JP4392324B2 - Method for producing case-hardened steel for cold forging - Google Patents

Description

  The present invention relates to a method for producing a case-hardened steel for cold forging having a low hardness after spheroidizing annealing and a uniform hardness. In addition, the steel materials described here refer to hot-rolled steel wire rods, steel bars, and the like.

  Conventionally, carbon steel materials for machine structures and low alloy steel materials for machine structures have been used as structural steel materials for producing machine structural parts such as automobile parts and construction machine parts. In order to manufacture machine structural parts such as drive system parts such as automobile bolts, lots, engine parts, and gears from these steel materials, conventionally, they have been mainly manufactured by hot forging and cutting processes. However, in recent years, switching to a cold forging process has been directed toward improving productivity and the like.

  In the cold forging process, cold forging is usually performed after spheroidizing annealing (SA) is performed on a steel material such as a steel wire drawn from a hot rolled wire to ensure cold workability. However, in cold forging, there is a problem that work hardening occurs in the steel material, ductility is reduced, and cracking occurs and the die life is reduced.

  In particular, in recent years, in order to reduce the extra space of cold forged parts as much as possible in order to reduce the weight of automobiles due to environmental problems, the degree of cold forging processing (processing rate) has increased, and the mold life has been steadily decreasing. It is in. Such a decrease in the mold life is a factor that greatly hinders the productivity of the cold forging process under the situation that the mold is enlarged due to the enlargement of forged parts.

For this reason, various techniques for improving the cold forgeability by reducing the hardness after spheroidizing annealing and softening have been proposed.
For example, a method for producing a case-hardened steel has been proposed in which the structure of a hot-rolled material is refined, the drawing process is performed, and the spheroidizing annealing is omitted (see Patent Document 1).

  In addition, the hot-rolled material has a martensite, bainite or martensite + bainite structure and is used in the mechanical structural parts manufacturing process of rough drawing, spheroidizing annealing, finish drawing, cold forging, carburizing and quenching and tempering. In addition, it has been proposed to provide a highly ductile spheroidizing material by omitting rough drawing (drawing) before spheroidizing annealing (see Patent Document 2).

Furthermore, the structure area ratio of the ferrite in the region from the surface to the depth of the rod wire radius x 0.15 of the steel rod wire material having a structure composed of one or more of martensite, bainite, pearlite is set to 10% or less. It has been proposed that only the surface layer is hardened and that the central portion has a soft structure to provide a bar wire for cold forging having excellent ductility after spheroidizing annealing (see Patent Document 3).
Japanese Patent Publication No. 6-99747 (Claims) JP 2000-336457 A (claims, paragraph 0004) JP 2001-240941 A (Claims)

  Each technique disclosed in each of the above publications, in particular, is still insufficient to reduce the hardness after spheroidizing annealing of the steel for cold forging, which is necessary for extending the die life for the enlargement of the die. is there.

  Furthermore, since Patent Document 1 remains a hot-rolled material in which spheroidizing annealing is omitted, there is a large variation in the hardness of the steel material within and between lots. Since the structure of martensite and bainite in Patent Document 2 is also sensitive to the cooling rate, and remains as a hot-rolled material in which spheroidizing annealing is omitted due to variations in wire diameter, rolling in hot rolling, and temperature during cooling, There are large variations in the hardness of steel materials within and between lots. The same applies to Patent Document 3.

  Conventionally, in order to improve cold forgeability, spheroidizing annealing is also repeated two or three times, but repeated spheroidizing annealing over a long period of time significantly reduces productivity. Let In this regard, although a number of techniques relating to the improvement of spheroidizing annealing have been proposed in the past, the spheroidizing of steel for cold forging, which is necessary for extending the die life for the enlargement of the die, as described above. The hardness reduction after annealing is insufficient, and it is not a technique for improving this.

  The present invention has been made in view of such a problem, and even in the case of only one spheroidizing annealing, the hardness after spheroidizing annealing is low, and this hardness is uniform and excellent in cold forgeability. An object of the present invention is to provide a method for producing a case-hardened steel.

