JP5576785B2 - Steel material excellent in cold forgeability and manufacturing method thereof - Google Patents

Description

  The present invention relates to a steel material for cold forging used in the manufacture of various parts such as automobile parts and construction machine parts, and in particular, a steel material having a low deformation resistance after spheroidizing annealing and excellent cold forgeability, And a manufacturing method thereof.

  Cold forging of steel is used in a wide range of fields because of its high productivity. For example, when manufacturing various parts such as automobile parts and construction machine parts, a spheroidizing annealing treatment is performed for the purpose of imparting cold forgeability to hot rolled materials such as carbon steel and alloy steel, and then cooling. After the forging is performed and then formed into a predetermined shape by performing cutting or the like, the final strength adjustment is performed by quenching and tempering.

  In recent years, the shape of parts tends to become complicated and large, and the steel that is cold forged accordingly undergoes severe deformation locally, so if the deformability is low, defects due to material cracking are likely to occur, Further, when the deformation resistance is high, there arises a problem that the tool life is shortened, for example, the tool die or the die is damaged due to excessive load. For these reasons, cold forging steel is required to have excellent properties (cold forgeability) in reducing deformation resistance and improving deformability.

As a technique for improving the cold forgeability of steel by spheroidizing annealing, for example, Patent Document 1 discloses that the ferrite grain size is refined to 15 μm or less and the number density of spherical cementite is increased (the number of spherical cementites per mm ^2). = 1.0 × 10 ⁶ × C content (%) or more) to reduce cold cracking caused by coarse cementite and improve cold workability has been proposed. However, in this technique, since the number density of spherical cementite is increased, deformation resistance is increased by dispersion strengthening of cementite, so that it is insufficient from the viewpoint of softening strength.

Further, Patent Document 2 uses B-added steel (B: 0.0003 to 0.007%) to form a metal structure composed of ferrite and spherical carbide, and has a ferrite grain size of 8 or more and the number of spherical carbides. A technique for softening is proposed by suppressing the above (number of spherical carbide per 1 mm ² = 1.5 × 10 ⁶ × C content (%) or less). However, the steel material that is the subject of this technology is B-added steel, and its application range is limited because it differs from the usual component composition of JIS regulations, and the production conditions also require low-temperature rolling, so a rolling mill There is a problem that the load on the vehicle increases. Moreover, although the metal structure of this patent document 2 contains many lamellar perlites, the problem of an increase in deformation resistance and a decrease in deformability occurs due to lamellar perlite.

JP 2009-275250 A JP 2001-11575 A

  The present invention has been made paying attention to such circumstances, and its purpose is to reduce the deformation resistance and improve the deformability, and to provide a steel material capable of exhibiting excellent cold forgeability, and a method for producing the same. It is to provide.

  The present invention that has achieved the above-mentioned object is as follows: C: 0.15-0.6% (meaning mass%, the same applies to chemical components), Si: 0.05-0.6%, Mn: 0.1 -1.5%, P: 0.05% or less (excluding 0%), S: 0.001-0.05%, Cr: 0.01-0.5%, Al: 0.01-0 0.1%, N: 0.01% or less (not including 0%), the balance being iron and unavoidable impurities, and the steel metal structure has cementite and ferrite, The total area ratio of cementite and ferrite is 95 area% or more, the aspect ratio of 90% or more of the cementite is 3 or less, and the average center-to-center distance of the cementite is 1.5 μm or more, and The gist is that the average grain size of ferrite is 5 to 20 μm. It is a steel material with excellent cold forgeability.

  Further, as other elements, Cu: 0.25% or less (not including 0%), Ni: 0.25% or less (not including 0%), and Mo: 0.25% or less (not including 0%) It is also a preferred embodiment to contain at least one element selected from the group consisting of:

  Further, as other elements, Ti: 0.2% or less (not including 0%), Nb: 0.2% or less (not including 0%), and V: 1.5% or less (including 0%) It is also a preferred embodiment to contain at least one element selected from the group consisting of:

  Further, as other elements, Mg: 0.02% or less (not including 0%), Ca: 0.05% or less (not including 0%), Li: 0.02% or less (not including 0%) And REM: at least one element selected from the group consisting of 0.05% or less (excluding 0%) is also a preferred embodiment.

  In addition, the present invention provides a steel that satisfies any of the above-described component compositions after hot working, cooled to room temperature, and then heated to a temperature range of A1 point to A1 point + 50 ° C. After the temperature, the temperature range from A1 point to A1 point + 50 ° C. is maintained for 0 to 1 hr, and the temperature range from the A1 point to A1 point + 50 ° C. to the A1 point−100 ° C. to A1 point−30 ° C. After performing the annealing process for cooling at an average cooling rate of 10 to 200 ° C./hr twice or more, the temperature is raised to a temperature range of A1 point to A1 point + 30 ° C., and in the temperature range of A1 point to A1 point + 30 ° C. When cooling after holding, when reaching A1 point at the time of temperature rise and then cooling after holding in the temperature range of A1 point to A1 point + 30 ° C, A1 point to A1 point + 30 ° C until reaching A1 point The residence time in the temperature range is 10 minutes to 2 hours, and the points A1 to A1 + 30 ° C. After cooling the cooling temperature range from the temperature range A1 point -100 ° C to A1 point -20 ° C at an average cooling rate of 10 to 100 ° C / hr, the temperature is maintained for 10 minutes to 5 hours in the cooling temperature range. It is a method for producing a steel material excellent in cold forgeability having a gist in cooling.

