Bolt steel

Abstract

A bolt steel having a predetermined chemical composition, wherein the metal structure includes ferrite and pearlite, the total of which accounts for 95.0 area% or more, the area ratio of the ferrite is 54.0 area% or more, and the following expressions (1) and (2) are satisfied.　(1): Hvf ≤ 200 (2): Hvp × (1-Vf / 100) < -0.7358 × Hvf × Vf / 100 +200.79, where Hvf is the Vickers hardness of ferrite, Hvp is the Vickers hardness of pearlite, and Vf is the area ratio of ferrite.

Description

Bolt steel

[0001] The present disclosure relates to a steel for bolts, and particularly to a steel for bolts having excellent cold forgeability.

[0002] Bolts are made by subjecting rolled wire material of bolt steel to softening annealing to improve workability, then drawing it to a specified wire diameter, followed by cold forging to form a head, processing such as thread formation, and quenching and tempering to obtain the desired strength required for the bolt.

[0003] Various methods have been proposed to improve the cold forgeability of steel for bolts. Patent Document 1 discloses a steel wire in which the proportion of cementite present at ferrite grain boundaries relative to the total number of cementite particles is increased, with the aim of reducing deformation resistance and improving cold workability such as cold forgeability.

[0004] Patent Document 2 discloses a steel for cold working machine structural use that can ensure excellent cold workability even when the time for softening annealing (spheroidizing annealing) is shortened by increasing the pro-eutectoid ferrite area ratio in the metal structure of the rolled material and refining pearlite blocks.

[0005] Patent Document 3 discloses a steel for machine structural use in which the grain size of pearlite, lamellar spacing and ferrite grains are controlled, and which has excellent cold forgeability and machinability.

[0006] Patent Document 4 discloses a high-strength wire rod for induction hardening, which has a DI value determined by the components and the austenite grain size within a predetermined range, thereby ensuring hardenability while reducing the strength of the rolled material and ensuring cold workability such as cold forgeability and impact resistance.

[0007] JP 2018-44235 A JP 2018-3106 A JP 2006-291237 A JP 2005-133152 A

[0008] Typically, when manufacturing bolts, steel for bolts in the form of rolled material or the like is softened by annealing (spheroidizing annealing, SA) to reduce its strength and improve its cold forgeability, after which cold forging (heading processing) is performed.

[0009] In recent years, the demand for carbon neutrality has increased, and there has been a growing desire to omit softening annealing. However, if softening treatment is omitted in conventional bolt steel, the wiredrawing process will be carried out in a high-strength state, and further, since the wiredrawing process causes work hardening, the strength at the time of forging will be excessive, resulting in a problem of reduced cold forgeability. In order to avoid such problems and ensure sufficient cold forgeability even when softening annealing is omitted, for example, it is necessary to sufficiently reduce the hardness in the state before wiredrawing, which is in the form of a rolled material, and specifically, to make the average hardness Hva 195 Hv or less in Vickers hardness.

[0010] The steel wire described in Patent Document 1 and the steel for cold-worked machine structural use described in Patent Document 2 both disclose only a mode in which softening annealing is performed before cold working such as forging, and omitting softening annealing is likely to result in excessive strength after wiredrawing and in insufficient cold forgeability. Furthermore, although Patent Document 3 mentions omitting softening annealing in the steel for machine structural use, it places some emphasis on improving machinability, and omitting softening annealing may prevent the average hardness Hva before wiredrawing from being reduced to 195 Hv or less in Vickers hardness, which could result in insufficient cold forgeability. Patent Document 4 also mentions omitting softening annealing in the high-strength wire rod for induction hardening, but because the austenite grain size is controlled to be quite fine, at 10 to 14 in grain size number, in order to keep the DI value within a specified range, it may prevent the average hardness Hva from being reduced to 195 Hv or less in Vickers hardness, which could result in insufficient cold forgeability.

[0011] The present disclosure has been made in view of the above circumstances, and aims to provide a steel for bolts that has a sufficiently low hardness even when softening annealing is omitted, and as a result, has sufficiently high cold forgeability.

