking temperature. However, when the skin baking temperature is raised, the crystal grains of the material become coarse and the amount of heat treatment strain increases.

  Then, the case hardening boron steel which added Ti is proposed as what improved the crystal grain coarsening characteristic of the case hardening steel (patent documents 1, 2). They fix free nitrogen (free-N) by adding about 0.1 to 0.2% by mass of Ti in the steel, and finely combine Ti carbide, composite carbide containing Ti, Ti nitride, etc. By precipitating, the coarsening of the austenite crystal grains at the time of heating for the skin baking treatment is suppressed.

  On the other hand, in case hardening steel, cold workability is an important required characteristic because cold work is performed when forming into a part shape. In addition, when cold working steel materials, soft annealing such as spheroidizing annealing is often performed before processing to improve cold workability, but softening annealing includes equipment costs, labor costs, energy costs, etc. Therefore, a steel material excellent in cold workability that can be processed without such soft annealing is desired.

  For this reason, even in case hardening steel to which Ti is added, steel materials with improved workability have been developed so that they can be cold worked without softening annealing (Patent Documents 3, 4, 5, etc.) ). In these inventions, the cold workability is improved by adjusting the chemical components that mainly affect the cold workability in the steel components. Patent Documents 3 and 4 disclose a method for appropriately controlling the cooling rate after hot rolling as a further measure for improving cold workability, and Patent Document 5 further discloses an improvement in cold workability. In another improvement, a method for controlling the metallographic structure of the hot rolled material is also disclosed.

However, these conventional case-hardening steels cannot be said to have sufficient cold workability without soft annealing when applied to parts that have complex shapes or undergo strong processing, and further improvements are desired. It is.
Japanese Patent Laid-Open No. 10-81938 JP-A-10-130720 JP-A-6-299241 JP-A-10-130777 Japanese Patent Laid-Open No. 11-43737

  The present invention has been made paying attention to the circumstances as described above, and its purpose is to further improve the characteristics of the case hardening steel disclosed in the above-mentioned prior art, and in particular, to further improve the cold workability. It is possible to eliminate the softening annealing, and further suppress the coarsening of the crystal grains due to the heat treatment for the case hardening process, and the case hardening steel that gives the case hardening parts with good physical characteristics and dimensional system. It is another object of the present invention to provide a manufacturing method capable of reliably obtaining a case-hardening steel having such characteristics.

The steel for case hardening, which is excellent in crystal grain coarsening characteristics and cold workability according to the present invention, which has been able to solve the above-mentioned problems, and which can omit the softening annealing, is in mass%,
C: 0.10 to 0.35%,
Si: 0.03-1.0%,
Mn: 0.20 to 2.0%,
S: 0.1% or less (including 0%),
N: 0.030% or less (including 0%),
Al: 0.2% or less (including 0%),
Ti: 0.03 to 0.30%,
Characterized in that the balance is substantially made of steel, the average value of Vickers hardness in the cross section is 180 or less, and the maximum standard deviation of Vickers hardness variation is 5 or less. Have.

  In the case-hardening steel according to the present invention, the main body of the metal structure in the cross section is ferrite + pearlite, and those structures account for 80% or more, the average value of the Vickers hardness is relatively low, In addition, it is preferable because the standard deviation of hardness variation is suppressed to a lower level.

In addition to the above-mentioned essential elements, it is also effective for the steel of the present invention to contain one or more elements selected from the groups shown in the following 1) to 4) according to the required properties.
1) Cu: 3.0% or less (not including 0%), Ni: 3.0% or less (not including 0%), Cr: 2.0% or less (not including 0%), Mo: 2 At least one selected from the group consisting of 0.0% or less (excluding 0%),
2) B: 0.0005 to 0.010%,
3) Nb: 0.2% or less (not including 0%), V: 0.3% or less (not including 0%),
Zr: at least one selected from the group consisting of 0.3% or less (not including 0%),
4) REM: 0.03% or less (not including 0%), Ca: 0.03% or less (not including 0%), Mg: 0.03% or less (not including 0%), Pb: 0 .3% or less (excluding 0%), Bi: 0.3% or less (not including 0%), Te: 0.3% or less (not including 0%), Se: 0.3% or less ( 0% is not included), Sn: at least one selected from the group consisting of 0.3% or less (not including 0%).

