WO2017033773A1 - 冷間加工用機械構造用鋼、およびその製造方法 - Google Patents
冷間加工用機械構造用鋼、およびその製造方法 Download PDFInfo
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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- the present disclosure relates to a machine structural steel for cold working and a manufacturing method thereof.
- the present invention relates to a steel for machine structure having low deformation resistance after spheroidizing annealing and excellent cold workability, and a useful method for producing the steel for machine structure.
- the machine structural steel for cold working of the present disclosure is suitably used for various parts such as automobile parts and construction machine parts manufactured by cold working such as cold forging, cold forging and cold rolling. It is done.
- the form of the steel is not particularly limited, and for example, a rolled wire is targeted.
- a wire is used in the meaning of a rolled wire and refers to a wire steel material cooled to room temperature after hot rolling.
- a steel wire refers to the linear steel material which performed the wire drawing and / or annealing etc. to the said rolling wire rod, and adjusted the characteristic.
- spheroidizing annealing treatment is usually applied to hot rolled wire rods such as carbon steel and alloy steel for the purpose of imparting cold workability. . Then, the steel wire after spheroidizing annealing is subjected to cold working, and then subjected to machining such as cutting to form a predetermined shape, and further subjected to quenching and tempering treatment, and final strength adjustment is performed.
- Patent Document 1 discloses a steel for cold-working machine structure that can be softened even by performing spheroidizing annealing for a relatively short time by controlling the metal structure before spheroidizing annealing, and its manufacture.
- a method is disclosed. Specifically, the total area ratio of pearlite and ferrite with respect to the entire structure is 95 area% or more, the ferrite area ratio is set to a predetermined value or more, and the bcc-Fe crystal grain size is controlled within an appropriate range.
- Structural steel is disclosed.
- the steel is cooled to a temperature range of 600 to 660 ° C. at an average cooling rate of 5 ° C./second or more, and then 1 ° C. It is disclosed that cooling is performed for 20 seconds or more at an average cooling rate of 10 seconds / second or less.
- Patent Document 2 discloses a steel wire material in which a metal structure contains a pro-eutectoid ferrite structure, a pearlite structure, and a bainite structure, and a manufacturing method thereof. It is disclosed that this steel wire can shorten the softening annealing time and can realize excellent cold forgeability after the softening annealing.
- a steel wire rod it is hot-rolled, wound, and then immersed in a molten salt bath at 500 ° C. or higher and 600 ° C. or lower for 10 seconds or longer, and then in a molten salt bath at 530 ° C. or higher and 600 ° C. or lower for 20 seconds or longer. It is disclosed that cooling is performed after holding at a constant temperature for 150 seconds or less.
- the ferrite grain size number is 9 or more
- the ferrite structure fraction is 30 area% or more
- the balance is pearlite, bainite, martensite, or a mixed structure thereof
- the bainite + martensite structure A hot-rolled wire for cold forging having a rate of 50% by area or more of the balance and a method for producing the same are disclosed.
- the method of manufacturing a hot-rolled wire rod for cold forging after finish rolling in the temperature range of Ar 3 point to Ar 3 point + 150 ° C, cooling between Ar 1 point and 300 ° C at a cooling rate of 5 to 40 ° C / sec. Is disclosed.
- JP 2013-7091 A Japanese Patent No. 5195209 Japanese Patent No. 4299744
- the techniques proposed so far are useful for shortening the spheroidizing annealing time, but it is hoped that a better spheroidized structure than the conventional technique can be obtained and the softening technique should be developed. It is rare.
- the embodiment of the present invention has been made under such circumstances, and its purpose is to make the spheroidization equivalent to or more than the conventional spheroidization even when the spheroidizing annealing time is made shorter than usual. It is intended to provide a steel for cold-working machine structure that can be made softer than before and a method for producing the same.
- the machine structural steel for cold working according to the embodiment of the present invention that can solve the above-mentioned problems is, by mass%, C: 0.07% or more and less than 0.3%, Si: 0.05 to 0.00. 5%, Mn: 0.2 to 1.7%, P: more than 0%, 0.03% or less, S: 0.001 to 0.05%, Al: 0.01 to 0.1%, and N :
- Each containing 0 to 0.015%, the balance consisting of iron and inevitable impurities, the metal structure of the steel contains pro-eutectoid ferrite and pearlite, and the total area ratio of pro-eutectoid ferrite and pearlite with respect to the whole structure is 90
- the area ratio Af of the pro-eutectoid ferrite satisfies the relationship of Af ⁇ A in relation to the A value represented by the following formula (1), and is equivalent to the average circle of bcc-Fe crystal grains.
- the diameter is 15 to 30 ⁇ m, and the average interval between pearlite lamellar is 0.20 ⁇ m or less.
- A (103 ⁇ 128 ⁇ [C (%)]) ⁇ 0.80 (%) (1)
- [C (%)] shows content of C in the mass%.
- the machine steel for cold working is further in mass%, Cr: more than 0%, 0.5% or less, Cu: more than 0%, 0.25% or less, Ni: Contains one or more selected from the group consisting of more than 0%, 0.25% or less, Mo: more than 0%, 0.25% or less, and B: more than 0%, 0.01% or less, and Formula (X) is satisfied.
- [Cr%], [Cu%], [Ni%], and [Mo%] respectively indicate the contents of Cr, Cu, Ni, and Mo expressed in mass%.
- the steel wire further contains, by mass%, Ti: more than 0% and 0.1% or less.
- finish rolling is performed at 950 ° C. or more and 1150 ° C. or less, and then the average cooling rate is 3 to the first cooling end temperature of 700 to 750 ° C.
- the steel for machine structural use for cold working appropriately adjusts the chemical composition, and makes the total area ratio of pro-eutectoid ferrite and pearlite and the area ratio of pro-eutectoid ferrite with respect to the whole structure equal to or more than a predetermined value.
- -Centered cubic, body-centered cubic lattice)-Average equivalent circle diameter of Fe crystal grains hereinafter sometimes referred to simply as "bcc-Fe average grain size”
- pearlite lamellar spacing are in appropriate ranges. .
- the machine structural steel for cold working according to the present disclosure has low deformation resistance when steel is processed into the above-mentioned various parts at room temperature or in a heat generating region after spheroidizing annealing, and the working mold and steel Since cracking of the (material) is suppressed, excellent cold workability can be exhibited.