  According to the present invention, the cold forgeability (deformation) is determined by appropriately controlling the steel composition ratio, cementite size and dispersion state, ferrite crystal grain size, etc. Steel materials with excellent resistance and deformability can be provided. That is, since the steel material of the present invention having excellent cold forgeability has reduced deformation resistance, it can suppress wear and breakage of jigs for plastic working such as dies, and the deformability is improved. Therefore, the occurrence of cracks during the forging process can also be suppressed. The steel material of the present invention is suitable for cold forging steel wires and rods.

It is a schematic explanatory drawing of the spheroidization heat processing pattern (heat pattern) of this invention. It is a related figure of the deformation resistance of each steel in an Example, and a carbon equivalent.

  The present inventors studied from various angles in order to improve the cold forgeability of steel materials. First, as for the component composition of the steel material, B-added steel has been proposed as described above. However, when B is added, hardenability increases, and when spheroidizing heat treatment is applied, a structure with high hardness such as layered pearlite is formed. Therefore, the deformation resistance is increased and it is difficult to sufficiently improve the cold forgeability. Therefore, in the present invention, the basic composition is such that B is not added.

  In addition, investigations were repeated based on the idea that appropriate control of the metal structure of steel is effective in improving cold forgeability. Although it is effective to improve the cold forgeability by using a mixed structure of cementite and ferrite, it is particularly necessary to further optimize the cold forgeability by optimizing the ferrite grain size and the aspect ratio of the cementite. I understood it. If the ratio of lamellar pearlite, bainite, and martensite in the metal structure increases, the strength of the steel may increase, so the ratio of these structures should be reduced to make the steel microstructure a mixed structure of cementite and ferrite. Is effective for improving the cold forgeability. However, even when cementite and ferrite are mixed, if the cementite is not sufficiently spheroidized, the strength of the steel cannot be reduced sufficiently, and even if the cementite is spheroidized, the cementite is dispersed. In some cases, however, precipitation strengthening occurs, so it is necessary to define the degree of cementite spheroidization and the distance between adjacent cementites. In addition, the size of the ferrite crystal grain also affects the cold forgeability, and the strength increases depending on the size of the crystal grain. Therefore, it is necessary to set the ferrite crystal grain size within a specific range. I understood. Furthermore, it has also been found that it is effective to perform heat treatment a plurality of times in order to appropriately control the metal structure.

  The present invention has been made on the basis of the above-mentioned findings, in that the area ratio of ferrite and cementite is appropriately controlled, the aspect ratio of the cementite crystal grains and the distance between adjacent cementite particles (average distance between the center of gravity) Is characterized in that it is controlled within a specific range, the average grain size of ferrite is controlled within a specific range, and the chemical composition of the steel material is appropriately controlled. In addition, the production method of the present invention for obtaining such a steel material is characterized in that a heat treatment is performed a plurality of times on the rolled material after hot rolling, and the heating temperature and cooling rate during the heat treatment are appropriately controlled. doing. Hereinafter, the present invention will be specifically described.

  First, the metal structure of steel will be described.

Metal structure: Having cementite and ferrite As described above, cementite and ferrite are metal structures that contribute to improving cold forgeability by reducing the deformation resistance of steel. However, as described above, since the desired softening cannot be achieved simply by making the metal structure containing spheroidized cementite and ferrite, as described in detail below, the area ratio of the metal structure, as well as cementite. The aspect ratio, the average distance between the center of gravity of cementite, the average grain size of ferrite crystal grains, and the like must also be satisfied.

Total area ratio of cementite and ferrite: 95 area% or more based on the entire structure The effect of improving the cold forgeability of the steel of the present invention is manifested by making the metal structure a mixed structure of cementite and ferrite. In order to obtain such an effect, the total area ratio of cementite and ferrite with respect to the entire structure is 95 area% or more, preferably 97 area% or more, more preferably 99 area% or more. Metal structures other than cementite and ferrite include, for example, martensite and bainite that can be produced in the manufacturing process. However, when the area ratio of these structures increases, the strength increases and cold forgeability deteriorates. Therefore, it may not be included at all. Therefore, the total area ratio of cementite and ferrite with respect to the entire structure is more preferably 100 area%.

  In addition, the ratio of cementite and ferrite is not particularly limited, but it is preferable that the ratio is generally controlled to cementite: ferrite = 2-20: 98-80.

Aspect ratio of 90% or more of cementite: 3 or less A cementite needs to have an aspect ratio (major axis / minor axis) of 3 or less. If this aspect ratio is larger than 3, it is difficult to sufficiently reduce the strength. For example, cracks are generated at the interface, and cracks are likely to occur during cold forging. Further, the increase in hardness after high strain becomes remarkable, and the life of jigs for plastic working such as dies is reduced. The smaller the aspect ratio, the better. The preferred cementite aspect ratio is 2.5 or less, more preferably 2.0 or less.

  Moreover, by increasing the cementite with an aspect ratio of 3 or less in the total cementite, the hardness of the steel can be reduced and the occurrence of local cracks due to coarse cementite (aspect ratio exceeding 3) can be suppressed. Therefore, the cold forgeability is improved. On the other hand, when cementite having an aspect ratio larger than 3 remains, the softening and ductility improvement of the steel material is insufficient. From this point of view, the cementite size of 90% or more, preferably 95% or more, more preferably 97% or more is set to 3 or less, preferably 2.5 or less in aspect ratio. The aspect ratio is the ratio of major axis / minor axis when the major axis is obtained for each cementite and the minor axis is the diameter perpendicular to the major axis direction.