Further, the production method of the present invention is positioned as a method capable of industrially and reliably producing a case-hardening steel having the above-described characteristics, and a steel satisfying the above-mentioned component composition requirements at 1250 ° C. After soaking at the above temperature, directly or after rolling and cooling to a temperature below the Ar ₁ transformation point, reheated to 1050 to 1200 ° C., directly or after rolling and below the Ar ₁ transformation point. It is characterized in that the treatment to cool to the temperature is performed once or more, then reheated to 850 to 1000 ° C. and then rolled, and the final rolling temperature is set to 700 to 850 ° C.

  According to the present invention, the chemical composition of steel is specified, in particular, the average value of Vickers hardness in the cross section is specified, and the standard deviation of the variation is suppressed as much as possible, more preferably the metal structure of the steel cross section. With a structure consisting mainly of ferrite and pearlite, it has excellent cold workability to withstand complex processing and strong processing without soft annealing, and is resistant to crystallization by case hardening heat treatment for surface hardening treatment. It is possible to provide a case-hardening steel that provides a case-hardening part that is excellent in grain coarsening properties and excellent in mechanical properties and dimensional accuracy.

  Under the prior art as described above, the present inventors focused on Ti-added case-hardening steel, affecting their performance to further improve the grain coarsening resistance and cold workability. Research has been conducted mainly on the composition, physical properties, crystal structure, etc. As a result, as described above, the steel composition is specified, the average value of Vickers hardness in the hot-rolled material cross section is kept low, the standard deviation of the hardness variation is reduced, or the metal structure is further reduced. It is known that if it is optimized, it has excellent cold workability even when softening annealing is omitted, and it can obtain a good case-hardening steel in terms of the grain coarsening resistance property due to heating for the case-hardening treatment. The present invention has been conceived.

  Hereinafter, the reason for determining the chemical composition of steel in the present invention will be clarified, and subsequently, the average value of Vickers hardness in the steel cross section, the standard deviation of hardness variation, and the reason for determining the metal structure will be clarified. Go.

  First, the reason for determining the chemical composition of steel will be described.

C: 0.10 to 0.35%;
C is an important element for securing the core hardness necessary for a machine part. If it is less than 0.10%, the static strength as a machine part becomes insufficient due to insufficient hardness. However, if the amount of C is too large, it becomes too hard and the toughness of the core part decreases and the cold workability also deteriorates, so it is necessary to keep it to 0.35% or less. A more preferable C content is 0.12% or more and 0.30% or less.

Si: 0.03-1.0%;
Si acts as a deoxidizer, has the effect of reducing the amount of oxide inclusions and improving internal quality, and is effective in ensuring the surface hardness of case-hardened parts by suppressing the decrease in hardness during tempering. It is an element and requires addition of 0.03% or more. However, if the amount of Si is too large, the material becomes too hard and the cold workability deteriorates, and the formation of a grain boundary oxide layer during the carburizing process is promoted, and the mechanical properties are adversely affected. Therefore, in order to suppress these obstacles, 1.0% is set as the upper limit. A more preferable Si content is 0.05% or more and 0.50% or less.

Mn: 0.20 to 2.0%;
Mn acts as a deoxidizer, has the effect of reducing the amount of oxide inclusions and improving the internal quality of the steel, and also has the effect of significantly increasing the hardenability during carburizing and quenching. Therefore, it is necessary to contain 0.2% or more. However, if it is too much, not only the deformation resistance during cold working increases and the workability decreases, but also the formation of grain boundary oxide layer during carburizing is promoted, and the mechanical properties are adversely affected. The upper limit is 2.0%. A more preferable content of Mn is 0.40% or more and 1.80% or less.

S: 0.1% or less;
S has the effect of reacting with Mn to form MnS to enhance machinability, but it also has an effect on impact properties and cold workability due to the formation of TiS inclusions, etc. It is preferable to suppress it, and at most 0.1% or less, preferably 0.03% or less.