- FIG. 1 is an explanatory diagram for illustrating a method for measuring a pearlite lamellar interval.
- the inventors can obtain a spheroidized structure equivalent to or higher than the conventional one even when the spheroidizing annealing time is shorter than usual (hereinafter referred to as “short-time spheroidizing annealing”).
- short-time spheroidizing annealing In order to realize a machine structural steel for cold working that can be softened more than before, we examined it from various angles. As a result, it has been found that softening of steel can be achieved by increasing the ferrite grain size and expanding the average interparticle distance of carbides in the metal structure (spheroidized structure) of the steel wire after spheroidizing annealing. It was.
- pre-structure control of the metal structure before spheroidizing annealing
- the pre-structure is made a structure mainly composed of pro-eutectoid ferrite and pearlite, and then the area ratio of pro-eutectoid ferrite is increased as much as possible.
- the bcc-Fe crystal grains are controlled to be coarser than before, and the interval between the pearlite lamellars may be set to a predetermined value or less, and the steel having such a pre-structure may be subjected to short-time spheroidizing annealing,
- the inventors have found that a spheroidized structure equivalent to or higher than that of the conventional spheroidized structure can be obtained, and that the spheroidized structure can be softened more than before, and the embodiment of the present invention has been completed.
- the steel microstructure of the embodiment of the present invention contains proeutectoid ferrite and pearlite.
- These structures are metal structures that contribute to the improvement of cold workability by reducing the deformation resistance of steel after spheroidizing annealing.
- the desired softening cannot be achieved simply by forming a metal structure containing proeutectoid ferrite and pearlite. Therefore, as described below, it is necessary to appropriately control the area ratio of these structures, the average particle diameter of bcc-Fe crystal grains, and the like.
- Total area ratio of pro-eutectoid ferrite and pearlite 90% or more
- bainite after spheroidizing annealing And / or the structure becomes locally fine due to the influence of martensite, and the softening of the steel becomes insufficient.
- the total area ratio of pro-eutectoid ferrite and pearlite with respect to the entire structure needs to be 90% or more.
- the total area ratio of pro-eutectoid ferrite and pearlite is preferably 95% or more, more preferably 97% or more, and most preferably 100%.
- metal structures other than proeutectoid ferrite and pearlite include martensite, bainite, and austenite. As described above, when the area ratio of these structures such as martensite increases, the strength of the steel increases. Therefore, these structures may not be included at all.
- the steel may contain carbides, nitrides, oxides, and / or sulfides other than cementite as other structure factors.
- Average equivalent circle diameter of bcc-Fe crystal grains 15-30 ⁇ m
- the average equivalent circular diameter of bcc-Fe crystal grains in the steel front structure that is, the bcc-Fe average particle diameter is set to 30 ⁇ m or less
- a good spheroidizing structure that is, the degree of spheroidization
- a small spheroidized structure is obtained.
- the average bcc-Fe particle diameter exceeds 30 ⁇ m, the spheroidized structure deteriorates (that is, the degree of spheroidization increases) by short-time spheroidizing annealing, and the desired spheroidized structure cannot be obtained.
- the bcc-Fe average particle diameter is preferably 29 ⁇ m or less, more preferably 28 ⁇ m or less.
- the bcc-Fe average particle diameter was set to 15 ⁇ m or more.
- the bcc-Fe average particle diameter is preferably 16 ⁇ m or more, and more preferably 17 ⁇ m or more.
- the circle equivalent diameter of a crystal grain means the diameter of a circle having the same area as each crystal grain.
- the structure to be controlled by the above-mentioned average bcc-Fe grain size is bcc-Fe crystal grains surrounded by a large-angle grain boundary in which the orientation difference between two adjacent crystal grains is larger than 15 °.
- the structure also includes small-angle grain boundaries having an orientation difference of 15 ° or less. However, these small-angle grain boundaries have little influence on the spheroidized structure obtained after spheroidizing annealing. In order to obtain a desired spheroidized structure after spheroidizing annealing, it is necessary to control the large-angle grain boundary of the structure before spheroidizing annealing.
- a good spheroidized structure that is, a spheroidized structure having a small degree of spheroidization
- a good spheroidized structure can be achieved even with short-time spheroidizing annealing.
- the above-mentioned “azimuth difference” is also referred to as “deviation angle” or “bevel angle”, and the EBSP method (Electron Back Scattering Pattern method) may be employed for measuring the orientation difference.
- bcc-Fe is intended to include ferrite contained in the pearlite structure in addition to pro-eutectoid ferrite.
- the metal structure of the steel of the embodiment of the present invention has proeutectoid ferrite and pearlite as described above.
- the interval between pearlite lamellar is narrowed (that is, the pearlite lamellar is refined)
- spheroidization of carbides mainly cementite in pearlite
- the interval between the pearlite lamellars in the previous tissue must be 0.20 ⁇ m or less on average (hereinafter simply referred to as “average lamellar interval”).
- the average lamellar spacing is preferably 0.18 ⁇ m or less, and more preferably 0.16 ⁇ m or less.
- the lower limit of the average lamellar interval is not particularly limited, but is usually about 0.05 ⁇ m.
- the “perlite lamellar spacing” refers to the distance between adjacent lamellar cementite layers. More precisely, it is the shortest distance from the center position of the thickness of the lamellar cementite layer to the center position of the thickness of the adjacent lamellar cementite layer.
- Proeutectoid ferrite area ratio Af ⁇ A Furthermore, when the area ratio of pro-eutectoid ferrite increases in the previous structure, the number of carbide precipitation sites during spheroidizing annealing decreases, the number density of carbides decreases, and the coarsening of the carbides is promoted. Thereby, the distance between the particles of the carbide is increased, and the metal structure can be further softened.
- the area ratio of pro-eutectoid ferrite varies depending on the carbon content. As the amount of carbon increases, the pro-eutectoid ferrite area ratio decreases.
- the appropriate pro-eutectoid ferrite area ratio for obtaining a good spheroidizing material also varies depending on the carbon content. As the amount of carbon increases, the area ratio of suitable pro-eutectoid ferrite decreases.
