JP6632281B2 - Case hardening steel for cold forging with excellent resistance to grain coarsening - Google Patents

Description

  The invention of the present application is to provide a carburized part used after carburizing and tempering after cold forging, if necessary, for example, gears, CVJs and shafts of automobiles, construction machines, machine tools and the like. The present invention relates to case hardening steel which is suitable as such a material.

  In recent years, there has been a strong demand for changing from hot forging to cold forging and realizing a process of carburizing a cold forged part without performing normalizing or the like (called a cold forging-carburizing process). I have. This process has the advantages of improving product yield, reducing cutting man-hours by using near net shape, eliminating forging heating by eliminating hot forging, and simplifying heat treatment after cold forging. Very large. On the other hand, cold forging has the disadvantage of significantly increasing the shaping load compared to hot forging during processing, and one of the issues is how to make the forging material soft.

  In practice, case hardening steel will be used for the cold forging-carburizing process, and spheroidizing annealing is usually selected as the optimum softening treatment. However, another problem is that if the steel in a spheroidized and annealed state is cold forged and then carburized as it is without performing normalization or annealing, crystal grain coarsening is likely to occur.

  Therefore, elements such as Ti and Nb that form fine precipitates that suppress the movement of crystal grain boundaries are actively used. In addition, there has been proposed a steel material for cold forging that does not include Ti and Nb as essential components and uses a relatively soft ferrite-pearlite structure. For example, 0.015 to 0.10% by mass of Nb is an essential addition, Mo is limited to 0.01% or less, N is less than 0.0080% by mass, and the content of ferrite and pearlite is reduced. A case hardening steel having a structure with a ratio of 80% or more and having excellent cold forgeability and crystal grain coarsening suppressing ability in which the size and number of Nb precipitates are limited has been proposed (for example, see Patent Document 1). .). However, the addition of Ti or Nb proposed here causes an increase in manufacturing cost, and therefore, a proposal for a method in which these elements are not added is desired.

  In addition, the amount of AlN precipitated after hot working is limited to 0.005% or less, the structure fraction of bainite is limited to 30% or less, and the score of the ferrite band having a structure parallel to the hot rolling direction is evaluated. Is 1 to 3, and a case hardening steel excellent in coarse grain prevention properties has been proposed (for example, see Patent Document 2). The ferrite band refers to a ferrite striped structure. If the ferrite band is remarkable, as in the case of a ferrite band rating of 4 or more, the pearlite structure is continuously connected. , Which may cause coarsening. However, in the present invention, in the evaluation of steel, since the working ratio in the cold upsetting is set to 50%, in recent years, cold forging with a working ratio of 70% or more is actually performed, and the performance is sufficient. Is gone.

  Further, the area ratio of the pearlite cluster is 2.0% or less, and the ratio (D / d) of the average diameter D of the largest pearlite cluster to the average diameter d of the pearlite grains is 5.5 or less. A case hardening steel having an excellent property of preventing crystal grain coarsening during carburizing has been proposed (Patent Document 3). The pearlite cluster referred to here is a texture having no continuity in the rolling direction, and refers to pearlite which is often formed over two pearlite bands. Here, the area ratio is an area ratio of a pearlite cluster having an equivalent circle diameter (diameter when converted into a circle having the same area) of 50 μm or more. The average diameter d of the pearlite particles indicates the average value of the diameters of the pearlite particles present in the pearlite band obtained by a cutting method. Then, if the generation of coarse pearlite clusters in the structure before carburizing is suppressed, the occurrence of abnormal grains during carburizing treatment is eliminated, and the prevention of coarsening of crystal grains can be realized. However, also in the present invention, since the working ratio of cold working is evaluated at 60%, it cannot be said that it is sufficient as the steel for recent cold forging similarly to the above.