[0012] Aspect 1 of the present invention is directed to a steel sheet comprising: C: 0.15 to 0.35 mass%, Si: less than 0.30 mass% (including 0 mass%), Mn: 0.30 to 1.50 mass%, P: 0.030 mass% or less (including 0 mass%), S: 0.030 mass% or less (including 0 mass%), Cr: 0.05 to 0.80 mass%, Ti: 0.01 to 0.08 mass%, B: 0.0005 to 0.0030 mass%, Al: 0.010 to 0.050 mass%, and N: 0.0100 mass% or less (including 0 mass%), with the balance consisting of Fe and inevitable impurities; the metallographic structure includes ferrite and pearlite, the total of which is 95.0 area% or more, and the area ratio of the ferrite is 54.0 area% or more; This steel for bolts satisfies the following formulas (1) and (2): Hvf≦200 (1) Hvp×(1−Vf / 100)<−0.7358×Hvf×Vf / 100+200.79 (2) where Hvf is the Vickers hardness of ferrite, Hvp is the Vickers hardness of pearlite, and Vf is the area fraction of ferrite.

[0013] A second aspect of the present invention is the bolt steel according to the first aspect, wherein the average hardness is 195 HV or less in Vickers hardness.

[0014] A third aspect of the present invention is the steel for bolts according to the first or second aspect, further containing one or more selected from the group consisting of Mo: 0.15% by mass or less (excluding 0% by mass), V: 0.10% by mass or less (excluding 0% by mass), Cu: 0.55% by mass or less (excluding 0% by mass), Ni: 0.55% by mass or less (excluding 0% by mass), and Sn: 0.55% by mass or less (excluding 0% by mass).

[0015] According to one embodiment of the present invention, it is possible to provide a steel for bolts that can have an average hardness Hva of 195 Hv or less in Vickers hardness without performing softening annealing, and that can ensure sufficient cold forgeability even when processing such as wire drawing is performed.

[0016] 1 is a graph showing the results of arranging the samples of the example and comparative example in terms of the value of Hvp×(1−Vf / 100) and the value of Hvf×Vf / 100.

[0017] The present inventors conducted extensive research to solve the above-mentioned problems. As a result, they found that not only the ranges of the contents of the individual elements were optimized, but also that the metallographic structure contained ferrite and pearlite, with the total containing ferrite and pearlite being 95.0 area % or more, and the ferrite area fraction Vf was 54.0 area % or more. Furthermore, in addition to the ferrite area fraction Vf, the Vickers hardness of ferrite, Hvf, and the Vickers hardness of pearlite, Hvp, satisfy the following formulas (1) and (2). By achieving these, they found a steel for bolts according to an embodiment of the present invention that can easily achieve a hardness (average hardness Hva) of 195 Hv or less, which ensures sufficient cold forgeability even after subsequent cold working such as wire drawing, without softening annealing.

[0018] Furthermore, the inventors have found that bolt steel having such characteristics can be produced by appropriately controlling the heating temperature during hot rolling, the final rolling temperature, and the placing temperature, which is the temperature at which the hot-rolled steel material is placed on a conveying means for controlled cooling, such as a Stelmor conveyor, as well as the cooling rate from the final rolling temperature to the placing temperature, so as to achieve slow cooling. Each of the requirements specified in the embodiments of the present invention will now be described in detail.

[0019] 1. Chemical Composition The steel for bolts according to the embodiment of the present invention has the chemical composition described below.

[0020] [C: 0.15% by mass or more, 0.35% by mass or less] C is an essential element for ensuring the strength of steel, and at least 0.15% by mass is required. The C content is preferably 0.18% by mass or more, more preferably 0.20% by mass or more. On the other hand, if the C content is excessive, the area ratio of pearlite increases and deformability cannot be ensured. Therefore, the C content is set to 0.35% by mass or less. The C content is preferably 0.33% by mass or less, more preferably 0.30% by mass or less.

[0021] [Si: Less than 0.30% by mass (including 0% by mass)] Si is an element used in deoxidation, but excessive Si content increases deformation resistance due to solid solution strengthening, which not only increases the forging load in cold forging but also causes a decrease in deformability, so it is desirable to keep the Si content as low as possible. Therefore, the Si content is set to less than 0.30% by mass. The Si content is preferably less than 0.20% by mass, more preferably less than 0.10% by mass.

[0022] In this specification, "including 0% by mass" means that the content in accordance with the embodiment is not intentionally added, for example, includes the case where the content is at the level of unavoidable impurities (it does not exclude the case where the element is intentionally added as long as it is within a predetermined range). On the other hand, in this specification, "not including 0% by mass" means that the element is intentionally added.

