Non-heat treated steel for bolt

Abstract

To provide a steel for a non-heat treated bolt having high strength and excellent cold forgeability.SOLUTION: A steel for a non-heat treated bolt, comprising C, Si, Mn, P, S, Cr, Al and N in predetermined ranges, with the balance being Fe and inevitable impurities, wherein a carbon equivalent Ceq represented by the following formula (1) is 0.81 or more and 1.20 or less, a steel microstructure is a dual phase microstructure of ferrite and pearlite, and a ferrite fraction in the total steel microstructure is 30% by area or more and less than 60% by area: Ceq = [C] + [Si] / 7 + [Mn] / 5 + [Cr] / 9 (1) In the formula (1), [C], [Si], [Mn] and [Cr] respectively represent the contents of C, Si, Mn and Cr in mass% in the steel.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to a non-heat treated steel for bolts. [Background technology]

[0002] In the manufacture of mechanical structural parts such as bolts, the steel used in the manufacture is required to exhibit, for example, excellent cold workability during part production, and the resulting parts are required to exhibit desired tensile strength, delayed fracture resistance, etc.

[0003] Patent Document 1 proposes a steel wire for machine structural parts that reduces deformation resistance during cold working, improves crack resistance, and exhibits excellent cold workability. The steel wire for machine structural parts has a predetermined chemical composition, and the metal structure of the steel is composed of ferrite and cementite, and the proportion of cementite present at ferrite grain boundaries is 40% or more of the total number of cementite particles.

[0004] The steel wire for machine structural parts described in Patent Document 1 is based on the premise that it will be subjected to heat treatment for tempering when manufacturing the parts. However, in recent years, the omission of this heat treatment, i.e., non-tempering, has been promoted in order to reduce CO2 emissions and save energy. Non-tempered bolts obtained without heat treatment can eliminate the heat treatment and the post-heat treatment process of straightening, thereby achieving cost reductions. Furthermore, this also contributes to shortening the lead time and improving the working environment in the manufacture of bolts, parts, etc.

[0005] As technology relating to non-heat treated bolts, for example, Patent Document 2 discloses wire rod for non-heat treated bolts, steel wire for non-heat treated bolts, methods for manufacturing the same, and non-heat treated bolts. Patent Document 2 discloses a non-heat treated bolt that has a predetermined chemical composition, a specified carbon equivalent Ceq of 0.60 to 0.80, a ferrite fraction of 60 area % or more in the total steel structure, an area fraction of pearlite of 5% or more in the total steel structure, and a ferrite grain size number of 10 or more, as a non-heat treated bolt that has the desired tensile strength and yield strength ratio, as well as the delayed fracture resistance required for a bolt, without the need for high-temperature heat treatment such as quenching and tempering or bluing.

[0006] Patent Document 3 discloses a steel for machine structural use that can exhibit excellent cold forgeability and machinability even without a spheroidizing annealing process. The steel for machine structural use satisfies a specified chemical composition, has a metallographic structure that is a two-phase structure of ferrite and pearlite, has an average lamellar spacing of 220 to 500 nm, an average grain size of pearlite of 30 μm or less, and has a difference of 1 or less between the ferrite grain size number in a region from D / 2 (D: steel cross-sectional diameter) to D / 8 in the cross section of the steel and the ferrite grain size number on the outermost surface.

[0007] Patent Document 4 discloses a steel material to be used for machine structural parts, which has a predetermined chemical composition, and in which the microstructure at a predetermined observation position of a cross section contains ferrite with the remainder consisting of pearlite and / or bainite, the arithmetic mean value of the area fraction of ferrite is 50 to 70%, the standard deviation of the area fraction of the ferrite is 4.0% or less, the ratio of the maximum average grain size to the minimum average grain size among the average grain sizes of ferrite is 2.00 or less, and in which the microstructure at a predetermined observation position of a longitudinal section contains ferrite with the remainder consisting of pearlite and / or bainite, the arithmetic mean value of the area fraction of ferrite is 50 to 70%, the standard deviation of the area fraction of the ferrite is 4.0% or less, and the ratio of the maximum average grain size to the minimum average grain size among the average grain sizes of ferrite is 2.00 or less. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 2018-44235 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-186652 [Patent Document 3] Japanese Patent Application Laid-Open No. 2006-291237 [Patent Document 4] International Publication No. 2022 / 071419 Summary of the Invention [Problem to be solved by the invention]

[0009] Non-tempered steels used in various fasteners and other components are required to have both the cold forgeability required for manufacturing the components and sufficient strength to obtain high-strength components. However, strength and cold forgeability (crack resistance) generally have a trade-off relationship, and as strength increases, cold forgeability deteriorates, presenting the challenge of difficulty in achieving both high strength and excellent cold forgeability. In particular, it is difficult to ensure high cold forgeability when softening annealing is omitted. Therefore, even non-tempered steels are required to exhibit sufficiently high cold forgeability.