JP 2012-237052 A JP-A-11-106866 JP-A-2005-127434

  The inventor has conducted intensive research on steels to which Ti and Nb are not added. As a pre-structure for cold forging in the cold forging-carburizing process, cold workability and crystallinity are utilized while utilizing ferrite-pearlite. The technology for highly balancing the particle size characteristics was sought. As a result, from the viewpoint of cold workability, coarse pearlite having a large lamellar interval in the pearlite structure (that is, soft in hardness) is desirable, but from the viewpoint of crystal grain size characteristics, dense pearlite (that is, hardness Stiffness) was found to be a contradictory event.

  Therefore, the problem to be solved by the invention of the present application is to control the lamellar interval in the pearlite structure to an appropriate range while maintaining the ferrite-pearlite structure as a main component, and to provide excellent resistance to crystal grain coarsening during carburization and The case hardening of machine structural steel that is easy to cold work, especially the pearlite structure control at the electron microscope level, can effectively suppress the grain coarsening in the cold forging and carburizing process. An object of the present invention is to provide a case hardening steel having excellent coarsening prevention properties.

  On the other hand, the invention which positively utilizes the element which forms the fine precipitate which suppresses the movement of the grain boundary, such as Ti or Nb, which is mentioned in the above background art, or the essential addition of Ti and Nb In the case of the invention using a relatively soft ferrite-pearlite structure as a steel material for cold forging without using the ferrite-pearlite structure control at the optical microscope level, the crystal grain coarsening characteristics are all limited. Stay inadequate.

  In the first means of the present invention for solving the above-mentioned problems, in the first means, C: 0.14 to 0.25%, Si: 0.05 to 0.50%, Mn: 0 by mass%. 0.10 to 1.00%, P: ≤ 0.025%, S: ≤ 0.040%, Cr: 0.70 to 2.30%, Mo: 0.10 to 0.25%, Cu: ≤ 0 .30%, Al: 0.015 to 0.040%, O: ≤0.0020%, N: 0.0120 to 0.0190%, and the balance is Fe and inevitable impurities. Further, the area ratio of the ferrite-pearlite structure is 90% or more, the lamella spacing in the pearlite structure is 0.20 μm or more on average, and 0.35 μm or less at the maximum, and the crystal grains at a cold working rate of 70% are further reduced. Excellent resistance to grain coarsening during carburization with a coarsening temperature of 930 ° C or higher It is a case hardening steel for machine structural steel that is easy to cold work.

  In the second means, C: 0.14 to 0.25%, Si: 0.05 to 0.50%, Mn: 0.10 to 1.00%, P: ≤ 0.025% by mass% , S: ≦ 0.040%, Cr: 0.70 to 2.30%, Mo: 0.10 to 0.25%, Cu: ≦ 0.30%, Al: 0.015 to 0.040%, O: ≦ 0.0020%, N: 0.0120 to 0.0190%, further contains Ni: 0.50 to 2.00%, the balance being Fe and unavoidable impurities. Further, the area ratio of the ferrite-pearlite structure is 90% or more, the lamella spacing in the pearlite structure is 0.20 μm or more on average, and 0.35 μm or less at the maximum, and the crystal grains at a cold working rate of 70% are further reduced. It has excellent resistance to grain coarsening during carburization with a coarsening temperature of 930 ° C or higher, It is a case hardened steel for machine structural use that is easy to work with.

  The addition of an element such as Ti or Nb that forms a carbonitride to greatly improve the grain size characteristics exerts a high ability to suppress grain coarsening in the cold forging-carburizing process. However, the addition of these elements inevitably increased the production cost. On the other hand, in steels that do not contain these elements, it is difficult to suppress grain coarsening in subsequent carburization unless normalizing or annealing is performed after cold forging. In recent years, there has been an invention in which a ferrite-pearlite structure is used as a pre-cold forging structure instead of a spheroidized annealed structure to be adapted to a cold forging-carburizing process. In recent years when such severe processing is performed, the ability to suppress crystal grain coarsening has not been sufficient. Under such circumstances, the invention of the present application uses steel to which Ti or Nb is not added in a process in which cold forging and carburization are combined, and furthermore, the structure before cold forging is made of ferrite. In addition to adjusting mainly to the pearlite structure, by regulating the lamella spacing of the pearlite in the ferrite-pearlite structure to an appropriate range, after severe cold forging, without normalizing or annealing. In the carburizing process, an unprecedented excellent effect that coarsening of crystal grains can be suppressed is achieved.