- the area ratio Af of pro-eutectoid ferrite with respect to the entire structure satisfies the relationship of A value represented by the following formula (1) and Af ⁇ A. Thus, it has been found that further softening can be achieved.
- A (103 ⁇ 128 ⁇ [C (%)]) ⁇ 0.80 (%) (1)
- [C (%)] shows content of C in the mass%.
- Af is preferably (103 ⁇ 128 ⁇ [C (%)]) ⁇ 0.85 or more, more preferably (103 ⁇ 128 ⁇ [C (%)]) ⁇ 0.90 or more.
- the upper limit of Af is not particularly limited. However, if Af is increased, the manufacturing cost increases. Therefore, considering productivity, Af is preferably (103 ⁇ 128 ⁇ [C (%)]) ⁇ 0.97 or less.
- An embodiment of the present invention is a machine structural steel for cold working, and its steel type may be any steel having a normal chemical composition as a steel for cold working mechanical structure, but C, Si, Mn, About P, S, Al, and N, it adjusts to the following suitable ranges.
- “%” for the chemical component composition means mass%.
- C 0.07% or more and less than 0.3%
- C is an element useful for securing the strength of steel, that is, the strength of the final product.
- the C content needs to be 0.07% or more.
- the C content is preferably 0.09% or more, more preferably 0.11% or more.
- the C content is preferably 0.28% or less, and more preferably 0.26% or less.
- Si 0.05 to 0.5% Si is useful as a deoxidizing element and as an element for improving the strength of the final product by solid solution hardening.
- the Si content was set to 0.05% or more.
- the Si content is preferably 0.07% or more, and more preferably 0.10% or more.
- the Si content is set to 0.5% or less.
- the Si content is preferably 0.45% or less, more preferably 0.40% or less.
- Mn 0.2 to 1.7%
- Mn is an effective element for increasing the strength of the final product through improvement of hardenability.
- the Mn content is set to 0.2% or more.
- the Mn content is preferably 0.3% or more, and more preferably 0.4% or more.
- the Mn content is set to 1.7% or less.
- the Mn content is preferably 1.5% or less, and more preferably 1.3% or less.
- P more than 0% and 0.03% or less
- P is an element inevitably contained in steel, causes segregation of grain boundaries in steel, and causes ductility deterioration. Therefore, the P content is set to 0.03% or less.
- the P content is preferably 0.02% or less, more preferably 0.017% or less, and still more preferably 0.01% or less.
- S 0.001 to 0.05%
- S is an element inevitably contained in the steel, and is present as MnS in the steel and deteriorates the ductility. Therefore, S is an element harmful to cold workability. Therefore, the S content is set to 0.05% or less.
- the S content is preferably 0.04% or less, and more preferably 0.03% or less. However, since S has an effect of improving machinability, it is useful to contain 0.001% or more.
- the S content is preferably 0.002% or more, and more preferably 0.003% or more.
- Al 0.01 to 0.1%
- Al is useful as a deoxidizing element and is useful for fixing solute N present in steel as AlN.
- the Al content is determined to be 0.01% or more.
- the Al content is preferably 0.013% or more, and more preferably 0.015% or more.
- the Al content is determined to be 0.1% or less. Al content becomes like this. Preferably it is 0.090% or less, More preferably, it is 0.080% or less.
- N 0 to 0.015%
- N is an element inevitably contained in the steel.
- the N content is preferably 0.013% or less, and more preferably 0.010% or less. The smaller the N content is, the more preferable it is, and the most preferable is 0%. However, there may be a case where approximately 0.001% remains due to restrictions on the manufacturing process.
- the balance is substantially iron.
- substantially iron means that the presence of trace components such as Sb and Zn, which do not inhibit the characteristics of the present disclosure other than iron, can be allowed, and other than P, S, and N. For example, it means that inevitable impurities such as O and H may be included.
- the following optional elements may be selectively contained as necessary. Depending on the type of optional element (selected component) selected, the properties of the steel can be further improved.
- Cr more than 0%, 0.5% or less
- Cu more than 0%, 0.25% or less
- Ni more than 0%, 0.25% or less
- Mo more than 0%, 0.25% or less
- B One or more selected from the group consisting of more than 0% and less than 0.01% Cr, Cu, Ni, Mo and B all increase the strength of the final product by improving the hardenability of the steel. It is an effective element. If necessary, these elements may be contained alone or in combination of two or more. Such effects increase as the content of these elements increases.
- a preferable content for effectively exhibiting the above-described effect is such that the Cr content is 0.015% or more, more preferably 0.020% or more.
- the preferable contents of Cu, Ni and Mo are all 0.02% or more, more preferably 0.05% or more.
- the preferable content of B is 0.0003% or more, more preferably 0.0005% or more.
- the Cr content is preferably 0.5% or less, and the Cu, Ni and Mo contents are preferably 0.25% or less.
- a more preferable content of Cr is 0.45% or less, and further preferably 0.40% or less.
- the more preferable contents of the Cu content, the Ni content, and the Mo content are all 0.22% or less, and more preferably 0.20% or less.
- the B content is preferably 0.01% or less.
- a more preferable content of the B content is 0.007% or less, and further preferably 0.005% or less.
- Ti more than 0% and 0.1% or less Ti forms a compound with N and reduces the solid solution N, thereby exhibiting the softening effect. Therefore, if necessary, Ti may be contained.
- a preferable Ti content for effectively exhibiting such an effect is 0.01 or more, more preferably 0.02 or more. However, when the Ti content is excessive, the formed compound causes an increase in hardness. Therefore, the preferable Ti content is 0.08% or less, more preferably 0.05 or less.
- the steel that satisfies the above-described component composition is adjusted to the finish rolling temperature when hot-rolling, and the subsequent cooling rate is adjusted. It is preferable to appropriately adjust the cooling rate and the temperature range as two stages.
- Finish rolling at 950 ° C. or higher and 1150 ° C. or lower
- a first cooling for cooling at an average cooling rate of 3 ° C./second or less from a temperature of 950 ° C. to 1150 ° C. to a first cooling end temperature of 700 to 750 ° C.
- the second cooling is performed in this order from the first cooling end temperature to the temperature range of at least 600 ° C. at an average cooling rate of 5 to 30 ° C./second.