N: 0.030% or less;
N combines with Al and Ti to form nitrides and carbonitrides, and has the effect of suppressing austenite grain growth during carburizing heating, but has a significant adverse effect on impact properties and fatigue properties. , At most 0.030% or less, preferably 0.025% or less.

Al: 0.2% or less;
Al is an element effective for deoxidation of steel, and also works effectively for adjusting crystal grains. However, if the Al content is too high, hard and coarse non-metallic inclusions (Al ₂ O ₃ ) are generated. Therefore, impact characteristics and cold workability are deteriorated, so it should be suppressed to 0.2% or less. A more preferable content of Al is 0.1% or less.

Ti: 0.03-0.30%;
Ti combines with free-N in steel to produce fine Ti nitrides, and precipitates as fine Ti carbides and Ti-containing composite carbides to suppress coarsening of austenite grains during carburizing heating It is an important element that must be contained in an amount of 0.03% or more in order to exhibit these functions effectively. However, if the amount of Ti is too large, the amount of Ti-containing precipitates produced becomes excessive and adversely affects cold workability, so 0.30% is made the upper limit. A more preferable Ti content is 0.05% or more and 0.20% or less.

  The essential constituent elements of the steel used in the present invention are as described above, and the balance is substantially Fe. “Substantially” means to allow the entry of unavoidable elements such as P (phosphorus) and O (oxygen), which are inevitably mixed. In order to suppress it, it is preferable to suppress P to 0.03 or less and O to 0.003% or less.

  By the way, P segregates at the grain boundaries and lowers the impact characteristics and cold workability of the parts. Therefore, it should be suppressed as much as possible, at most 0.03% or less, more preferably 0.010% or less. It is good. Further, O (oxygen) lowers the strength characteristics of the steel material, so 0.003% or less, more preferably 0.001% or less is preferable.

  In addition to the above essential elements, the steel material used in the present invention is also effective to contain the following selective elements in order to give further additional characteristics as desired. What added the element is also contained in the technical scope of this invention.

Cu: 3.0% or less (not including 0%), Ni: 3.0% or less (not including 0%), Cr: 2.0% or less (not including 0%), Mo: 2.0 % Or less (excluding 0%), at least one selected from the group consisting of:
Cu, Ni, Cr, and Mo are all effective elements in that they all contribute to improvement in hardenability, and among these, Cu also contributes to improvement in corrosion resistance. Ni and Mo also contribute to improving the toughness of the steel material, and Cr also has the effect of increasing the carburizing hardenability. However, since the effects of these elements are saturated near the above upper limit values, addition beyond this is not economical, excessive amounts of Cr adversely affect toughness, Mo is toughness and cold workability Addition exceeding the upper limit value should be avoided.

  Of these elements, Cu, in particular, tends to deteriorate the hot workability of the steel material when it is added alone. However, it is preferable to use an appropriate amount of Ni together with Cu, because the adverse effects of such Cu addition can be avoided.

B: 0.0005 to 0.010%;
B has the effect of significantly increasing the hardenability of the steel material in a small amount, and also has the effect of enhancing the impact strength by strengthening the grain boundaries. Such an effect is effectively exhibited by adding 0.0005% or more. However, these effects saturate at about 0.010%, and if the amount of B is too large, B nitride is easily formed and significantly affects cold workability, so at most 0.010% Should be kept below. A more preferable B content is 0.0007% or more and 0.0050% or less.

Nb: 0.2% or less (not including 0%), V: 0.3% or less (not including 0%), Z
r: at least one selected from the group consisting of 0.3% or less (not including 0%);
Nb, V, and Zr all have a function of suppressing the coarsening of austenite crystal grains by forming precipitates made of carbides and nitrides. Since it will adversely affect sex, each should be kept below the upper limit.

REM: 0.03% or less (not including 0%), Ca: 0.03% or less (not including 0%), Mg: 0.03% or less (not including 0%), Pb: 0.3 % Or less (excluding 0%), Bi: 0.3% or less (not including 0%), Te: 0.3% or less (not including 0%), Se: 0.3% or less (0% At least one selected from the group consisting of Sn: 0.3% or less (not including 0%);
Any of these elements effectively works to improve the machinability of the steel material, but if it is too much, the toughness is remarkably deteriorated, so even if it is added, it should be kept below the upper limit value.