[0010] As mentioned above, Patent Document 1 is based on the premise of tempering, and does not consider ensuring cold forgeability in the case of non-tempered steel. Patent Document 2 considers cold forgeability, but does not consider the combination of high strength and high cold forgeability. Patent Document 3 is a technology that omits softening annealing, i.e., is based on the premise of non-tempered steel, but places some emphasis on improving machinability, and does not consider the combination of high strength and high cold forgeability. Patent Document 4 also focuses on improving machinability, but does not consider the combination of high strength and high cold forgeability.

[0011] The present disclosure has been made in view of the above circumstances, and an object of the present disclosure is to provide a steel for non-heat treated bolts that exhibits high strength and good cold forgeability without softening annealing. [Means for solving the problem]

[0012] Aspect 1 of the present invention is C: 0.15% by mass to 0.45% by mass, Si: 1.10% by mass to 2.50% by mass, Mn: more than 1.50 mass% and 2.50 mass% or less, P: more than 0% by mass and 0.030% by mass or less, S: more than 0% by mass and 0.030% by mass or less, Cr: more than 0% by mass and less than 0.50% by mass, Al: 0.010% by mass to 0.100% by mass, and N: more than 0 mass% and 0.020 mass% or less, The balance is Fe and unavoidable impurities, The carbon equivalent Ceq represented by the following formula (1) is 0.81 or more and 1.20 or less, This steel is for use in non-tempered bolts, and has a two-phase structure of ferrite and pearlite, with the ferrite fraction of the total steel structure being 30% by area or more and less than 60% by area. Ceq=[C]+[Si] / 7+[Mn] / 5+[Cr] / 9 (1) In formula (1), [C], [Si], [Mn], and [Cr] represent the contents of C, Si, Mn, and Cr in the steel, respectively, expressed in mass%.

[0013] Aspect 2 of the present invention is The non-heat treated steel for bolts according to aspect 1 further contains one or more selected from the group consisting of Cu: more than 0 mass % and not more than 0.20 mass %, and Ni: more than 0 mass % and not more than 0.20 mass %. [Effects of the Invention]

[0014] According to the present disclosure, it is possible to provide a non-heat treated bolt steel that exhibits high strength and excellent cold forgeability. DETAILED DESCRIPTION OF THE INVENTION

[0015] As a result of extensive research, the inventors have found that by suppressing the specified chemical composition, particularly the C content, and increasing the Si and Mn contents relatively, and by keeping the carbon equivalent Ceq expressed by the specified formula (1) within a certain range, and by keeping the ferrite fraction in the steel structure within a certain range, it is possible to obtain a non-tempered bolt steel that ensures a high tensile strength of 730 MPa or more and exhibits good cold forgeability without softening annealing.

[0016] In the non-thermal-treated bolt steel according to this embodiment, high strength and excellent cold forgeability are achieved by balancing the carbon equivalent (Ceq) and the ferrite fraction. The elements that make up the carbon equivalent (Ceq), namely, C, Si, Mn, and Cr, are all solid-solution strengthening elements, and containing appropriate amounts of these elements improves the strength of the steel. However, excessive C among these elements reduces the ferrite fraction (increases the pearlite fraction), degrading cold forgeability. Therefore, in this embodiment, the C content is reduced to ensure cold forgeability, while the Si and Mn contents are increased compared to conventional steels, thereby improving strength. In particular, increasing the Si content improves both strength and cold forgeability. Since Si is a ferrite solid-solution strengthening element, it is generally known that a high Si content increases deformation resistance and degrades cold forgeability. However, in hypoeutectoid steels such as those described in this embodiment, the effect of the balance between the Si content and the ferrite fraction on cold forgeability has not been investigated to date. According to this embodiment, if the Si content is set to a predetermined high level within a predetermined range of ferrite fraction, it is possible to achieve both high strength and excellent cold forgeability.

