JP2006307270A - Case hardening steel having excellent crystal grain coarsening resistance and cold workability, and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a case hardening steel which exhibits excellent crystal grain coarsening resistance even when subjected to carburizing at a temperature higher than that in the conventional example so as to be case-hardened such as carburized and carbonitrided in a shorter period of time as the stock for rod-shaped machine parts such as a pulley for a CVT requiring carburizing depth and excellent workability and which also has excellent cold workability. <P>SOLUTION: The case hardening steel having excellent crystal grain coarsening resistance and cold workability is composed of 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 case hardening steel is characterized in that carbides and/or carbonitrides satisfying inequality (1): (Ti)/(Nb)≥0.05 are present in an amount of ≥2.0×10<SP>7</SP>pieces/mm<SP>2</SP>in the cross-section, and further, the maximum value of the standard deviation in the Vickers hardness in the cross-section is ≤10. Alternatively, the steel has a metallic structure where the area ratio of (ferrite+pearlite) is ≥80%. <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 used as a material for gears, gears with shafts, etc., the characteristics such that the crystal grains do not become coarse even when subjected to the case hardening treatment at a relatively high temperature (hereinafter referred to as crystal grain coarsening resistance) The present invention relates to a case hardening steel excellent in cold workability and a useful manufacturing method thereof.

  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 defined by JIS standards such as SCr, SCM, SNCM, etc. are usually used. After forming into the desired part shape by machining such as forging and cutting, carburizing and carbonitriding It is manufactured through a finishing process such as polishing.

  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 austenite (γ) crystal grains of the raw material become coarse, resulting in problems such as deterioration of mechanical characteristics and an increase in heat treatment strain.

  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, and 3). 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 γ crystal grains at the time of heating for the skin baking treatment is suppressed.

  In addition, Nb and Al are positively added to the steel, and a predetermined amount of Nb-based precipitates and Al and Nb-based composite precipitates [Nb (CN), AlN] are precipitated. There has also been proposed a case-hardening steel that exhibits the effect of preventing the growth (pinning effect) (Patent Document 4).

  On the other hand, the case-hardening steel is cold worked when it is formed into a part shape, so that cold workability is also an important required characteristic. And the steel materials which improved cold workability are developed also in the steel for case hardening to which Ti was added (patent documents 5-8 etc.). In these inventions, the cold workability is improved by appropriately adjusting the types and amounts of components mainly affecting the cold workability in the steel components. In addition, Patent Documents 5 and 7 disclose a method for appropriately controlling the cooling rate after hot rolling as a further cold workability improvement measure, and Patent Document 6 discloses cold workability. For further 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 when applied to parts that have complicated shapes or undergo strong processing, and further improvements are desired.
Japanese Patent Laid-Open No. 10-81938 JP-A-10-130720 JP 2001-303174 A JP-A-9-78184 JP-A-6-299241 JP-A-10-130777 Japanese Patent Laid-Open No. 11-43737 JP 2003-89818 A

  The technology proposed so far is to suppress the growth of crystal grains by finely dispersing AlN, Nb (CN), Ti (CN), etc. as much as possible. Even when the carburizing / nitriding temperature is increased to about 1000 ° C., the size of the crystal grains can be suppressed to some extent.

  However, increasing the skin baking temperature is important not only for shortening the skin baking time but also for increasing the carburization depth. For example, a conventional gear satisfies a required characteristic with a carburization depth of about 0.5 to 1 mm, but a carburizing depth of about 2 mm is used for a pulley for winding a metal belt used in a CVT (unauthorized transmission). Therefore, it is necessary to further raise the carburizing temperature in order to cope with these requirements.

  However, as described above, in the conventional grain coarsening prevention technology, when the carbonitriding treatment is performed in a high temperature range exceeding 1000 ° C., the precipitates as crystal nuclei described above are dissolved to prevent the grain coarsening. Lost, causing abnormal growth of γ grains.