  In the present invention, in addition to the restriction of the steel components described above, as an important physical characteristic as a rolled material, the average value of Vickers hardness in the cross section is 180 or less, the maximum value of the standard deviation of the hardness variation Is required to be 5 or less. That is, the present inventors have conducted research from various angles on the influence of physical properties on the cold workability and crystal grain coarsening characteristics during heat treatment for rolled steel that satisfies the above-mentioned component composition requirements. As described above, the average value of Vickers hardness and the standard deviation of the hardness variation in the cross section of the test steel material have a significant effect on their properties, and the average value of the Vickers hardness is more preferably 180 or less. Is not more than 170 and the standard deviation of the maximum deviation of 5 or less has stable and excellent cold workability even when softening annealing is omitted, and at the time of heat treatment for case hardening It was confirmed that excellent performance was also exhibited in the grain coarsening resistance characteristics.

  The theoretical reason why such a tendency can be obtained has not been clarified yet, but the following can be considered. That is, a relatively low average value of Vickers hardness means that it is relatively soft and easily deformed, and by suppressing this value to a predetermined value or less, it is possible to improve cold workability. It is done. Specifically, by suppressing the value to 180 or less, more preferably 170 or less, it is possible to obtain excellent cold workability without softening annealing. However, even if the average value of the Vickers hardness is 180 or less, if the standard deviation of the hardness variation becomes large, a region with a small deformability exists locally, and that region becomes the starting point of the fracture. Since the cold workability of the steel as a whole is reduced, the standard deviation of the hardness variation is required to suppress the workability deterioration due to the hardness variation and ensure excellent cold workability without soft annealing. It is necessary to suppress the maximum value of 5 to 5 or less, and it is more preferable to suppress the maximum value to 4 or less.

  Furthermore, as a result of studying the metal structure of the rolled steel material that gives the average value of the Vickers hardness and the maximum standard deviation of the variation, the higher the total area ratio of ferrite and pearlite in the cross-sectional metal structure, the higher the ratio. The average value of the Vickers hardness is stabilized at a relatively low value, the maximum value of the standard deviation of the hardness variation is small, and the total area ratio is at least 80%, preferably 90% or more, more preferably It was confirmed that 95% or more exhibits excellent cold workability so that soft annealing after processing can be omitted.

  By the way, the fact that the total area ratio of ferrite + pearlite is large means that there are few other structures such as bainite and martensite, and since the metal structure is entirely homogeneous, Vickers hardness However, it is considered that the hardness is generally uniform and the hardness variation is reduced.

  As described above, according to the present invention, the steel composition is specified, and the average value of the Vickers hardness of the steel cross section and the maximum standard deviation of the hardness variation are kept low. By securing 80% or more of the total of ferrite and pearlite, excellent cold workability is ensured even when softening annealing is omitted, and excellent crystal grain coarsening characteristics due to heating for case hardening treatment, It is possible to provide steel for case hardening that provides case hardening parts with good strength characteristics and dimensional accuracy.

Next, in order to obtain a case-hardening steel having the above-described characteristics, a steel material satisfying the above-mentioned chemical composition requirements is soaked at a temperature of 1250 ° C. or higher and directly or rolled, and then Ar ₁ transformation is performed. After cooling to a temperature below the point, reheat to 1050 to 1200 ° C., directly or directly after rolling and cool to a temperature below the Ar ₁ transformation point, and then reheat to 850 to 1000 ° C. It is very effective to roll after heating and to control the final rolling temperature to 700 to 850 ° C.

After soaking at 1250 ° C. or higher and cooling to below the Ar ₁ transformation point, the austenite coarsened during heating is transformed into ferrite, and the austenite grains are then refined when reversely transformed into austenite by subsequent heating. At the same time, the Ti-containing precipitate is made to be able to appropriately adjust the size of the Ti precipitate by reheating in the next step.