[0017] First, the chemical composition of the non-heat treated bolt steel of the present disclosure will be described below.

[0018] 1.Chemical composition C: 0.15% by mass ~ 0.45% by mass C is a solid solution strengthening element necessary for ensuring strength. If the C content is too low, the strength will decrease. Also, if the C content is too low, the ferrite fraction will exceed 60% by area, resulting in an excessive ferrite fraction. From this perspective, the C content must be 0.15% by mass or more. The C content is preferably 0.16% by mass or more, and more preferably 0.17% by mass or more. However, if the C content is excessive, the strength will be excessively high and the cold forgeability will decrease. From this perspective, the C content must be 0.45% by mass or less. The C content is preferably 0.43% by mass or less, and more preferably 0.40% by mass or less.

[0019] Si: 1.10% by mass ~ 2.50% by mass Si is an important element in the non-heat treated bolt steel according to the present disclosure. Si can ensure the strength of the steel material through solid solution strengthening. From this viewpoint, the Si content must be 1.10% by mass or more. The Si content is preferably 1.30% by mass or more, and more preferably 1.50% by mass or more. However, if the Si content is excessive, the deformation resistance increases, causing a decrease in cold forgeability. From this viewpoint, the Si content must be 2.50% by mass or less. The Si content is preferably 2.30% by mass or less, and more preferably 2.10% by mass or less.

[0020] Mn: More than 1.50 mass% and 2.50 mass% or less Mn is an element that can ensure the strength of steel material through solid solution strengthening. Therefore, the Mn content is set to more than 1.50 mass%. The Mn content is preferably 1.60 mass% or more. However, if the Mn content is excessive, the hardness increases and the cold forgeability deteriorates. From this perspective, the Mn content must be set to 2.50 mass% or less. The Mn content is preferably 2.30 mass% or less, and more preferably 2.10 mass% or less.

[0021] P: More than 0% by mass and 0.030% by mass or less 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% 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. However, since it is practically difficult to achieve a P content of 0% by mass, the lower limit of the P content is more than 0% by mass, and there is a tendency for about 0.001% by mass or more of P to remain due to constraints in the manufacturing process, etc.

[0022] S: More than 0% by mass and 0.030% by mass or less 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. However, since it is practically difficult to achieve 0% by mass of S, the lower limit of the S content is more than 0% by mass, and there is a tendency for S to remain at about 0.001% by mass or more due to constraints in the manufacturing process, etc.

[0023] Cr: More than 0% by mass but not more than 0.50% by mass Cr is an effective element for promoting the ferrite + pearlite transformation during hot rolling and precipitating carbides without increasing strength more than necessary. However, excessive Cr content not only increases tensile strength more than necessary, but also increases hardenability and makes pearlite with narrow lamellar spacing more likely to form. As a result, cold forgeability deteriorates. Therefore, the Cr content is set to 0.50% by mass or less. The Cr content is preferably 0.49% by mass or less, and more preferably 0.48% by mass or less.

[0024] Al:0.010 mass%~0.100 mass% 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, Al2O3 is produced in excess, which deteriorates cold forgeability. Therefore, the Al content is set to 0.100% by mass or less. The Al content is preferably 0.080% by mass or less.

[0025] N: More than 0% by mass and 0.020% by mass or less 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 0.020 mass% or less. The lower the N content, the better, with 0 mass% being the most preferable, but there is a tendency for N to remain at approximately 0.0010 mass% or more due to constraints in the manufacturing process, etc.

[0026] Remainder: Fe and unavoidable impurities In a preferred embodiment, the balance is Fe and inevitable impurities. The inevitable impurities include trace elements (e.g., As, Sb, Sn, etc.) that are introduced depending on the conditions of raw materials, materials, manufacturing facilities, etc. For example, there are elements such as P, S, and N, whose content is usually the lower the better, and therefore they are inevitable impurities, but whose composition ranges are separately specified as above. Therefore, in this specification, the term "unavoidable impurities" that make up the balance refers to a concept that excludes elements whose composition ranges are separately specified.

[0027] The chemical composition in this embodiment does not necessarily contain any of the optional elements described below. As long as the desired properties can be maintained, the optional elements described below may also be contained. By including the optional elements described below as necessary, the properties can be further improved.