In order to achieve this object, the gist of the method for producing a case-hardened steel for cold forging that is low in hardness after spheroidizing annealing of the present invention and is homogeneous is mass%, and C: 0.25% or less, Si: 0.3% or less, Mn: 1.5% or less, P: 0.02% or less, S: 0.02% or less, Al: 0.08% or less, N: 0.100% or less (any of these) Element is also 0%), Cr: 0.2 to 1.5% , and a steel material composed of the remaining Fe and unavoidable impurities, and has a structure in an arbitrary cross section from the surface to the center of the steel material. , Spheroidizing annealing is performed on a steel material having a bainite volume fraction of 50% or less (including 0%) on average, after wire drawing with a reduction in area of 28% or more. It is.

  Normally, the structure of the hot-rolled wire is composed of ferrite and lamellar pearlite, and the hardness does not fall below a certain level even when direct spheroidizing annealing is performed. For this reason, conventionally, for example, as disclosed in Patent Document 2 (paragraph 0004), the pearlite lamellar (cementite) is divided by performing a rough drawing process (drawing process) before spheroidizing annealing. And drawing distortion is given and spheroidizing annealing is enabled.

  However, in a low carbon steel material having C of 0.25% or less, the higher the rough drawing rate, the smaller the ferrite grain size after spheroidizing annealing, and thus it was considered difficult to soften. For this reason, the rough drawing-drawing rate of the low carbon steel material having C of 0.25% or less has a surface reduction rate of less than about 28%.

  However, in the present invention, prior to spheroidizing annealing, a strong wire drawing process (drawing process) with a surface reduction rate of 28% or more is intentionally performed to promote cementite spheroidization during spheroidizing annealing for cold forging. To soften case-hardened steel.

  In the present invention, the steel material structure before spheroidizing annealing is composed of a ferrite pearlite structure having an average bainite volume fraction of 50% or less (including 0%). When the steel structure before spheroidizing annealing is mainly bainite or martensite, even if the wire drawing with a reduction in area of 28% or more is performed before spheroidizing annealing, the cementite It cannot be coarsened, and it is difficult to soften the case-hardened steel for cold forging. In addition, the deformation resistance during wire drawing becomes excessive, and the adverse effect of shortening the life of the wire drawing dies becomes greater.

  In addition, it is known that medium carbon steel material is softened by increasing the spheroidizing of cementite at the time of spheroidizing annealing as the drawing rate is higher. However, the medium carbon steel material has a larger pearlite fraction and a lower ferrite fraction than the low carbon steel material. On the other hand, in the low carbon steel material with C of 0.25% or less, the ferrite fraction is larger than that of the medium carbon steel material. For this reason, as described above, the higher the rough drawing rate, the smaller the ferrite grain size after spheroidizing annealing, so it is considered difficult to soften, and avoid increasing the rough drawing rate. It was done.

(Steel structure)
In the present invention, the volume fraction of bainite, which is likely to occur in the cooling process after hot rolling, is preferably as small as possible for softening by spheroidizing annealing. Therefore, the average bainite volume fraction is 50% or less (including 0%), preferably 30% or less, more preferably 10% or less. And it shall consist of a ferrite pearlite structure | tissue with a structure | tissue as a main phase. Thus, when strong wire drawing is performed, spheroidizing and coarsening of cementite can be promoted by a single spheroidizing annealing, and the case-hardening steel for cold forging can be softened. Can extend the mold life,

  As described above, when the steel material structure before spheroidizing annealing is bainite exceeding 50%, or in the case of mainly martensite, even if the wire drawing with a reduction in area of 28% or more is performed before spheroidizing annealing, It is difficult to promote softening of the case-hardened steel for cold forging by promoting spheroidization and coarsening of cementite by one spheroidizing annealing. In the case of a bainite or martensite-based structure, the dispersion of cementite is uniform and spheroidization by annealing is easy, but the degree of softening is low due to the short average interparticle distance. For this reason, in order to soften to the required level, it is necessary to grow the cementite by Ostwald. For this purpose, it is necessary to repeat the spheroidizing annealing at the expense of production efficiency.

