Cold-working machine structure steel and its manufacturing method

Abstract

A cold-working machine structural steel comprising 0.30-0.45 mass% C, 0.10-0.40 mass% Si, 0.50-1.00 mass% Mn, 0.050 mass% or less P, 0.050 mass% or less S, 0.80-1.30 mass% Cr, 0.01-0.10 mass% Al, and the remainder being iron and unavoidable impurities; wherein the area ratio of proeutectoid ferrite is 10% or more and 70% or less, the steel contains at least one selected from the group consisting of bainite, martensite, and pearlite; and wherein the steel has a dislocation density of 3.5×10 14 m ‑2 above.

Description

Technical Field

[0001] The present invention relates to cold-working machine structure steel and a method for manufacturing the same. Background Art

[0002] When manufacturing various parts, such as automotive components and construction machinery parts, spheroidizing annealing is often performed to impart cold workability to hot-rolled materials such as carbon steel and alloy steel. The rolled material, whose cold workability has been enhanced by spheroidizing annealing, is then cold worked and, if necessary, further machined, such as cutting, into the desired shape. The material is then quenched and tempered for final strength adjustment.

[0003] In recent years, from the perspective of energy conservation, the conditions of spheroidizing annealing have been re-evaluated, and in particular, a reduction in spheroidizing annealing time is required. If the spheroidizing annealing treatment time can be reduced, it is expected that the energy consumption and CO2 emissions will be reduced accordingly.

[0004] However, when the spheroidizing annealing time (hereinafter sometimes referred to as "spheroidizing annealing time") is significantly shortened using conventional hot-rolled materials, it is found that the spheroidization degree, which is an indicator of the degree of cementite spheroidization, deteriorates, making it difficult to fully soften the steel and deteriorating the cold workability. Therefore, shortening the spheroidizing annealing time is not easy. Therefore, research is underway to develop a technology that can fully soften the steel even when the spheroidizing annealing time is shortened.

[0005] For example, Patent Document 1 discloses a steel for machine structures, which ensures cold formability even when the spheroidizing annealing time is shortened by making the area ratio of proeutectoid ferrite 30% or more and 70% or less, and making the average grain size of ferrite grains 5 to 15 μm in a rolled material having a specified composition.

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent Application Laid-Open No. 2020-125538 Summary of the Invention

[0009] Problems to be solved by the invention

[0010] By using the machine structural steel described in Patent Document 1, the spheroidizing annealing time (the sum of the holding time at the specified holding temperature and the cooling time from the holding temperature to the specified air cooling start temperature), which is currently about 15 hours, can be shortened to about 10 hours. However, the desire to shorten the spheroidizing annealing time is ever stronger, and even the machine structural steel described in Patent Document 1 cannot meet this demand.

[0011] In view of such circumstances, the present invention proposes to provide a cold-working machine structural steel and a method for producing the same, which can achieve sufficient softening even at a relatively low spheroidizing annealing temperature of, for example, about 750°C, and with a spheroidizing treatment time significantly shorter than conventional methods, for example, significantly shorter than 10 hours.

[0012] Means of solving the problem

[0013] A first embodiment of the present invention is a cold working machine structural steel comprising

[0014] C: 0.30-0.45 mass%,

[0015] Si: 0.10-0.40 mass%,

[0016] Mn: 0.50-1.00 mass%,

[0017] P: 0.050 mass% or less,

[0018] S: 0.050 mass% or less,

[0019] Cr: 0.80-1.30 mass%,

[0020] Al: 0.01-0.10 mass%,

[0021] Balance: Iron and inevitable impurities,

[0022] The area ratio of proeutectoid ferrite is 10% or more and 70% or less, and the ferrite contains at least one selected from the group consisting of bainite, martensite, and pearlite.

[0023] The dislocation density is 3.5×10 14 m -2 above.

[0024] A second aspect of the present invention is the cold working machine structural steel according to the first aspect, wherein the average grain size of the proeutectoid ferrite is 6 μm or less.

[0025] A third aspect of the present invention is the cold working machine structural steel according to the first or second aspect, further comprising one or more elements selected from the group consisting of the following elements:

[0026] Cu: 0.25 mass% or less (excluding 0 mass%),

[0027] Ni: 0.25 mass% or less (excluding 0 mass%), and

[0028] Mo: 0.40 mass% or less (excluding 0 mass%).

[0029] A fourth aspect of the present invention is the cold working machine structural steel according to any one of aspects 1 to 3, further comprising one or more elements selected from the group consisting of the following elements:

[0030] Ti: 0.20 mass% or less (excluding 0 mass%),

[0031] Nb: 0.20 mass% or less (excluding 0 mass%), and

[0032] V: 1.50 mass % or less (excluding 0 mass %).

[0033] A fifth aspect of the present invention is the cold working machine structural steel according to any one of aspects 1 to 4, further comprising one or more elements selected from the group consisting of the following elements:

[0034] N: 0.01 mass % or less (excluding 0 mass %),

[0035] Mg: 0.02 mass% or less (excluding 0 mass%),

[0036] Ca: 0.05 mass% or less (excluding 0 mass%),

[0037] Li: 0.02 mass% or less (excluding 0 mass%), and

[0038] REM: 0.05 mass% or less (excluding 0 mass%).

[0039] A sixth aspect of the present invention is a method for producing the cold-working machine structural steel according to any one of aspects 1 to 5, comprising:

[0040] (a) a step of performing hot working at a processing temperature T0 of higher than 800° C. and lower than 1000° C. with a compression rate of 20% or more;

[0041] (b) After the step (a), cooling to a first cooling temperature T1 of 670° C. to 730° C. at a first cooling rate CR1 of 5° C. / s or more;

[0042] (c) after step (b), maintaining the first cooling temperature T1 for a holding time t1 between 10 and 600 seconds;

[0043] (d) After the step (c), the step of cooling the steel sheet to a second cooling temperature T2 of 550° C. or lower at a second cooling rate CR2 of 5° C. / second or higher.