  In addition, these conventional techniques also have a problem that the steel material is hardened by the precipitation strengthening action of the precipitates generated for preventing the coarsening of the crystal grains and the cold workability is deteriorated.

  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, excellent cold workability. In addition to providing a case-hardening steel that can effectively suppress coarsening of crystal grains even when the case baking temperature is increased, and provides a case-hardening part with good physical characteristics and dimensional accuracy, Furthermore, it is providing the manufacturing method which can obtain the steel for case hardening provided with such a characteristic reliably.

The steel for case hardening excellent in crystal grain coarsening characteristics and cold workability according to the present invention, which was able to solve the above-mentioned problems, is mass%,
C: 0.10 to 0.35%,
Si: 0.03-1.0%,
Mn: 0.2 to 2.0%,
S: 0.1% or less (including 0%),
Nb: 0.025 to 0.20%
Ti: 0.025 to 0.12%,
N: 0.020% or less (including 0%),
Al: 0.13% or less (including 0%),
The balance is made of steel substantially consisting of Fe, and there are 2.0 × 10 ⁷ pieces / mm ² or more of carbides and / or carbonitrides satisfying the following formula (1) in the cross section, A gist exists where the maximum value of the standard deviation of the Vickers hardness in the cross section is 10 or less.
(Ti) / (Nb) ≧ 0.05 (1)
[However, (Ti) and (Nb) represent respective contents (mass%) of Ti and Nb in the carbide and / or carbonitride. ]

In the case hardening steel according to the present invention, the steel further satisfies the relationship of the following formula (2):
[Ti] -47.9 [N] /14≧0.0050 (mass%) (2)
{However, [Ti] and [N] represent each content (mass%) of Ti and Nb in steel. }
Alternatively, it is preferable that 80% or more of the metal structure in the cross section is “ferrite + pearlite” because it has further excellent workability.

  Further, the case hardening steel according to the present invention may contain one or more elements selected from the groups shown in the following a) to d) according to the required properties.

a) 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 element selected from the group consisting of 0.0% or less (excluding 0%),
b) B: 0.0005 to 0.010%,
c) V: 0.3% or less (not including 0%), Zr: 0.3% or less (not including 0%)
Hf: 0.4% or less (not including 0%), Ta: 0.3% or less (not including 0%), at least one element selected from the group consisting of
d) 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 ( At least one element selected from the group consisting of Sn: 0.3% or less (not including 0%).

Further, the production method of the present invention is positioned as a method for industrially producing a case-hardening steel having the above-mentioned characteristics, and after casting the steel satisfying the requirements of the above component composition, it is cast. The slab obtained was reheated to a temperature of 1250 ° C. or higher, cooled to a temperature below the Ar ₁ transformation point, reheated to 850 to 1000 ° C., and then rolled. The final rolling temperature was 700 to 850 ° C. It has a feature.

  According to the present invention, the chemical composition of the steel is specified, and in particular, the content ratio of Ti and Nb in the carbide and / or carbonitride present in the cross section is specified, and the number thereof is specified, Furthermore, the standard deviation of the Vickers hardness is suppressed, more preferably, the content ratio of Ti and N in the steel is controlled to a specific range, and further, the metal structure of the cross section is made a structure mainly composed of ferrite and pearlite. Has excellent cold workability to withstand complex shapes and strong processing, and exhibits excellent grain coarsening resistance even when the case baking temperature for surface hardening treatment is increased, It is possible to provide a case hardening steel that provides case hardening parts with excellent characteristics and dimensional accuracy.

  Under the conventional technology as described above, the present inventors further improve the grain coarsening resistance and cold workability, and the steel composition and the presence form of precipitates that affect their performance, I have been researching mainly physical properties and crystal structures. As a result, as described above, the composition of steel is specified, the composition, size and number of precipitates observed in the cross section are specified, and the standard deviation of the Vickers hardness of the cross section is reduced. Alternatively, if the metal structure was further optimized, it was found that a steel for case hardening having stable and excellent crystal grain coarsening characteristics and cold workability could be obtained, and the present invention was completed.

