JP4464864B2 - Case-hardening steel with excellent grain coarsening resistance and cold workability that can be omitted for soft annealing. - Google Patents

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., it is excellent in crystal grain coarsening prevention properties, has good cold workability and can eliminate 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 increased, the austenite (γ) crystal grains of the raw material become coarse and the amount of heat treatment strain increases.

  Then, the case hardening steel which added Ti and Nb is proposed as what improved the grain coarsening prevention characteristic (henceforth a crystal grain coarsening resistance characteristic) of steel for case hardening (patent document). 1-3). These materials fix free nitrogen (free-N) by adding about 0.1 to 0.2% by mass of Ti or Nb in steel, and Ti or Nb carbides, composite carbides containing them, nitriding By precipitating an object or the like finely, the coarsening of the γ crystal grains during 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. Also, when cold working steel, 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 or Nb is added, steel materials having improved workability have been developed so that cold work can be performed without softening annealing (Patent Documents 4 to 9, etc.) ). In these inventions, the cold workability is improved by appropriately adjusting the types and contents of components mainly affecting the cold workability in steel. The above-mentioned patent documents also disclose a method for appropriately controlling the cooling rate after hot rolling, a method for controlling the metal structure of the hot rolled material, and the like as further measures for improving the cold workability.

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 2001-303174 A JP-A 63-4042 JP-A-6-299241 JP-A-10-130777 Japanese Patent Laid-Open No. 11-43737 JP-A-9-78184 JP 2003-89818 A

  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. Providing a case-hardening steel that makes it possible to omit softening annealing and further suppresses the coarsening of crystal grains due to heat treatment for skin hardening, giving a case-hardened part with good physical properties and dimensional accuracy. Furthermore, another object of the present invention is to provide a production method capable of reliably obtaining a case-hardening steel having such characteristics.

The steel for case hardening, which has excellent crystal grain coarsening characteristics and cold workability according to the present invention, which has been able to solve the above-mentioned problems, and has made it possible to omit softening annealing, is in mass%.
C: 0.05 to 0.30%
Si: 0.05-2.0%,
Mn: 0.2 to 2.0%,
S: 0.002 to 0.2%,
N: 0.003-0.030%,
Al: 0.01 to 0.12%,
Nb: 0.01-0.20%,
Ti: 0.005 to 0.12%,
With the balance being substantially Fe, with an average value of Vickers hardness in the cross section of 180 or less and a maximum standard deviation of Vickers hardness of 5 or less. is doing.

  In the case-hardening steel according to the present invention, the main component of the metal structure in the cross section is ferrite + pearlite, and these structures occupy 80% or more, the standard deviation of the hardness is suppressed lower This is preferable.

  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) V: 0.3% or less (excluding 0%) and / or Zr: 0.3% or less (0%
Not included),
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, in addition to specifying the chemical composition of steel, in particular, the average value of Vickers hardness in the cross section is specified, and the standard deviation of the hardness is suppressed as much as possible, more preferably the metal of the steel cross section By making the structure mainly composed of ferrite + pearlite, it has excellent cold workability to withstand complex processing and strong processing without softening annealing, and it has resistance to heat treatment by surface hardening heat treatment for surface hardening treatment. It is possible to provide a case hardening steel that provides a case hardening component 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 further improve the grain coarsening resistance and cold workability, and the composition and physical properties of hot rolled materials that affect their performance. Research has been conducted mainly on crystal structures. As a result, as described above, in addition to specifying the component composition of steel, the average value of Vickers hardness in the cross section of the hot rolled material is kept low, the standard deviation of the hardness 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, and further the reason for determining the metal structure will be clarified. .

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

C: 0.05-0.30%;
C is an important element for securing the core hardness necessary for a machine part. If it is less than 0.05%, 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 is lowered and the cold workability is also deteriorated, so it is necessary to keep it to 0.30% or less. A more preferable C content is 0.10% or more and 0.25% or less.

Si: 0.05-2.0%;
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. Element, and requires addition of 0.05% 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, 2.0% is set as the upper limit in order to suppress these obstacles. A more preferable Si content is 0.10% or more and 1.5% 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 1.8% or less.

S: 0.002 to 0.2%;
S combines with Mn, Ti, etc. to form MnS inclusions, TiS inclusions, etc., and adversely affects the impact characteristics of the parts. Therefore, it is preferable to suppress as much as possible. In the present invention where impact characteristics are required, the upper limit is set. Set to 0.2%. However, S, on the other hand, has an effect of improving the machinability, and when the machinability is strongly required, it should be contained in an appropriate amount, and more preferably about 0.002% or more. In normal steel for machine structural use, it is preferably about 0.01% or more and about 0.07% or less.

N: 0.003-0.030%;
N combines with Al, Ti, Nb to form nitrides and carbonitrides, and has the effect of suppressing the growth of γ crystal grains during carburizing heating. It must be contained by 0.003% or more, preferably 0.005% or more. However, if the amount of N is too large, the workability and impact properties will be adversely affected, so at most 0.030% or less, more preferably 0.025% or less, and even more preferably 0.020% or less. Is good.