- the finish rolling temperature, the first cooling, and the second cooling will be described in detail.
- the finish rolling temperature is preferably 1150 ° C. or lower.
- the finish rolling temperature is preferably 970 ° C. or higher, more preferably 990 ° C. or higher.
- the finish rolling temperature is preferably 1130 ° C or lower, more preferably 1110 ° C or lower.
- the first cooling starts from the finish rolling temperature of 950 ° C. or more and 1150 ° C. or less, and the first cooling end temperature of 700 to 750 ° C. End with.
- the average cooling rate of the first cooling is set to 3 ° C./second or less.
- the average cooling rate of the first cooling is preferably 2.5 ° C./second or less, more preferably 2 ° C./second or less.
- the lower limit of the average cooling rate of the first cooling is not particularly limited. However, as a practical range, it is preferably 0.01 ° C./second or more. In the first cooling, the cooling rate may be changed as long as the average cooling rate is 3 ° C./second or less.
- the second cooling starts from a temperature range of 700 to 750 ° C. and ends at least at 600 ° C. In the second cooling, if the average cooling rate is slower than 5 ° C./second, it becomes difficult to set the average lamellar spacing of pearlite to 0.20 ⁇ m or less.
- the average cooling rate of the second cooling is preferably 7 ° C./second or more, more preferably 10 ° C./second or more.
- the average cooling rate of the second cooling is preferably 28 ° C./second or less, more preferably 25 ° C./second or less. In the second cooling, the cooling rate may be changed as long as the average cooling rate is 5 to 30 ° C./second.
- the end temperature of the second cooling for cooling at the above average cooling rate is 600 ° C. at the maximum.
- the reason for setting “600 ° C.” is that the average lamella spacing of pearlite and the total area ratio of pro-eutectoid ferrite and pearlite defined in the present disclosure are substantially determined in the cooling process up to 600 ° C. This is because it is hardly influenced by the cooling rate after °C. Therefore, the end temperature of the second cooling is not limited to 600 ° C., and may be room temperature as in an example described later. Alternatively, for example, the end temperature of the second cooling may be set to 400 ° C., and then normal cooling such as cooling may be performed to cool to room temperature. In general, the average cooling rate during cooling is often slower than the average cooling rate of the second cooling described above.
- spheroidizing annealing for a short time for example, spheroidizing annealing for about 1 to 3 hours in a temperature range of about Ac 1 to Ac 1 + 30 ° C.
- the spheroidization degree can be made equal to or lower than the target spheroidization degree described later, and the hardness can be made lower than the target hardness described later.
- Ac 1 is a value calculated from the following equation. In the following formula, (% element name) means the content of each element in mass%.
- Ac 1 (° C.) 723-10.7 (% Mn) ⁇ 16.9 (% Ni) +29.1 (% Si) +16.9 (% Cr)
- Embodiments of the present invention are not limited by the following examples, and can be implemented with modifications within a range that can be adapted to the spirit of the present disclosure described above and below, all of which are within the technical scope of the present disclosure. Is included.
- Rolling was performed using steel having the chemical composition shown in Table 1 to obtain a wire having a diameter of ⁇ 17.0 mm, and further, a test piece for processing for master having a diameter of ⁇ 8.0 mm ⁇ 12.0 mm was obtained by machining.
- a machining heat treatment test was carried out with a machining for master testing machine under the conditions shown in Table 2.
- the processing conditions listed in Table 2 simulate the rolling conditions in an actual machine. In Table 2, the processing temperature corresponds to the finish rolling temperature.
- test piece after performing the heat treatment test under the conditions shown in Table 2 was evaluated according to the following procedures (1) to (3).
- the spheroidizing and hardness of the test piece that was further spheroidized and annealed were measured as described in (4) and (5) below.
- the test piece after the heat treatment or spheroidizing annealing was cut along a plane (axial center cross section) passing through the central axis of the test piece and parallel to the central axis.
- the cut specimen may be referred to as a “longitudinal section sample”.
- the vertical cross section sample was resin-filled so that the axial center cross section of the test piece could be observed.
- FIG. 1A is a schematic diagram of a pearlite lamellar structure 1
- FIG. 1B is an enlarged view of the pearlite lamellar structure 1.
- FIG. The pearlite lamellar structure 1 is a structure in which lamellar ferrite 3 and lamellar cementite 2 are arranged in layers (lamellar shape) as shown in FIG.
- the lamellar spacing defined in the present disclosure is the lamellar cementite 2 spacing.
- the structure was revealed by picral etching of a longitudinal section sample whose mirror surface was mirror polished. Thereafter, tissue observation at the D / 4 position was performed using FE-SEM, and a total of 5 fields were photographed in a region of 42 ⁇ m ⁇ 28 ⁇ m at a magnification of 3000 ⁇ or a region of 25 ⁇ m ⁇ 17 ⁇ m at a magnification of 5000 ⁇ . At this time, at least one perlite was included in each field of view. The perlite with the finest lamellar spacing (that is, the narrowest lamellar spacing) in each field of view of the photograph taken was selected and used as the measurement object.
- the length L of the line segment 4 and the number n of lamellar cementite 2 included in the line segment 4 were measured.
- the number n includes lamellar cementite where the start and end of the line segment are located.
- the lamellar interval ⁇ was calculated from the length L and the number n using Equation (2).
- the lamellar interval ⁇ was obtained for each visual field, and the average value of the five visual fields was calculated.
- the line segment 4 is drawn so that the number n of lamellar cementite 2 intersecting the line segment 4 is 5 or more.
- ⁇ L / (n ⁇ 1) (2)
- a sample having a martensite structure precipitated in the metal structure and a total area ratio of pro-eutectoid ferrite and pearlite of less than 90% was not measured because it was difficult to calculate lamellar spacing.
- Example A processing formaster test was performed using the steel types A to U shown in Table 1 above and changing the processing temperature (corresponding to the finish rolling temperature) and the cooling rate as shown in Table 2 below. Thereby, the processing for master test piece which has a different front organization was produced, respectively.
- Steel type O has an amount of Mn exceeding 1.7%, which is outside the scope of the present invention.
- steel type P the amount of Ti exceeds 0.1%, which is outside the scope of the present invention.
- [Cr%] + [Cu%] + [Ni%] + [Mo%] is 0.75 mass% or less and satisfies the above-described formula (X). Yes.