  Hereinafter, the reason why the chemical composition of the steel is determined in the present invention will be clarified, and subsequently, the reason why the component, size, number of precipitates in the steel cross section, standard deviation of Vickers hardness, metal structure, etc. are determined. Make it clear.

  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 0.40% or less. The C content is more preferably 0.15% or more and 0.30% or less, still more preferably 0.17% or more and 0.25% 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, not only the material becomes too hard and cold workability deteriorates, but also the formation of a grain boundary oxide layer is promoted during the carburizing heat treatment, which deteriorates the mechanical properties. In order to suppress this, 1.0% was set as the upper limit. A more preferable Si content is 0.05% or more and 0.8% or less.

Mn: 0.2-2.0%;
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 0.4% or more and 1.5% or less.

S: 0.1% or less;
S reacts with Mn to form MnS and enhances machinability. However, S acts as a source of inclusions such as TiS and adversely affects impact properties and cold workability. It is good to keep it at most 0.1% or less, preferably 0.05% or less.

Ti: 0.025 to 0.12%;
Ti becomes fine (Nb, Ti) (CN) and suppresses the growth of γ crystal grains, and Ti dissolves in Nb (CN), thereby reducing the grain growth of carbonitride during carburizing. It also has the effect of suppressing the growth of γ crystal grains, and in order to exhibit these functions effectively, it must be contained by 0.025% or more. However, if the amount of Ti is too large, the Nb—Ti-containing precipitate becomes coarse and adversely affects cold workability and fatigue characteristics, so the upper limit is made 0.12%. A more preferable Ti content is 0.030% or more and 0.10% or less.

Nb: 0.025 to 0.20%;
Nb forms fine (Nb, Ti) (CN) and has an action of suppressing the growth of γ crystal grains, and is one of the most important elements in the present invention together with Ti. In order to exhibit such an action effectively, Nb must be contained by 0.025% or more. However, when the Nb content exceeds 0.20%, not only the effect is saturated, but coarse carbides are generated and the cold workability and fatigue characteristics are deteriorated, so 0.20% was set as the upper limit. . The preferable lower limit of Nb is 0.040%, more preferably 0.050% or more, and the preferable upper limit is 0.15%, more preferably 0.12% or less.

N: 0.020% or less;
N combines with Al and Ti to form nitrides and carbonitrides and exerts an effect of suppressing austenite (γ) grain growth during carburizing heating, but has a significant adverse effect on impact properties and fatigue properties. Therefore, it should be suppressed to 0.020% or less, preferably 0.010% or less at most.

Al: 0.13% or less;
Al is an element effective for deoxidation of steel, and also acts effectively on the adjustment of γ crystal grains. However, if the Al content is too high, hard and coarse non-metallic inclusions (Al ₂ O ₃ ) are generated and the impact characteristics and cold workability are deteriorated, so they should be suppressed to 0.13% or less. A more preferable content of Al is 0.10% 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 is preferable to keep it as small as possible, and at most 0.03% or less, more preferably 0.01% or less. It is good. Further, O (oxygen) adversely affects the strength characteristics of the steel material, so it is preferable to keep it at 0.003% or less, more preferably 0.001% or less.

  In addition to the above essential elements, the steel materials used in the present invention are also effective to contain the following selective elements in order to give further additional characteristics as desired. Are also included in the technical scope of the present 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 characteristics by strengthening the grain boundaries. Such an effect is effectively exhibited by adding 0.0005% or more. However, these effects are saturated at about 0.010%, and if the amount of B is too large, B nitride is easily formed and adversely affects cold workability, so at most 0.010% or less. Should be suppressed. A more preferable B content is 0.0007% or more and 0.0050% or less.