[0023] [Mn: 0.30% by mass or more, 1.50% by mass or less] Mn is an element effective in improving hardenability and increasing the strength of the final product. To effectively exert this effect, the Mn content is set to 0.30% by mass or more. The Mn content is preferably 0.33% by mass or more, and more preferably 0.35% by mass or more. On the other hand, excessive Mn content increases hardness and deteriorates cold workability. Therefore, the Mn content is set to 1.50% by mass or less. The Mn content is preferably 1.40% by mass or less, and more preferably 1.30% by mass or less.

[0024] [P: 0.030% by mass or less (including 0% by mass)] P is an element inevitably contained in steel, and causes grain boundary segregation in the steel, which leads to deterioration of ductility. Therefore, the P content is set to 0.030% by mass or less. The P content is preferably 0.020% by mass or less, and more preferably 0.015% by mass or less. The lower the P content, the better, but there is a tendency for about 0.001% by mass or more to remain due to constraints in the manufacturing process, etc.

[0025] [S: 0.030% by mass or less (including 0% by mass)] S is an element that is inevitably contained in steel. It exists as MnS in steel and deteriorates ductility, so it is an element that is harmful to cold forgeability. Therefore, the S content is set to 0.030% by mass or less. The S content is preferably 0.020% by mass or less, and more preferably 0.010% by mass or less. The lower the S content, the better, but there is a tendency for about 0.001% by mass or more to remain due to constraints in the manufacturing process, etc.

[0026] [Cr: 0.05% by mass or more, 0.80% by mass or less] Cr is an element that promotes ferrite + pearlite transformation during hot rolling and is effective in precipitating carbides without increasing strength more than necessary. To achieve this effect, the Cr content is set to 0.05% by mass or more. The Cr content is preferably 0.08% by mass or more, and more preferably 0.10% by mass or more. However, an excessive Cr content not only increases hardness more than necessary, but also increases hardenability, making it more likely that pearlite with narrow lamellar spacing will form. Therefore, the Cr content is set to 0.80% by mass or less. The Cr content is preferably 0.78% by mass or less, and more preferably 0.75% by mass or less.

[0027] [Ti: 0.01 mass% or more, 0.08 mass% or less] Ti is an element that forms TiN and thereby exerts the same effect as BN. Furthermore, when added simultaneously with B, the hardenability-improving effect of B can be fully exerted. To exert these effects, the Ti content is set to 0.01 mass% or more. The Ti content is more preferably 0.015 mass% or more. However, if the Ti content is too high, the excess Ti forms TiC, which increases the precipitation strengthening effect, so it is preferable to keep the Ti content to 0.08 mass% or less. The Ti content is more preferably 0.075 mass% or less.

[0028] [B: 0.0005% by mass or more, 0.0030% by mass or less] B is an element that combines with N to form BN, thereby suppressing dynamic strain aging during cold working and effectively suppressing solid-solution strengthening by solute N. It also has the effect of suppressing an increase in strength due to excessive precipitation of AlN, which significantly refines ferrite grains. Furthermore, when added in combination with Ti, B fixes N as TiN, effectively allowing B to function as a hardenable element in a solid-solution state, making it a useful element for adjusting strength during quenching and tempering in the final process of parts such as bolts. To achieve these effects, the B content is set to 0.0005% by mass or more. The B content is preferably 0.0010% by mass or more. However, since an excessive B content may deteriorate toughness, the B content is set to 0.0030% by mass or less.

[0029] [Al: 0.010% by mass or more, 0.050% by mass or less] Al is useful as a deoxidizing element and is also useful for fixing solute N present in steel as AlN. To effectively exert these effects, the Al content is set to 0.010% by mass or more. The Al content is preferably 0.013% by mass or more, and more preferably 0.015% by mass or more. However, if the Al content is excessive, AlN is formed in excess, which deteriorates cold forgeability. Therefore, the Al content is set to 0.050% by mass or less. The Al content is preferably 0.045% by mass or less.

[0030] [N: 0.0100% by mass or less (including 0% by mass)] N is an element that is inevitably contained in steel. Excessive solute N in steel leads to increased hardness and decreased ductility due to strain aging, deteriorating cold forgeability. Therefore, the N content is set to 0.0100% by mass or less. The lower the N content, the better, with 0% by mass being the most preferable. However, due to constraints in the manufacturing process, there is a tendency for about 0.0010% by mass or more to remain.