  Even if the cementite has an aspect ratio of 3 or less, the presence of extremely coarse cementite may deteriorate the cold forgeability, so the average particle size is preferably 0.05 μm or more, more preferably 0.00. It is 1 μm or more, preferably 3.0 μm or less, more preferably 2.0 μm or less. The average particle diameter of cementite is obtained as an average value of diameter values of perfect circles having the areas of individual cementites obtained by image analysis.

The average distance between the center of gravity of cementite: 1.5μm or more As mentioned above, reducing the aspect ratio of cementite is effective for improving cold forgeability, but if the distance between adjacent cementites is too close, the steel will be sufficient. Cannot be softened. Therefore, the distance between the average centers of gravity of adjacent cementite is set to 1.5 μm or more. The thickness is preferably 1.7 μm or more, more preferably 1.9 μm or more. Thus, the cold forgeability is improved by appropriately securing the distance between the center of gravity and keeping adjacent cementite grains independent of each other. On the other hand, since the effect is saturated even if the average distance between the center of gravity of the cementite is too far, the preferable upper limit is 10 μm or less, more preferably 8 μm or less.

  The average center-of-gravity distance is determined based on the image analysis (1) the connected lines do not pass on other images, and (2) the adjacent lines are constrained to leave the shorter line when the connecting lines intersect. The center of gravity of (cementite grains) is connected by a straight line, the length of the line segment is measured, and the average value is obtained. At this time, the cementite to be measured is a distance between adjacent cementites at the shortest distance among all cementites contained in the metal structure, and the measurement object is not limited to cementite having an aspect ratio of 3 or less.

Average crystal grain size of ferrite: 5 to 20 μm
If the average crystal grain size of ferrite is too large, softening cannot be achieved and the deformability decreases, so that sufficient cold forgeability cannot be ensured. Therefore, the average crystal grain size of ferrite is 20 μm or less, more preferably 18 μm or less, and still more preferably 16 μm or less. On the other hand, if the average crystal grain size of ferrite is too fine, the strength may be increased, and it is difficult to make the grain size less than 5 μm in terms of manufacturing conditions. It was. The thickness is preferably 7 μm or more, more preferably 9 μm or more.

  The steel of the present invention having properties excellent in cold forgeability is required not only to satisfy the metal structure of the steel but also to satisfy the chemical composition of the steel.

C: 0.15-0.6%
C is an element necessary for ensuring the product strength after cold forging. By containing 0.15% or more, C can ensure the strength necessary for a part. C is preferably 0.20% or more, more preferably 0.25% or more. However, if the amount of C is excessive, the steel becomes too hard and cold forgeability deteriorates. Therefore, the C content is 0.6% or less. The amount of C is preferably 0.55% or less, more preferably 0.50% or less.

Si: 0.05-0.6%
Si acts as a deoxidizing element and is an element necessary for ensuring product strength by solid solution hardening. When there is too little Si, deoxidation will become inadequate and it will become easy to generate | occur | produce a gas defect at the time of melting. Therefore, Si is 0.05% or more, preferably 0.06% or more, more preferably 0.07% or more. However, when the amount of Si becomes excessive, the strength of the steel becomes excessively high and the cold forgeability deteriorates. Therefore, Si is 0.6% or less, preferably 0.55% or less, more preferably 0.50% or less.

Mn: 0.1 to 1.5%
Mn is an element necessary for improving the hardenability and improving the product strength, and is 0.1% or more, preferably 0.12% or more, more preferably 0.15% or more. However, when Mn is excessive, the hardenability is excessively improved and bainite is excessively generated or martensite is easily generated, the strength is increased, and the cold forgeability is deteriorated. Therefore, Mn is 1.5% or less, preferably 1.2% or less, more preferably 1.0% or less.

P: 0.05% or less (excluding 0%)
P is an impurity element inevitably contained in the steel. If the amount of P is excessive, grain boundary segregation occurs and causes ductile deterioration, so it is necessary to reduce it as much as possible. Therefore, P is 0.05% or less, preferably 0.045% or less, more preferably 0.040% or less. In addition, it is industrially difficult to make P amount 0%.

S: 0.001 to 0.05%
S is an impurity inevitably contained in the steel, but is an element that effectively acts to improve the machinability of the steel by combining with Mn in the steel to form MnS inclusions. 001% or more, preferably 0.003% or more, more preferably 0.005% or more. However, when the amount of S becomes excessive, the amount of MnS inclusions increases and causes ductile deterioration, so it is necessary to reduce it as much as possible. Therefore, S is 0.05% or less, preferably 0.045% or less, more preferably 0.040% or less.

Cr: 0.01 to 0.5%
Cr is an element that effectively acts to enhance the hardenability of the steel and improve the strength of the product. In order to exhibit such an effect, Cr is 0.01% or more, preferably 0.03% or more, more preferably 0.05% or more. However, if the Cr amount is excessive, the strength becomes too high and the cold forgeability deteriorates, so the Cr amount is 0.5% or less, preferably 0.45% or less, more preferably 0.40% or less. It is.

Al: 0.01 to 0.1%
Al is useful as a deoxidizing agent, and Al is an element that binds to N and precipitates AlN to prevent the crystal grains from growing abnormally during processing and reducing the strength. Al also acts as a deoxidizer. In order to exhibit such an effect, Al is 0.01% or more, preferably 0.015% or more, more preferably 0.020% or more. However, when Al is excessive, Al ₂ O ₃ is excessively generated and the cold forgeability is deteriorated. Therefore, Al is 0.1% or less, preferably 0.090% or less, more preferably 0.080% or less.