[0028] One or more selected from the group consisting of Cu: more than 0 mass% and 0.20 mass% or less, and Ni: more than 0 mass% and 0.20 mass% or less Cu and Ni are elements that are effective in improving the corrosion resistance and delayed fracture resistance of steel. The effects of these elements increase as their contents increase. However, excessive amounts of each element cause the following problems. That is, when the Cu content is excessive, the above effects saturate, and hot ductility decreases, resulting in a decrease in steel productivity. From this perspective, when Cu is contained, the Cu content is preferably 0.20% by mass or less, and more preferably 0.15% by mass or less. Furthermore, when the Ni content is excessive, the above effects saturate, leading to an increase in manufacturing costs. From this perspective, when Ni is contained, the Ni content is preferably 0.20% by mass or less, and more preferably 0.15% by mass or less.

[0029] The non-heat treated steel for bolts according to the present disclosure further satisfies the carbon equivalent Ceq represented by the following formula (1) of 0.81 or more and 1.20 or less. Ceq=[C]+[Si] / 7+[Mn] / 5+[Cr] / 9 (1) In formula (1), [C], [Si], [Mn], and [Cr] represent the contents of C, Si, Mn, and Cr in the steel, respectively, expressed in mass%.

[0030] Formula (1) represents the carbon equivalent of a steel material. In formula (1), C, Si, Mn, and Cr are all solid-solution strengthening elements, and adding appropriate amounts improves the strength of the steel material. As described above, if C is included in the elements included in formula (1) in excess, the ferrite fraction decreases (the pearlite fraction increases), and cold forgeability deteriorates. Therefore, the C content is reduced to ensure cold forgeability. In this embodiment, the Si content and Mn content are set relatively higher than those of conventional steels in order to improve strength.

[0031] If the carbon equivalent Ceq is too small, the strength (TS) of the steel material will decrease. Furthermore, if the carbon equivalent Ceq is too small, even if the average cooling rate in a predetermined temperature range after hot rolling, as described below, is increased, the ferrite fraction may become excessive, making it difficult to ensure high strength. From these viewpoints, the carbon equivalent Ceq must be 0.81 or more. The carbon equivalent Ceq is preferably 0.82 or more, and more preferably 0.83 or more. On the other hand, if the carbon equivalent Ceq is too large, the strength of the steel material will be too high, resulting in a decrease in cold forgeability. Therefore, the carbon equivalent Ceq must be 1.20 or less. The carbon equivalent Ceq is preferably 1.10 or less.

[0032] 2.Organization The non-heat-treated bolt steel of the present disclosure has a steel structure (microstructure) that is a two-phase structure of ferrite and pearlite, and the fraction of ferrite in the total steel structure (also referred to as the "ferrite fraction") must be 30 area % or more and less than 60 area %. It is preferable that there is no supercooled structure (martensite and bainite), and the fraction of the supercooled structure is 0 area %.

[0033] By making the steel structure a ferrite-pearlite dual-phase structure and by controlling the ferrite fraction to between 30 and 60 area% of the total steel structure, it is possible to achieve a balanced improvement in tensile strength and cold forgeability. If the ferrite fraction is below 30 area%, the pearlite fraction becomes relatively high, resulting in an excessive increase in strength and a deterioration in cold forgeability. The ferrite fraction is preferably 32 area% or more, more preferably 34 area% or more. On the other hand, if the fraction of ferrite, a softening phase, increases and the ferrite fraction exceeds 60 area%, the strength decreases and high strength cannot be achieved. The ferrite fraction is preferably 58 area% or less, more preferably 56 area% or less. To achieve a structure with a ferrite fraction of less than 60 area%, it is recommended to reduce the carbon content and, as described below, perform controlled rolling with the average cooling rate from 800°C to 500°C after hot rolling within a certain range.

[0034] 3.Characteristics (1) Tensile strength (TS) The non-heat treated steel for bolts of the present disclosure has sufficient strength. In this specification, "sufficient strength" means that the tensile strength (TS) evaluated in the examples described below is 730 MPa or more. The tensile strength (TS) is preferably 750 MPa or more, and more preferably 800 MPa or more. Because the non-heat treated steel for bolts has sufficient strength, the non-heat treated bolts obtained using this non-heat treated steel for bolts can also achieve high strength.

[0035] (2) Cold forgeability As described above, the non-heat treated bolt steel of the present disclosure has excellent cold forgeability. In this specification, "excellent cold forgeability" means that the crack occurrence rate at 65% compression is 0% in an end-restraint compression test evaluated in the Examples described later.