[0031] The basic components of the steel for bolts according to the embodiment of the present invention are as described above, and in one preferred embodiment, the balance is iron and inevitable impurities. Elements (e.g., As, Sb, Ca, Cu, Ni, O, H, etc.) that are introduced due to the conditions of raw materials, materials, manufacturing equipment, etc. are permitted as inevitable impurities. Note that, for example, P and S, the lower the content, the better, and therefore they are inevitable impurities, but there are elements whose composition ranges are separately specified as described above. Therefore, in this specification, the "unavoidable impurities" that make up the balance are a concept that excludes elements whose composition ranges are separately specified.

[0032] Other Optional Elements In another preferred embodiment of the present invention, elements other than those described above may be added as needed, as long as the effects of the present invention are not impaired. Examples of such optional elements include Mo, V, Cu, Ni, and Sn. The appropriate content of each element when adding one or more elements selected from the group consisting of Mo, V, Cu, Ni, and Sn is shown below.

[0033] [Mo: 0.15% by mass or less (excluding 0% by mass)] Mo is an element effective in improving the hardenability of steel. To ensure this effect, the Mo content is preferably 0.01% by mass or more. On the other hand, if the Mo content is excessive, the strength becomes too high and the cold forgeability deteriorates. Therefore, the Mo content is set to 0.15% by mass or less, preferably 0.14% by mass or less.

[0034] [V: 0.10% by mass or less (excluding 0% by mass)] V is an element effective in improving the hardenability of steel. To ensure this effect, the V content is preferably 0.01% by mass or more. On the other hand, if the V content is excessive, the strength becomes too high and the cold forgeability deteriorates. Therefore, the V content is set to 0.10% by mass or less, preferably 0.08% by mass or less.

[0035] [Cu: 0.55% by mass or less (excluding 0% by mass)] Cu is an element effective in improving the corrosion resistance of bolts, but an excessive content of Cu results in excessive hardness and deteriorates cold forgeability. Therefore, when Cu is added, its content is set to 0.55% by mass or less, preferably 0.30% by mass or less. Note that Cu is an element that is often present in steel as an unavoidable impurity. Since the content level of Cu as an unavoidable impurity is usually 0.02% by mass or less, when Cu is intentionally added, its content is preferably set to a level exceeding 0.02% by mass.

[0036] [Ni: 0.55% by mass or less (excluding 0% by mass)] Ni is an element effective in improving the corrosion resistance of bolts, but an excessive content of Ni results in excessive hardness and deteriorates cold forgeability. Therefore, when Ni is added, its content is set to 0.55% by mass or less, preferably 0.30% by mass or less. Note that Ni is an element that is often present in steel as an unavoidable impurity. Since the content level of Ni as an unavoidable impurity is usually 0.02% by mass or less, when Ni is intentionally added, its content is preferably set to a level exceeding 0.02% by mass.

[0037] [Sn: 0.55% by mass or less (excluding 0% by mass)] Sn is an element effective in improving the corrosion resistance of a bolt, but an excessive content of Sn makes the bolt too hard and deteriorates its cold forgeability. Therefore, when Sn is added, its content is set to 0.55% by mass or less, preferably 0.30% by mass or less.

[0038] 2. Metallographic Structure 2-1. Ferrite and Pearlite, Total of 95.0 Area% or More If the metallographic structure contains excessive amounts of bainite and martensite, the hardness of the bolt steel after rolling increases, making it impossible to achieve an average hardness (Hva) of 195 HV or less before wiredrawing without softening annealing. Therefore, the metallographic structure is primarily composed of ferrite and pearlite, which can suppress an increase in strength. Specifically, the metallographic structure contains both ferrite and pearlite, with the total of ferrite and pearlite being 95.0 area% or more. That is, the proportion of structures other than ferrite and pearlite, such as bainite, is 5.0 area% or less. It is preferable that the total of ferrite and pearlite be 100 area%, i.e., the metallographic structure is preferably composed only of ferrite and pearlite.

[0039] 2-2. Ferrite area fraction Vf is 54.0 area% or more. Furthermore, in the metal structure primarily composed of ferrite and pearlite, the ferrite ratio (area fraction Vf) is increased. Specifically, the ferrite ratio in the entire metal structure is set to 54.0 area% or more. This makes it possible to set the average hardness Hva of the rolled material to 195 Hv or less in Vickers hardness. As a result, excellent cold forgeability can be ensured even when cold working such as wire drawing is performed on the rolled material without softening annealing. The ferrite area fraction Vf is preferably 55.5 area% or more, more preferably 56.0 area% or more.