V: 0.3% or less (not including 0%), Zr: 0.3% or less (not including 0%), H
f: at least one selected from the group consisting of 0.4% or less (not including 0%);
V, Zr, and Hf all have the effect of suppressing the coarsening of the γ 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 above-described restriction of the steel components, carbide and / or carbonitriding satisfying the above formula (1), that is, “(Ti) / (Nb) ≧ 0.05” in the cross section of the rolled material. It is an essential requirement that the number of objects is 2.0 × 10 ⁷ pieces / mm ² or more and that the maximum value of the standard deviation of Vickers hardness in the cross section is 10 or less.

  That is, the present inventors have conducted research from various angles on the effect on the cold workability and the grain coarsening resistance property at the time of heat treatment of the rolled steel material satisfying the above-mentioned component composition requirements. It has been found that characteristics become a very important factor in ensuring their physical properties.

“The number of carbides and / or carbonitrides satisfying (Ti) / (Nb) ≧ 0.05 is 2.0 × 10 ⁷ / mm ² or more”;
As the ratio of Ti to Nb in the carbide and / or carbonitride increases, the growth suppressing effect on the γ crystal grains of the precipitate is improved, and the value of (Ti) / (Nb) is 0.05 or more The effect is effectively exhibited, but if it is less than 0.05, the effect is hardly exhibited. Further, the presence of as many carbides and carbonitrides as possible makes the γ crystal grains finer, and if the number is less than 2.0 × 10 ⁷ particles / mm ² , the effect of suppressing crystal grain growth is greatly reduced. Therefore, in the present invention, the lower limit of the number of carbides and / or carbonitrides is set to 2.0 × 10 ⁷ pieces / mm ² . More preferably, it is 1.0 × 10 ⁸ pieces / mm ² or more, and further preferably 5.0 × 10 ⁸ pieces / mm ² or more.

  More preferably, in the present invention, in addition to the (Ti) / (Nb) value, the Ti content and N content in the steel are expressed by the above formula (2), that is, “[Ti] -47.9 [N] / 14 ≧ 0.0050 (mass%) ”is satisfied.

  By the way, Ti reacts preferentially with N in the melting stage of steel. Therefore, in order to suppress grain growth during carburizing by solidly dissolving Ti in Nb (CN), after Ti reacts with N In this case, it is necessary to leave a predetermined amount of Ti. For this purpose, it is preferable to satisfy the above relational expression. That is, even after Ti reacts with N, 0.050% or more of Ti is preferably left, more preferably 0.010% or more, and further preferably 0.015% or more.

“Maximum standard deviation of Vickers hardness in the cross section is 10 or less”;
Further, the present inventors have confirmed that the standard deviation of Vickers hardness in the cross section of the test steel also has a significant effect on the required characteristics, and the maximum value of the standard deviation is 10 or less. It has been confirmed that it has excellent cold workability stably and exhibits excellent crystal grain coarsening characteristics even when heat treatment is performed at a high temperature for skin hardening.

  The theoretical reason why such a tendency can be obtained has not been clarified yet, but the following can be considered. That is, when the maximum value of the standard deviation of Vickers hardness is large, the presence (dispersion state, size, etc.) of precipitates (carbide, nitride or carbonitride) present in steel is inhomogeneous. On the contrary, the fact that the maximum value is small seems to mean that the precipitation state is homogeneous. Therefore, since the state where the precipitate is present seems to be homogeneous in the case where the maximum value is small, it is a factor to improve the cold workability after spheroidizing annealing and the grain coarsening resistance property during heat treatment. I believe that.

  Such a tendency changes suddenly when the maximum value of the standard deviation is around 10, and when this value exceeds 10, the cold workability is clearly poor and it is 10 or less, more preferably 8 or less. Was confirmed to exhibit excellent cold workability.