  Prior to the embodiments for carrying out the invention, first, the reasons for limiting the chemical composition of steel in the means of the present invention will be described below. In addition,% in a chemical component is a mass%.

C: 0.14 to 0.25%, desirably 0.15 to 0.23%
C is an element necessary for securing the core strength of the steel material after carburizing as a component for machine structure. For that purpose, C needs to be 0.14% or more. On the other hand, if the C content is too large, the cold workability of the steel is impaired, and the crystal grains are coarsened during carburization through the action of refining the ferrite recrystallized grains during the process of cold forging the steel to the carburizing temperature. C is set to 0.25% or less in order to promote Therefore, C is set to 0.14 to 0.25%, and desirably, C is set to 0.15 to 0.23%.

Si: 0.05 to 0.50%, desirably Si: 0.05 to 0.30%
Si is an element necessary for deoxidation in steel refining, and for that purpose, 0.05% or more of Si is required. On the other hand, since Si has an effect of strengthening ferrite, an excessive addition impairs the cold workability, so the upper limit of Si is 0.50%. Therefore, Si is set to 0.05 to 0.50%, and desirably, Si is set to 0.05 to 0.30%.

Mn: 0.10-1.00%, desirably Mn: 0.25-0.90%
Mn is an element necessary for ensuring hardenability. However, if Mn is less than 0.10%, the effect on hardenability is not sufficiently obtained, and if Mn exceeds 1.00%, cold workability is reduced. Therefore, Mn is 0.10 to 1.00%, preferably Mn is 0.25 to 0.90%.

P: ≦ 0.025%
Although P is an unavoidable element contained in scrap, it is necessary to regulate P to 0.025% or less because it strengthens ferrite and lowers cold workability. Therefore, P is set to 0.025% or less.

S: ≦ 0.040%
S combines with Mn to form MnS, which is a nonmetallic inclusion that improves machinability. However, since MnS promotes cracking during cold working, it is necessary to add S to 0.040% or less. Therefore, S is set to 0.040% or less.

Cr: 0.70 to 2.30%, preferably Cr: 1.00 to 2.00%, more preferably Cr: 1.20 to 1.80%
Cr is an element that affects the lamella spacing of pearlite and is important for forming moderately dense pearlite which is indispensable for improving crystal grain size characteristics. To obtain this effect, Cr needs to be 0.70% or more. On the other hand, if Cr is added excessively, the cold workability is impaired and the carburizing property is impaired. Therefore, the content of Cr is set to 2.30% or less. Therefore, Cr is 0.70 to 2.30%, preferably Cr is 1.00 to 2.00%, and more preferably Cr is 1.20 to 1.80%.

Mo: 0.10 to 0.25%
Mo is not only an element that improves hardenability and toughness, but also has an effect of coarsening crystal grains when carburizing cold-worked steel by a solute drag effect (ie, solute drag action due to interface segregation). It is an important element that suppresses odors. It is presumed that the reason for this effect is explained as follows. When a steel mainly composed of a ferrite-pearlite structure is cold forged, a large amount of dislocations are introduced into a ferrite portion. At this time, when a severe cold working of a working rate of 70% is performed, the dislocations are entangled with each other to form a fine dislocation cell structure. When the temperature of the steel in which the fine dislocation cell structure is introduced is increased for carburization, ferrite recrystallization occurs during the heating process while being affected by the size of the dislocation cell structure. At this time, the recrystallized grains gradually grow in the process of raising the temperature, or the growth is suppressed by the presence of carbides in pearlite, which will be described later, but in any case, compared to the ferrite grain size before processing. Become fine. By the way, since Mo is hardly diffused in steel at low temperature, it does not easily affect the development of the dislocation cell structure and the growth stage of recrystallization. However, when the temperature reaches around the austenitizing temperature at which the recrystallization process has progressed to some extent, Mo tends to segregate near the recrystallized ferrite grain boundaries. Then, in the process of maintaining the carburizing temperature via austenitization, the crystal grains try to grow in an attempt to take a more energy stable state, but Mo segregated near the interface due to the movement of the grain boundaries. The grain growth is suppressed by the so-called solution drag effect, which causes resistance. Unlike the case where the lamellar interval in the pearlite structure is wide as described later, Mo does not strongly affect the refining of the recrystallized ferrite grains, so that the crystal grain size after austenitization is excessively reduced. Therefore, it is considered that the alloy has an effect of effectively suppressing grain growth in the carburizing temperature range. For this effect, Mo must be added at least 0.10% or more. However, when Mo is contained in excess of 0.25%, the formation of bainite is promoted to lower the cold workability, so that Mo is set to 0.25% or less. Therefore, Mo is set to 0.10 to 0.25%.