- [Cr%] + [Cu%] + [Ni%] + [Mo%] exceeds 0.75 mass% and does not satisfy the formula (X).
- first cooling starts from the processing temperature, ends in a temperature range of 700 to 750 ° C. that is the first cooling end temperature
- second cooling The cooling starts from the first cooling end temperature and ends at room temperature.
- No. 10, 20, and 44 are cooling at a constant average cooling rate from the processing temperature at the start of the first cooling to the end temperature of the second cooling, and therefore distinguish between “first cooling” and “second cooling”.
- No. No. 44 was cooled in the range from 850 ° C. to 300 ° C. at an average cooling rate of 40.0 ° C./second, and then allowed to cool to room temperature.
- the end temperature of “first cooling” was set to 650 ° C.
- the end temperature of “second cooling” was set to 550 ° C., and then cooled to room temperature.
- the processed formaster specimen was divided into four equal parts in a cross section orthogonal to the central axis. One of them was used as a sample for structure investigation, and the other was used as a sample for spheroidizing annealing.
- Spheroidizing annealing was performed by vacuum-sealing each test piece and performing heat treatment in an atmospheric furnace. The spheroidizing annealing was carried out at 730 ° C. for 2 hours, then cooled to 710 ° C. at an average cooling rate of 30 ° C./hour, then cooled to 680 ° C. at an average cooling rate of 10 ° C./hour, and then allowed to cool.
- Table 3 shows the structure before spheroidizing annealing, the degree of spheroidizing after spheroidizing annealing, and the hardness evaluated in the manner of (1) to (5) above.
- the required degree of spheroidization depends on the C content. Therefore, the target spheroidization degree (described as “target spheroidization degree” in Table 3) was a value obtained by the following equation (3).
- the required hardness varies depending on the C, Si and Mn contents. Therefore, the target hardness (described as “target hardness” in Table 3) was a value obtained by the following equation (4).
- Target degree of spheroidization 5 ⁇ [C%] + 1.5 (3)
- Target hardness 88.4 ⁇ Ceq + 86.0 (4)
- Ceq [C%] + 0.2 ⁇ [Si%] + 0.2 ⁇ [Mn%], and [C%], [Si%] and [Mn%] are C and Si in mass%, respectively. And the Mn content.
- No. 9 used the steel type A in Table 1 that satisfies the composition of the embodiment of the present invention, but the processing temperature corresponding to the finish rolling temperature was low. Therefore, the average particle diameter of bcc-Fe has been reduced, and the hardness after spheroidizing annealing has been hard.
- No. No. 10 used the steel type A in Table 1 that satisfies the composition of the embodiment of the present invention, but the cooling rate of the second cooling was slow. For this reason, the pearlite average lamellar spacing was increased, and the degree of spheroidization after spheroidizing annealing was poor.
- No. No. 11 used the steel type A in Table 1 that satisfies the composition of the embodiment of the present invention, but the cooling rate of the first cooling was fast. For this reason, the area ratio of pro-eutectoid ferrite has decreased, and the hardness after spheroidizing annealing has been hard.
- No. No. 12 used the steel type A in Table 1 that satisfies the composition of the embodiment of the present invention, but the processing temperature was high. Therefore, the average particle diameter of bcc-Fe was increased, and the degree of spheroidization after spheroidizing annealing was increased (that is, the spheroidized structure was poor).
- No. No. 14 used the steel type B in Table 1 that satisfies the composition of the embodiment of the present invention, but the cooling rate of the second cooling was slow. Therefore, the pearlite average lamellar interval was increased, and the degree of spheroidization after spheroidizing annealing was increased (that is, the spheroidized structure was poor).
- No. No. 18 used the steel type D in Table 1 that satisfies the composition of the embodiment of the present invention, but the processing temperature was high. Therefore, the average particle diameter of bcc-Fe was increased, and the degree of spheroidization after spheroidizing annealing was increased (that is, the spheroidized structure was poor).
- No. No. 20 used the steel type E in Table 1 that satisfies the composition of the embodiment of the present invention, but the cooling rate of the first cooling was fast. For this reason, the area ratio of pro-eutectoid ferrite has decreased, and the hardness after spheroidizing annealing has been hard.
- No. No. 22 used the steel type F in Table 1 that satisfies the composition of the embodiment of the present invention, but the cooling rate of the second cooling was fast. Therefore, a martensite structure was precipitated, and the total area ratio of pro-eutectoid ferrite and pearlite and the area ratio of pro-eutectoid ferrite were reduced. As a result, the hardness after spheroidizing annealing has been hard.
- No. No. 25 used the steel type H in Table 1 that satisfies the composition of the embodiment of the present invention, but the processing temperature was low. Therefore, the average particle diameter of bcc-Fe has been reduced, and the hardness after spheroidizing annealing has been hard.
- No. No. 28 used the steel type I in Table 1 that satisfies the composition of the embodiment of the present invention, but the cooling rate of the first cooling was fast. For this reason, the area ratio of pro-eutectoid ferrite has decreased, and the hardness after spheroidizing annealing has been hard.
- No. No. 33 used the steel type L in Table 1 that satisfies the composition of the embodiment of the present invention, but the cooling rate of the first cooling was fast. For this reason, the area ratio of pro-eutectoid ferrite has decreased, and the hardness after spheroidizing annealing has been hard.
- No. No. 34 used the steel type L in Table 1 that satisfies the composition of the embodiment of the present invention, but the cooling rate of the second cooling was fast. Therefore, a martensitic structure was precipitated, and the total area ratio of pro-eutectoid ferrite and pearlite and the area of pro-eutectoid ferrite were reduced. As a result, the hardness after spheroidizing annealing has been hard.
- No. No. 44 used the steel type R in Table 1 that satisfies the composition of the embodiment of the present invention, but the processing temperature was low, the cooling rate of the first cooling was fast, and the cooling rate of the second cooling was also fast.
- the average particle size of bcc-Fe was reduced, the area ratio of pro-eutectoid ferrite was reduced, and the martensite structure was precipitated to reduce the total area ratio of pro-eutectoid ferrite and pearlite.
- the hardness after spheroidizing annealing has been hard.
