JP5407178B2 - Steel wire rod for cold forging excellent in cold workability and manufacturing method thereof - Google Patents

Description

  The present invention is used as a material for machine parts such as bolts, screws, and nuts, and relates to a steel wire formed by cold working such as cold forging and rolling, and a manufacturing method thereof, and particularly capable of suppressing forming cracks. The present invention relates to a steel wire rod excellent in cold workability and a manufacturing method thereof. In addition, the steel wire material made into object by this invention shall also contain the "burn-in coil" which wound the hot-rolled steel bar in the shape of a coil.

  Since cold forging is excellent in dimensional accuracy and productivity of products, the switch from hot forging, which has been conventionally performed, is expanding when forming mechanical parts such as steel bolts, screws, and nuts. Parts such as bolts and nuts are often used for structural purposes. For this reason, steel is added with alloying elements such as C, Si, and Mn, and strength is applied by heat treatment such as quenching after processing. doing. However, as the alloying element content increases, the deformation resistance of the steel material increases, so that the mold load increases during cold forging, causing wear and damage to the mold, and processing cracks in the product. There are issues such as to do. In order to avoid these, the steel for cold forging is required to be soft and extremely ductile. For this reason, conventionally, hot rolled material has been softened by spheroidizing annealing to improve workability. Thus, in recent years, steel materials used for cold forging have been required to have improved workability due to reduction in component manufacturing costs and higher functionality of components.

  Conventionally, various methods have been proposed for improving the cold forgeability of steel materials. Patent Document 1 discloses a steel for cold forging in which Si is limited to 0.15% or less and Mn is limited to 0.60% or less, and Ti and B are added to improve cold forgeability. However, in this method, the degree of softening is not sufficient, the volume fraction of ferrite in the structure is high, and the ductility is lowered due to the uneven structure, and sufficient workability is not obtained.

  In Patent Document 2, the area where the average particle diameter of ferrite is 2 to 5.5 μm, the major axis is 3 μm or less, and the ratio of cementite whose aspect ratio is 3 or less is 70% or more of the total cementite is It is disclosed that the cold workability is improved by setting it to 10% or more. This method is effective in processing where the crack generation position is in the vicinity of the surface of the rolled wire, but the effect of improving workability is small for processing where the crack generation position is inside the rolled wire. In actual cold forging, since the cold wire forging is performed after the rolled wire is cut, the vicinity of the surface of the rolled wire often does not become a crack generation position, and the effect is limited.

  In Patent Document 3, when the value obtained by dividing the standard deviation of the distance between cementites by the average value of the distance between cementites is 0.50 or less, the distance between the cementites becomes almost uniform, and as a result, the deformation resistance during cold forging decreases. In addition, it is disclosed that cracking is reduced. In this method, in order to spheroidize cementite, it is necessary to increase the annealing temperature, and there is a problem that the heat treatment cost increases. Further, as a result of increasing the annealing temperature, the number density of cementite decreases and the size increases. As a result, during cold forging, cracks are generated from coarse cementite, so that ductility is not sufficiently improved. Moreover, the space | interval between cementite becomes large and there exists a subject that the structure | tissue after hardening becomes non-uniform | heterogenous.

JP-A-60-230960 JP 2000-73137 A JP 2006-316291 A

An object of this invention is to provide the steel wire for cold forging which has the outstanding cold workability, and can implement | achieve the structure | tissue and intensity | strength homogenization after a quenching process, and its manufacturing method.