  Moreover, it is set as the said structure | tissue in the arbitrary cross sections from the surface of steel materials so that the low hardness after spheroidizing annealing may be homogenized over the site | part of steel materials. Even if the structure of the present invention is partly present, if there is a bainite exceeding 50% or a structure mainly composed of martensite, the variation in hardness increases. For this reason, depending on the part of the steel material, the hardness becomes too high and the mold life cannot be extended.

  The degree of softening effective in extending the mold life is as follows. When the C content is less than 0.2% as the average hardness of the steel material after spheroidizing annealing, the Cr content is about 1.0%. When the Cr content is about 1.1%, 425 MPa or less, when the Cr content is about 1.2%, 450 MPa or less, and when the C content is about 0.2%, the Cr content is 1 For about 1%, 440 MPa or less is a standard.

(Composition of steel material)
The composition (unit: mass%) of the steel material of the present invention will be described below including the reasons for limiting each element.

As a premise of obtaining the above structure of the steel of the present invention and obtaining excellent cold forgeability, the composition of the steel of the present invention is C: 0.25% or less, Si: 0.3% or less, Mn: 1.5% or less P: 0.02% or less, S: 0.02% or less, Al: 0.08% or less, N: 0.100% or less (all of these elements do not include 0%), Cr: 0.2 It is made into the steel material which contains -1.5% , and consists of remainder Fe and an unavoidable impurity.

  And, if necessary, in the above component composition, if necessary, Mo: 0.4% or less (not including 0%), or V: 1.5% or less, Ti: 0.2% or less, Nb: Contains one or more of 0.2% or less (both not including 0%).

C: 0.25% or less (excluding 0%).
C is contained to ensure the strength of the steel. If the C content is less than 0.02%, the strength of the steel may be insufficient, and preferably 0.02% or more. On the other hand, when the C content exceeds 0.25%, the strength and the hardness of the ferrite are excessively increased, and the cold forgeability is deteriorated. For this reason, C is 0.25% or less of low carbon, preferably 0.02 to 0.25%.

Si: 0.3% or less (excluding 0%).
Si is contained as a deoxidizing element and for the purpose of increasing the strength of the final product by solid solution hardening. If the Si content is less than 0.01%, the strength due to these effects may be insufficient, and preferably 0.01% or more. On the other hand, when Si exceeds 0.3%, this effect is saturated, and rather, the hardness is increased and the ductility is deteriorated. For this reason, Si is 0.3% or less, preferably 0.01 to 0.3%.

Mn: 1.5% or less (excluding 0%).
Mn is an element effective for increasing the strength of the final product through improvement in hardenability. If Mn is less than 0.2%, the strength of the final product may be insufficient. On the other hand, when Mn exceeds 1.5%, this effect is saturated, but rather the hardness is increased and the ductility is deteriorated. For this reason, Mn is 1.5% or less, preferably 0.2 to 1.5%.

P: 0.02% or less (excluding 0%).
P is a component inevitably contained in the steel, but P causes grain boundary segregation and center segregation in the steel and causes ductile deterioration, so it is suppressed to 0.02% or less.

S: 0.02% or less (excluding 0%).
S is a component inevitably contained in the steel, but is present as MnS in the steel and is a harmful element that deteriorates ductility for cold forging, so it is suppressed to 0.02% or less. .

Al: 0.08% or less (excluding 0%).
Al is useful as a deoxidizer and is useful for fixing solute N present in steel as AlN. If the Al content is less than 0.01%, these effects may be insufficient, and preferably 0.01% or more. However, if the Al content exceeds 0.08% and Al ₂ O ₃ is excessively generated, internal defects increase and cold forgeability deteriorates. For this reason, Al is made into 0.08% or less, preferably 0.01 to 0.08%.

N: 0.100% or less (excluding 0%).
N is a component inevitably contained in the steel, and is a harmful element that forms nitrides with other elements to lower the cold forgeability, so it is suppressed to 0.100% or less.

Cr: 0.2 to 1.5%.
Cr increases the strength of the final product by increasing hardenability. If Cr is less than 0.2%, the strength of the final product may be insufficient. On the other hand, if Cr exceeds 1.5%, bainite and martensite structures are formed on a part of the steel material (such as a surface with a high cooling rate or a central part where alloy elements are easily segregated) as hot rolled, and the hardness is high. Increase. For this reason, Cr is 0 . The range is 2 to 1.5%.