N: 0.01% or less (excluding 0%)
N is an impurity inevitably contained in the steel, but if solid solution N is contained in the steel, the hardness increases due to strain aging and the ductility decreases, and the cold forgeability deteriorates. Therefore, N is 0.01% or less, preferably 0.009% or less, more preferably 0.008% or less.

  The component composition of the steel according to the present invention is as described above, and the balance is iron and inevitable impurities. As inevitable impurities, mixing of trace elements (for example, B, As, Sb, Sn, etc.) brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. is allowed.

  In addition, Cu, Ni, Mo, Ti, Nb, V, Mg, Ca, Li, REM, and the like may be actively included as other elements within a range not impairing the effects of the present invention.

Selected from the group consisting of Cu: 0.25% or less (not including 0%), Ni: 0.25% or less (not including 0%), and Mo: 0.25% or less (not including 0%) At least one kind of element Cu, Ni, and Mo are elements that effectively act to improve the hardenability and the product strength. Such an effect increases as the content of these elements increases, but in order to effectively exhibit, Cu, Ni, and Mo are each preferably 0.05% or more, more preferably 0.08% or more. More preferably, it is 0.10% or more. However, if it is contained excessively, the supercooled structure is excessively generated, the strength becomes too high, and the cold forgeability is lowered. Therefore, Cu, Ni, and Mo are each preferably 0.25% or less. More preferably, it is 0.22% or less, More preferably, it is 0.20% or less. In addition, Cu, Ni, and Mo may each be contained independently, and may contain 2 or more types, and the content in the case of containing 2 or more types is arbitrary in the above range. The amount is sufficient.

Selected from the group consisting of Ti: 0.2% or less (not including 0%), Nb: 0.2% or less (not including 0%), and V: 1.5% or less (not including 0%) At least one kind of element Ti, Nb, V is an element that combines with N to form a compound (nitride) and reduce the amount of solute N in the steel, thereby obtaining an effect of reducing deformation resistance. In order to exhibit such an effect, Ti, Nb, and V are each preferably 0.05% or more, more preferably 0.06% or more, and further preferably 0.08% or more. However, if these elements are contained excessively, the amount of nitride increases, the deformation resistance increases, and the cold forgeability deteriorates. Therefore, Ti and Nb are each preferably 0.2% or less, more preferably 0.8. 18% or less, more preferably 0.15% or less, and V is preferably 1.5% or less, more preferably 1.3% or less, and still more preferably 1.0% or less. In addition, Ti, Nb, and V may be contained independently and may contain 2 or more types chosen arbitrarily.

Mg: 0.02% or less (not including 0%), Ca: 0.05% or less (not including 0%), Li: 0.02% or less (not including 0%), and REM: 0.0. At least one element selected from the group consisting of 05% or less (excluding 0%) Mg, Ca, Li, and REM spheroidize sulfide compound inclusions such as MnS and improve the deformability of steel It is an effective element to make it. Such an action increases as the content thereof increases. However, in order to exhibit the effect effectively, Mg, Ca, Li and REM are each preferably 0.0001% or more, more preferably 0.0005% or more. However, even if contained excessively, the effect is saturated and an effect commensurate with the content cannot be expected. Therefore, Mg and Li are each preferably 0.02% or less, more preferably 0.018% or less, and still more preferably 0.00. 015% or less, Ca and REM are each preferably 0.05% or less, more preferably 0.045% or less, and still more preferably 0.040% or less. In addition, Ca, Mg, Li, and REM may be contained independently and may contain 2 or more types chosen arbitrarily.

  As described above, in the present invention, it is assumed that B is not added, but B may be inevitably mixed as an impurity. If the B content increases, the cold forgeability deteriorates. Therefore, it is better that B is as small as possible. Even if it is inevitably mixed, the upper limit is made less than 0.0003%. B is preferably 0.0002% or less, more preferably 0.0001% or less.

  In order to obtain a steel material having the metal structure, it is effective to perform heat treatment a plurality of times after hot rolling and appropriately control the heat treatment conditions.

  The steel material of the present invention is subjected to a heat treatment after hot working the steel satisfying the above component composition and then cooling it to room temperature. The specific heat treatment condition is that the temperature is raised from room temperature to the temperature range of A1 point to A1 point + 50 ° C., and after the temperature rise, the temperature is maintained in the temperature range of A1 point to A1 point + 50 ° C. for 0 to 1 hr. After performing the annealing process which cools the temperature range from a point-A1 point +50 degreeC to the temperature range from A1 point -100 degreeC-A1 point -30 degreeC with the average cooling rate of 10-200 degreeC / hr 2 times or more, A1 When the temperature is raised to the point A1 to the point + 30 ° C. and the temperature is kept at the point A1 to the point A1 + 30 ° C. and then cooled, the point A1 to the point A1 after the point A1 is reached during the temperature rise. When cooling after holding in the temperature range of + 30 ° C., the stay in the temperature range of A1 point to A1 point + 30 ° C. until reaching the A1 point is 10 minutes to 2 hours, and the temperature range of the A1 point to A1 point + 30 ° C. The cooling temperature range from A1 point to -100 ° C to A1 point to -20 ° C from 10 to 10 ° C ° C. / After cooling at an average cooling rate of hr, can be prepared by further cooling was held in the cooling temperature range 10 minutes to 5 hours.