(1) By mass%, C: 0.005 to 0.60%, Si: 0.01 to 0.40%, Mn: 0.20 to 1.80%, P: 0.040% or less, S: 0.050% or less, Al: 0.001 to 0.060%, N: 0.0005 Containing ~ 0.0300%, the balance is made of Fe and inevitable impurities, and the ferrite structure having an average particle size of 15 μm or less, and the spherical cementite having an average aspect ratio of 3 or less and an average particle size of 0.6 μm or less A steel wire rod for cold forging excellent in cold workability, characterized in that the number of spherical cementite satisfying these is 1.0 × 10 ⁶ × C content (%) or more per 1 mm ² .
(2) The steel wire rod for cold forging excellent in cold workability according to (1), further containing B: 0.0001 to 0.0060% by mass% in addition to the above components.
(3) In addition to the above components, the composition further comprises one or two of Ti: 0.002 to 0.050% and Nb: 0.005 to 0.100% by mass%. (1) to (2) A steel wire for cold forging excellent in cold workability according to any one of the items.
(4) Bloom steel wire or billet having the composition described in any one of (1) to (3) is hot-rolled, wound, and then put into a molten salt bath at 400 ° C to 600 ° C. After being immersed for 10 seconds or more, it is cooled after being kept at a constant temperature in a molten salt bath of 450 ° C. or more and 600 ° C. or less for 20 seconds or more and 150 seconds or less, and then annealed at 600 ° C. or more and 700 ° C. or less (1) The manufacturing method of the steel wire for cold forging excellent in the cold workability as described in any one of the items (3).
(5) The steel wire according to any one of (1) to ( 3 ) is wound after hot rolling, and then immersed in a molten salt bath at 400 ° C. or higher and 600 ° C. or lower for 10 seconds or longer, and then 450 Hold at a constant temperature for 20 seconds to 150 seconds in a molten salt bath of ℃ ℃ 600 ℃, cool, and after wire drawing with a surface area reduction of 40% or less, anneal at 600 ℃ to 700 ℃ The manufacturing method of the steel wire for cold forging excellent in the cold workability as described in any one of the items (1) to ( 3 ).

  According to the present invention, by improving the deformability of a steel material, it becomes possible to form a complex shaped part by cold forging, and there is an effect of reducing the cost of parts processing by improving the steel material yield and improving the productivity. .

  As a result of studying the improvement of the metal structure in order to improve the deformability of the steel material, the inventors have made it possible to uniformly disperse cementite and further increase the number of cementite in order to improve cold workability. Was found to be effective. Conventionally, even if the number density of cementite increases, the occurrence of molding cracks cannot be suppressed. Conventional steel has a large variation in cementite size, and coarse cementite is included even if the number density is increased. Coarse cementite becomes a starting point of cracking during cold working, and cold workability deteriorates. Therefore, the present inventors reduced the ferrite particle size of the substrate and increased the number density of cementite, thereby making the size of the cementite uniform, suppressing coarse cementite, and forming cracks. As a result, the present inventors have found that the reduction is achieved.

  The reason why molding cracks are reduced by increasing the number of cementite can be estimated as follows. When the size of cementite is uniform, the particle diameter of cementite becomes finer as the number density increases. During deformation, cracks are generated at the interface between ferrite and cementite when the strain increases, but when the size of cementite is small, the generation of cracks is suppressed. In addition, the cementite is uniformly dispersed in the structure by increasing the number density of cementite. As a result, local unevenness in strength is reduced, and distortion is not concentrated locally during deformation. These effects are thought to suppress molding cracks.

  In the present invention, the steel wire needs to have a specific structure. The reasons for limiting the organization are explained below.

  Finer ferrite grains improve ductility and improve deformability. When the average particle size exceeds 15 μm, the ductility decreases and the frequency of occurrence of cold forging cracks increases. For this reason, the upper limit of the average grain size of ferrite is set to 15 μm.

  In addition, the occurrence of molding cracks correlates with the average particle diameter of spherical cementite, and when the average particle diameter becomes coarse, cracks are generated from the periphery of strained cementite, and cracks are likely to occur. When the average particle diameter of spherical cementite exceeds 0.6 μm, ductility is lowered and cold forging cracks are likely to occur. For this reason, it is desirable that the average particle diameter of spherical cementite be 0.6 μm.

  A structure having a large aspect ratio of cementite has high deformation resistance, and the mold load during cold forging increases. When the average aspect ratio exceeds 3, the cold deformation resistance increases, and tool wear of the mold tends to occur. For this reason, it is desirable to set the upper limit of the average aspect ratio to 3.

When the number of spherical cementite per 1 mm ² is less than 1.0 × 10 ⁶ × C content (%), the distribution of cementite is non-uniform, and non-uniform strength is generated. If there is a non-uniform portion of strength, cold forging cracks may occur due to local concentration of deformation during forging. For this reason, the lower limit of the number of spherical cementite was set to 1.0 × 10 ⁶ × C content (%) per 1 mm ² . A more preferable range is 1.6 × 10 ⁶ × C content (%) or more per 1 mm ² .

  A method for measuring the ferrite particle diameter, the average particle diameter of spherical cementite, the aspect ratio, the number, and the distance between the spherical cementites in the present invention will be described below.