[0036] 4. Manufacturing method The non-heat treated bolt steel according to this embodiment can be produced by the following method. First, a steel satisfying the above-described chemical composition is melted and cast. The casting method is not particularly limited, and a commonly used method may be employed. For example, an ingot casting method or a continuous casting method may be employed. After casting, hot rolling is performed to obtain the non-heat treated bolt steel. After casting and before hot rolling, hot blooming may be performed as necessary. Blooming may include a soaking treatment before blooming. The blooming conditions are not particularly limited, and a commonly used method may be employed.

[0037] The hot rolling process is carried out so as to satisfy the following conditions: (Heating temperature during hot rolling: 1000℃~1200℃) In order to dissolve the carbides present in the steel and form a uniform austenite structure, the heating temperature during hot rolling is set to 1000°C or higher. The heating temperature is preferably 1010°C or higher, and more preferably 1015°C or higher. On the other hand, if the heating temperature is too high, the crystal grains become coarse and the mechanical properties of the bolt deteriorate, so the upper limit is set to 1200°C. The heating temperature is preferably 1190°C or lower, and more preferably 1185°C or lower. After heating at the above heating temperature, hot rolling is performed. The rolling conditions for hot rolling are not particularly limited, and general conditions can be used. After hot rolling, the material is cooled under the following conditions.

[0038] (Average cooling rate from 800°C to 500°C after hot rolling: 1.0-3.0°C / sec) Even if the chemical composition, including the carbon equivalent Ceq, is within the specified range, if the cooling after hot rolling is too rapid, hard structures such as bainite and martensite will form, reducing cold forgeability, and the ferrite fraction will likely fall below 30% by area, making it impossible to obtain excellent cold forgeability. Therefore, the average cooling rate from 800°C to 500°C after hot rolling is set to 3.0°C / sec or less. The average cooling rate is preferably 2.5°C / sec or less, more preferably 2.0°C / sec or less, and even more preferably less than 2.0°C / sec.

[0039] On the other hand, even if the chemical composition including the carbon equivalent Ceq is within the specified range, if the cooling after hot rolling is too slow, the ferrite fraction will be 60 area % or more, and the desired strength will not be obtained. Therefore, from the viewpoint of ensuring strength, the average cooling rate is set to 1.0°C / sec or more. The average cooling rate is preferably 1.2°C / sec or more, and more preferably 1.4°C / sec or more. The cooling from 500°C to room temperature is not particularly limited, and for example, natural cooling can be used. The cooling step conditions shown in Patent Document 4 are an average cooling rate of 0.10 to 1.00°C / sec from 800 to 300°C, which is slower than that of the present embodiment. [Example]

[0040] The present invention will be described in more detail below with reference to examples. The present disclosure is not limited to the following examples, and appropriate modifications can be made within the scope of the above-mentioned and below-mentioned aims, and all such modifications are included in the technical scope of the present disclosure.

[0041] 1. Sample Preparation For Nos. 1 to 7, a small melting furnace (capacity 150 kg / ch) was used to smelt steel materials satisfying the chemical compositions shown in Table 1 below, and the steel materials were cast to obtain ingots. The ingots were then heated at 1150 to 1300°C for 60 minutes or more and then hot forged to obtain steel slabs with a target size of 155 mm x 155 mm and dimensional tolerances of 153 to 156 mm x 153 to 156 mm. The steel slabs were hot rolled under the conditions shown in Table 2 to obtain rolled material samples with the rolled diameters shown in Table 2.

[0042] For Nos. 8 and 9, using actual equipment, the following processes were carried out in the usual manner: converter melting, casting, blooming, and hot rolling under the conditions shown in Table 2, to obtain rolled material samples with the rolled diameters shown in Table 2.

[0043] [Table 1]

[0044] [Table 2]

[0045] 2. Observation of steel structure and measurement of ferrite fraction The cross section (cross section perpendicular to the hot rolling direction) of the obtained rolled material sample was polished and etched with picral solution, and then the metal structure was observed using an optical microscope. 2The area was observed at 400x magnification. As a result, only ferrite and pearlite were observed as the steel structure. In other words, the total of ferrite and pearlite was 100% by area. Based on this result, the ferrite fraction was then calculated. In the steel structure observation, the white areas are ferrite and the dark areas are pearlite, so for the area where the metal structure observation was performed above, the image was binarized based on the color level value to determine the ferrite fraction. The results are shown in Table 3. In this example, a sample with a ferrite fraction of 30% by area or more and less than 60% by area was considered to be acceptable.