  In particular, in order to obtain a steel that satisfies the above metal structure, it is desirable to appropriately control the heat treatment conditions after hot working. Hereinafter, the heat treatment conditions of the present invention will be described with reference to FIG. 1, but the heat treatment pattern of the present invention is not limited to FIG. 1 and can be changed as appropriate. For example, in FIG. 1, the first annealing treatment is performed in the range of A1 point to A1 point + 30 ° C., and the second annealing treatment is performed in the range of A1 point to A1 point + 50 ° C., but the present invention is not limited to this. It can be changed within the range of the invention (A1 point to A1 point + 50 ° C.). In FIG. 1, the second annealing is performed immediately after the first annealing. However, the present invention is not limited to this, and the first and second annealing intervals can be controlled within a range of 0 to 2 hours, for example. .

  The heat treatment specified in the present invention is performed after hot working steel that satisfies the above component composition, but the hot working conditions are not particularly limited, and a desired temperature (for example, about 800 to 1300 ° C.). You just have to process it.

  The steel material after hot working (here, the temperature of the steel material is room temperature) is heated to a temperature range of A1 point to A1 point + 50 ° C. (1 in FIG. 1). By heating the steel material in the above temperature range (heating rate is 30 ° C./hr to 100 ° C./hr, for example), the cementite can be partially left while dissolving and dividing the layered cementite. This residual cementite can be used as a nucleus to grow spheroidized cementite and to increase the distance between particles (average distance between the center of gravity) while preventing the precipitation of cementite (layered cementite, rod-like pearlite) with a large aspect ratio. Can do. When the heating temperature is lower than the A1 point, the layered cementite cannot be sufficiently dissolved and divided, so that cementite having a large aspect ratio remains and the hardness becomes high. On the other hand, when heating exceeds the point A1 + 50 ° C., all cementite dissolves, so there is no cementite as the core of spherical cementite, and spherical cementite with a desired aspect ratio cannot be obtained, and layered cementite is precipitated by subsequent cooling. As a result, the strength is increased. Also, the crystal grain size of ferrite becomes too large. A preferable heating temperature is A1 point + 5 ° C. or higher, more preferably A1 point + 10 ° C. or higher, preferably A1 point + 40 ° C. or lower, more preferably A1 point + 30 ° C. or lower.

  Subsequently, the heated steel material after being heated is held in the temperature range (A1 point to A1 point + 50 ° C.) for 0 minute to 1 hour (2 in FIG. 1). “Holding” here refers to the time for maintaining substantially the same temperature in the temperature range, and refers to the period from the temperature rise to the start of cooling (H in FIG. 1). By holding the steel material in the temperature range, it is possible to promote the dissolution and fragmentation of cementite. If the holding time is too long, the dissolution of cementite proceeds too much, and spherical cementite with an aspect ratio of 3 or less may not be obtained. Therefore, the holding time is 1 hr or less, preferably 50 minutes or less, more preferably 40 minutes or less. The lower limit of the holding time is not particularly limited, and if the layered cementite is sufficiently dissolved and divided by the above heating and remains to the extent that it becomes a core of spherical cementite, it is held for a certain period of time without being held for a certain period of time. You may perform the cooling mentioned later immediately without doing. Therefore, the lower limit of the holding time is 0 minute, preferably 0.5 minute or more, more preferably 1 minute or more. In addition, when heating the steel material to a temperature range of A1 point + 30 ° C. to A1 point + 50 ° C., since the cementite is in a temperature range in which it is easy to dissolve, if the holding time is long, the cementite is excessively dissolved, and the aspect ratio is 3 or less. Since it is difficult to obtain cementite, the holding time is desirably 20 minutes or less, more preferably 15 minutes or less.

  Next, the temperature range from the temperature range (A1 point to A1 point + 50 ° C.) to A1 point −100 ° C. to A1 point −30 ° C. is cooled at an average cooling rate of 10 to 200 ° C./hr (in FIG. 1, 3). By cooling from the above temperature range to a temperature range of A1 point-100 ° C to A1 point-30 ° C, solid solution C is aggregated into the cementite remaining by the heating, and spheroidized cementite can be grown. At this time, if the average cooling rate is too high, layered cementite is reprecipitated rather than growing cementite, which is not desirable. Therefore, an average cooling rate shall be 200 degrees C / hr or less. A preferable average cooling rate is 100 ° C./hr or less, and a more preferable average cooling rate is 80 ° C./hr or less. On the other hand, if the average cooling rate is too slow, the ferrite grain size becomes coarse and the treatment takes too much time, which is not desirable. Therefore, the average cooling rate is 10 ° C./hr or more, preferably 15 ° C./hr or more, more preferably 20 ° C./hr or more.

  In the present invention, it is necessary to perform heat treatment under the above conditions twice or more (in FIG. 1, 1, 2, 3 and 1 ', 2', 3 'represent two times). A single heat treatment cannot sufficiently dissolve and divide the layered cementite, and the aspect ratio of the obtained spherical cementite remains larger than 3, so that the strength is not sufficiently lowered. In addition, the distance between the average centers of gravity between adjacent spherical cementites cannot be sufficiently increased, and the ferrite grain size remains large, so that the desired cold forgeability cannot be obtained. When the above heat treatment is performed a plurality of times, the cementite melts by heating and gradually decreases in size, especially the refined spherical cementite melts and disappears with each heating, so the average distance between the centers of gravity can be increased. . Further, by cooling at an appropriate cooling rate, spherical cementite grows by agglomeration of solute C, so that an aspect ratio with a desired ratio can be obtained. Further, the ferrite diameter can be set in a desired range. Note that the heat treatment performed a plurality of times may be performed under different conditions as long as they are within the range specified in the present invention.