  An EBSP apparatus was used to measure the ferrite particle size. A region of 275 μm × 165 μm was measured at the surface layer vicinity portion, 1 / 4D portion, and 1/2 portion of the cross section in the longitudinal direction of the wire. From the ferrite crystal orientation map measured by EBSP, the boundary where the orientation difference was 15 degrees or more was defined as the ferrite grain boundary. The block size was determined by the method of Johnson-Saltykov (“Metromorphology” Uchida Otsukuru, S47.7.30, original work: R.T.DeHoff, F.N.Rhiness.P189).

  The average particle diameter, aspect ratio, and number of spherical cementite were determined by image analysis of scanning electron micrographs. The visual field of 25 μm × 20 μm was observed at five magnifications at a magnification of 5000 in the vicinity of the surface layer, 1 / 4D portion, and 1 / 2D portion of the cross section in the longitudinal direction of the wire, and the obtained photograph was obtained by image analysis. The average particle diameter was the equivalent circle diameter, and the aspect ratio was (major axis length) / (minor axis length).

  The distance between the spherical cementite was taken at five magnifications and 5 fields of 25 μm × 20 μm area was photographed. A circle was drawn in the area not containing 0.1 μm or more of cementite, and the maximum diameter of the circle was defined as the maximum distance between the cementites.

The steel wire rod for cold forging of the present invention is composed mainly of C: 0.005 to 0.60%, Si: 0.01 to 0.40%, Mn: 0.20 to 1.80%, P: 0.040% or less, and S: 0.050% or less. And the balance Fe and inevitable impurities. The reason why the range of these elements is limited will be described below.

  C is added to ensure strength as a machine part. If it is less than 0.005%, the strength required for machine parts cannot be secured, and if it exceeds 0.60%, ductility and toughness deteriorate, so 0.005 to 0.60% was set.

  Si functions as a deoxidizing element and is an element effective for imparting necessary strength and hardenability to steel and improving resistance to temper softening. If the content is less than 0.01%, these effects are insufficient. If the content exceeds 0.40%, the toughness and ductility deteriorate, and the hardness increases and the cold forgeability deteriorates, so the content was made 0.01 to 0.40%.

  Mn is an element necessary for imparting necessary strength and hardenability to steel. If the content is less than 0.20%, the effect is insufficient. If the content exceeds 1.80%, the toughness deteriorates and the hardness increases and the cold forgeability deteriorates, so the content was made 0.20 to 1.80%.

  P increases deformation resistance during cold forging and degrades toughness. Further, it is desirable to reduce the grain boundary because it segregates and embrittles the crystal grain boundary after quenching and tempering to deteriorate toughness. Therefore, the upper limit was made 0.040%.

  S reacts with an alloy element such as Mn and exists as a sulfide. These sulfides improve machinability. If added over 0.050%, the cold forgeability deteriorates, and the grain boundaries after quenching and tempering become brittle and the toughness deteriorates. For this reason, it was made into 0.050% or less.

Further, the steel wire for cold forging of the present invention is mass% for the purpose of improving the properties described below, Al: 0.001 to 0.060%, N: 0.0005 to 0.0300%, B: 0.0001 to 0.0060%, Ti: One or two or more of 0.002 to 0.050% and Nb: 0.005 to 0.10% can be contained.

  Al is added for the purpose of deoxidation and austenite grain refinement. Al functions as a deoxidizing element, forms AlN and functions as pinning particles, and refines the grain size to improve workability. Moreover, solid solution N is fixed, and dynamic strain aging is suppressed and deformation resistance is reduced. If the content is less than 0.001%, these effects do not function. If the content exceeds 0.060%, the toughness deteriorates, so the upper limit was made 0.060%.

  N is added for the purpose of refining austenite crystal grains. N combines with Al, Ti and the like to form nitrides, which function as pinning particles and make the crystal grains finer. If it is less than 0.0005%, the amount of deposited nitride is insufficient, the crystal grains become coarse and the ductility deteriorates. If added over 0.0300%, deformation resistance increases due to dynamic strain aging due to solute N and deteriorates workability, so 0.0005 to 0.0300% was set.

  B is added for the purpose of improving hardenability. If it is less than 0.0001%, the effect is insufficient, and even if added over 0.0060%, the effect is saturated, so 0.0001 to 0.0060% was set.

  Ti and Nb form carbonitrides. These carbonitrides are dispersed in steel and function as pinning particles, suppress the coarsening of crystal grains and improve workability.

