Steel for bolt

Abstract

To provide a steel for bolt that attains sufficiently low hardness even when omitting softening annealing, and thus enables adequately high cold workability.SOLUTION: A steel for bolt having a predetermined chemical composition, wherein the metallographic structure includes ferrite and pearlite, the total of those being 95.0 area% or more, and the ferrite area fraction being 54.0 area% or more. The steel satisfies the following formulas (1) and (2): Hvf≤200 (1) and 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.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[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 bolt steel. 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, which is 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. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 2018-44235 [Patent Document 2] Japanese Patent Application Publication No. 2018-3106 [Patent Document 3] Japanese Patent Application Laid-Open No. 2006-291237 [Patent Document 4] Japanese Patent Application Laid-Open No. 2005-133152 Summary of the Invention [Problem to be solved by the invention]

[0008] Typically, when manufacturing bolts, the steel for bolts, in the form of rolled material, 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, there has been an increasing demand for carbon neutrality, and a growing desire to omit softening annealing. However, if softening treatment is omitted from conventional bolt steel, the wiredrawing process will be carried out in a high-strength state, and the wiredrawing process will cause work hardening, resulting in excessive strength during forging and reduced cold forgeability. To avoid such problems and ensure sufficient cold forgeability even if softening annealing is omitted, it is necessary to make the hardness of the material in the state before wiredrawing, which is in the form of a rolled material, sufficiently low, 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 only disclose a mode in which cold working such as forging is performed after softening annealing. If softening annealing is omitted, the strength after wiredrawing becomes excessively high, and there is a high probability that cold forgeability cannot be ensured. Furthermore, although Patent Document 3 mentions omitting softening annealing in the steel for machine structural use, since a certain degree of emphasis is placed on improving machinability, omitting softening annealing may result in the average hardness Hva before wiredrawing not being able to be 195 Hv or less in Vickers hardness, and there is a risk that sufficient cold forgeability may not be ensured. Although Patent Document 4 also mentions omitting softening annealing in the high-strength wire rod for induction hardening, the austenite grain size is controlled to be quite fine, at a grain size number of 10 to 14, in order to keep the DI value within a predetermined range. Therefore, the average hardness Hva cannot be reduced to 195 Hv or less in Vickers hardness, and there is a risk that sufficient cold forgeability cannot be ensured.

[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. [Means for solving the problem]

[0012] Aspect 1 of the present invention is C: 0.15~0.35% by mass, Si: less than 0.30 mass% (including 0 mass%) Mn: 0.30~1.50% by mass, P: 0.030% by mass or less (including 0% by mass), S: 0.030 mass% or less (including 0 mass%), Cr:0.05~0.80% by mass, Ti:0.01~0.08% by mass, B:0.0005~0.0030% by mass, Al: 0.010 to 0.050 mass%; and N: 0.0100 mass% or less (including 0 mass%), with the remainder being Fe and unavoidable impurities, The metal structure contains 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 satisfies the following formulas (1) and (2). Hvf≦200 (1) 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 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 a steel for bolts according to the first or second aspect, further containing one or more selected from the group consisting of 0.15% by mass or less (excluding 0% by mass) Mo, 0.10% by mass or less (excluding 0% by mass) V, 0.55% by mass or less (excluding 0% by mass) Cu, 0.55% by mass or less (excluding 0% by mass) Ni, and 0.55% by mass or less (excluding 0% by mass) Sn. [Effects of the Invention]

[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. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a graph showing the results of sorting samples of Examples and Comparative Examples by the value of Hvp×(1−Vf / 100) and the value of Hvf×Vf / 100. DETAILED DESCRIPTION OF THE INVENTION

[0017] The present inventors conducted extensive research to solve the above-mentioned problems. As a result, they found that not only the content ranges of each element were optimized, but also that the metallographic structure contained ferrite and pearlite, with a total of 95.0 area % or more, and that the ferrite area fraction Vf was 54.0 area % or more. Furthermore, in addition to this 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, without softening annealing, which ensures sufficient cold forgeability even after subsequent cold working such as wire drawing.