[0044] A seventh aspect of the present invention is a method for producing a steel wire, wherein the cold-working machine structural steel produced by the method described in the sixth aspect is subjected to one or more steps of annealing, spheroidizing annealing, wire drawing, upsetting, and quenching and tempering.

[0045] Effects of the Invention

[0046] One embodiment of the present invention can provide a cold-working machine structural steel and its production, which can achieve sufficient softening even at a relatively low spheroidizing annealing temperature and a significantly shorter spheroidizing annealing time than conventional steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 This is a schematic diagram showing a working heat treatment pattern (working heat treatment history) of a steel material in the method for producing cold-working structural steel of the present invention.

[0048] Figure 2 It is a schematic diagram showing the spheroidizing annealing conditions (SA1). DETAILED DESCRIPTION

[0049] The present inventors have conducted research from various perspectives and have found that, in a cold working machine structural steel having a predetermined composition, by containing an appropriate amount of proeutectoid ferrite of not less than 10% and not more than 70% by area ratio, the metal structure contains at least one selected from the group consisting of bainite, martensite, and pearlite, thereby achieving a dislocation density of 3.5×10 14 m -2 As described above, it is possible to realize a cold-working machine structural steel that can be sufficiently softened during spheroidizing annealing even at a relatively low temperature and for a significantly short time.

[0050] It was also discovered that such cold-working machine structural steel can be produced by a method comprising the following steps: for steel having a specified composition, (a) hot working at a working temperature T0 of higher than 800°C and lower than 1000°C at a reduction ratio of 20% or more; (b) after step (a), cooling at a first cooling rate CR1 of higher than 5°C / second to a first cooling temperature T1 of higher than 670°C and lower than 730°C; (c) after step (b), holding at the first cooling temperature T1 for a holding time t1 of 10 to 600 seconds; and (d) after step (c), cooling at a second cooling rate CR2 of higher than 5°C / second to a second cooling temperature T2 of lower than 550°C.

[0051] Hereinafter, details of embodiments of the present invention will be described.

[0052] In this specification, the term "wire rod" refers to rolled wire rod, meaning a linear steel material that has undergone a hot rolling process and then a cooling process to room temperature. Furthermore, the term "steel wire" refers to a linear steel material that has undergone annealing or other adjustments to the properties of the rolled wire rod.

[0053] <1. Chemical composition>

[0054] The cold working machine structural steel according to an embodiment of the present invention contains 0.30-0.45 mass% C, 0.10-0.40 mass% Si, 0.50-1.00 mass% Mn, 0.050 mass% or less P, 0.050 mass% or less S, 0.80-1.30 mass% Cr, and 0.01-0.10 mass% Al.

[0055] Each element is described below in detail.

[0056] (C: 0.30-0.45 mass %)

[0057] C is an element that provides strength. If the content is less than 0.30 mass%, the required strength of the final product cannot be achieved. On the other hand, if the content is greater than 0.45 mass%, the cold workability and toughness of the steel decrease. Therefore, the C content is 0.30-0.45 mass%. Furthermore, the C content is preferably 0.43 mass% or less, and more preferably 0.40 mass% or less. This is because it allows for the precipitation of more proeutectoid ferrite.

[0058] (Si: 0.10-0.40 mass%)

[0059] Si is useful as a deoxidizing element and as a strength-enhancing element, added to increase the strength of the final product through solid strengthening. To effectively achieve these effects, the Si content is set to 0.10% by mass or greater. On the other hand, excessive Si content can lead to excessive hardness increases and deterioration of the steel's cold workability. Therefore, the Si content is set to 0.40% by mass or less.

[0060] (Mn: 0.50-1.00 mass%)

[0061] Mn is an effective element in increasing the strength of the final product by improving hardenability. To effectively exert this effect, the Mn content is set to 0.50% by mass or more. On the other hand, if Mn is contained in excess, the hardness increases and the cold workability of the steel deteriorates. Therefore, the Mn content is set to 1.00% by mass or less.

[0062] (P: 0.050 mass % or less)

[0063] P is an element inevitably contained in steel and segregates at grain boundaries in steel, causing deterioration in the ductility of the steel. Therefore, the P content is set to 0.050 mass % or less.

[0064] (S: 0.050 mass % or less)

[0065] S is an element inevitably contained in steel. It exists in steel as MnS, which degrades the ductility of steel and is therefore a harmful element that degrades the cold workability of steel. Therefore, the S content is set to 0.050 mass% or less.

[0066] (Cr: 0.80 mass% or more and 1.30 mass% or less)

[0067] Cr is an effective element in increasing the strength of the final product by improving the hardenability of steel. To effectively exert this effect, the Cr content should be at least 0.80 mass%. This effect increases with increasing Cr content. However, excessive Cr content can lead to excessively high strength and deteriorate the cold workability of the steel. Therefore, the Cr content should be below 1.30 mass%.

[0068] (Al: 0.01 mass% or more and 0.10 mass% or less)

[0069] Al is useful as a deoxidizer and, in combination with nitrogen, precipitates AlN, preventing abnormal grain growth and reduced strength during processing. To effectively achieve these effects, the Al content is 0.01% by mass or greater, preferably 0.015% by mass or greater, and more preferably 0.020% by mass or greater. However, excessive Al content leads to excessive formation of Al₂O₃, degrading cold forgeability. Therefore, the Al content is 0.10% by mass or less, preferably 0.090% by mass or less, and more preferably 0.080% by mass or less.

[0070] The basic components are as described above. In one preferred embodiment, the balance is iron and inevitable impurities. As inevitable impurities, the inclusion of elements (e.g., B, As, Sn, Sb, Ca, O, H, etc.) doped according to the conditions of raw materials, materials, manufacturing equipment, etc. is permitted.

[0071] Furthermore, for example, elements such as P and S are generally preferred to be contained in smaller amounts, and although they are considered unavoidable impurities, their compositional ranges are separately specified as described above. Therefore, in this specification, the term "unavoidable impurities" that constitute the remainder is used to refer to elements other than those whose compositional ranges are separately specified.