- Japanese Patent Application No. 2015-166030 and a Japanese patent application filed on June 23, 2016, Japanese Patent Application No. 2016-124959. Accompanied by claiming priority as a basic application. Japanese Patent Application No. 2015-166030 and Japanese Patent Application No. 2016-124959 are incorporated herein by reference.
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Abstract
Description
A=(103-128×[C(%)])×0.80 (%) ・・・(1)
但し、上記式(1)中、[C(%)]は、質量%でCの含有量を示す。
[Cr%]+[Cu%]+[Ni%]+[Mo%]≦0.75 ・・・(X)
但し、[Cr%]、[Cu%]、[Ni%]および[Mo%]は、それぞれ、質量%で示したCr、Cu、NiおよびMoの含有量を示す。
鋼の前組織にベイナイトおよびマルテンサイト等の微細な組織が多い場合には、一般的な球状化焼鈍を行っても、球状化焼鈍後はベイナイトおよび/またはマルテンサイトの影響によって組織が局部的に微細となり、鋼の軟質化が不十分となる。こうした観点から、鋼を十分に軟質化するためには、全組織に対する初析フェライトとパーライトの合計面積率は90%以上とする必要がある。初析フェライトとパーライトの合計面積率は、好ましくは95%以上であり、より好ましくは97%以上であり、最も好ましくは100%である。なお、初析フェライトとパーライト以外の金属組織としては、マルテンサイト、ベイナイトおよびオーステナイトが挙げられる。前述の通り、マルテンサイト等のこれら組織の面積率が高くなると鋼の強度が高くなるため、これらの組織は全く含まれていなくても良い。鋼は、他の組織因子としてセメンタイト以外の炭化物、窒化物、酸化物、および/または硫化物を含有してよい。
鋼の前組織におけるbcc-Fe結晶粒の平均円相当直径、即ちbcc-Fe平均粒径を30μm以下にしておくと、短時間球状化焼鈍後にも良好な球状化組織(つまり、球状化度の小さい球状化組織)が得られる。bcc-Fe平均粒径が30μmを超えると、短時間球状化焼鈍では球状化組織が劣化し(つまり、球状化度が大きくなり)、所望の球状化組織が得られない。bcc-Fe平均粒径は、好ましくは29μm以下であり、より好ましくは28μm以下である。しかしながら、前組織におけるbcc-Fe平均粒径が小さくなり過ぎると、球状化焼鈍後のフェライト結晶粒径の微細化により強化され、鋼の軟質化が困難となる。そこで、bcc-Fe平均粒径を15μm以上とした。bcc-Fe平均粒径は、好ましくは16μm以上であり、より好ましくは17μm以上である。なお、結晶粒の円相当直径とは、各結晶粒と同一面積を有する円の直径を意味する。
本発明の実施形態の鋼の金属組織は、前述した通り、初析フェライトとパーライトを有する。パーライトラメラーの間隔を狭くする(つまり、パーライトラメラーを微細化する)と、短時間球状化焼鈍でも炭化物(主に、パーライト中のセメンタイト)の球状化が促進され、良好な球状化組織が得られる。こうした観点から、前組織におけるパーライトラメラーの間隔は平均で(以下、単に「平均ラメラー間隔」と呼ぶ)0.20μm以下とする必要がある。平均ラメラー間隔は、好ましくは0.18μm以下であり、より好ましくは0.16μm以下である。平均ラメラー間隔の下限は特に限定されないが、通常0.05μm程度である。
更に、前組織において、初析フェライトの面積率が増加すると、球状化焼鈍中の炭化物析出サイトが減少することにより炭化物の数密度が減少して、炭化物の粗大化が促進される。これにより、炭化物の粒子間距離が広くなり、金属組織を更に軟質化することができる。一方、初析フェライトの面積率は、含有炭素量に影響を受けて変化する。炭素量が増加すると、初析フェライト面積率は減少する。同様に、良好な球状化材を得るための適切な初析フェライト面積率も、含有炭素量に応じて変化する。炭素量が多いほど、適切な初析フェライトの面積率は減少する。こうした観点から数多くの実験結果を解析したところ、前組織において、全組織に対する初析フェライトの面積率Afが、下記式(1)で表されるA値と、Af≧Aの関係を満足することによって、更なる軟質化を図ることができることを見出した。
A=(103-128×[C(%)])×0.80(%) ・・・(1)
但し、上記式(1)中、[C(%)]は、質量%でCの含有量を示す。
Afは、好ましくは(103-128×[C(%)])×0.85以上であり、より好ましくは(103-128×[C(%)])×0.90以上である。なお、上記観点からはAfの上限は特に限定されない。しかしながら、Afを高くすると製造コストが増加するため、生産性を考慮すると、Afは(103-128×[C(%)])×0.97以下であることが好ましい。
Cは、鋼の強度、即ち最終製品の強度を確保する上で有用な元素である。こうした効果を有効に発揮させるため、C含有量は0.07%以上とする必要がある。C含有量は、好ましくは0.09%以上であり、より好ましくは0.11%以上である。しかしながら、Cが過剰に含有されると強度が高くなって冷間加工性が低下するので、0.3%未満とする必要がある。C含有量は、好ましくは0.28%以下であり、より好ましくは0.26%以下である。
Siは、脱酸元素として、および固溶体硬化による最終製品の強度向上元素として有用である。このような効果を有効に発揮させるため、Si含有量を0.05%以上と定めた。Si含有量は、好ましくは0.07%以上であり、より好ましくは0.10%以上である。一方、Siが過剰に含有されると硬度が過度に上昇して冷間加工性を劣化させる。そこでSi含有量を0.5%以下と定めた。Si含有量は、好ましくは0.45%以下であり、より好ましくは0.40%以下である。