[0046] 3. Characterization (1) Tensile test The rolled material samples were subjected to a tensile test in accordance with JIS Z2241:2022 using tensile test specimens with lengths of 200 mm to 400 mm to determine the tensile strength. The results are shown in Table 3. In this example, specimens with a tensile strength TS of 730 MPa or more were evaluated as having high strength (passing).

[0047] (2) Cold forging test Test samples measuring 8 mm in diameter (φ) and 12 mm in length were prepared from the rolled material samples and subjected to end-constrained compression tests as described below. In the end-constrained compression tests, the test samples were placed in a forging press and subjected to cold forging tests at room temperature, with a strain rate of 5 s to 10 s, and a compression ratio of 65%. After the cold forging tests, the surfaces were observed at 20x magnification using a stereomicroscope to check for the presence or absence of cracks. This test and surface observation were performed on five samples for each of Nos. 1 to 9. The crack occurrence rate (%) was calculated as 100 x (number of test samples with cracks) / (total number of test samples = 5). The results are shown in Table 3. In this example, a crack occurrence rate of 0% was evaluated as "excellent cold forgeability," and a crack occurrence rate of more than 0% was evaluated as "poor cold forgeability."

[0048] [Table 3]

[0049] The results in Tables 1 to 3 reveal the following: Nos. 1 to 4 are inventive examples that satisfy all of the requirements of the embodiments of the present invention. That is, because they satisfy the predetermined chemical composition and carbon equivalent Ceq and have the specified steel structure, all of them have a TS of 730 MPa or more, and the crack occurrence rate at 65% compression in the end-constrained compression test was 0%.

[0050] In contrast, Nos. 5 to 9 did not satisfy at least one of the chemical composition, carbon equivalent Ceq, and steel structure, and either could not ensure high strength or were inferior in cold forgeability.

[0051] No. 5 did not achieve the desired strength due to a low carbon equivalent Ceq and an excessive ferrite fraction, but its cold forgeability was ensured due to its low strength.

[0052] In No. 6, the desired strength was not obtained due to the low carbon equivalent Ceq. However, since the range of each element and the manufacturing conditions were the same as those of the inventive examples, the ferrite fraction was within the specified range. Furthermore, because the strength was low, cold forgeability was ensured.

[0053] No. 7 did not achieve the desired strength due to a low carbon equivalent Ceq and an excessive ferrite fraction. However, because of its low strength, cold forgeability was ensured.

[0054] In No. 8, the average cooling rate after hot rolling was high, resulting in a significantly smaller ferrite fraction and poor cold forgeability. Although No. 8 had a low carbon equivalent (Ceq), the ferrite fraction was also low, resulting in sufficient strength.

[0055] No. 9 did not achieve the desired strength due to its low carbon equivalent Ceq. However, because of its low strength, cold forgeability was ensured.

Claims

1. C: 0.15% by mass to 0.45% by mass, Si: 1.10% by mass to 2.50% by mass, Mn: more than 1.50% by mass and not more than 2.50% by mass, P: more than 0% by mass and not more than 0.030% by mass, S: more than 0% by mass and not more than 0.030% by mass, Cr: more than 0% by mass and not more than 0.50% by mass, Al: 0.010% by mass to 0.100% by mass, and N: more than 0 mass% and 0.020 mass% or less; The balance is Fe and unavoidable impurities, The carbon equivalent Ceq represented by the following formula (1) satisfies 0.81 or more and 1.20 or less, A steel for non-tempered bolts, the steel structure of which is a two-phase structure of ferrite and pearlite, and the proportion of ferrite in the total steel structure is 30 area % or more but less than 60 area %. Ceq=[C]+[Si] / 7+[Mn] / 5+[Cr] / 9 (1) In formula (1), [C], [Si], [Mn], and [Cr] represent the contents of C, Si, Mn, and Cr in the steel, respectively, expressed in mass %.

2. The non-heat treated steel for bolts according to claim 1, further containing one or more selected from the group consisting of Cu: more than 0 mass% and 0.20 mass% or less, and Ni: more than 0 mass% and 0.20 mass% or less.

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