[0072] (Other selected elements)

[0073] In other preferred embodiments of the present invention, elements other than those listed above may be included as needed, as long as the effects of the embodiments of the present invention are not impaired. Examples of such selected elements are shown below. Depending on the included components, the properties of the steel are further improved.

[0074] The description "excluding 0 mass %" in other optional elements means that the amount inevitably contained as impurities (impurity level amount) is not included and is intentionally added.

[0075] (One or more selected from the group consisting of Cu: 0.25 mass % or less (excluding 0 mass %), Ni: 0.25 mass % or less (excluding 0 mass %), and Mo: 0.40 mass % or less (excluding 0 mass %))

[0076] Cu: 0.25 mass% or less (excluding 0 mass%), Ni: 0.25 mass% or less (excluding 0 mass%)

[0077] Cu and Ni are elements that improve hardenability and effectively play a role in improving the strength of the product. This effect increases with the increase in the content of these elements, but in order to effectively play their role, Cu and Ni are preferably 0.05% by mass or more, more preferably 0.08% by mass or more, and further preferably 0.10% by mass or more. However, when excessively contained, supercooled tissue is excessively generated, the strength becomes too high, and the cold forgeability is reduced. Therefore, Cu and Ni are preferably 0.25% by mass or less, respectively. More preferably, 0.22% by mass or less, and further preferably 0.20% by mass or less. In addition, Cu and Ni can be contained separately or in combination. In addition, when both Cu and Ni are contained, the content can be any content within the above range.

[0078] Mo: 0.40 mass% or less (excluding 0 mass%)

[0079] Mo is an element effective in increasing the strength of the final product by improving the hardenability of steel, and therefore can be intentionally added. This effect increases with increasing Mo content. However, excessive Mo content leads to excessive strength and deterioration of the steel's cold workability. In particular, the inclusion of Mo in steel along with Cr can make it difficult to significantly soften the steel after spheroidizing annealing. Therefore, Mo is preferably not more than 0.40 mass%.

[0080] (One or more selected from the group consisting of Ti: 0.20 mass% or less (excluding 0 mass%), Nb: 0.20 mass% or less (excluding 0 mass%), and V: 1.50 mass% or less (excluding 0 mass%))

[0081] Ti, Nb, and V are elements that combine with N to form compounds (nitrides), reducing the amount of dissolved N in steel and achieving a reduction in deformation resistance. In order to achieve this effect, Ti, Nb, and V are preferably contained in an amount of 0.05% by mass or more, more preferably 0.06% by mass or more, and even more preferably 0.08% by mass or more. However, if these elements are contained in excess, the amount of nitrides increases, deformation resistance increases, and cold forgeability deteriorates. Therefore, Ti and Nb are preferably contained in an amount of 0.20% by mass or less, more preferably 0.18% by mass or less, and even more preferably 0.15% by mass or less, and V is preferably contained in an amount of 1.50% by mass or less, more preferably 1.30% by mass or less, and even more preferably 1.00% by mass or less. In addition, Ti, Nb, and V may be contained individually or in combination. In addition, when two or more elements are contained, the content may be any content within the above-mentioned ranges.

[0082] (One or more selected from the group consisting of N: 0.01 mass% or less (excluding 0 mass%), Mg: 0.02 mass% or less (excluding 0 mass%), Ca: 0.05 mass% or less (excluding 0 mass%), Li: 0.02 mass% (excluding 0 mass%), and rare earth elements (REM): 0.05 mass% or less (excluding 0 mass%))

[0083] Nitrogen is an unavoidable impurity in steel. However, if dissolved N is present in steel, strain aging leads to increased hardness, decreased ductility, and deteriorated cold forgeability. Therefore, the N content is preferably 0.01% by mass or less, more preferably 0.009% by mass or less, and even more preferably 0.008% by mass or less. Mg, Ca, Li, and REM are also effective elements for spheroidizing sulfide inclusions such as MnS, thereby improving the deformability of steel. These effects increase with increasing content, but to effectively exert them, the contents of Mg, Ca, Li, and REM are each preferably 0.0001% by mass or more, and more preferably 0.0005% by mass or more. However, even if excessively contained, these effects are saturated, and effects commensurate with the content cannot be expected. Therefore, the contents of Mg and Li are each preferably 0.02% by mass or less, more preferably 0.018% by mass or less, and even more preferably 0.015% by mass or less. The contents of Ca and REM are each preferably 0.05 mass% or less, more preferably 0.045 mass% or less, and even more preferably 0.040 mass% or less. Furthermore, N, Ca, Mg, Li, and REM may be contained alone or in combination. When containing two or more, the contents of each may be within any range described above.

[0084] The REM content means the total content of 17 elements, namely, two elements Sc and Y and 15 elements from La to Lu. The REM content means that one or more elements selected from these 17 elements are contained.

[0085] <2. Metal Structure>

[0086] The cold-working machine structural steel according to an embodiment of the present invention contains proeutectoid ferrite in an area ratio of 10% to 70%. Proeutectoid ferrite contributes to the softening of the steel after spheroidizing annealing. However, simply containing proeutectoid ferrite does not achieve steel that can be sufficiently softened after spheroidizing annealing at a relatively low temperature and for a short period of time.

[0087] Therefore, the inventors of the present application have found that by increasing the dislocation density, for example, even by spheroidizing annealing at a relatively low temperature and for a short time, the hardness and hardness variation can be suppressed, and the steel can be sufficiently softened.

[0088] Specifically, the portion other than the proeutectoid ferrite (the remainder of the metal structure) contains one or more selected from the group consisting of bainite, martensite, and pearlite. As described in detail below, the dislocation density within bainite, martensite, and pearlite can be increased by performing appropriate processing heat treatment. As a result, the dislocation density can be increased to 3.5×10 14 m -2 above.