Al: 0.01 to 0.12%;
Al is an element effective for adjusting the crystal grains of the steel structure. That is, Al combines with N in the steel to form a nitride, which exhibits the action of suppressing the growth of γ crystal grains during the heat treatment. In addition, when Al is added in combination with Nb or Ti, which will be described later, a composite precipitate of Al nitride and Ti carbonitride that is more stable than an Al-based single precipitate, or a composite of Al nitride and Nb carbonitride A precipitate or a composite precipitate of Al nitride and Nb—Ti composite carbonitride is formed, and the grain coarsening resistance characteristics during high-temperature carburization are enhanced. In order to exhibit these effects effectively, it is necessary to contain 0.01% or more. However, if the Al content is too high, hard and coarse non-metallic inclusions (Al ₂ O ₃ ) are generated and impact properties are deteriorated, so the upper limit was set to 0.12%. A more preferable content of Al is 0.015% or more, more preferably 0.02% or more, and 0.10% or less, and further preferably 0.07% or less.

Nb: 0.01-0.20%;
Nb is an element that plays a particularly important role in the present invention, and combines with N and C in steel to form nitrides, carbides or carbonitrides, and contributes to suppression of grain coarsening during carburizing heating. If it is less than 0.01%, nitrides, carbides, or carbonitrides that are stable at high temperatures are not generated, so that the crystal grain coarsening resistance is insufficient. Moreover, Nb is added in combination with Al or Ti, so that it is more stable than a single precipitate containing Nb and is a more stable composite of Al nitride and Nb carbonitride, Nb-Ti composite carbonitride, or Al nitride. A composite precipitate of Nb—Ti composite carbonitride is formed and contributes to improvement of the grain coarsening resistance resistance during high temperature carburization.

  However, if the Nb content is too large, coarse precipitates containing Nb are generated to accelerate the Ostwald grain growth and reversely deteriorate the crystal grain coarsening characteristics. Therefore, it should be suppressed to 0.20% or less. A more preferable content of Nb is 0.02% or more, more preferably 0.03% or more, 0.15% or less, and further preferably 0.10% or less.

Ti: 0.005 to 0.12%;
Ti is also an element that plays an important role in the present invention. That is, Ti in steel is combined with N and C to form carbides, nitrides, carbonitrides, and suppresses coarsening of γ crystal grains during high-temperature carburization. Also, by adding Al and Nb in combination, a more stable Al nitride and Ti carbonitride composite precipitate, Nb-Ti composite carbonitride, or Al nitride and Nb than a single precipitate containing Ti. -Forms composite precipitates of Ti composite carbonitrides and contributes to the improvement of grain coarsening resistance. When the Ti content is less than 0.005%, the number of precipitated Ti carbonitrides and composite carbonitrides with other elements becomes insufficient, and satisfactory grain coarsening resistance characteristics cannot be obtained. On the other hand, if the Ti content exceeds 0.12% and becomes excessively large, coarse Nb—Ti carbonitrides are generated to promote Ostwald growth and deteriorate the anti-crystal grain coarsening characteristics. The more preferable content of Ti is 0.008% or more, 0.10% or less, and more preferably 0.05% 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 curability. However, since the effect of each element is saturated near the upper limit, adding more than that is not economical, excessive amount of Cr adversely affects 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 addition of Cu 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 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.

V: 0.3% or less (not including 0%) and / or Zr: 0.3% or less (not including 0%);
V and Zr both have the effect of suppressing the coarsening of the γ crystal grains by forming precipitates made of carbides and nitrides, but if the amount is too large, the amount of the precipitates will increase so much that the moldability will be improved. Each of them should be kept at 0.3% or less because it will have an adverse effect.

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, and the maximum value of the standard deviation of the hardness is An essential requirement is 5 or less. That is, the present inventors have conducted research from various angles on the cold workability and the effect on the grain coarsening resistance property during heat treatment of rolled steel that satisfies the above-mentioned component composition requirements. In addition, the average value of the Vickers hardness and the standard deviation of the hardness 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 180 or less, more preferably 170 or less. And having a maximum hardness standard deviation of 5 or less has stable and excellent cold workability even when softening annealing is omitted, and is resistant to crystal grains during heat treatment for skin hardening. It was confirmed that excellent performance was exhibited in the coarsening 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, when the standard deviation of the hardness is large, there is a locally small region of deformability, and that region becomes a starting point of destruction, The cold workability of the steel as a whole decreases. Therefore, in order to suppress workability deterioration due to hardness variation and ensure excellent cold workability without softening annealing, it is necessary to suppress the maximum standard deviation of hardness to 5 or less. Preferably, the maximum value is suppressed to 4 or less.

  Furthermore, as a result of studying the metal structure of the rolled steel material to give the average value of such Vickers hardness and the maximum value of the standard deviation, the higher the total area ratio of ferrite and pearlite in the cross-sectional metal structure, the above The average value of Vickers hardness is stabilized at a relatively low value, and the maximum value of the standard deviation of the hardness becomes small, and the total area ratio is at least 80%, preferably 90% or more, more preferably 95%. Those described above were confirmed to exhibit excellent cold workability to such an extent that softening annealing after processing could 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 Is generally uniform and the standard deviation of hardness is considered to be small.