[0018] Furthermore, the present 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 placement 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 placement temperature so as to achieve slow cooling. Hereinafter, each requirement defined in the embodiment of the present invention will 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 materials and requires at least 0.15% by mass. 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 mass% (including 0 mass%)] Although Si is an element used for deoxidation, excessive Si content increases deformation resistance due to solid solution strengthening, which not only increases the forging load in cold forging but also reduces 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, and more preferably less than 0.10% by mass.

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

[0023] [Mn: 0.30 mass% or more, 1.50 mass% or less] Mn is an element that is 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, if Mn is contained in excess, the hardness increases and the cold workability deteriorates. 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 that is 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 mass% or less. The P content is preferably 0.020 mass% or less, and more preferably 0.015 mass% or less. The lower the P content, the better, but there is a tendency for about 0.001 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 in steel as MnS, which deteriorates ductility and is therefore 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 mass% or more, 0.80 mass% or less] Cr is an element that promotes the 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 and makes 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 exhibits the same effect as BN by forming TiN. Furthermore, when added simultaneously with B, the hardenability-improving effect of B can be fully exhibited. To exhibit 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 best 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 mass% or more, 0.0030 mass% or less] B is an element that inhibits dynamic strain aging during cold working by bonding with N to form BN and is also effective in inhibiting solid-solution strengthening due to solute N. It also inhibits the increase in strength that would occur if excessive AlN were to precipitate, significantly refining ferrite grains. Furthermore, when added in combination with Ti, B fixes N as TiN and effectively acts as a hardenable element while retaining the solute B 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 can degrade toughness, the B content is set to 0.0030% by mass or less.

[0029] [Al: 0.010 mass% or more, 0.050 mass% or less] Al is useful as a deoxidizing element and 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 produced 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, and 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, but there is a tendency for N to remain at approximately 0.0010% by mass or more due to constraints in the manufacturing process, etc.

[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 unavoidable impurities. As unavoidable impurities, elements (e.g., As, Sb, Ca, Cu, Ni, O, H, etc.) that are introduced due to the conditions of raw materials, materials, manufacturing facilities, etc. are permitted to be mixed in. For example, there are elements such as P and S, whose content is usually the lower the better and therefore they are unavoidable impurities, but whose composition ranges are separately defined as above. Therefore, in this specification, the "unavoidable impurities" that make up the balance are a concept that excludes elements whose composition ranges are separately defined.

[0032] Other selective elements Furthermore, in another preferred embodiment of the present invention, elements other than those described above may be added as needed to the extent that 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 mass% or less (excluding 0 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 effective element for 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 mass% or less (excluding 0 mass%)] Cu is an element effective in improving the corrosion resistance of bolts, but excessive content increases hardness and deteriorates cold forgeability. Therefore, when Cu is added, its content should be 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 should preferably be greater than 0.02% by mass.

[0036] [Ni: 0.55 mass% or less (excluding 0 mass%)] Ni is an element effective in improving the corrosion resistance of bolts, but excessive content leads to excessive hardness and deterioration of cold forgeability. Therefore, when Ni is added, its content should be 0.55 mass% or less, preferably 0.30 mass% or less. 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 mass% or less, when Ni is intentionally added, its content should preferably be more than 0.02 mass%.

[0037] [Sn: 0.55 mass% or less (excluding 0 mass%)] Sn is an element that is effective in improving the corrosion resistance of bolts, but if the content is excessive, the hardness becomes too high and the cold forgeability deteriorates. Therefore, if Sn is added, the content is set to 0.55 mass% or less, preferably 0.30 mass% or less.