[0040] The area ratio of metal structures such as ferrite and pearlite is measured by observing the cross section of the rolled wire rod etched with picral under an optical microscope. In the case of a steel material with a circular cross section such as rolled wire rod, the metal structure is observed at the D / 4 position (D: wire diameter, i.e., the position on the cross section that is one-fourth the wire diameter from the surface of the rolled material toward the center) where a typical metal structure can be observed. Observation is performed at 400x magnification. The field area to be observed is, for example, 0.0413 mm ２After the metallographic structure that has appeared is confirmed, image processing software may be used to measure the area ratio of each metallographic structure. For example, when the observed metallographic structure is only ferrite and pearlite, the white areas are ferrite and the dark areas are pearlite, and the image may be binarized based on the level value of the color tone to determine the area ratios of ferrite and pearlite.

[0041] 3. Hardness of Ferrite and Hardness of Pearlite The Vickers hardness Hvf of ferrite satisfies the following formula (1): Hvf≦200 (1) Hvf is the Vickers hardness of ferrite. That is, the Vickers hardness of ferrite (hardness determined by micro Vickers hardness measurement) Hvf is 200 or less. Even if the amount of ferrite (area fraction Vf) is sufficient, if the hardness of ferrite is too high, the average hardness Hva will be too high. For this reason, the Vickers hardness Hvf of ferrite is set to 200 or less, preferably 180 or less. Note that in the bolt steel according to the embodiment of the present invention having the above-described chemical composition, the Vickers hardness Hvf of ferrite is usually often 100 or more. Therefore, the Vickers hardness Hvf of ferrite may be set to 100 or more.

[0042] Furthermore, if the hardness of pearlite is significantly higher than that of ferrite, the average hardness Hva will be excessively large even if the area ratio of ferrite is larger than that of pearlite, as described above. For this reason, it is necessary to satisfy the following formula (2), which was derived using the results of the Examples and Comparative Examples described below. Formula (2) is derived from the results of the Examples and Comparative Examples described below. Formula (2) is a relationship between the hardness of pearlite in the entire material, which is the product of the hardness Hvp of pearlite and (1-Vf / 100), which corresponds to the area ratio of pearlite, and the hardness of ferrite in the entire material, which is the product of the hardness Hvf of ferrite and the area ratio of ferrite. Using the results of the Examples and Comparative Examples, formula (2) was used to determine the range in which the average hardness Hva of the rolled material is 195 Hv or less in Vickers hardness. Hvp×(1−Vf / 100)<−0.7358×Hvf×Vf / 100+200.79 (2) Here, Hvf is the Vickers hardness of ferrite, Hvp is the Vickers hardness of pearlite, and Vf is the area ratio of ferrite.

[0043] The Vickers hardness Hvf of ferrite and the Vickers hardness Hvp of pearlite are determined by micro-Vickers hardness measurement under a load of 10 gf. Measurements are made on cross sections etched with picral, as with the metallographic structure described above. In the case of steel materials with a circular cross section, such as rolled wire rods, measurements are made at a representative location, the D / 4 position on the cross section (D: wire diameter; that is, a position on the cross section that is one-quarter of the wire diameter from the surface of the rolled material toward the center). Measurements are made at multiple locations, and the average value is used. For example, measurements are made at five locations each for ferrite and pearlite, and the average values ​​for each are used.

[0044] A bolt steel satisfying the above-described chemical composition, metal structure, ferrite hardness, and pearlite hardness can have an average hardness Hva of 195 HV or less, preferably 194 HV or less, and more preferably 193 HV or less, in the rolled state. This means that the steel has excellent cold forgeability even after wire drawing without softening annealing. Here, the "average hardness Hva" does not refer to the hardness of a specific structure portion, i.e., the local hardness, such as the Vickers hardness Hvf of ferrite and the Vickers hardness Hvp of pearlite, but rather refers to the hardness representative of the entire bolt steel in the form of a rolled material, etc. The average hardness Hva is the Vickers hardness measured under a load of 10 kgf (i.e., 1000 times the Vickers hardness measured for the Vickers hardness Hvf of ferrite and the Vickers hardness Hvp of pearlite). Measurement is performed on a cross section. In the case of steel materials with a circular cross section, such as rolled wire rod, measurement is made at the D / 4 position (D: wire diameter; that is, the position on the cross section that is a quarter of the wire diameter from the surface of the rolled material toward the center) which is a representative location on the cross section. Measurements are made at multiple locations, and the average value is used. For example, measurements are made at two or more locations, and the average value is used.