[0089] [2-1. Area ratio of proeutectoid ferrite: 10% or more and 70% or less]

[0090] By having a large amount of proeutectoid ferrite, the aggregation and spheroidization of carbides such as cementite can be promoted during spheroidizing annealing, resulting in a reduction in the hardness of the steel. From this perspective, the area fraction of proeutectoid ferrite needs to be 10% or greater. The area fraction of proeutectoid ferrite is preferably 20% or greater, more preferably 30% or greater, and even more preferably 40% or greater. On the other hand, obtaining proeutectoid ferrite with an area fraction exceeding 70% requires special treatments such as slow cooling and holding for very long periods of time, making it difficult to use conventional mass production equipment. Therefore, the upper limit of the area fraction of proeutectoid ferrite is set at 70%.

[0091] The area ratio of a specific metal structure, such as proeutectoid ferrite, can be determined by drawing a grid pattern on a metal structure photograph, counting the number of intersections (grid points) where that structure exists, and then calculating the ratio of the statistical value to the total number of intersections. In this case, if the intersection is at the boundary between the target metal structure, such as proeutectoid ferrite, and another metal structure, the intersection is counted as 0.5 points.

[0092] The metal structure is observed at the midpoint between the center and the surface, that is, in the case of a wire, at a position one-quarter of the wire diameter D from the surface (D / 4 position).

[0093] [2-2. Contains one or more selected from the group consisting of bainite, martensite, and pearlite]

[0094] In addition to the above-mentioned proeutectoid ferrite, it also includes one or more selected from the group consisting of bainite, martensite and pearlite.

[0095] As will be described later, by performing appropriate heat treatment on bainite, martensite and pearlite, the dislocation density formed inside the bainite, martensite and pearlite accompanying the phase transformation can be increased. Moreover, by forming a metal structure with a high dislocation density, a total dislocation density of up to 3.5×10 14 m -2 above dislocation density.

[0096] Bainite, martensite and pearlite may exist alone or in combination.

[0097] In addition, if the total amount (area ratio) of bainite, martensite and pearlite can be 3.5×10 14 m -2 The above dislocation density can be any value. The total of bainite, martensite and pearlite (the total of bainite, martensite and pearlite) is preferably 50% or more, more preferably 70% or more, in terms of area ratio relative to the total metal structure (remaining metal structure) other than the aforementioned proeutectoid ferrite.

[0098] More preferably, the remaining metal structure as a whole is preferably composed of at least one of bainite, martensite, and pearlite. This is because the desired dislocation density can be more easily achieved. Furthermore, the phrase "the remaining metal structure as a whole is composed of at least one of bainite, martensite, and pearlite" may include the following: when observing a relatively narrow field of view, no metal structures other than bainite, martensite, and pearlite are confirmed in the remaining metal structure, but when observing a wider field of view, a small amount of metal structures other than bainite, martensite, and pearlite are confirmed.

[0099] The term "pearlite" used in this specification encompasses not only a structure in which a so-called lamellar structure is clearly observed, but also so-called "fine pearlite" in which cementite is divided and does not have a pure lamellar structure.

[0100] Pearlite is preferably composed of fine pearlite because a desired dislocation density can be more easily obtained.

[0101] [2-3. The dislocation density is 3.5×10 14 m -2 above]

[0102] The cold working machine structural steel according to the embodiment of the present invention has a dislocation density of 3.5×10 14 m -2 More than 5×10 14 m -2 By achieving a high dislocation density, the carbide segmentation and solid solution can be promoted during spheroidizing annealing. As a result, even when spheroidizing annealing is performed at a relatively low temperature and for a short time, variations in hardness can be suppressed, allowing for sufficient softening.

[0103] The dislocation density is more preferably 1×10 16 m -2 This is because if the dislocation density is higher than 1×10 16 m -2 , depending on the heat treatment conditions of spheroidizing annealing, the dislocation density after spheroidizing annealing becomes relatively high, and the hardness may become higher.

[0104] Such a high dislocation density cannot be achieved simply by the presence of one or more of bainite, martensite, and pearlite, but can be achieved by increasing the dislocations introduced during phase transformation by performing appropriate working heat treatment as will be described later.

[0105] As described in detail in the Examples, the dislocation density can be determined from the values ​​of the strain (lattice strain) and the Burgers vector obtained by the Williamson-Hall (WH) method in X-ray diffraction.

[0106] [2-4. The average grain size of proeutectoid ferrite is 6 μm or less]

[0107] The cold-working machine structural steel according to the embodiment of the present invention preferably has an average proeutectoid ferrite grain size of 6 μm or less. This is because hardness variation after spheroidizing annealing can be more reliably suppressed by setting the average proeutectoid ferrite grain size to 6 μm or less.

[0108] <3. Manufacturing Method>

[0109] As described in detail below, the cold-working machine structural steel according to the embodiment of the present invention can be produced by performing predetermined hot working in a predetermined temperature range and then performing a work heat treatment with cooling and holding under predetermined conditions.

[0110] Figure 1 This is a schematic diagram showing the processing heat treatment pattern (processing heat treatment history) of steel materials in the method for manufacturing cold-working structural steel of the present invention. Figure 1In the manufacturing method shown, a steel material such as a wire rod having the above-mentioned chemical composition is subjected to a working heat treatment including the following steps (a) to (d).

[0111] (a) A process of hot working at a processing temperature T0 of more than 800°C and less than 1000°C with a compression rate of 20% or more

[0112] (b) After step (a), cooling the substrate at a first cooling rate CR1 of 5°C / s or higher to a first cooling temperature T1 of 670°C to 730°C or lower

[0113] (c) After step (b), a step of maintaining the first cooling temperature T1 for a holding time t1 of 10 to 600 seconds

[0114] (d) After step (c), the step of cooling the substrate at a second cooling rate CR2 of 5°C / s or higher to a second cooling temperature T2 of 550°C or lower is performed.

[0115] Each step is described below.

[0116] [Step (a): a step of hot working at a processing temperature T0 of higher than 800°C and lower than 1000°C at a compression ratio of 20% or higher]

[0117] like Figure 1 The steel material (e.g., wire rod) having the above chemical composition is heated to a temperature T0 (processing temperature T0) and hot-processed. The processing temperature T0 is higher than 800°C and lower than 1000°C. In addition, the compression rate of the hot processing is higher than 20%.