  The number of repetitions of the heat treatment is not particularly limited, and may be performed so as to obtain a target metal structure. However, in order to further promote softening of the spheroidized heat treatment material, the number of repetitions is preferably 3 times or more, more preferably 4 times or more. On the other hand, since the effect may be saturated when the number of repetitions is too large, it is preferably 80 times or less, more preferably 60 times or less.

  After the heat treatment is performed a plurality of times (the heat treatment other than the final heat treatment may be collectively referred to as a pre-stage heat treatment), the final heat treatment is performed in a temperature range of A1 point to A1 point + 30 ° C. The final heat treatment is performed for the purpose of controlling the microstructure of cementite, such as the aspect ratio of cementite, the distance between the average centers of gravity, and the crystal grain size of ferrite, as in the previous heat treatment. That is, although these metal structures are already in a desirable state by the above-described heat treatment, there are cementite aspect ratios and average center-of-gravity distances outside the specified range, average crystal grain size of ferrite, etc. For the purpose of controlling within the prescribed range, the final heat treatment is performed by more strictly controlling the heat treatment conditions.

  First, after the pre-stage heat treatment, the steel material is heated to a temperature range of A1 point to A1 point + 30 ° C. (10 in FIG. 1). If the heating temperature in the final heat treatment is lower than the A1 point, unnecessary cementite cannot be sufficiently dissolved and remains, so it is difficult to make the average distance between the center of gravity of the cementite within a desired range. On the other hand, if heating exceeds the point A1 + 30 ° C., the cementite formed by the previous heat treatment is melted, and cementite having a desired aspect ratio cannot be obtained. A preferable heating temperature is A1 point + 5 ° C. or higher, more preferably A1 point + 10 ° C. or higher, preferably A1 point + 25 ° C. or lower, more preferably A1 point + 20 ° C. or lower.

  Also, the steel staying time from reaching the A1 point to maintaining the temperature range (A1 point to A1 point + 30 ° C.) and again below the Al point is 10 minutes to 2 hours (in FIG. 1, H2 And the bold line part indicated by R is the stay time target area). If the staying time (A1 point → A1 point to A1 point + 30 ° C → A1 point) in the above temperature range (A1 point to A1 point + 30 ° C.) is shorter than 10 minutes, fine cementite is not sufficiently dissolved, and the average center of gravity It becomes difficult to make the distance sufficiently large. On the other hand, if the residence time in the above temperature range exceeds 2 hours, most of the cementite is dissolved, and cementite having a large aspect ratio such as layered cementite is likely to precipitate during subsequent cooling, and the crystal grain size of ferrite is coarse. Therefore, the desired cold forgeability cannot be obtained. The residence time is preferably 15 minutes or more, more preferably 20 minutes or more, preferably 1.5 hours or less, more preferably 60 minutes or less.

  In addition, the holding time in the above temperature range (the holding time in the temperature range of A1 to A1 + 30 ° C., the portion indicated by H2 in FIG. 1) is not particularly limited, and the stay time in the above temperature range is 10 minutes to 2 hours. As long as it is within the range.

  After holding in the temperature range of A1 point to A1 point + 30 ° C., it is cooled to A1 point−100 ° C. to A1 point−20 ° C. (30 in FIG. 1). At this time, depending on the cooling rate, layered cementite may precipitate, or a lot of fine cementite may precipitate to shorten the average distance between the center of gravity and increase the hardness. Therefore, it is necessary to appropriately control the cooling rate from the viewpoint of promoting the growth of spherical cementite. When the average cooling rate exceeds 100 ° C./hr, the growth of spherical cementite becomes insufficient. On the other hand, when the average cooling rate is less than 10 ° C./hr, coarse ferrite precipitates. Therefore, the average cooling rate is controlled within the range of 10 to 100 ° C./hr. It is preferably 15 ° C./hr or more, more preferably 20 ° C./hr or more, preferably 80 ° C./hr or less, more preferably 60 ° C./hr or less.

  The reason for setting the average cooling temperature from A1 point to -100 ° C to A1 point to -20 ° C is that, by cooling to this temperature range at the above average cooling rate, solid solution C can be agglomerated to promote the growth of cementite. is there.

  And steel materials are hold | maintained in the said temperature range (A1 point-100 degreeC-A1 point-20 degreeC) for 10 minutes-5 hours (In FIG. 1, it is a 40 part and the thick line part shown by H3). This is because by maintaining the temperature in the temperature range for a certain period of time, a larger amount of solid solution C can be aggregated into cementite, and the ratio of cementite having the desired aspect ratio can be 90% or more. If the holding time is too short, such an effect cannot be obtained, and the hardness increases after cooling. Accordingly, the holding time needs to be 10 minutes or longer, preferably 20 minutes or longer, more preferably 30 minutes or longer. On the other hand, even if the holding time is too long, the effect obtained is saturated, so the time is 5 hours or less. Preferably it is 4 hours or less, More preferably, it is 3 hours or less, More preferably, it is 2 hours or less.

  After holding the said predetermined time in the temperature range of said A1 point-100 degreeC-A1 point-20 degreeC, steel materials are cooled. As a cooling condition, the temperature of the steel material may be lowered to a desired temperature such as room temperature by allowing it to cool in air (particularly temperature control is not performed) (50 in FIG. 1). Here, the term “cooling” refers to cooling within a range where the average cooling rate is generally 5 to 20 ° C./s.