[0045] 4. Manufacturing Method The steel for bolts according to the embodiment of the present invention can be manufactured by appropriately controlling the cooling rate from the final rolling temperature to the placing temperature, in addition to the heating temperature during hot rolling, the final rolling temperature, and the placing temperature, which is the temperature at which the hot-rolled steel is placed on a conveying means for controlled cooling, such as a Stelmor conveyor, of a steel material having the above-described chemical composition, as will be described in detail below.

[0046] (1) Melting and Blooming The steelmaking raw materials are melted to satisfy the above-mentioned chemical composition, and the resulting molten steel is cast to obtain a cast material, which is then bloomed and rolled to obtain a billet or other slab (rolled base material). The billet can be obtained by any of the usual methods used in the production of wire rod and steel bar. Casting can be performed by batch processing to obtain an ingot, or by continuous casting. The billet (e.g., billet) can be subjected to processing such as facing, as necessary.

[0047] (2) Hot Rolling The steel slab is heated under the conditions detailed below, hot rolled, and then subjected to controlled cooling to obtain a rolled material.

[0048] - Heating of the slab The slab is heated at a temperature of 1000°C or higher and 1200°C or lower. To reduce the strength of the rolled material, it is necessary to refine the prior austenite grain size and promote ferrite transformation. For this reason, the slab heating temperature must be lowered; specifically, the slab heating temperature (soaking temperature) is set to 1200°C or lower. To more reliably prevent the prior austenite grain size from coarsening, the slab heating temperature is preferably 1180°C or lower, and more preferably 1150°C or lower. On the other hand, from the viewpoint of stable operation, the slab heating temperature is set to 1000°C or higher, preferably 1020°C or higher, and more preferably 1050°C or higher.

[0049] Final Rolling Temperature The heated steel slab is hot-rolled. At this time, the final rolling temperature (finish rolling temperature) is set to 800°C or higher and 910°C or lower. In order to refine the prior austenite grain size and to suppress the precipitation of TiC, which contributes to an increase in strength, the final rolling temperature is set to 910°C or lower, preferably 908°C or lower, and more preferably 905°C or lower. On the other hand, from the viewpoint of stable operation, the final rolling temperature is set to 800°C or higher, preferably 802°C or higher, and more preferably 805°C or higher.

[0050] Placement temperature and cooling rate from final rolling temperature to placement temperature The rolled material rolled at the above-mentioned predetermined final rolling temperature is subjected to controlled cooling by a method such as Stelmor (air blast cooling) in order to control properties such as the metal structure. In this case, in order to obtain the above-mentioned predetermined total fraction (area %) of ferrite and pearlite and the predetermined ferrite area fraction Vf, as well as the ferrite Vickers hardness Hvf, pearlite Vickers hardness Hvp, and ferrite area fraction Vf that satisfy the above-mentioned formulas (1) and (2), not only the placement temperature, which is the temperature at which the material is placed on a conveying means (placement location) for controlled cooling, such as a Stelmor conveyor, but also the cooling rate from the final rolling temperature (finish rolling temperature) to the placement temperature is controlled, as will be described in detail below.

[0051] The placing temperature is set to 800°C or higher and 850°C or lower. The placing temperature is preferably 845°C or lower, more preferably 840°C or lower. The placing temperature is also preferably 802°C or higher, more preferably 805°C or higher. By reducing the cooling rate from the final rolling temperature to the placing temperature, it is possible to promote coarsening of the ferrite grain size and suppress an increase in strength (or hardness). The cooling rate from the final rolling temperature to the placing temperature is 30°C / sec or lower, preferably 29.5°C / sec or lower.

[0052] Typically, the steel material that has left the finish rolling rolls is water-cooled before reaching the placing temperature. Under normal water-cooling conditions, the cooling rate from the final rolling temperature to the placing temperature exceeds 30°C / second. For this reason, the cooling rate from the final rolling temperature to the placing temperature may be set to 30°C / second or less by, for example, reducing the amount of water used to effect slow water cooling, or by using a means other than water cooling that allows slow cooling, such as air cooling. Furthermore, the temperatures such as the above-mentioned final rolling temperature and placing temperature may be measured using a known method, such as measurement with a radiation thermometer.

[0053] By using the manufacturing method of steel for bolts according to the embodiment of the present invention described above, it is possible to make the average hardness Hva of the obtained rolled material 195 Hv or less, preferably 194 Hv or less, and more preferably 193 Hv or less in Vickers hardness, and excellent cold forgeability can be obtained even when cold working such as wire drawing is performed on this rolled material without softening annealing. Note that a person skilled in the art who has come into contact with the manufacturing method of steel for bolts according to the embodiment of the present invention described above may be able to find a manufacturing method different from the above-mentioned manufacturing method by trial and error.