[0118] To ensure the required amount of proeutectoid ferrite, the working temperature T0 is set to 1000°C or lower and the hot working reduction ratio is set to 20% or higher. Furthermore, setting the working temperature T0 to 1000°C or lower and the hot working reduction ratio to 20% or higher also has the effect of making the proeutectoid ferrite grains finer.

[0119] If the processing temperature T0 is below 800°C, the phase transformation in the high temperature range is accelerated during the subsequent cooling, and the dislocation density cannot reach 3.5×10 14 m -2 Therefore, the processing temperature T0 is higher than 800°C.

[0120] Hot working may be any method as long as it can achieve a compression ratio of 20% or more. Examples of hot working include press working and rolling working.

[0121] The compression ratio is calculated as follows.

[0122] <Compression ratio during stamping (in this case, compression ratio is also called reduction ratio)>

[0123] Compression ratio (%) = (h1-h2) / h1×100

[0124] h1: The height of the steel before processing, h2: The height of the steel after processing

[0125] <Compression ratio when producing wire rod through rolling (in this case, the compression ratio is also called area reduction ratio)>

[0126] Compression ratio (%) = (S1 - S2) / S1 × 100

[0127] S1: Steel cross-sectional area before processing, h2: Steel cross-sectional area after processing

[0128] The compression rate of a single hot working may be 20% or higher. Alternatively, multiple hot workings may be performed while maintaining the temperature T0 so that the total compression rate is 20% or higher.

[0129] [Step (b): After step (a), cooling to a first cooling temperature T1 of 670° C. to 730° C. at a first cooling rate CR1 of 5° C. / s or higher]

[0130] After step (a), if Figure 1 As shown, the steel is cooled to a first cooling temperature T1 at a first cooling rate CR1. The first cooling temperature T1 is 670°C or higher and 730°C or lower. The first cooling rate CR1 is 5°C / second or higher. By cooling to the first cooling temperature T1 at a rate of 5°C / second or higher, the dislocation density of the obtained cold-working structural steel can be reduced to 3.5×10 14 m -2 Furthermore, by setting the first cooling rate CR1 to 5° C. / s or more, the proeutectoid ferrite grains can be refined.

[0131] The cooling rate can be measured by bringing a contact thermometer such as a thermocouple into contact with the steel material. Alternatively, as a simple method, a non-contact thermometer can be used to measure the surface temperature of the steel material.

[0132] [Step (c): After step (b), a step of maintaining the first cooling temperature T1 for a holding time t1 of 10 to 600 seconds]

[0133] After step (b), if Figure 1 As shown, the first cooling temperature T1 is maintained during the holding time t1.

[0134] The holding time t1 is 10 to 600 seconds, preferably 10 to 400 seconds, and more preferably 10 to 200 seconds. In order to obtain a proeutectoid ferrite amount of 10 to 70% in terms of area ratio, the holding time t1 at the first cooling temperature T1 is 10 seconds or longer. On the other hand, if the holding time t1 exceeds 600 seconds, the dislocation density generated by the phase transformation during further cooling from the first cooling temperature T1 may be less than 3.5×10 14 m -2 If the holding time t1 is too long, C and other alloying elements will be concentrated in the austenite, and the growth of ferrite generated during the subsequent cooling process will be inhibited, making it difficult to ensure a sufficient ferrite area ratio. Therefore, the holding time t1 is 600 seconds or less. The holding time t1 is preferably 400 seconds or less, and more preferably 200 seconds or less.

[0135] [Step (d): After step (c), cooling at a second cooling rate CR2 of 5°C / s or higher to a second cooling temperature T2 of 550°C or lower]

[0136] After step (c), if Figure 1 As shown, the second cooling rate CR2 is used to cool the steel to the second cooling temperature T2. The second cooling temperature T2 is 550°C or less. In addition, the second cooling rate CR2 is 5°C / second or more. The second cooling rate CR2 is preferably 50°C / second or less. In order to obtain a dislocation density of 3.5×10 14 m -2 The above is performed by cooling the steel sheet between the first cooling temperature T1 and the temperature T2 of 550° C. or lower at a cooling rate of 5° C. / s or higher.

[0137] Regarding the cooling to a temperature lower than the second cooling temperature T2 after step (d), Figure 1 In the illustrated embodiment, the second cooling temperature T2 is maintained during the holding time t2 and cooled to room temperature at a third cooling rate CR3 (eg, furnace cooling, air cooling, or rapid cooling (eg, gas rapid cooling)).

[0138] However, the present invention is not limited thereto, and any cooling method may be used. As an example of such cooling, the second cooling temperature T2 may be set to room temperature, and cooling may be performed from the first cooling temperature T1 to room temperature at a second cooling rate CR2.

[0139] When the second cooling temperature T2 is maintained during the holding time t2, it is preferred that the second cooling temperature T2 be 400°C to 550°C and the holding time t2 be 100 to 3000 seconds. By setting the second cooling temperature T2 to 400°C or higher, the desired ferrite area ratio can be more easily obtained. The second cooling temperature T2 is more preferably 500°C or higher. By setting the second cooling temperature T2 to 550°C or lower, a high dislocation density can be more easily obtained. The second cooling temperature T2 is more preferably 540°C or lower. By setting the holding time t2 to 100 seconds or higher, the desired ferrite area ratio can be more easily obtained. The holding time t2 is more preferably 150 seconds or higher, and further preferably 210 seconds or higher. By setting the holding time t2 to 3000 seconds or lower, high productivity can be ensured while a high dislocation density can be more easily obtained. The holding time t2 is more preferably 1500 seconds or lower.

[0140] In addition, after cooling to the second cooling temperature T2 by the second cooling rate CR2, it is possible not to hold (i.e., the holding time t2 is 0 seconds), and then cooling to room temperature from the second cooling temperature T2 at a third cooling rate CR3 that is different from the second cooling rate CR2. At this time, the third cooling rate CR3 may be faster or slower than the second cooling rate CR2. As cooling methods for obtaining the third cooling rate CR3, furnace cooling, cooling or rapid cooling (e.g., gas rapid cooling) can be exemplified. In this case, the second cooling rate CR2 and the third cooling rate CR3 are preferably 1 to 25°C / second. If the second cooling rate CR2 and the third cooling rate CR3 are 1°C / second or more, a high dislocation density can be obtained more easily, and if the second cooling rate CR2 and the third cooling rate CR3 are 25°C / second or less, a desired ferrite area ratio can be obtained more easily.