Mo: 0.4% or less (excluding 0%)
Mo, like Cr, increases the strength of the final product by increasing hardenability. In the case of selective inclusion, if Mo is less than 0.05%, this effect may not be obtained. On the other hand, when Mo exceeds 0.4%, similarly to Cr, bainite and martensite structure are formed in a part of the steel material in the hot rolled state, and the hardness is increased. For this reason, when it contains selectively, Mo is 0.4% or less, Preferably, you may be 0.05 to 0.4% of range.

V: 1.5% or less, Ti: 0.2% or less, Nb: 0.2% or less (all do not include 0%).
V, Ti, and Nb have the effect of making the crystal grains of the steel material finer, and are selectively contained for adjusting the crystal grain size. When selectively contained, if Ti and Nb contents are each less than 0.005% and the V content is less than 0.03%, this effect may not be obtained. On the other hand, when V exceeds 1.5%, Ti: 0.2%, and Nb: 0.2%, the effect is saturated and the ductility is rather deteriorated. Therefore, when one or more of these are selectively contained, V: 1.5% or less, Ti: 0.2% or less, and Nb: 0.2% or less, preferably V: 0.0. It is set as the range of 03-1.5%, Ti: 0.005-0.2%, Nb: 0.005-0.2%.

(Production method)
Preferred production conditions for the steel of the present invention will be described below.

  The process itself of the steel of the present invention is based on a conventional method. That is, in the case of a wire rod, the steel having the above composition is melted and cast, the steel slab (slab) is heated, cooled after hot rolling, and coiled. Thereafter, the wire coil is pickled, drawn and drawn, and spheroidized annealing is performed to obtain a steel wire for cold forging. In the case of a steel bar, the steel bar is manufactured in the same process except for the process specific to the wire.

  When hot-rolling a steel slab, the steel material structure after hot rolling is the structure in an arbitrary cross section from the surface of the steel material to the center, and the average bainite volume fraction is 50% or less (including 0%). And a composite structure of ferrite and pearlite. Hot rolling consists of rough rolling, intermediate rolling, and finish rolling.

  For this purpose, it is preferable to set the steel slab heating temperature during hot rolling to at least 850 ° C. or higher. The heating temperature of the billet is measured when the billet leaves the furnace.

  It is also effective to set the subsequent hot rolling temperature to the austenite region.

  And control of the cooling rate after this hot rolling (after finish rolling) is also important. The cooling control of the stealmore line after hot rolling is effective for making the structure in an arbitrary cross section from the surface to the center of the steel material a composite structure of ferrite and pearlite with a small bainite volume fraction.

  When the cooling rate after this hot rolling is fast (too fast), the steel structure becomes more than 50% bainite or martensite mainly, and the wire drawing has a surface reduction rate of 28% or more before spheroidizing annealing. Even if the processing is performed, it is difficult to promote the spheroidization and coarsening of cementite by one spheroidizing annealing and to soften the case-hardened steel for cold forging. Further, it becomes difficult to homogenize the structure of the present invention in an arbitrary cross section from the surface to the center of the steel material having the structure.

  In the present invention, the composite structure of ferrite and pearlite having a low bainite volume fraction can be obtained by further reducing the cooling rate after rolling. As for the cooling rate after this hot rolling, when cooling the hot-rolled steel wire with a stealmore line, an average cooling rate V (° C / ° C) from immediately after being placed on the stealmore line to at least 500 ° C. It is preferable to cool s) at 2.0 ° C./s or less, preferably 1 ° C./s or less. “Substantially placed” means placing at the first location where there is an air cooling facility. Strictly speaking, the cooling rate of the wire rod when cooled by the stealth conveyor is different depending on the sparse part and the dense part of the wire coil coil, but means an average cooling rate of these cooling rates.