Mnは、焼入れ性の向上を通じて、最終製品の強度を増加させるのに有効な元素である。このような効果を有効に発揮させるため、Mn含有量を0.2%以上と定めた。Mn含有量は、好ましくは0.3%以上であり、より好ましくは0.4%以上である。一方、Mnが過剰に含有されると硬度が上昇して冷間加工性を劣化させる。そこでMn含有量を1.7%以下と定めた。Mn含有量は、好ましくは1.5%以下であり、より好ましくは1.3%以下である。
Pは、鋼中に不可避的に含まれる元素であり、鋼中で粒界偏析を起こし、延性の劣化の原因となる。そこで、P含有量は0.03%以下と定めた。P含有量は、好ましくは0.02%以下であり、より好ましくは0.017%以下、更に好ましくは0.01%以下である。P含有量は少なければ少ない程好ましく、0%であることが最も好ましいが、製造工程上の制約などにより0.001%程度残存してしまう場合もある。
Sは、鋼中に不可避的に含まれる元素であり、鋼中でMnSとして存在して延性を劣化させるので、冷間加工性に有害な元素である。そこでS含有量を0.05%以下と定めた。S含有量は、好ましくは0.04%以下であり、より好ましくは0.03%以下である。但し、Sは被削性を向上させる作用を有するので、0.001%以上含有させることは有用である。S含有量は、好ましくは0.002%以上であり、より好ましくは0.003%以上である。
Alは、脱酸元素として有用であると共に、鋼中に存在する固溶NをAlNとして固定するのに有用である。こうした効果を有効に発揮させるため、Al含有量を0.01%以上と定めた。Al含有量は、好ましくは0.013%以上であり、より好ましくは0.015%以上である。しかしながら、Al含有量が過剰になると、Al2O3が過剰に生成し、冷間加工性を劣化させる。そこでAl含有量を0.1%以下と定めた。Al含有量は、好ましくは0.090%以下であり、より好ましくは0.080%以下である。
Nは、鋼中に不可避的に含まれる元素であり、鋼中に固溶Nが含まれると、歪み時効による硬度上昇、延性低下を招き、冷間加工性を劣化させる。そこでN含有量を0.015%以下と定めた。N含有量は、好ましくは0.013%以下であり、より好ましくは0.010%以下である。N含有量は少なければ少ない程好ましく0%であることが最も好ましいが、製造工程上の制約などにより0.001%程度残存してしまう場合もある。
Cr、Cu、Ni、MoおよびBは、いずれも鋼材の焼入れ性を向上させることによって最終製品の強度を増加させるのに有効な元素である。必要によって、それらの元素を単独で又は2種以上で含有してもよい。このような効果は、これら元素の含有量が増加するに従って大きくなる。前記した効果を有効に発揮させるための好ましい含有量は、Cr量が0.015%以上、より好ましくは0.020%以上である。Cu量、Ni量およびMo量の好ましい含有量は、いずれも0.02%以上、より好ましくは0.05%以上である。B量の好ましい含有量は、0.0003%以上、より好ましくは0.0005%以上である。
本発明の実施形態に係る鋼線は、Cr、Cu、NiおよびMoの1種以上を上述した範囲で含有する場合、下記式(X)を満足することが好ましい。
[Cr%]+[Cu%]+[Ni%]+[Mo%]≦0.75 ・・・(X)
但し、[Cr%]、[Cu%]、[Ni%]および[Mo%]は、それぞれ、質量%で示したCr、Cu、NiおよびMoの含有量を示す。
Cr、Cu、NiおよびMoの含有量が上記式(X)を満たすことにより、鋼の強度が高くなり過ぎることを抑制して、冷間加工性を向上できる。
Tiは、Nと化合物を形成し、固溶Nを低減することで、軟質化の効果を発揮する。よって必要によって、Tiを含有してもよい。このような効果を有効に発揮させるための好ましいTi含有量は、0.01以上、より好ましくは0.02以上である。しかしながら、Tiの含有量が過剰になると、形成される化合物が硬さ増加を招く。そこで、好ましいTi含有量は0.08%以下、より好ましくは0.05以下である。
950℃以上、1150℃以下で仕上圧延し、その後、
950℃以上、1150℃以下から700~750℃の第1冷却終了温度まで平均冷却速度:3℃/秒以下で冷却する第1冷却と、
前記第1冷却終了温度から少なくとも600℃の温度範囲まで平均冷却速度:5~30℃/秒で冷却する第2冷却とをこの順で行う。
仕上圧延温度、第1冷却および第2冷却について、それぞれ詳しく説明する。
bcc-Fe平均粒径を15~30μmにするためには、仕上圧延温度を適切に制御する必要がある。仕上圧延温度が1150℃を超えると、bcc-Fe平均粒径を30μm以下にすることが困難となる。よって、仕上圧延温度は1150℃以下とすることが好ましい。但し、仕上圧延温度が950℃未満となると、bcc-Fe平均粒径を15μm以上にすることが困難となる。よって、仕上圧延温度は950℃以上とすることが好ましい。仕上圧延温度は、好ましくは970℃以上であり、より好ましくは990℃以上である。仕上圧延温度は、好ましくは1130℃以下であり、より好ましくは1110℃以下である。
第1冷却の平均冷却速度:3℃/秒以下
第1冷却は、仕上圧延温度である950℃以上、1150℃以下から開始し、700~750℃の第1冷却終了温度で終了する。第1冷却において、冷却速度が速くなると初析フェライト面積率Afが小さくなり、Af≧Aの関係が満足出来なくなる可能性がある。そこで、第1冷却の平均冷却速度を3℃/秒以下とする。第1冷却の平均冷却速度は好ましくは2.5℃/秒以下であり、より好ましくは2℃/秒以下である。第1冷却の平均冷却速度の下限は特に限定されない。しかしながら、現実的な範囲として、0.01℃/秒以上とするのが好ましい。なお、第1冷却では、平均冷却速度が3℃/秒以下である限り、冷却速度を変化させても良い。
第2冷却の平均冷却速度:5~30℃/秒
第2冷却は、700~750℃の温度範囲から開始し、少なくとも600℃で終了する。第2冷却において、平均冷却速度が5℃/秒より遅いとパーライトの平均ラメラー間隔を0.20μm以下とすることが困難となる。第2冷却の平均冷却速度は、好ましくは7℃/秒以上であり、より好ましくは10℃/秒以上である。一方、30℃/秒より速いと、ベイナイトやおよび/またはマルテンサイトのような組織が生じて、初析フェライトおよびパーライトの合計面積率を90%以上とすることが困難となる。第2冷却の平均冷却速度は、好ましくは28℃/秒以下であり、より好ましくは25℃/秒以下である。なお、第2冷却では、平均冷却速度が5~30℃/秒である限り、冷却速度を変化させてもよい。