[0038] 2.Metal structure 2-1. Ferrite and pearlite are included and the total is 95.0% or more by area 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 increases in strength. Specifically, the metallographic structure contains both ferrite and pearlite, with the total of ferrite and pearlite being 95.0 area % or more. In other words, 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% or more Furthermore, in the metal structure mainly composed of ferrite and pearlite, the proportion of ferrite (area fraction Vf) is increased. Specifically, the proportion of ferrite 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 rolled wire material etched with picral under an optical microscope. In the case of steel materials with a circular cross section such as rolled wire material, the metal structure is observed at the D / 4 position on the cross section (D: wire diameter; that is, the position on the cross section that is one-quarter of 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 of view area to be observed is, for example, 0.0413 mm 2 Examples include: Furthermore, after the metallographic structure that has appeared has been confirmed, image processing software may be used to measure the area ratio of each metallographic structure. For example, if the observed metallographic structure is only ferrite and pearlite, the white areas represent ferrite and the dark areas represent pearlite, and the image may be binarized based on the level values ​​of the color tones to determine the area ratios of ferrite and pearlite.

[0041] 3.Hardness of ferrite and 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 (hardness determined by micro Vickers hardness measurement) Hvf of the ferrite is 200 or less. Even if the amount of ferrite (area fraction Vf) is sufficient, if the hardness of the ferrite is too high, the average hardness Hva will be too high. For this reason, the Vickers hardness Hvf of the 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-mentioned chemical composition, the Vickers hardness Hvf of the ferrite is usually often 100 or more, so the Vickers hardness Hvf of the 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) determines the range in which the average hardness Hva of the rolled material is 195 Hv or less in Vickers hardness, based on the 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. 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 fraction of ferrite.

[0043] The Vickers hardness Hvf of ferrite and the Vickers hardness Hvp of pearlite are determined by micro-Vickers hardness measurements at a load of 10 gf. Measurements are made on cross sections etched with picral, just like the metallographic structure described above. In the case of steel products with a circular cross section, such as rolled wire, measurements are made at a representative location on the cross section, the D / 4 position (D: wire diameter; that is, the 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 value of each is used.

[0044] A bolt steel that satisfies all of the above-mentioned 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" is not the hardness of a specific structure portion, such as the above-mentioned Vickers hardness Hvf of ferrite and the Vickers hardness Hvp of pearlite, i.e., the hardness of a local portion, but is a hardness that represents the entire bolt steel in the form of a rolled material or the like. The average hardness Hva is the Vickers hardness measured under a load of 10 kgf (i.e., 1000 times the load when measuring the Vickers hardness Hvf of ferrite and the Vickers hardness Hvp of pearlite mentioned above). Measurements are taken on a cross section. In the case of steel products with a circular cross section, such as rolled wire rod, measurements are taken at a representative location, the D / 4 position on the cross section (D: wire diameter; i.e., a position on the cross section that is one-quarter of the wire diameter from the surface of the rolled material towards the center). Measurements are taken at multiple locations, and the average value is used. For example, measurements are taken 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 produced 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-mentioned 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 subjected to blooming and rolling to obtain billets or other steel slabs (rolled base material). The billets can be obtained by any of the usual methods used in the manufacture of wire rods and steel bars. Casting can be carried out by batch processing, as in the case of ingots, or by continuous casting. Furthermore, the billets (e.g., billets) 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 steel billets 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] - Loading temperature and cooling rate from the final rolling temperature to the loading 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) to control properties such as the metal structure. In this process, 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 placing temperature, which is the temperature at which the material is placed on a conveying means (placing location) for controlled cooling, such as a Stelmor conveyor, but also the cooling rate from the final rolling temperature (finish rolling temperature) to the placing 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 preferably 802° C. or higher, more preferably 805° C. or higher. By reducing the cooling rate from the final rolling temperature to the loading 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 loading temperature is 30°C / sec or less, preferably 29.5°C / sec or less.