  The wire coil after the controlled hot rolling and controlled cooling is subjected to a pretreatment such as pickling or coating as necessary, and then subjected to a drawing process with a surface reduction rate of 28% or more. By this strong wire drawing, strain is introduced into the wire, spheroidizing annealing of one time promotes spheroidization and coarsening of cementite, softening of case-hardened steel for cold forging, mold life Can be stretched. In addition, ferrite recrystallization generated during spheroidizing annealing becomes uniform, and variations in hardness are suppressed. In this respect, the area reduction rate is preferably as high as possible, and the area reduction rate is 28% or more, preferably 30% or more, more preferably 35% or more.

  On the other hand, if the area reduction rate of the wire drawing process is less than 28%, even if the steel material after hot rolling is composed of a composite structure of ferrite and pearlite with a low bainite volume fraction, in one spheroidizing annealing, It is difficult to promote the spheroidization of cementite and to soften the case-hardening steel for cold forging. In addition, ferrite recrystallization that occurs during spheroidizing annealing becomes insufficient or non-uniform, which causes a large variation in hardness.

  As described above, when the steel material structure before spheroidizing annealing is bainite exceeding 50%, or in the case of mainly martensite, even if the wire drawing with a reduction in area of 28% or more is performed before spheroidizing annealing, It is difficult to promote softening of the case-hardened steel for cold forging by promoting spheroidization and coarsening of cementite by one spheroidizing annealing.

  A steel material for cold forging such as a wire rod is subjected to a bondage treatment or the like, if necessary, and is subjected to cold forging.

  Examples of the present invention will be described below. Steel wire rods of each component composition were produced by the above-mentioned conventional method, except that the cooling conditions after hot rolling were changed, and the volume fraction of bainite was changed in the structure after hot rolling, in particular. . Then, these steel wires are drawn at a drawing rate shown in Table 2 to obtain a steel wire having a wire diameter of Φ10 to 55 mm, then spheroidizing annealing, softening and homogenization after annealing. From this evaluation, the effect on the die life in cold forging was predicted.

  Specifically, the steel slab having the composition shown in Table 1 is heated and hot-rolled. As shown in Table 2, after changing the cooling conditions after the hot-rolling, the wire diameter of Φ15 mm is commonly used. A steel wire was produced. In addition, the cooling rate after rolling shown in Table 2 shows an average cooling rate when the steel wire is cooled to 500 ° C. after the steel wire is placed on the stealmore conveyor after finish rolling. The cooling rate after the hot rolling was appropriately controlled by combining the ring pitch of the coiled wire, the use of a slow cooling cover, the air volume at the time of air cooling, the wind direction, and the like.

The volume fraction of the bainite structure of the steel wire after hot rolling was measured from the surface of the steel wire using a SEM (scanning electron microscope: JSM-5410, manufactured by JEOL Co., Ltd.) 5000 times that of each test piece of the steel wire. The structures in 10 cross sections up to the center were measured one field at a time. This is an image analysis software (Image-Pro Prus manufactured by MEDIA CYBERNETICS ^™ ), and the bainite structure is the total measurement area of bainite in the field of view observed with the SEM, with respect to the measurement area totaled with other structures such as ferrite and pearlite. The area fraction (%) of each was obtained and the results at 10 locations (10 fields of view) were averaged, and then the volume fraction of the bainite structure was obtained. These results are shown in Table 2.

  In addition, in the said structure observation result of each manufactured steel wire, all except the said bainite was a mixed structure of a ferrite and pearlite.

  Then, these steel wires are drawn and drawn at the respective drawing rates shown in Table 2 to obtain steel wires having a wire diameter of Φ11 to 15 mm, and then spheroidized and annealed only once, each of about 2 Ton steel wire coil was obtained. There are two types of spheroidizing annealing patterns in Table 2. After pattern A is soaked at 760 ° C. × 5 hr, it is gradually cooled at a cooling rate of 13 ° C./hr, allowed to cool after 685 ° C., and pattern B is soaked. After being treated at 760 ° C. × 7 hr, it was gradually cooled at a cooling rate of 11 ° C./hr, and after 680 ° C., it was allowed to cool and cool.