After soaking at a temperature of 1250 ° C. or higher, the cooling to a temperature below the Ar ₁ transformation point is performed by transforming austenite coarsened during heating into ferrite and then reverting to austenite by heating before rolling. This is to transform and refine the austenite grains and to precipitate Ti-containing precipitates at the time of ferrite transformation. For this purpose, it is essential to cool to a temperature below the Ar ₁ transformation point after soaking.

  After that, re-heating in the temperature range of 1050 to 1200 ° C. is to precipitate Ti-containing precipitates of an appropriate size in the austenite, and to coarsen the austenite grains during heating for the skin baking treatment. In addition to reducing the solid solution amount of Ti during heating before rolling and suppressing precipitation strengthening due to Ti-containing precipitates that precipitate in the cooling process after rolling, variations in the average hardness and hardness of rolled steel materials are achieved. Is to keep the value low. Incidentally, if the re-heating temperature exceeds 1200 ° C, the solid solution amount of the Ti-containing precipitate increases, and if it is lower than 1050 ° C., the Ti-containing precipitate does not grow to an appropriate size. The purpose of heat cannot be fulfilled.

Therefore, if this re-heating and cooling to the temperature below the Ar ₁ transformation point are performed once or more, or rolling is performed during that time, the hardness variation of the rolled material cross section is further reduced and the cold workability is further enhanced. Will be.

  Then, after reheating to 850-1000 degreeC, it rolls and it controls so that the final rolling temperature may be in the range of 700-850 degreeC. The reason why the reheating temperature before rolling is set to 850 ° C. or more is that if it is less than 850 ° C., the deformation resistance during rolling is too large and the load on the rolling mill becomes excessive. The reason why the reheating temperature is suppressed to 1000 ° C. or less is to improve cold workability by refining the austenite grains after rolling and making the metal structure of the rolled material mainly composed of fine ferrite and fine pearlite. A more preferable reheating temperature before rolling is 950 ° C. or less.

  Moreover, if the final rolling temperature at the time of rolling performed after reheating is less than 700 ° C., precipitation of ferrite occurs during the rolling process, the deformation resistance further increases, and the rolling load becomes large, making it unsuitable for actual operation. Conversely, if the final rolling temperature exceeds 850 ° C., the austenite grains after rolling become coarse, and it becomes difficult to obtain a fine ferrite + fine pearlite structure suitable for cold workability.

  Other manufacturing conditions are not particularly limited, and an optimum condition may be appropriately selected and applied from a known condition range.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and is suitable as long as it can meet the purpose described above and below. It is also possible to carry out the invention with modifications, and these are all included in the technical scope of the present invention.

Example 1
Steel materials having the chemical composition shown in Table 1 were melted in a small melting furnace, and after casting and soaking, hot forging was performed to obtain a steel piece having a side of 155 mm square. Using this steel slab, as shown in Tables 2 and 3, the steel was soaked at 1300 ° C or 1200 ° C for 60 minutes and then cooled to room temperature. Next, a part of each soaking material was reheated to each temperature in the range of 1000 to 1250 ° C. and then air-cooled to room temperature. Thereafter, the reheat soaking material was heated to each temperature in the range of 870 ° C. to 1100 ° C. as shown in the same table, and rolled at the final rolling temperature shown in the same table to obtain a steel bar having a diameter of 30 mm.

  A sample that can observe the cross section of each rolled steel bar was cut out, polished to a mirror surface, treated with a corrosive solution “ethanol + 3% nital”, and then, as shown in FIG. / 8 position (D represents the diameter of the steel bar), 4 positions from 4/4 position, 3D / 8 position arbitrarily, and a total of 16 positions were observed with an optical microscope at 400 times magnification. (Α) + Perlite (P) area ratio was determined. All the remaining structures were bainite. Moreover, the Vickers hardness of the same cross-sectional position as the above was measured in each of three cross sections, and the maximum value of the standard deviation of the hardness variation was obtained. The Vickers hardness was measured with a load of 10 kg.

  The grain coarsening resistance characteristics of each test material are defined in JIS G 0551 for the crystal grain size of each test bar steel after cold forging at a reduction rate of 70% and heating at 1000 ° C. for 3 hours. It measured according to the austenite grain size test method, and evaluated by the area ratio of coarse grains having a grain size number of 5 or less. More than 5%: Defect (x), 5% or less: Good (◯).