[0141] According to the manufacturing method described above, the cold working machine structural steel according to the embodiment of the present invention can be obtained.

[0142] The cold-working machine structural steel according to the embodiment of the present invention is assumed to be subjected to spheroidizing annealing thereafter, but other processing (such as wire drawing) may be performed before or after spheroidizing annealing depending on circumstances.

[0143] As shown in the examples described below, cold-working machine structural steel according to an embodiment of the present invention can be sufficiently softened even at a relatively low temperature of 750°C by significantly shortening the spheroidizing annealing time (the sum of the holding time at the specified holding temperature and the cooling time from the holding temperature to the specified air cooling start temperature) to approximately 5 hours or less compared to the conventional method (approximately 11 hours in Patent Document 1). Furthermore, in the present invention, steel wire can be produced by subjecting the steel material (cold-working structural steel) obtained under the above-described manufacturing conditions to one or more of the following steps: annealing, spheroidizing annealing, wire drawing, upsetting, and quenching and tempering. The steel wire referred to herein refers to a wire-shaped steel material whose properties have been adjusted by subjecting the steel material obtained under the above-described manufacturing conditions to annealing, spheroidizing annealing, wire drawing, upsetting, quenching and tempering, etc. However, this also includes wire-shaped steel material that has undergone steps such as annealing, wire drawing, upsetting, and quenching and tempering, as well as steps such as the above-described annealing.

[0144] As described above, the method for manufacturing the cold-working machine structural steel according to the embodiment of the present invention has been described. However, a person skilled in the art who understands the desired properties of the cold-working machine structural steel according to the embodiment of the present invention may, through trial and error, find a method other than the above-described manufacturing method to manufacture the cold-working machine structural steel having the desired properties according to the embodiment of the present invention.

[0145] Example

[0146] The present invention is not limited to the following examples, and can be implemented with appropriate modifications within the scope of the above and below-mentioned principles, which are all included in the technical scope of the present invention.

[0147] Table 1

[0148]

[0149] 8 mm x 12 mm test pieces for Formaster machining were prepared using rolled materials of steel grade 1 (SCM435), steel grade 2 (SCM440), and steel grade 3 (SCR440) shown in Table 1. SCM435, SCM440, and SCR440 are steel grades specified in Japanese Industrial Standard JIS G4053.

[0150] As shown in Table 1, Steel Types 1 and 2 contain Cu and Ni, but these are at impurity levels. In other words, Cu and Ni are unavoidable impurities and not intentionally added. Furthermore, Steel Type 3 contains 0.01 mass% Mo, which is at an impurity level. In other words, the Mo in Steel Type 3 is an unavoidable impurity and not intentionally added.

[0151] The above test pieces for Formaster were tested using a Formaster testing machine. Figure 1 The working heat treatment shown is used to produce specimens of structural steel for cold working.

[0152] The specimen was heated at 10°C / second to the processing temperature T0. After reaching the processing temperature T0 and holding for 300 seconds, two presses were performed as hot working. The first press was performed at a strain rate of 50 / second to reduce the height of the specimen from 12 mm to 7 mm (ε = 0.54). Five seconds later, the second press was performed at a strain rate of 50 / second to reduce the height of the specimen from 7 mm to 3 mm (ε = 0.85).

[0153] Table 2 shows the processing temperature T0, the first cooling temperature T1, the first cooling rate CR1, the holding time t1, the second cooling temperature T2, and the second cooling rate CR2. In addition, Table 2 also shows the holding time t2 and the third cooling rate CR3 for reference.

[0154] Samples No. 1-3 and No. 1-4 were cooled from the first cooling temperature T1 to room temperature at the second cooling rate CR2, since the second cooling temperature T2 was room temperature. Samples No. 1-5, No. 2-2, and No. 3-4 were hot worked at the working temperature T0 and then cooled to room temperature at 30°C / s.

[0155] Note that conditions that deviate from those shown in the manufacturing method of the embodiment of the present invention are underlined.

[0156] Table 2

[0157]

[0158] The heat-treated sample was cut along its central axis and then quartered to obtain four samples, including a longitudinal cross-section. One of the samples was not subjected to spheroidizing annealing (hereinafter sometimes referred to as the sample before spheroidizing annealing), while the other was subjected to spheroidizing annealing (hereinafter sometimes referred to as the sample after spheroidizing annealing). Spheroidizing annealing was performed on each sample in a vacuum tube.

[0159] Figure 2 It is a schematic diagram showing the spheroidizing annealing conditions (SA1).

[0160] The spheroidizing annealing was performed by heating to 750° C. at 80° C. / hour, holding for 1 hour, cooling to 660° C. at a cooling rate of 30° C. / hour, and then standing to cool.

[0161] Specifically, the spheroidizing annealing temperature is a relatively low 750°C, the spheroidizing annealing time is a significantly short 4.7 hours, and the holding time is also a significantly short 1 hour.

[0162] The samples before spheroidizing annealing were embedded in resin in a manner that allowed observation of the longitudinal section, and (1) the area ratio of proeutectoid ferrite and the observation of structures other than proeutectoid ferrite, (2) the average grain size of proeutectoid ferrite, and (3) the dislocation density were measured.

[0163] In addition, the samples after spheroidizing annealing were also embedded in resin in a manner such that the longitudinal section could be observed, as described above, and (4) the hardness and its variation after spheroidizing annealing were measured.

[0164] In any of the measurements and observations (1) to (4), the diameter of the sample is assumed to be D, and the measurement is performed at the position D / 4 from the surface of the sample toward the central axis.