Ac1(℃)=723-10.7(%Mn)-16.9(%Ni)+29.1(%Si)+16.9(%Cr)
軸中心断面を鏡面研磨した縦断面サンプルを、ナイタールエッチングによって組織を現出させた。その後、D/4位置の組織を、光学顕微鏡にて倍率400倍で、220μm×165μmの領域を5視野撮影した。得られた写真に対し、等間隔の10本の縦線、横線を格子状に引き、100個の交点上に存在する初析フェライトおよびパーライトの点数を測定した。各視野において各組織の面積率(%)を求めて、5視野の平均値を算出した。
bcc-Fe平均粒径の測定には、EBSP解析装置およびFE-SEM(Field-Emission Scanning Electron Microscope、電界放出型走査電子顕微鏡)を用いた。結晶方位差(斜角)が15°を超える境界、すなわち、大角粒界を結晶粒界として「結晶粒」を定義し、bcc-Fe平均粒径を決定した。このとき、測定領域は200μm×400μm、測定ステップは1.0μm間隔として測定した。測定方位の信頼性を示すコンフィデンス・インデックス(Confidence Index)が0.1以下の測定点は解析対象から削除した。また、金属組織中にマルテンサイト組織が析出したサンプルは、適切なbcc-Fe平均粒径が得られないため、測定を行わなかった。
図1(a)はパーライトラメラーの組織1の模式図を、図1(b)はパーライトラメラーの組織1の拡大図を示す。パーライトラメラーの組織1は、図1(b)に示すように、ラメラーフェライト3とラメラーセメンタイト2が層状(ラメラー状)に並んだ組織である。本開示で規定するラメラー間隔とはラメラーセメンタイト2の間隔である。
λ=L/(n-1) ・・・(2)
また、金属組織中にマルテンサイト組織が析出して、初析フェライトおよびパーライトの合計面積率が90%未満のサンプルは、ラメラー間隔の算出が困難であるため、測定を行わなかった。
球状化焼鈍後した試料片の縦断面サンプルについて、軸中心断面を鏡面研磨した後に、ピクラールエッチングによって組織を現出させた。D/4位置の組織を、光学顕微鏡を用いて倍率400倍で5視野観察した。各視野の球状化度をJIS G3539:1991の付図によってNo.1~No.4で評価して、5視野の平均値を算出した。なお、球状化度が小さいほど、良好な球状化組織であることを意味する。
球状化焼鈍後した試料片の縦断面サンプルについて、軸中心断面を鏡面研磨した縦断面サンプルのD/4位置の硬度を測定した。硬度測定には、ビッカース硬度計を用いて、荷重1kgfで測定した。D/4位置にある5つの異なる点で測定を行い、その平均値(HV)を求めた。
上記表1に示した鋼種A~Uを用い、加工温度(仕上圧延温度に相当)および冷却速度を下記表2のように変化させて、加工フォーマスタ試験を実施した。これにより、異なる前組織を有する加工フォーマスタ試験片を夫々作製した。なお、鋼種Oは、Mnの量が1.7%を超えており、本願発明の範囲外である。鋼種Pは、Tiの量が0.1%を超えており、本願発明の範囲外である。また、鋼種A~OおよびQ~Uでは、[Cr%]+[Cu%]+[Ni%]+[Mo%]が0.75質量%以下であり、上述の式(X)を満たしている。鋼種Pでは、[Cr%]+[Cu%]+[Ni%]+[Mo%]が0.75質量%を超えており、式(X)を満たさない。
目標球状化度=5×[C%]+1.5 ・・・(3)
目標硬さ=88.4×Ceq+86.0 ・・・(4)
ただし、Ceq=[C%]+0.2×[Si%]+0.2×[Mn%]であり、[C%]、[Si%]および[Mn%]は、それぞれ質量%でC、SiおよびMnの含有量を示す。
2 ラメラーセメンタイト
3 ラメラーフェライト
4 線分(層状組織に直交し、かつ始端および終端がラメラーセメンタイトの厚さ中心に位置している)
Claims (4)
- 質量%で、
C :0.07%以上、0.3%未満、
Si:0.05~0.5%、
Mn:0.2~1.7%、
P :0%超、0.03%以下、
S :0.001~0.05%、
Al:0.01~0.1%、および
N :0~0.015%を夫々含有し、残部が鉄および不可避不純物からなり、
鋼の金属組織が、初析フェライトおよびパーライトを含有し、全組織に対する初析フェライトおよびパーライトの合計面積率が90%以上であると共に、前記初析フェライトの面積率Afが、下記式(1)で表されるA値との関係で、Af≧Aの関係を満足し、
bcc-Fe結晶粒の平均円相当直径が15~30μmであり、且つ、
パーライトラメラーの間隔が平均で0.20μm以下であることを特徴とする冷間加工用機械構造用鋼。
A=(103-128×[C(%)])×0.80(%) ・・・(1)
但し、上記式(1)中、[C(%)]は、質量%でCの含有量を示す。 - 更に、質量%で、
Cr:0%超、0.5%以下、
Cu:0%超、0.25%以下、
Ni:0%超、0.25%以下、
Mo:0%超、0.25%以下、および
B :0%超、0.01%以下よりなる群から選択される1種以上を含有し、かつ
下記式(X)を満足する請求項1に記載の冷間加工用機械構造用鋼。
[Cr%]+[Cu%]+[Ni%]+[Mo%]≦0.75 ・・・(X)
但し、[Cr%]、[Cu%]、[Ni%]および[Mo%]は、それぞれ、質量%で示したCr、Cu、NiおよびMoの含有量を示す。 - 更に、質量%で、
Ti:0%超、0.1%以下を含有する請求項1または2に記載の冷間加工用機械構造用鋼。 - 請求項1~3のいずれか1項に記載の冷間加工用機械構造用鋼を製造するに当たり、
950℃以上、1150℃以下で仕上圧延し、
次いで700~750℃の第1冷却終了温度まで平均冷却速度:3℃/秒以下で冷却する第1冷却と、前記第1冷却終了温度から少なくとも600℃の温度範囲まで平均冷却速度:5~30℃/秒で冷却する第2冷却とを順次行うことを特徴とする冷間加工用機械構造用鋼の製造方法。
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