  Ti forms a compound with C or N and exists as TiC, TiN, or Ti (CN). These carbonitrides are effective as pinning particles. In addition, it is added to fix N in steel in order to make the effect of improving hardenability by adding B function effectively. If the content is less than 0.002%, the effect does not appear. If the content exceeds 0.050%, the effect is saturated and the hardness is increased and the cold forgeability is deteriorated, so the content is set to 0.002 to 0.050%.

  Nb combines with N or C to form NbN, NbC, or a composite inclusion Nb (CN) thereof, and effectively functions to suppress coarsening of austenite crystal grains. If less than 0.005%, the effect is insufficient, and even if added over 0.10%, the effect is saturated, so 0.005 to 0.10% was set.

  O is inevitably contained in steel and exists as oxides such as Al and Ti. If the O content is high, a coarse oxide is formed, which causes fatigue failure. As the deoxidizing element, 0.0001 to 0.01% of Ca, or 0.0001 to 0.01% of Mg, or both of them can be contained. These elements are effective for deoxidation and have the effect of improving fatigue strength by refining oxides.

  Next, the manufacturing method of this invention is demonstrated below.

  A bloom or billet having the above component composition is rolled into a wire by hot rolling and wound up after hot rolling. The coiling temperature is not particularly limited, but is generally 750 ° C. or higher and 1000 ° C. Then, it is immersed in a molten salt bath at 400 ° C. or higher and 600 ° C. or lower for 10 seconds or longer. The cooling rate is not particularly limited, but in the case of a wire of about φ12, the cooling rate after winding up to 600 ° C. is about 40-50 ° C./s. When the temperature of the molten salt bath is less than 400 ° C., a martensite structure is generated, the strength after annealing is increased, and workability is deteriorated. If it exceeds 600 ° C, the molten salt will decompose and impair operability. When the immersion time is less than 10 seconds, the inside of the wire becomes insufficiently cooled, and the volume fraction of pro-eutectoid ferrite after cooling increases, increasing the spacing between cementite after annealing, and improving hardenability and workability. Deteriorate.

  Next, the mixture is kept in a molten salt bath at 450 ° C. or more and 600 ° C. or less for 20 seconds to 150 seconds and then cooled. When the constant temperature holding temperature is lower than 450 ° C., the transformation completion time is prolonged and the productivity is hindered. When the temperature exceeds 600 ° C., the lamella spacing of pearlite becomes coarse, the average particle size of cementite after annealing may exceed 0.6 μm, and the molten salt is decomposed to inhibit productivity. When the isothermal holding time is less than 20 seconds, the lower limit is set to 20 seconds to reduce the workability by increasing the strength after annealing due to cooling without completing the transformation. Therefore, the upper limit was set to 150 seconds. In the case of using a molten salt bath of 530 ° C. or higher and 600 ° C. or lower, the immersion in the molten salt bath may be performed for 30 seconds or longer in one bath without being immersed in two baths. Cooling after immersion in the molten salt bath is usually cooled to room temperature, then reheated and annealed, or reheated after wire drawing and annealed, but the cooling end temperature is not particularly limited, 600 It may be in the range of room temperature below ℃.

  When the aspect ratio of the carbide after annealing is reduced to further improve the workability, wire drawing with a surface reduction rate of 40% or less can be performed. By dividing the cementite by wire drawing, spheroidization after annealing is promoted and the aspect ratio of the carbide is reduced. If the area reduction ratio of the wire drawing process exceeds 40%, the strength after annealing increases and the workability deteriorates, so the upper limit was made 40%.

  Softening annealing is performed at 600 ° C to 700 ° C. If the annealing temperature is less than 600 ° C, the cementite is insufficiently spheroidized and the average aspect ratio becomes 3 or more, which deteriorates the workability. On the other hand, when the temperature exceeds 700 ° C., the cementite becomes coarse and the average particle diameter exceeds 0.6 μm, and the number of cementite also decreases. For this reason, the lower limit of the annealing temperature was set to 600 ° C., and the upper limit was set to 700 ° C. A more preferable range is 600 ° C. or higher and 680 ° C. or lower.