  In addition, the cold workability was made by preparing notched test pieces with a length of 41.3 mm as shown in FIG. 2 from the material obtained by drawing each hot-rolled material to a diameter of 27.5 mm, and each of the five end face complete restraint tests. And evaluated by the number of test pieces in which cracking occurred when the reduction was reduced to 40%. A: No crack, B: One crack, X: Two or more cracks.

  The results are collectively shown in Tables 2 and 3.

  From Tables 1 to 3, it can be considered as follows.

  No. Examples 1 to 8 and 15 to 37 are examples that satisfy all of the prescribed requirements of the present invention. Both the grain coarsening resistance property and the cold workability are good, and good results are obtained by comprehensive judgment. . No. No. 8 has a ferrite + pearlite area ratio slightly lower than the recommended range of the present invention. Therefore, although the cold workability is slightly inferior to other examples, the overall judgment is good.

  In contrast, no. 9 to 14, since any one of the soaking temperature, re-soaking, rolling heating and final rolling temperature before hot rolling is outside the preferred range, the maximum value of the hardness variation standard deviation is a value defined by the present invention (5 ) And one or both of the grain coarsening resistance and cold workability have not reached the target. No. No. 38 to 43, because the chemical composition of steel is outside the specified requirements, the average value of hardness or the standard deviation of hardness variation exceeds the specified value, or the maximum value temporarily satisfies the specified requirement. Even if it satisfies, one of the grain coarsening resistance and cold workability is poor, and the object of the present invention cannot be achieved.

It is explanatory drawing which shows the measurement structure of the metal structure and Vickers hardness of the steel bar cross section after rolling. It is a figure which shows the test piece for cold workability evaluation employ | adopted in experiment.

Claims (7)

% By mass
C: 0.10 to 0.35%,
Si: 0.03-1.0%,
Mn: 0.20 to 2.0%,
S: 0.1% or less (including 0%),
N: 0.030% or less (including 0%),
Al: 0.2% or less (including 0%),
Ti: 0.03 to 0.30%,
The balance is made of steel substantially consisting of Fe, the average value of Vickers hardness in the cross section is 180 or less, and the maximum standard deviation of Vickers hardness variation is 5 or less. This is a steel for skin hardening that can omit softening annealing and has excellent grain coarsening resistance and cold workability.   The steel for case hardening according to claim 1, wherein 80% or more of the metal structure in the cross section is ferrite + pearlite.   Still other elements of steel are Cu: 3.0% or less (excluding 0%), Ni: 3.0% or less (excluding 0%), Cr: 2.0% or less (0% The steel for skin hardening according to claim 1 or 2, which contains at least one element selected from the group consisting of Mo: 2.0% or less (not including 0%).   The steel for case hardening according to any one of claims 1 to 3, wherein the steel further contains B: 0.0005 to 0.010% as another element.   Still other elements of steel are Nb: 0.2% or less (not including 0%), V: 0.3% or less (not including 0%), Zr: 0.3% or less (0%) The case hardening steel according to any one of claims 1 to 4, which contains at least one element selected from the group consisting of:   Still other elements of steel are REM: 0.03% or less (excluding 0%), Ca: 0.03% or less (not including 0%), Mg: 0.03% or less (0% Not included), Pb: 0.3% or less (not including 0%), Bi: 0.3% or less (not including 0%), Te: 0.3% or less (not including 0%), Se Any one element selected from the group consisting of: 0.3% or less (not including 0%), Sn: 0.3% or less (not including 0%) The steel for case hardening described in 1. After steel that satisfies the requirements of the component composition described in any one of claims 1 to 6 is soaked at a temperature of 1250 ° C. or higher, and directly cooled or cooled to a temperature below the Ar ₁ transformation point after rolling. , Reheated to 1050 to 1200 ° C., directly or directly after rolling and then cooled to a temperature below the Ar ₁ transformation point, then reheated to 850 to 1000 ° C. and then rolled, A method for producing a case-hardening steel capable of omitting softening annealing, which is excellent in crystal grain coarsening characteristics and cold workability, characterized in that the rolling temperature is 700 to 850 ° C.

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