  Furthermore, as a result of examining the influence of the physical properties of the rolled steel on the maximum standard deviation of Vickers hardness, the higher the total area ratio of ferrite and pearlite in the metal structure in the rolled material cross section The maximum value of the standard deviation is small, and the total area ratio is at least 80%, preferably 90% or more, more preferably 95% or more. It was confirmed to show sex. 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 carbide and / or carbonitride satisfying the relationship of “(Ti) / (Nb) ≧ 0.05” is specified in the transverse section while specifying the component composition of steel. 0 × 10 ⁷ pieces / mm ² or more, and the maximum standard deviation of Vickers hardness in the cross section is 10 or less, preferably “[Ti] −47.9 [N] / 14 ≧ 0” .0050 (mass%) "and by making the cross-sectional metal structure 80% or more in terms of the sum of ferrite and pearlite, heat for skin hardening treatment while ensuring excellent cold workability It is possible to provide a case-hardening steel that provides a case-hardening part that has excellent crystal grain coarsening characteristics due to, and has good strength characteristics and dimensional accuracy.

Next, in order to obtain a case-hardening steel having the above-described characteristics, a steel material that satisfies the above-described chemical component requirements is reheated to a temperature of 1250 ° C. or higher, and then cooled to a temperature of the Ar ₁ transformation point or lower. And it is good to employ | adopt the method of rolling, after reheating to 850-1000 degreeC, and controlling the final rolling temperature in the range of 700-850 degreeC.

  The soaking temperature is 1250 ° C. or higher because the coarse Nb—Ti-containing precipitates present in the steel are once dissolved in austenite, and the Nb—Ti-containing precipitates precipitated in the subsequent steps are uniform and fine. This is to suppress the coarsening of the austenite crystal grains during heating for the case baking treatment and to suppress the deterioration of cold workability due to the coarse Nb—Ti-containing precipitates.

  By the way, if this temperature (reheating temperature) is less than 1250 ° C or as-cast without reheating, it remains separated into coarse TiN and relatively fine NbC, and NbC becomes coarse during subsequent carburizing. On the other hand, once carbonitride is dissolved by holding at a high temperature, Nb and Ti in the matrix are re-dissolved, and Nb-Ti composite precipitates are re-precipitated. This is because grain growth is remarkably suppressed.

After soaking at the temperature of 1250 ° C. or higher, it is once cooled to a temperature below the Ar ₁ transformation point. The reason is that austenite coarsened at the time of heating is transformed into ferrite, then transformed back to austenite by heating before rolling, and the austenite crystal grains are refined and the precipitated Nb-Ti-containing precipitates are refined, This is for enhancing the crystal grain coarsening characteristics during the case baking treatment. For that purpose, after soaking, it is essential to cool to a temperature not higher than the Ar ₁ transformation point.

  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.

  The carburizing steel of the present invention is assumed to be used after carburizing and quenching, and exhibits the effect that no coarsening of crystal grains occurs even when the temperature during carburizing is relatively high. However, it is of course applicable to a carbonitriding method in which nitriding is performed simultaneously with carburizing. In addition, the carburizing steel of the present invention is carburized after cold forging or hot forging after reheating as described above, but the effect is not exhibited only at a limited forging temperature, The effect is demonstrated regardless of the forging temperature. Prior to cold forging or hot forging (that is, after reheating), it is possible to perform a normalizing process in order to make the structure uniform.

  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 chemical compositions shown in Tables 1 and 2 were melted in a small smelting furnace, and after casting and soaking, hot forging was performed to obtain a steel piece having a side of 155 mm square. This steel slab was used, soaked at 1300 ° C. or 1200 ° C. for 60 minutes as shown in Tables 3 and 4, and then cooled to room temperature. Each soaking material was heated to temperatures ranging from 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 steel bar), D / 4 position, 3D / 8 position each arbitrarily selected 4 locations, a total of 16 locations were observed with an optical microscope at 400 times magnification, and the point counting method was used for ferrite (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 hardness 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 1025 ° 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 (◯).

  The cold workability of each hot-rolled material was determined from each sample material that had been subjected to spheroidizing annealing that was “heated at 770 ° C. × 5 hours and then cooled at 15 ° C./s” and then drawn into a diameter of 27.5 mm. As shown in FIG. 2, test pieces with notches having a length of 41.3 mm were prepared, 5 end face complete restraint tests were performed, and evaluation was performed based on the number of test pieces in which cracking occurred when the reduction was reduced to 30%. did. A: No crack, B: One crack, X: Two or more cracks.