[0054] The following examples are provided to more specifically describe the embodiments of the present invention. The embodiments of the present invention are not limited to the following examples, and may be modified as appropriate within the scope of the above-described and below-described aims, and all such modifications are within the technical scope of the embodiments of the present invention.

[0055] 1. Sample Preparation For the samples of steel types A to E, steel materials were melted using a general method used in the mass production of steel for bolts, and the process was carried out up to blooming and rolling, to obtain steel slabs having the chemical compositions shown in Table 1. As shown in Table 1, the Cu and Ni contents of steel types A to E were all at the unavoidable impurity levels. The obtained steel slabs were then heated, hot rolled and controlled cooled (Stelmor) using mass production equipment under the conditions shown in Table 2, to obtain rolled materials (wire rods) having the wire diameters shown in Table 2.

[0056] For the samples of steel types F and O, ingots were melted using an experimental furnace, and then the ingots were hot forged to obtain square bars (steel billets) measuring 155 mm in length and width and having the chemical compositions shown in Table 1. Next, using mass production equipment, the obtained square bars were heated, hot rolled, and controlled cooled under the conditions shown in Table 2 to obtain rolled materials (wire rods) having the wire diameters shown in Table 2.

[0057] For the samples of steel grades G to N, ingots were melted using an experimental furnace. The resulting ingots were then hot forged to produce round bar forgings with diameters of 13 mm to 14 mm and the chemical compositions shown in Table 1. These round bar forgings were then processed to obtain cylindrical test pieces with diameters of 6.0 mm and lengths of 9.0 mm. These cylindrical test pieces were then subjected to a Formasta test. In the Formasta test, processing and heat treatment were performed under the temperature conditions shown in Table 2. The processing simulated wire rolling. Specifically, the samples were processed from a height of 9 mm to a height of 6 mm at a processing strain rate of 50 / s at the "final rolling temperature" in Table 2, and then further processed from a height of 6 mm to a height of 3 mm, thereby simulating wire rolling. Therefore, the "Rolled wire diameter" column in Table 2 is marked with "-".

[0058]

[0059]

[0060] 2. Sample Evaluation The obtained rolled material samples or rolled simulated samples were subjected to metallographic observation, and measurements of the ferrite area fraction Vf, the ferrite Vickers hardness Hvf, the pearlite Vickers hardness Hvp, and the average hardness Hva were performed by the methods detailed below.

[0061] (Metal structure observation and ferrite area ratio measurement) The cross section of the obtained sample was polished and etched with picral solution, and then the metal structure was observed with an optical microscope. ２ The area was observed at a magnification of 400 times. As a result, as shown in Table 3, only ferrite and pearlite were observed in the metal structure of all samples. In other words, the total area of ​​ferrite and pearlite was 100%.

[0062] Based on these results, the ferrite area ratio Vf was then calculated. Since the white areas in the metallographic structure observation represent ferrite and the dark areas represent pearlite, the image of the region where the metallographic structure observation was performed was binarized based on the color tone level value to determine the ferrite area ratio. More specifically, using the image processing software "Adobe Photoshop" (Adobe Inc.), of the color tone level values ​​of 0 to 255, 235 to 255 were determined to be white areas, i.e., ferrite. The metallographic structure photograph was imported into Adobe Photoshop, and the number of pixels with color tone level values ​​of 235 or more and 255 or less was added up, and the result divided by the total number of pixels was determined as the ferrite area ratio Vf. The results obtained are shown in Table 3.

[0063] (Vickers Hardness of Ferrite and Pearlite) The Vickers hardness Hvf of ferrite and the Vickers hardness Hvp of pearlite were determined by micro-Vickers hardness measurement at a load of 10 gf. The measurement was performed at the D / 4 position of a cross section etched with picral, as in the metallographic structure described above, by identifying the ferrite portion and the pearlite portion, respectively. Five locations were measured for both ferrite and pearlite, and the average value was used as the measurement result. The left side of equation (2), i.e., the value of "Hvp × (1 - Vf / 100)", calculated using the obtained Vickers hardness Hvf of ferrite, the Vickers hardness Hvp of pearlite, and the ferrite area fraction Vf, and the right side of equation (2), i.e., the value of "-0.7358 × Hvf × Vf / 100 + 200.79", are shown in Table 3. Furthermore, Table 3 also indicates whether or not formula (1) and formula (2) are satisfied. If formula (1) is satisfied, "pass" is entered in the "Pass / fail of formula (1)" column of Table 3, and if formula (1) is not satisfied, "fail" is entered in the "Pass / fail of formula (1)" column. If formula (2) is satisfied, "pass" is entered in the "Pass / fail of formula (2)" column of Table 3, and if formula (2) is not satisfied, "fail" is entered in the "Pass / fail of formula (2)" column.