[0165] (1) Measurement of the area ratio of proeutectoid ferrite

[0166] Before spheroidizing annealing, a longitudinal section of the sample was etched with Nital to reveal the structure. A photograph was taken at position D / 4 using an optical microscope at 400x magnification (field of view: 220 μm horizontal x 165 μm vertical). The resulting photograph was then drawn in a network pattern with 15 equally spaced vertical lines and 10 equally spaced horizontal lines. The number of proeutectoid ferrite points at each of the 150 intersections was measured, and the value divided by 150 was used as the area ratio (%) of proeutectoid ferrite.

[0167] At this time, if the grid is the boundary between proeutectoid ferrite and other structures, it is set to 0.5.

[0168] Furthermore, the portion other than the proeutectoid ferrite (the remaining metal structure) is also observed to identify which phase it is.

[0169] (2) Measurement of the average grain size of proeutectoid ferrite

[0170] Longitudinal cross-sections of the samples before spheroidizing annealing were etched with Nital to reveal the structure. Photographs were taken at the D / 4 position using an optical microscope at either 400x magnification (field of view: 220 μm wide × 165 μm long) or 1000x magnification (field of view: 147 μm wide × 110 μm long). The size (equivalent circle diameter) of each proeutectoid ferrite grain in the field of view was then calculated using image analysis software (Image-Pro Plus ver. 7.0), and the average of these values ​​was used as the average proeutectoid ferrite grain size.

[0171] In addition, proeutectoid ferrite grains adjoining the end of the photograph (proeutectoid ferrite grains whose original grain size could not be measured) were excluded from the statistical objects.

[0172] (3) Measurement of dislocation density

[0173] The sample before spheroidizing annealing was electrolytically ground to prepare a sample for measuring dislocation density. This sample was subjected to X-ray diffraction using a horizontal X-ray diffractometer SmartLab manufactured by Rigaku Corporation.

[0174] The X-ray diffraction profile was measured using Co as the target metal by the θ / 2θ diffraction method with 2θ ranging from 40° to 130°.

[0175] The obtained diffraction profile was used to determine the strain according to the Williamson-Hall (WH) method. The WH method uses the following formula.

[0176] βcosθ / λ=0.9 / D+2εsinθ / λ (Formula 1)

[0177] β 2 =β m 2 -β s 2 (Formula 2)

[0178] Here, β is the true full width at half maximum (rad), θ is the Bragg angle (rad), λ is the wavelength of incident X-rays (nm) (0.1789 nm is used as λ), D is the size of the crystallite (nm), and ε is the lattice strain.

[0179] In addition, the diffraction line width expansion caused by the device constant is corrected by the approximate formula (Formula 2). m is the measured half-height width, β s is the full width at half maximum of the unstrained sample (device function). As the unstrained sample, Si640d manufactured by NIST was used.

[0180] More specifically, the diffraction peaks of the (110), (211), and (220) planes of the proeutectoid ferrite (α-Fe) of the sample were measured to obtain the diffraction angle 2θ and the half-maximum width β. m .

[0181] Then, the measurement results of each of the above crystal planes are plotted by taking sinθ / λ on the horizontal axis and βcosθ / λ on the vertical axis.

[0182] For the plot, a linear function (y=ax+b) was used to approximate the curve. The strain (ε) and crystallite size (D) were determined from the slope and intercept of the straight line, and the strain (ε) was determined from these.

[0183] The dislocation density ρ can be expressed as in (Equation 3) using the strain ε and the Burgers vector b.

[0184] ρ=14.4ε 2 / b 2 (Formula 3)

[0185] Here, the size of the Burgers vector b is 0.25×10 －9 m.

[0186] From this, the dislocation density ρ is calculated.

[0187] (4) Hardness and deviation after spheroidizing annealing

[0188] To confirm the softening effect of spheroidizing annealing, the hardness of the sample after spheroidizing annealing was measured at five locations (5 points) using a Vickers hardness tester at the D / 4 position with a load of 1 kgf. The average value (HV) was used as the sample hardness (HV), and the standard deviation was calculated from the measured values ​​and used as the hardness deviation (HV). For the sample of steel type 1 (SCM435), the hardness HV was below 165, and if the hardness deviation HV was below 7.0, it was judged that the steel was sufficiently softened. On the other hand, for the samples of steel types 2 (SCM440) and 3 (SCR440), which have a higher carbon content, the hardness HV was below 180, and if the hardness deviation HV was below 7.0, it was judged that the steel was sufficiently softened.

[0189] Table 3 shows the area ratio of proeutectoid ferrite and structures other than proeutectoid ferrite, the average grain size of proeutectoid ferrite, the dislocation density, the hardness after spheroidizing annealing, and the variation of the hardness obtained by the above-mentioned method.

[0190] In Table 3, the conditions not meeting the requirements shown in the embodiment of the present invention and the softening evaluation criteria are underlined.

[0191] In addition, the so-called "main body" in the structure other than proeutectoid ferrite means that no metal structure other than this metal structure is confirmed within the above-mentioned observation field of view (220μm horizontally × 165μm vertically) (however, when observing a wider field of view area, the possibility of confirming a small amount of other metal structures is not ruled out).

[0192] In Sample No. 2-1, the pearlite confirmed in the structure other than the proeutectoid ferrite is fine pearlite.

[0193] Table 3

[0194]

[0195] From Tables 2 and 3, we can make the following observations.

[0196] Samples No. 1-1, 1-2, 1-3, 2-1, and 3-1 to 3-3 all meet all the requirements specified in the embodiments of the present invention. Furthermore, after spheroidizing annealing at a relatively low temperature of 750°C and for a relatively short time (holding time of 1 hour and spheroidizing annealing time of approximately 4.7 hours), both the hardness and hardness variation were good, indicating sufficient softening.

[0197] On the other hand, Samples No. 1-4, 1-5, 1-6, 2-2, and 3-4 do not satisfy one or more requirements specified in the present invention, and at least one of the hardness after spheroidizing annealing and the hardness variation is poor, that is, softening is insufficient.