[0052] Typically, the steel material that leaves 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 make the water-cooling slow, or by using a means other than water-cooling that allows for slow cooling, such as air-cooling. The temperatures such as the final rolling temperature and the placing temperature may be measured by a known method such as measurement using 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 this rolled material is subjected to cold working such as wire drawing without softening annealing. It should be noted 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, through trial and error, a manufacturing method different from the above-described manufacturing method. [Example]

[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 method commonly 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 level. 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 rod rolling. Specifically, the samples were processed from a height of 9 mm to a height of 6 mm, and then from a height of 6 mm to a height of 3 mm, at the "final rolling temperature" in Table 2 and with a processing strain rate of 50 / s. Therefore, the "rolled material wire diameter" column in Table 2 is marked with "-".

[0058] [Table 1]

[0059] [Table 2]

[0060] 2. Sample Evaluation The obtained rolled material samples or simulated rolled samples were subjected to metallographic observation, and measurements of the ferrite area ratio 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 using an optical microscope. 2 The area was observed at 400x magnification. 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 fraction Vf was then calculated. In metallographic observation, the white areas represent ferrite and the dark areas represent pearlite. Therefore, for the region where the metallographic observation was performed, the image was binarized based on the color tone level value to determine the ferrite area fraction. More specifically, using the image processing software "Adobe Photoshop" (Adobe Inc.), of the color tone level values ​​of 0 to 255, 235 to 255 was determined to be white, i.e., ferrite. The metallographic 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 this was divided by the total number of pixels to determine the ferrite area fraction Vf. The results obtained are shown in Table 3.

[0063] (Vickers hardness of ferrite and pearlite) The Vickers hardness of ferrite, Hvf, and the Vickers hardness of pearlite, Hvp, were determined by micro-Vickers hardness measurements under a load of 10 gf. Measurements were performed at the D / 4 position of a cross-section etched with picral, as in the metallographic microstructure described above, to identify the ferrite and pearlite regions, respectively. Measurements were performed at five locations for both ferrite and pearlite, and the average values ​​were used as the measurement results. The Vickers hardness of ferrite, Hvf, and the Vickers hardness of pearlite, Hvp, and the ferrite area fraction, Vf, were used to calculate the left side of equation (2), i.e., "Hvp × (1 - Vf / 100)," and the right side of equation (2), i.e., "-0.7358 × Hvf × Vf / 100 + 200.79," as 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] [Table 3]

[0066] As can be seen from Tables 1 to 3, Samples Nos. 1 to 27 have chemical compositions defined by the embodiments of the present invention and are samples produced according to the manufacturing methods of the embodiments of the present invention described above. The total amount (area fraction) of ferrite and pearlite and the ferrite area fraction 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 a state simulating the rolled material is 195 Hv or less. In other words, the samples have excellent cold forgeability even when subjected to cold working such as wire drawing without softening annealing.

[0067] On the other hand, in Samples Nos. 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 Nos. 29 to 31 do not satisfy both formulas (1) and (2), and all of the samples have an average hardness Hva exceeding 195 Hv. In other words, without softening annealing, sufficient cold forgeability cannot be ensured after cold working such as wire drawing. In addition, in 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, Sample No. 32 does not satisfy formula (2), 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] Figure 1 is a graph showing the above-mentioned samples Nos. 1 to 32, with the horizontal axis representing part 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.

Claims

1. C: 0.15 to 0.35% by mass, Si: less than 0.30% by mass (including 0% by mass), Mn: 0.30 to 1.50% by mass, P: 0.030% by mass or less (including 0% by mass), S: 0.030% by mass or less (including 0% by mass), Cr: 0.05 to 0.80% by mass, Ti: 0.01 to 0.08% by mass, B: 0.0005 to 0.0030% by mass, Al: 0.010 to 0.050 mass%; and N: 0.0100% by mass or less (including 0% by mass), and the balance being Fe and unavoidable impurities, The metal structure contains 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, A steel for bolts that satisfies the following formulas (1) and (2). Hvf≦200 (1) 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 fraction of ferrite.

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

3. 3. The steel for bolts according to claim 1 or 2, 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).

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