  Since the final heat treatment is an indispensable step, in the present invention, as shown in FIG. 1, the heat treatment is performed at least three times including at least the preceding heat treatment (two times or more) and the final heat treatment. By performing the above series of heat treatments, the total area ratio of cementite and ferrite to the steel metal structure can be 95% by area or more, the aspect ratio of 90% or more of the cementite is 3 or less, and the cementite A steel material having an average center-to-center distance of 1.5 μm or more and an average crystal grain size of ferrite of 5 to 20 μm can be obtained.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  20 kg of steel having the chemical composition shown in Table 1 below (the balance is iron and inevitable impurities) was melted in a vacuum induction furnace and cast into an ingot. The ingot was hot forged to 1200 ° C. to obtain a billet and then cooled. . Subsequently, the billet was heated to 900 ° C. and then hot forged to form a round bar having a diameter of 17 mm and then air-cooled. The obtained round bar was subjected to spheroidizing annealing under the conditions shown in Table 2 below to prepare a test piece.

  Based on the following test, the obtained test piece was examined for the metal structure, the area ratio of the metal structure, the ratio of cementite having an aspect ratio of 3 or less, the average distance between the center of gravity of cementite, the average crystal grain size of ferrite, and the deformation resistance. The results are shown in Tables 3 and 4.

(Observation of metal structure)
About each said test piece, the metal structure and the area ratio of the metal structure were measured in the procedure shown below.

  Each test piece after spheroidizing annealing (cut perpendicularly to the longitudinal direction (rolling direction), embedded in resin so that the longitudinal section can be observed), and cut at D / 4 position (D is the wire diameter) As a result of nitriding the surface and observing with an optical microscope, it was confirmed that almost 100% of the samples prepared in this example were cementite and ferrite.

(Aspect ratio of spherical cementite)
In the same manner as in the observation of the metal structure, the test piece was cut vertically, embedded in a resin, and then the cut surface was mirror-polished by emery paper, diamond buffing, or electrolytic polishing. Then, after etching with picral, the mirror-polished surface of the specimen was observed and imaged at a field emission scanning electron microscope (FE-SEM: observation magnification was 2000 to 5000 times depending on the tissue size. A photograph of each observation point was taken and the image was analyzed using an image analyzer (Media Cybernetics: Image-Pro Plus) to determine the aspect ratio of spherical cementite and the aspect ratio. The proportion of spherical cementite of 3 or less was determined.

(Average distance between center of gravity of spherical cementite)
The test piece was mirror-polished in the same manner as described above, photographed for 10 fields of view at 2000 to 5000 times, and the average distance between the center of gravity was obtained by image analysis and averaged.

(Grain size of ferrite)
In the same measurement region (D / 4 position in the longitudinal section) as in the observation of the above metal structure, the ferrite grain size number N is obtained by a comparative method (JIS G0552 “Ferrite grain size test method for steel”) using an image analysis device. The particle diameter _Dα was determined from the equation.
D _α = 0.254 / [2 ^{(N−1) / 2} ] × 100

(Deformation resistance by compression test)
A cylindrical compression test piece having a diameter of 10 mm and a length of 15 mm was cut out from each of the test pieces and subjected to an end face high-speed compression test at room temperature for measurement. The strain rate during compression was 10 s ⁻¹ , the compression rate was 70%, and the deformation resistance value of the steel at 70% was measured. Further, it was visually confirmed whether or not cracks occurred in each test piece after 70% compression.

  From the above results, No. in Tables 3 and 4 satisfying the requirements of the present invention. Nos. 16 to 27, 31 to 43, 53 and 54 had reduced cold resistance, and had excellent cold forgeability without causing cracks. On the other hand, examples that do not satisfy the requirements of the present invention have the following problems.

  No. which does not satisfy the production conditions of the present invention (heating conditions such as heating temperature, holding temperature, cooling rate). 1-15, 28-30, 44-52, 55, 56, the requirements of the metal structure defined in the present invention (area ratio, proportion of spherical cementite with an aspect ratio of 3 or less, average distance between center of gravity of spherical cementite, ferrite grains (Diameter) was not satisfied, and the deformation resistance was inferior.

  In detail, No. which performed the heat processing only once. In 1-15, since the ratio of spherical cementite having an aspect ratio of 3 or less was only about 25 to 63%, the deformation resistance was high.

  No. which does not satisfy the component composition of the present invention. No. 28 has high deformation resistance. Nos. 29 and 30 did not satisfy the requirements of the component composition and metal structure specified in the present invention (the ratio of spherical cementite having an aspect ratio of 3 or less, the distance between the average centers of gravity of cementite) and the deformation resistance.

  Of the examples where the heat treatment was performed several times, No. No. 44 is an example in which the cooling rate after the first heating is 300 ° C./hr which is outside the scope of the present invention. In this example, the proportion of spherical cementite having an aspect ratio of 3 or less was low, and the average distance between the center of gravity of the spherical cementite was insufficient.

  No. No. 45 is an example in which the heating temperature during the heat treatment deviates from the temperature range of A1 point to A1 point + 50 ° C. and the cooling rate during the final heat treatment was slow. In this example, the proportion of spherical cementite having an aspect ratio of 3 or less was low, and the average distance between the center of gravity of the spherical cementite was insufficient. Moreover, the ferrite particle size was coarsened.

  No. No. 46 is an example in which the residence time in the heating temperature range (A1 point to A1 point + 30 ° C.) during the final heat treatment is short (8 minutes) and the cooling rate is too fast. In this example, the average distance between the center of gravity of the spheroidized cementite was insufficient.

  No. No. 47 is an example in which the residence time in the temperature range is longer than the heating temperature range (A1 point to A1 point + 30 ° C.) at the time of the final heat treatment. In this example, the ratio of spherical cementite having an aspect ratio of 3 or less was low, and the ferrite grain size was coarsened.