Cu: ≦ 0.30%
Cu is an unavoidable element contained in scrap, but has aging properties and increases strength. However, when Cu exceeds 0.30%, the hot workability deteriorates. Therefore, Cu is set to 0.30% or less.

Al: 0.015 to 0.040%, desirably Al: 0.0025 to 0.040%
Al is an element used as a deoxidizer in steel refining, and as described below, combines with N to precipitate finely as AlN, and plays an important role in suppressing the crystal grain coarsening. To obtain this effect, Al is added in an amount of 0.015% or more. On the other hand, if Al exceeds 0.040%, the effect of suppressing coarsening of crystal grains is reduced by decreasing fine AlN contrary to increasing coarse AlN, so the upper limit is made 0.040%. . Therefore, Al is set to 0.015 to 0.040%, and preferably Al is set to 0.0025 to 0.040%.

O: ≦ 0.0020%
O is an element inevitably contained. However, if O is contained in excess of 0.0020%, workability and fatigue strength are reduced due to an increase in oxides. Therefore, O is set to 0.0020% or less.

N: 0.0120 to 0.0190%, desirably N: 0.0120 to 0.0170%
N is finely precipitated as AlN in steel and exhibits an effect of preventing crystal grain coarsening. However, when the N content is less than 0.0120%, the effect is small, and when the N content exceeds 0.0190%, the fine AlN decreases in contrast to the increase in the coarse AlN, whereby the effect of suppressing the grain coarsening is reduced. Spoil. Therefore, N is 0.0120 to 0.0190%, and desirably, N is 0.0120 to 0.0170%.

Ni: 0.50 to 2.00%
Ni in the second means is an element for improving hardenability and toughness. For that purpose, an addition of 0.50% or more is necessary. However, if the Ni content exceeds 2.00%, the machinability decreases and the cost increases. Therefore, Ni is set to 0.50 to 2.00%.

Area ratio of ferrite-pearlite structure: 90% or more In the present invention, from the viewpoint of ensuring cold forgeability, the area ratio of ferrite-pearlite structure is set to 90% or more. In the present invention, since Mo is an essential addition, bainite may be formed. However, bainite does not lower the crystal grain size characteristics, but increases the hardness and impairs cold forgeability. Therefore, the area ratio of ferrite to pearlite is set to 90% or more.

Lamella spacing in pearlite structure: 0.20 μm or more on average and 0.35 μm or less in maximum In the present invention, the lamellar spacing in pearlite structure is 0.20 μm or more on average from the viewpoint of achieving both cold forgeability and crystal grain size characteristics. And at most 0.35 μm. The reason for controlling the average of the lamellar intervals in the pearlite structure to 0.20 μm or more is that the hardness of the pearlite structure is softened and the cold forgeability is improved. The reason why the maximum lamellar interval in the pearlite structure is 0.35 μm or less is that such a pearlite structure naturally contributes to the improvement of cold forgeability, but on the other hand, promotes the coarsening of crystal grains. is there.