[0198] In Sample No. 1-4, the working temperature T0 was too high, the first cooling temperature T1 was too low, and the holding time t1 was too long. As a result, the dislocation density was too low, resulting in poor hardness and hardness variation after spheroidizing annealing.

[0199] In Sample No. 1-5, the first cooling temperature T1 was too low, i.e., room temperature. Therefore, the holding time t1 could not be maintained at the appropriate first cooling temperature T1 (670°C to 730°C). As a result, sufficient proeutectoid ferrite could not be obtained, resulting in poor hardness after spheroidizing annealing.

[0200] In Sample No. 1-6, the working temperature T0 was too high, the first cooling temperature T1 was too high, and the second cooling rate CR2 was too slow. Consequently, the dislocation density was too low. While the hardness after spheroidizing annealing was good due to the sufficient amount of proeutectoid ferrite, the low dislocation density resulted in poor hardness variation.

[0201] In Sample No. 2-2, the first cooling temperature T1 was too low, i.e., room temperature. Therefore, the holding time t1 could not be maintained at the appropriate first cooling temperature T1 (670°C to 730°C). As a result, sufficient proeutectoid ferrite could not be obtained, resulting in poor hardness after spheroidizing annealing.

[0202] In Sample No. 3-4, the first cooling temperature T1 was too low, i.e., room temperature. Therefore, the holding time t1 could not be maintained at the appropriate first cooling temperature T1 (670°C to 730°C). As a result, sufficient proeutectoid ferrite could not be obtained, resulting in poor hardness after spheroidizing annealing.

[0203] The cold-working machine structural steel of the present invention is suitable as a raw material for various parts manufactured by cold working such as cold forging, cold heading, or cold rolling. The form of the steel is not particularly limited and may be, for example, a rolled material such as a wire rod or a steel bar.

[0204] Such parts include, for example, automotive parts and construction machinery parts, and specifically include bolts, screws, nuts, sockets, ball joints, inner tubes, torsion bars, clutch housings, retainers, housings, hubs, covers, casings, washers, tappets, saddles, valves, inner housings, clutches, sleeves, outer rings, sprockets, stators, anvils, cross shafts, rocker arms, vehicle bodies, flanges, rollers, joints, connectors, pulleys, metal fittings, forks, bases, valve lifters, spark plugs, pinions, steering shafts, and common rails. The cold-working machine structural steel of the present invention is industrially useful as a raw material for such parts. After spheroidizing annealing, it exhibits low deformation resistance and excellent cold workability when manufactured into the various parts described above at room temperature and within the exothermic working range.

[0205] This application claims priority based on Japanese Patent Application No. 2021-30472, filed on February 26, 2021, and Japanese Patent Application No. 2021-209428, filed on December 23, 2021. Japanese Patent Application Nos. 2021-30472 and 2021-209428 are incorporated herein by reference.

Claims

1. A cold working mechanical structure steel containing C: 0.30-0.45% by mass Si: 0.10-0.40 mass%, Mn: 0.50-1.00 mass%, P: 0.050 mass% or less, S: 0.050 mass% or less, Cr: 0.80-1.30 mass%, Al: 0.01-0.10 mass%, Balance: Iron and inevitable impurities, The area ratio of proeutectoid ferrite is 31% or more and 70% or less, and the ferrite contains at least one selected from the group consisting of bainite, martensite, and pearlite. The dislocation density is 3.5×10 14 m -2 above.

2. The cold-working machine structure steel according to claim 1, wherein: The average grain size of the proeutectoid ferrite is 6 μm or less.

3. The cold working machine structure steel according to claim 1 or 2, wherein: Also contains at least one of the following (A) to (C): (A) one or more selected from the group consisting of Cu: 0.25 mass % or less and excluding 0 mass %, Ni: 0.25 mass % or less and excluding 0 mass %, and Mo: 0.40 mass % or less and excluding 0 mass %; (B) one or more selected from the group consisting of Ti: 0.20 mass % or less and excluding 0 mass %, Nb: 0.20 mass % or less and excluding 0 mass %, and V: 1.50 mass % or less and excluding 0 mass %; (C) N: 0.01 mass % or less (excluding 0 mass %), Mg: 0.02 mass % or less (excluding 0 mass %), Ca: 0.05 mass % or less (excluding 0 mass %), Li: 0.02 mass % or less (excluding 0 mass %), and REM: One or more selected from the group consisting of 0.05 mass % or less and excluding 0 mass %.

4. A method for producing cold-working machine structural steel according to claim 1 or 2, wherein: include: (a) a step of hot working at a processing temperature T0 of higher than 800°C and lower than 1000°C with a compression ratio of 20% or higher; (b) after the step (a), cooling to a first cooling temperature T1 of 670° C. to 730° C. at a first cooling rate CR1 of 5° C. / s or more; (c) after step (b), maintaining the first cooling temperature T1 for a holding time t1 between 10 and 600 seconds; (d) After the step (c), cooling is performed at a second cooling rate CR2 of 10° C. / s or higher to a second cooling temperature T2 of 550° C. or lower.

5. A method for manufacturing a steel wire, wherein: The cold-working machine structural steel produced by the method according to claim 4 is subjected to one or more steps selected from the group consisting of annealing, spheroidizing annealing, wire drawing, upsetting, and quenching and tempering.

6. A method for manufacturing cold-working machine structural steel according to claim 3, wherein: include: (a) a step of hot working at a processing temperature T0 of higher than 800°C and lower than 1000°C with a compression ratio of 20% or higher; (b) after the step (a), cooling at a first cooling rate CR1 of 5°C / s or higher to a first cooling temperature T1 of 670°C to 730°C; (c) after step (b), maintaining the first cooling temperature T1 for a holding time t1 between 10 and 600 seconds; (d) After the step (c), cooling is performed at a second cooling rate CR2 of 10° C. / s or higher to a second cooling temperature T2 of 550° C. or lower.

7. A method for manufacturing a steel wire, wherein: The cold-working machine structural steel produced by the method according to claim 6 is subjected to one or more steps selected from the group consisting of annealing, spheroidizing annealing, wire drawing, upsetting, and quenching and tempering.

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