  The average tensile strength (average TS: MPa) of the manufactured steel wire and the difference ΔTS (MPa) between the maximum and minimum tensile strength were measured. The average TS of the steel wire was obtained by calculating the average of the TS of three links in total, one for each of the top, middle, and bottom among the manufactured steel wire coils. For ΔTS, the difference between the maximum TS and the minimum TS selected from these TSs was obtained. These results are shown in Table 2.

  The acceptance criteria for the softening evaluation of the average TS of the steel wire by spheroidizing annealing are set for each steel type shown in Table 1, steel type 1 is 430 MPa or less, steel type 2 is 425 MPa or less, steel type 3 is 440 MPa or less, and steel type 4 is 450 MPa or less. , And. Moreover, as for the homogenization standard of the steel wire by spheroidizing annealing, ΔTS was set to 20 MPa or less.

  As is clear from Table 2, the steel wire rods of Invention Examples 3-6, 9-12, and 15 have the composition of the present invention, and the structure in an arbitrary cross section from the surface to the center of the steel wire rod has a bainite volume fraction. Is composed of a ferrite pearlite structure having an average of 50% or less. And this steel wire has been drawn to a reduction in area of 28% or more. As a result, a steel wire (hardened steel for cold forging) having a low hardness after the spheroidizing annealing is obtained. Therefore, it is expected that the service life of the cold forging die can be extended significantly.

  On the other hand, each of Comparative Examples 1, 2, 7, 8, 13, 14, and 16 has a hardness after the spheroidizing annealing that is too high or varies in hardness as compared with the inventive examples. For this reason, it is expected that the service life of the cold forging die will be reduced.

  For example, in Comparative Example 1, the wire drawing rate is 0%, and no wire drawing is performed. For this reason, although it consists of a ferrite pearlite structure | tissue whose bainite volume fraction is 0% or less, the hardness of the steel wire after the said spheroidizing annealing is too high, and hardness varies.

  In Comparative Example 2, the wire drawing rate is too low. For this reason, although it consists of a ferrite pearlite structure whose bainite volume fraction is 0%, as in Comparative Example 1, the hardness of the steel wire after the spheroidizing annealing is too high, and the hardness varies. .

  In Comparative Example 7, Comparative Example 13, and Comparative Example 16, the cooling rate after rolling may be relatively high, and the bainite volume fraction of the steel wire is too high. For this reason, the steel wire after the spheroidizing annealing is too high and the hardness varies even though the wire drawing with a reduction in area of 28% or more is performed.

  In Comparative Example 8 and Comparative Example 14, the wire drawing rate is too low. For this reason, although it consists of a ferrite pearlite structure whose bainite volume fraction is 0%, as in Comparative Example 1, the hardness of the steel wire after the spheroidizing annealing is too high, and the hardness varies. .

  From the above results, the critical significance of the requirements of the present invention can be understood.

  As described above, according to the present invention, even after spheroidizing annealing only once, the hardness after spheroidizing annealing is low, and this hardness is uniform and excellent in cold forgeability. A method for producing case-hardened steel can be provided. For this reason, the steel material of the present invention is a component that places importance on machinability, and is useful for screws, nipples, and the like, which are mainly small components manufactured in large quantities by cutting.

Claims (3)

In mass%, C: 0.25% or less, Si: 0.3% or less, Mn: 1.5% or less, P: 0.02% or less, S: 0.02% or less, Al: 0.08% Hereinafter, N: 0.100% or less (all of these elements do not include 0%), Cr: 0.2-1.5% , a steel material composed of the balance Fe and inevitable impurities, Steel with a pearlite structure with an average bainite volume fraction of 50% or less (including 0%) in an arbitrary cross section from the surface to the center of steel is drawn with a reduction in area of 28% or more. A method for producing a case-hardened steel for cold forging having a low hardness after spheroidizing annealing and homogeneous, characterized by performing spheroidizing annealing after processing.   The manufacturing method of the case hardening steel for cold forging of Claim 1 in which the said steel materials contain Mo: 0.4% or less (excluding 0%) further.   The steel material further contains one or more of V: 1.5% or less, Ti: 0.2% or less, Nb: 0.2% or less (both not including 0%). Or the manufacturing method of the case hardening steel for cold forging of 2.