  No. No. 48 is an example in which the final heat treatment is performed without performing the heat treatment (pre-stage heat treatment) only once, and the cooling rate during the final heat treatment is high. In this example, the ratio of spherical cementite having an aspect ratio of 3 or less was low, and the average distance between the center of gravity of spherical cementite was insufficient.

  No. No. 49 is an example in which the residence time in the heating temperature range during the final heat treatment is long and the holding time in the temperature range after cooling (A1 point−100 ° C. to A1 point−20 ° C.) is short. In this example, the ratio of spherical cementite having an aspect ratio of 3 or less was low, and the average distance between the center of gravity of spherical cementite was insufficient.

  No. 50 is an example in which the heating temperature during the heat treatment was lower than the temperature range of A1 point to A1 point + 50 ° C. In this example, the ratio of spherical cementite having an aspect ratio of 3 or less was low, and the average distance between the center of gravity of spherical cementite was insufficient.

  No. No. 51 is an example in which the cooling temperature during the heat treatment was higher than the temperature range of A1 point−100 ° C. to A1 point−20 ° C. In this example, the ratio of spherical cementite having an aspect ratio of 3 or less was low, and the average distance between the center of gravity of spherical cementite was insufficient.

  No. No. 52 is an example in which the holding time in the temperature range after cooling at the time of final heat treatment (A1 point−100 ° C. to A1 point−20 ° C.) was short. In this example, the ratio of spherical cementite having an aspect ratio of 3 or less was low, and the average distance between the center of gravity of spherical cementite was insufficient.

  No. No. 55 is an example in which the residence time in the heating temperature range during the final heat treatment is short and the cooling rate is too fast. In this example, the ratio of spherical cementite having an aspect ratio of 3 or less was low.

  No. No. 56 is an example in which the final heat treatment is performed without performing the heat treatment (pre-stage heat treatment) only once, and the cooling rate during the final heat treatment is high. In this example, the ratio of spherical cementite having an aspect ratio of 3 or less was low.

  No. above. 44-52, 55, and 56 all had high deformation resistance.

  For reference, FIG. 2 shows the relationship between the deformation resistance and carbon equivalent of each steel in the above examples. When the steel component (carbon equivalent) is different, the standard deformation resistance value is also different, so the deformation resistance by component (carbon equivalent) is shown in FIG. As shown in FIG. 2, it can be seen that the steel of the present invention has a low deformation resistance. Carbon equivalent (Ceq) is “data sheet on deformation resistance and ductility of carbon steel wire for cold forging” Journal of the JSTP vol. 27 no. 304 (1986-5) This is a value calculated based on “C + 1/5 (Si + Mn) /%” described in FIG. 7 on page 571.

Claims (5)

C: 0.15-0.6% (meaning mass%, the same applies to chemical components below),
Si: 0.05 to 0.6%,
Mn: 0.1 to 1.5%
P: 0.05% or less (excluding 0%),
S: 0.001 to 0.05%,
Cr: 0.01 to 0.5%
Al: 0.01 to 0.1%,
N: 0.01% or less (excluding 0%)
And the balance consists of iron and inevitable impurities,
The steel metal structure has cementite and ferrite, the total area ratio of cementite and ferrite to the entire structure is 95 area% or more, the aspect ratio of 90% or more of the cementite is 3 or less, and A steel material excellent in cold forgeability, characterized in that the average center-to-center distance of cementite is 1.5 μm or more, and the average crystal grain size of the ferrite is 5 to 20 μm. As other elements,
Cu: 0.25% or less (excluding 0%),
2. The composition contains at least one element selected from the group consisting of Ni: 0.25% or less (not including 0%) and Mo: 0.25% or less (not including 0%). Steel materials described in 1. As other elements,
Ti: 0.2% or less (excluding 0%),
2. The composition contains at least one element selected from the group consisting of Nb: 0.2% or less (excluding 0%), and V: 1.5% or less (excluding 0%). Or the steel material of 2. As other elements,
Mg: 0.02% or less (excluding 0%),
Ca: 0.05% or less (excluding 0%),
2. At least one element selected from the group consisting of Li: 0.02% or less (not including 0%), and REM: 0.05% or less (not including 0%). The steel material in any one of -3. It is a manufacturing method of the steel materials in any one of Claims 1-4,
The steel satisfying the component composition according to any one of claims 1 to 4 is hot-worked, cooled to room temperature, and then heated to a temperature range of A1 point to A1 point + 50 ° C. After the temperature, the temperature range from A1 point to A1 point + 50 ° C. is maintained for 0 to 1 hr, and the temperature range from the A1 point to A1 point + 50 ° C. to the A1 point−100 ° C. to A1 point−30 ° C. After performing the annealing process which cools with the average cooling rate of 10-200 degreeC / hr twice or more,
When the temperature is raised from the A1 point to the A1 point + 30 ° C. and is cooled after being held in the temperature range of the A1 point to the A1 point + 30 ° C., the A1 point to the A1 after reaching the A1 point during the temperature rise. When cooling after maintaining in the temperature range of the point + 30 ° C., the residence time in the temperature range from the A1 point to the A1 point + 30 ° C. until reaching the A1 point is 10 minutes to 2 hours, and the A1 point to the A1 point + 30 ° C. After cooling the cooling temperature range from the temperature range A1 point -100 ° C to A1 point -20 ° C at an average cooling rate of 10 to 100 ° C / hr, the temperature is maintained for 10 minutes to 5 hours in the cooling temperature range. A method for producing a steel material excellent in cold forgeability characterized by further cooling.