  The inventors speculate as follows about the mechanism in which the lamella spacing of the pearlite structure contributes to the coarsening of crystal grains. In the process of cold forging a steel having a ferrite-pearlite structure and then raising the temperature for carburization, first, as described above, ferrite recrystallization occurs. At this time, pearlite also attempts to recrystallize because it contains a ferrite structure as a constituent element. However, the reaction of the carbide, which is a component of the other pearlite, being spheroidized by raising the temperature from the state of being processed by cold forging, proceeds simultaneously. The simultaneously formed spherical carbides pin the recrystallized grain boundaries and remain fine. Reflecting such a recrystallization process, crystal grains at the time of subsequent austenitization are formed finely, and such fine austenite grains have a high driving force for crystal grain growth. The process of crystal grain coarsening occurs. However, even in the state of ferrite-pearlite structure before cold forging, if the lamella spacing in the pearlite structure is restricted to a maximum of 0.35 μm or less, carbides spheroidized in the same process as described above are relatively hard to form. It stays in a fine state and, as a result, gradually dissolves in the process of raising the temperature to promote the growth of recrystallized ferrite grains. For this reason, in the subsequent austenitizing process, austenite grains are formed relatively large, and the driving force for crystal grain growth decreases. Therefore, this makes it difficult for the crystal grains to be coarsened during the holding at the carburizing temperature.

  Next, embodiments for carrying out the invention will be described. This will be described with reference to a table. First, No. 1 of the embodiment of the present invention shown in Table 1. Nos. 1 to 13 and Nos. Of Comparative Examples. Each steel containing 14 to 29 chemical components was melted in a 100 kg vacuum induction melting furnace (VIM) and cast into ingots. In Table 1, in Comparative Example No. The addition amounts of the elements indicated by hatching of 15 to 22 are outside the scope of the invention of the present application. In the steel materials of these examples and comparative examples, in order to temporarily dissolve AlN effective for pinning the crystal grain boundaries, the steel material was held at 1250 ° C. for 2 hours, then forged to a diameter of 34 mm, and air-cooled. In addition, such heating can be replaced by a heating step in which a continuously cast bloom or a steel ingot by casting is once formed into an intermediate product in a usual steel manufacturing method.

  The steels of the examples and comparative examples forged to a diameter of 34 mm were cut to a length of 100 mm. Subsequently, the steel of the example was subjected to a heat treatment by a slow cooling method or an isothermal annealing. These heat treatments were performed to obtain an appropriate ferrite-pearlite area fraction and a lamellar interval in the pearlite structure. The slow cooling method is performed by a method of continuous cooling after holding at an austenitizing temperature. In this case, an austenitizing temperature of 860 to 1000 ° C, preferably 870 to 960 ° C was selected. Further, the cooling rate from the holding in the austenitizing temperature range to the temperature at which the pearlite transformation was completed was selected from 50 to 450 ° C / h. The holding time was set at 1.5 hours in consideration of the time required for completing the austenitization of the steel material charged into the furnace. In practical use, the holding time is preferably determined in consideration of the furnace charging amount and dimensions of the steel material. In addition, in order to raise the temperature to the austenitizing temperature, in practice, the microstructure of a rolled steel bar or the microstructure obtained when hot forging for rough forming is performed prior to cold forging, etc. It is intended to homogenize by heating to the homogenization temperature. Here, No. 1 of the invention example. Nos. 1 to 4 and Nos. Numeral 11 indicates a heat treatment by a slow cooling method.