The fine carbonitride evaluation method is to produce an extraction replica for a transmission electron microscope from a sample of 900 ° C. normalizing material, and for 20 arbitrary carbides / carbonitrides at an observation magnification of 100,000 times, (Ti ) / (Nb) is analyzed by EDX (energy dispersive X-ray analyzer) to obtain a ratio of 0.05 or more, and then 20 views of a photograph with a measurement area of 0.75 μm ² at an observation magnification of 150,000 times. Observe, count the number, and multiply by the previously obtained ratio of (Ti) / (Nb) ≧ 0.05 to obtain carbide / carbonitride of (Ti) / (Nb) ≧ 0.05. The number was determined.

  The results are collectively shown in Tables 3 and 4.

  From Tables 1 to 4, the following can be considered.

  No. 1 to 8, 11 to 30 are examples that satisfy all the requirements of the present invention, both the grain coarsening resistance characteristics and the cold workability are good, and good results are obtained by comprehensive judgment. .

  No. No. 9, the steel composition is appropriate, but the soaking temperature and rolling heating temperature are inappropriate, and the number of precipitates of “(Ti) / (Nb) ≧ 0.05” is insufficient. The characteristic is bad. No. Nos. 10 and 11 lack the cold forgeability because the final rolling temperature is outside the preferred range and the ferrite + pearlite structure fraction is outside the recommended range. No. Nos. 31 to 38 are comparative examples in which the steel components deviate from the prescribed requirements, and the crystal grain coarsening resistance is insufficient or the workability (cold forgeability) 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 (8)

% By mass
C: 0.10 to 0.35%,
Si: 0.03-1.0%,
Mn: 0.2 to 2.0%,
S: 0.1% or less (including 0%),
Nb: 0.025 to 0.20%
Ti: 0.025 to 0.12%,
N: 0.020% or less (including 0%),
Al: 0.13% or less (including 0%),
The balance is made of steel substantially consisting of Fe, and there are 2.0 × 10 ⁷ pieces / mm ² or more of carbides and / or carbonitrides satisfying the following formula (1) in the cross section, A case hardening steel excellent in crystal grain coarsening characteristics and cold workability, wherein the maximum value of the standard deviation of Vickers hardness in the cross section is 10 or less.
(Ti) / (Nb) ≧ 0.05 (1)
However, (Ti) and (Nb) represent the respective contents (mass%) of Ti and Nb in the carbide and / or carbonitride. The case hardening steel according to claim 1, wherein the steel further satisfies the relationship of the following formula (2).
[Ti] -47.9 [N] /14≧0.0050 (mass%) (2)
However, [Ti] and [N] represent each content (mass%) of Ti and N in steel.   The steel for case hardening according to claim 1 or 2, 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 case hardening according to any one of claims 1 to 3, 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 4, wherein the steel further contains B: 0.0005 to 0.010% as another element.   Still other elements of steel are V: 0.3% or less (excluding 0%), Zr: 0.3% or less (excluding 0%), Hf: 0.4% or less (0% The steel for case hardening according to any one of claims 1 to 5, which contains at least one element selected from the group consisting of Ta: 0.3% or less (not including 0%).   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: not more than 0.3% (not including 0%), Bi: not more than 0.3% (not including 0%), Te: not more than 0.3% (not including 0%), Se 7: at least one element selected from the group consisting of 0.3% or less (not including 0%) and Sn: 0.3% or less (not including 0%) The steel for case hardening described in 1. A slab obtained by melting and casting steel satisfying the requirements of the component composition described in any one of claims 1 to 7 is reheated to a temperature of 1250 ° C or higher, and then a temperature not higher than the Ar ₁ transformation point. The steel is cooled to 850 to 1000 ° C. and rolled, and the final rolling temperature is set to 700 to 850 ° C. The manufacturing method.

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