[0064] (Average Hardness) The average hardness Hva was determined by measuring Vickers hardness under a load of 10 kgf. The measurement was performed at the D / 4 position on the cross section. For Samples Nos. 1 to 5, 9, 10, 28, and 32, measurements were taken at four locations, and the average value was used as the average hardness Hva. For the other samples, measurements were taken at two locations, and the average value was used as the average hardness Hva.

[0065]

[0066] As can be seen from Tables 1 to 3, Samples No. 1 to 27 have chemical compositions defined by the embodiments of the present invention and are samples produced according to the manufacturing method of the embodiments of the present invention described above. The total amount (area ratio) of ferrite and pearlite and the ferrite area ratio Vf are within the ranges defined by the embodiments of the present invention, and satisfy formulas (1) and (2). As a result, the average hardness Hva of the rolled material or the simulated rolled material is 195 Hv or less. In other words, they have excellent cold forgeability even when cold working such as wire drawing is performed without softening annealing.

[0067] On the other hand, for Samples No. 28 to 31, the heating temperature during rolling and the final rolling temperature were too high, and the cooling rate from the final rolling temperature to the setting temperature was too high. Therefore, Sample No. 28 does not satisfy formula (2), and Samples No. 29 to 31 do not satisfy both formulas (1) and (2), and the average hardness Hva of all samples exceeds 195 Hv. In other words, without softening annealing, sufficient cold forgeability cannot be ensured after cold working, such as wire drawing. Furthermore, for Sample No. 32, the final rolling temperature during rolling was too high, and the cooling rate from the final rolling temperature to the setting temperature was too high. Therefore, for Sample No. 32, formula (2) is not satisfied, and the average hardness Hva exceeds 195 Hv. In other words, without softening annealing, sufficient cold forgeability cannot be ensured after cold working, such as wire drawing.

[0068] 1 is a graph showing the results for the above-mentioned samples No. 1 to 32, with the horizontal axis representing a portion of the right-hand side of formula (2), "Hvf × Vf / 100," and the vertical axis representing the left-hand side of formula (2), "Hvp × (1 - Vf / 100)." The validity of formula (2) can also be understood from this graph.

[0069] This application claims priority from Japanese Patent Application No. 2024-055468, filed March 29, 2024, and Japanese Patent Application No. 2025-007101, filed January 17, 2025. Japanese Patent Application Nos. 2024-055468 and 2025-007101 are incorporated herein by reference.

Claims

1. C: 0.15 to 0.35 mass%, Si: less than 0.30 mass% (inclusive of 0 mass%), Mn: 0.30 to 1.50 mass%, P: 0.030 mass% or less (inclusive of 0 mass%), S: 0.030 mass% or less (inclusive of 0 mass%), Cr: 0.05 to 0.80 mass%, Ti: 0.01 to 0.08 mass%, B: 0.0005 to 0.0030 mass%, Al: 0.010 to 0.050 mass%, and N: 0.0100 mass% or less (inclusive of 0 mass%), with the balance being Fe and inevitable impurities, the metal structure including ferrite and pearlite, the total of which is 95.0 area% or more, and the area ratio of the ferrite being 54.0 area% or more, Bolt steel that satisfies the following formulas (1) and (2): Hvf≦200 (1) Hvp×(1−Vf / 100)<−0.7358×Hvf×Vf / 100+200.79 (2) where Hvf is the Vickers hardness of ferrite, Hvp is the Vickers hardness of pearlite, and Vf is the area fraction of ferrite.

2. Steel for bolts according to claim 1, having an average Vickers hardness of 195 HV or less.

3. A steel for bolts according to claim 1 or 2, further containing one or more selected from the group consisting of Mo: 0.15 mass% or less (excluding 0 mass%), V: 0.10 mass% or less (excluding 0 mass%), Cu: 0.55 mass% or less (excluding 0 mass%), Ni: 0.55 mass% or less (excluding 0 mass%), and Sn: 0.55 mass% or less (excluding 0 mass%).

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