  As for the isothermal annealing, a steel material having a diameter of 34 mm and a length of 100 mm was heated to the same austenitizing temperature range as in the slow cooling method and held for 1.5 hours, followed by another furnace held at a lower temperature. And the air was cooled after the furnace temperature reached the set temperature. The purpose of heating to the austenitizing temperature is for the same reason as described above. As for the selection of the holding time at the austenitizing temperature, the amount and size of the steel material to be charged into the furnace are taken into consideration in the same manner as in the case of the slow cooling method. The subsequent holding temperature on the low temperature side was selected from 750 ° C to 550 ° C. Those having a holding temperature exceeding 660 ° C. and below 610 ° C. are out of the scope of the present invention. The holding time was adjusted in consideration of the time required to complete the transformation from austenite to a structure mainly composed of ferrite-pearlite. Here, No. 1 of the invention example. No. 5 to 10, no. 12, No. 13 was subjected to a heat treatment by isothermal annealing. In applying the isothermal annealing to the actual manufacturing process, for example, it is preferable to use a furnace in which the primary side heating on the high-temperature side and the secondary side heating on the low-temperature holding side are separated, and the primary-side furnace and the secondary-side furnace are used. A cooling chamber for cooling the temperature range between the furnace and the furnace relatively quickly may be provided. As means for cooling the temperature range between the primary furnace and the secondary furnace, for example, a cooling fan or a device for applying hot air may be introduced. As a comparative example, the steel No. satisfying the components of the present invention, which was forged to a diameter of 34 mm and cut to a length of 100 mm. 14 was heated at 900 ° C. for 1.5 hours and then air-cooled for normalization. The cooling rate at the time of the air cooling is considerably higher than 450 ° C./h which is a standard of the cooling rate at the time of performing the above-mentioned slow cooling method. Further, the steel No. of the comparative example, which was forged to a diameter of 34 mm and cut to a length of 100 mm, wherein any of the components was outside the scope of the claims of the present invention. 15 to 22 were subjected to a heat treatment in accordance with the above-described slow cooling method or isothermal annealing conditions. Further, the chemical composition satisfying the claims of the present invention has the following chemical composition. For the steels of Nos. 23 to 29, the heat treatment was performed under conditions outside the above-described range of the slow cooling method or the isothermal annealing.

  For the steel material subjected to the heat treatment as described above, the area ratio of ferrite-pearlite and the lamella interval in the pearlite structure were measured. First, a cross section of about 10 mm square was cut out from the steel material forging direction, mirror-polished, and then corroded with 5% nital. The ferrite-pearlite area ratio was calculated based on the area of the remaining ferrite-pearlite portion by taking an 800 μm square structure photograph with an optical microscope, excluding the bainite area from the total area of 800 μm square by image analysis. . The measurement of the lamellar spacing in the pearlite structure was performed as follows. First, using a scanning electron microscope, three visual fields in which lamella spacing in the pearlite structure could be easily measured were photographed. Subsequently, the average lamella spacing at nine points was measured from the photographs by the line segment method, and the average was further defined as the average lamella spacing in the present invention. On the other hand, the entire pearlite structure of arbitrary 200 μm square including several pearlite grains was similarly observed with a scanning electron microscope, and the maximum lamella interval in the visual field was defined as the maximum lamella interval.

  The steel of the invention example and the comparative example subjected to the heat treatment described above has a size of 14 mm in diameter and 21 mm in length, and further has a notch for restraining the upper and lower end surfaces during processing. Processed to. Subsequently, after performing cold upsetting at a height ratio of 70%, a pseudo carburizing test was performed. According to this method, a cold forging-carburizing process in which carburization is performed without performing normalizing or annealing after cold forging is imitated. At this time, a value obtained by indexing the deformation resistance at the time of 70% cold working (the standard is the deformation resistance at the time of 70% cold working of No. 10 steel was 1.00) is an index of the cold forgeability. And

  In addition, pseudo carburizing is a method of giving only the heat history of carburizing without actually carburizing, but since carburizing does not usually directly affect the grain coarsening behavior, Thus, it is possible to appropriately evaluate the crystal grain coarsening temperature. In the pseudo carburizing, the temperature was raised at 300 ° C./h assuming a heating rate in a general carburizing process, the temperature was maintained at 850 ° C. to 970 ° C. for 5 hours, and then water cooling was performed. Thereafter, the cross section of the upsetting test piece was indexed, and after the mirror polishing, the crystal grain boundaries were corroded in a saturated picric acid solution kept at 50 ° C., and then the presence or absence of crystal grain coarsening was confirmed by microscopic observation. As a criterion for crystal grain coarsening, a cross section of a test piece was observed in a region where a shear strain of more than 1.0 was introduced by cold upsetting, and when a particle exceeding 70 μm was observed in the region, the crystal was observed. It was determined that grain coarsening had occurred. In the region where the shear strain due to cold upsetting is 1.0 or less, recrystallization ferrite grains are not finely formed in the pseudo carburizing heating process due to insufficient cold working. Since the crystal grains after austenitization occurring on the high-temperature side become larger reflecting the size of the recrystallized ferrite grains, crystal grains larger than 70 μm may be observed although the crystal grains are not coarsened, Therefore, crystal grain coarsening was determined only in the above region. Table 2 summarizes the above results.