  As described above, according to the present invention, the component composition of the steel is specified, and the average value of the Vickers hardness and the maximum standard deviation of the hardness of the steel cross section are suppressed to a low value. + By securing 80% or more of the total pearlite, excellent cold workability is ensured even when softening annealing is omitted, and it has excellent resistance to crystal grain coarsening due to heating for case hardening, and strength. It is possible to provide steel for case hardening that provides case hardening parts with good 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, cooling to below the Ar ₁ transformation point is because γ crystal grains when austenite (γ) coarsened during heating is transformed into ferrite and then transformed back to austenite by heating. This is because the Ti-containing material is precipitated and the size of the Ti precipitate can be appropriately adjusted 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 reversely transforming to austenite by heating before rolling. In order to refine the γ crystal grains and to precipitate Nb—Ti-containing precipitates at the time of ferrite transformation, it is essential to cool to a temperature below the Ar ₁ transformation point after soaking. .

  After that, reheating in the temperature range of 1050 to 1200 ° C. is because the NB—Ti-containing precipitates having an appropriate size are precipitated in austenite, and the γ crystal grains are coarsened during heating for the skin baking treatment. Rolled steel by suppressing graining and suppressing precipitation strengthening due to Nb-Ti-containing precipitates that precipitate in the cooling process after rolling by reducing the solid solution amount of Nb and Ti during heating before rolling. This is to keep the average hardness and standard deviation of hardness low. Incidentally, when the re-soaking temperature exceeds 1200 ° C, the solid solution amount of the Nb-Ti-containing precipitate increases, and when it is less than 1050 ° C, the Ti-containing precipitate does not grow to an appropriate size. The purpose of re-heating is not fulfilled.

Therefore, if the reheating is performed at least once to the temperature below the Ar ₁ transformation point, or if rolling is performed during that time, the standard deviation of the hardness of the cross section of the rolled material is further reduced, and the cold workability is It will be further enhanced.

  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. Moreover, the reason why the reheating temperature is suppressed to 1000 ° C. or less is to improve cold workability by refining the γ crystal 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. On the other hand, if the final rolling temperature exceeds 850 ° C., the γ crystal 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 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. Next, a portion 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 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 (F) + 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 average value and the maximum value of the standard deviation of the hardness were obtained. The Vickers hardness was measured with a load of 10 kg.

  The grain coarsening resistance characteristics of each test material are as follows: γ crystal grain size defined in JIS G 0551 for 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 particle size test method, and evaluated by the area ratio of coarse particles having a crystal particle 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 to 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 made based on the number of test pieces that had cracks when reduced to a reduction ratio of 50%. did. A: No crack, B: One crack, X: Two or more cracks.

  The results are shown in Tables 3 and 4.

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

  No. Examples 1 to 8 and 15 to 40 are examples that satisfy all of the prescribed requirements of the present invention, and both the grain coarsening resistance characteristics and the cold workability are good, and good results are obtained by comprehensive judgment. . No. Nos. 9 and 10 have a re-soaking temperature outside the specified range, so the average value of Vickers hardness exceeds 180 and the maximum standard deviation of the same hardness exceeds 5, and formability (cold forging) Is bad. No. No. 11 has poor crystal grain coarsening characteristics because the initial soaking temperature is too low. No. 12 has a low initial soaking temperature, the maximum standard deviation of hardness exceeds the specified value, and the ferrite + pearlite structure fraction is also relatively low. Is also bad.

  No. No. 13, because the rolling heating temperature is too high, the maximum value of the standard deviation of hardness exceeds the specified value, and the ferrite + pearlite structure fraction is also low, so the crystal grain coarsening characteristics are good, but cold forging The nature is bad. No. No. 14, because the final rolling temperature is too high, the maximum value of the standard deviation of hardness exceeds the specified value, and the ferrite + pearlite structure fraction is also low. The nature is bad.

  No. 41 to 51 are comparative examples in which the chemical composition of the steel used lacks any of the prescribed requirements, and either one of the average value of the Vickers hardness and the maximum value of the standard deviation of the hardness is outside the prescribed requirements. Therefore, either the grain coarsening resistance property or the 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 (6)

% By mass
C: 0.05 to 0.30%
Si: 0.05-2.0%,
Mn: 0.2 to 2.0%,
S: 0.002 to 0.2%,
N: 0.003-0.030%,
Al: 0.01 to 0.12%,
Nb: 0.01-0.20%,
Ti: 0.005 to 0.12%,
Is made of steel consisting of the remainder Fe and inevitable impurities , the average value of Vickers hardness in the cross section is 180 or less, and the maximum value of the standard deviation of Vickers hardness is 5 or less, 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.   The steel further contains V: 0.3% or less (not including 0%) and / or Zr: 0.3% or less (not including 0%) as other elements. Crazing steel for crab.   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 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.

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