Table 1 shows the components of the test steel. Steel K and steel L are comparative examples outside the scope of the present invention. For the purpose of evaluating the relationship between workability and structure, hot rolling and isothermal transformation treatment were performed under the conditions shown in Table 2, and then annealing treatment was performed under various conditions shown in Table 3 to change the structure. For the evaluation of workability, a tensile test was performed, and the drawing (RA) was evaluated as an index of ductility. Table 2 and Fig. 1 show the relationship between microstructure and workability evaluated using steel type H. Tables (1), (2), and (3) are examples that satisfy the scope of the present invention. (4) is a comparative example in which the annealing temperature is set to a temperature exceeding the upper limit of the present invention, and the average particle diameter of cementite and the number of cementites are outside the scope of the present invention. As a result, it can be seen that RA is significantly lower than that of the present invention. (6) is prepared in the conditions isothermal transformation conditions do not satisfy the scope of the present invention, the ferrite grain diameter and the average particle diameter of cementite and number the number of cementite are comparative examples departing from the scope of the present invention. RA is low compared to the present invention. From the above, it can be seen that when the average particle diameter of ferrite, the average aspect ratio and average particle diameter of cementite, and the number of cementites satisfy the range of the present invention, RA is remarkably improved and workability is excellent.

Wire rod rolling was performed using each steel type shown in Table 1, and the relationship between the manufacturing conditions and the structure after annealing was evaluated. In the annealing, heating was performed to a predetermined temperature described in Table 4 at a temperature increase rate of 180 ° C./h, and air cooling was performed after holding for 5 hours. As shown in Table 4, 17 is a condition in which both the cooling bath holding time and the constant temperature layer holding time are outside the scope of the present invention . Also, 2,4,7,9,13,21 is a comparative example was produced in a conventional production conditions after coiling is not carried out isothermal holding. In any of these, any of the average aspect ratio of cementite, the average particle diameter of cementite, and the number of cementite does not satisfy the scope of the present invention. From the above, it can be seen that the structure after annealing falls outside the scope of the present invention under conditions that do not satisfy the production conditions of the present invention.

  Table 5 shows the RA of each Example and Comparative Example shown in Table 4. Comparing RA of the same steel type, it can be seen that the RA of the present invention is remarkably improved as compared with the comparative example, and the workability is excellent.

The figure which shows the relationship between the average particle diameter (micrometer) of cementite, and RA (%).

Claims (5)

In mass%, C: 0.005 to 0.60%, Si: 0.01 to 0.40%, Mn: 0.20 to 1.80%, P: 0.040% or less, S: 0.050% or less, Al: 0.001 to 0.060%, N: 0.0005 to 0.0300% The balance is made of Fe and inevitable impurities, and the ferrite structure has an average particle size of 15 μm or less, and the average aspect ratio is 3 or less, and the spherical particle cementite has an average particle size of 0.6 μm or less. A steel wire rod for cold forging excellent in cold workability, characterized in that the number of spherical cementite satisfying the requirement is 1.0 × 10 ⁶ × C content (%) or more per 1 mm ² . In addition to the above components, further contains, by mass%, B: cold workability excellent cold forging steel wire rod according to claim 1, characterized in that it contains from 0.0001 to 0.0060 percent.   The cold working according to claim 1 or 2, further comprising one or two of Ti: 0.002 to 0.050% and Nb: 0.005 to 0.100% in mass% in addition to the above components. Steel wire for cold forging with excellent properties.   The bloom steel wire or billet having the component composition according to any one of claims 1 to 3 is hot-rolled, wound, and then immersed in a molten salt bath at 400 ° C or higher and 600 ° C or lower for 10 seconds or longer. Thereafter, holding at a constant temperature in a molten salt bath at 450 ° C. or higher and 600 ° C. or lower for 20 seconds or longer and 150 seconds or lower, cooling, and then annealing at 600 ° C. or higher and 700 ° C. or lower. The manufacturing method of the steel wire for cold forging excellent in the cold workability of 1 item | term. The steel wire according to any one of claims 1 to 3 is wound after hot rolling, and then immersed in a molten salt bath at 400 ° C to 600 ° C for 10 seconds or more, and then 450 ° C to 600 ° C. Maintained at a constant temperature in a molten salt bath at 20 ° C or lower for 20 seconds or more and 150 seconds or less, and then cooled, annealed at 600 ° C or more and 700 ° C or less after wire drawing with a surface area reduction rate of 40% or less. The manufacturing method of the steel wire for cold forging excellent in the cold workability of any one of Claims 1-3 .

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