  In Table 2, the shaded portion in the column of Comparative Example in the area ratio (%) of ferrite pearlite, average pearlite interval (μm), maximum pearlite interval (μm), and crystal grain coarsening temperature (° C.) Indicates that it deviates. Further, the shaded portion in the column of the comparative example in the deformation resistance at the time of 70% processing is the invention example No. When the steel No. 10 is set to the reference value of 1.00, the values exceeding the reference value are shown.

  No. of Invention Example The steels of Nos. 1 to 13 have a component range satisfying the claims of the present application, and the ferrite-pearlite area ratio, the average pearlite interval in the pearlite structure, and the maximum pearlite interval satisfy the regulation range. No. of the steel of the invention example. The deformation resistance at the time of 70% cold working indexed on the basis of 10 is 1.00 or less, and the crystal grain coarsening temperature is at least 930 ° C. or more of the practical carburizing temperature, This steel has both forgeability and resistance to coarsening of crystal grains.

  No. of the comparative example. In the steels Nos. 14 to 29, any one or more of the chemical composition, the ferrite-pearlite area ratio, the average pearlite interval and the maximum pearlite interval in the pearlite structure are out of the regulation range. No. of the comparative example. 14, No. 18, No. 19, no. 23, no. The steels of Nos. 26 to 29 have a deformation resistance at the time of 70% cold working exceeding 1.00, and at least 1.15 or more, and are inferior in cold forgeability. In addition, No. of the comparative example. 15-17, No. 20 to 22, No. 1; 24, no. Steel No. 25 has a maximum crystal grain coarsening temperature of 890 ° C., which is lower than the practical carburizing temperature, and is inferior in resistance to crystal grain coarsening. That is, it is clear that none of the steels of the comparative examples has both the cold forgeability and the crystal grain coarsening resistance.

Claims (2)

  In mass%, C: 0.14 to 0.25%, Si: 0.05 to 0.50%, Mn: 0.10 to 1.00%, P: ≦ 0.025%, S: ≦ 0. 040%, Cr: 0.70 to 2.30%, Mo: 0.10 to 0.25%, Cu: ≤ 0.30%, Al: 0.015 to 0.040%, O: ≤ 0.0020 %, N: 0.0120 to 0.0190%, the balance being Fe and unavoidable impurities, the area ratio of the ferrite-pearlite structure is 90% or more, and the lamella interval in the pearlite structure is 0.20 μm or more on average. And a maximum of 0.35 μm or less, and a crystal grain coarsening temperature at a cold working rate of 70% of 930 ° C. or more, which is excellent in resistance to crystal grain coarsening during carburization and is excellent in cold working. Easy case hardening steel for machine structural steel.   In mass%, C: 0.14 to 0.25%, Si: 0.05 to 0.50%, Mn: 0.10 to 1.00%, P: ≦ 0.025%, S: ≦ 0. 040%, Cr: 0.70 to 2.30%, Mo: 0.10 to 0.25%, Cu: ≤ 0.30%, Al: 0.015 to 0.040%, O: ≤ 0.0020 %, N: 0.0120 to 0.0190%, Ni: 0.50 to 2.00%, and the balance is Fe and inevitable impurities, and the area ratio of the ferrite-pearlite structure is 90% or more. Wherein the lamellar spacing in the pearlite structure is 0.20 μm or more on average and 0.35 μm or less at maximum, and the crystal grain coarsening temperature at a cold working rate of 70% is 930 ° C. or more. Of steel for machine structural use that has excellent resistance to crystal grain coarsening during Case hardened steel.