Patent ID: 12203149

DETAILED DESCRIPTION

The steel for non-heat-treated bolts disclosed herein will be specifically described below. First, the reasons for limitations on each component in the chemical composition will be explained. When components are expressed in “%”, this refers to “mass %” unless otherwise specified. Also, percentages of each microstructure are area fractions unless otherwise noted.

C: 0.18% to 0.24%

Carbon (C) is a beneficial element that can dissolve or form carbides in steel and improve the strength of the steel. C also becomes cementite when the steel forms a bainitic microstructure, and is also a source of dislocation generation. C is also an element that significantly improves the quench hardenability of the steel. To obtain these effects, C needs to be contained in an amount of 0.18% or more, and preferably 0.20% or more. On the other hand, C is an element that increases the quench hardenability of steel, and if contained above 0.24%, it increases the quench hardenability of the steel to the extent that it causes martensitic transformation instead of bainitic transformation, making the steel unsuitable for non-heat-treated bolts. In other words, if the steel has a martensitic microstructure, the dislocation density is too high that it inhibits dislocation migration and reduces the room for pile-up, resulting in inability to obtain a sufficient Bauschinger effect. As a result, not only is a sufficient Bauschinger effect not achieved, but also the drawability of the steel wire after wiredrawing is significantly reduced, making it unsuitable for use in bolts. Therefore, the upper limit of C content is set at 0.24%, and preferably at 0.22% or less.

Si: 0.10% to 0.22%

Silicon (Si) is an important element that can dissolve in iron and increase the strength of steel, yet it also has the effect of significantly increasing deformation resistance. In addition, Si is an effective element for adjusting the quench hardenability of steel and widening the range of cooling rates at which bainite can be obtained with an appropriate amount of Si added. To obtain this effect, Si needs to be contained in an amount of 0.10% or more, and preferably 0.13% or more. On the other hand, Si is an element that accelerates work hardening when added unnecessarily, deformation resistance after wiredrawing becomes so large that it cancels out the Bauschinger effect of bainite. Therefore, the upper limit of Si content is set at 0.22%. It is more preferably 0.20% or less.

Mn: 0.60% to 1.00%

Manganese (Mn) is an element that promotes the formation of bainite during steel cooling. To obtain this effect, Mn needs to be contained in an amount of 0.60% or more, preferably 0.65% or more, and more preferably 0.70% or more. On the other hand, Mn is an element that increases the quench hardenability of steel, and if contained in excess, it increases the quench hardenability of the steel to the extent that it causes martensitic transformation, making the steel unsuitable for use in non-heat-treated bolts. Therefore, the upper limit of Mn content is set at 1.00%. It is preferably 0.95% or less, and more preferably 0.90% or less.

Al: 0.010% to 0.050%

Aluminum (Al) combines with nitrogen (N) at or below about 1000° C. to form a precipitate as MN (aluminum nitride), which suppresses the coarsening of austenite crystal grains during heating for hot rolling. Al also has the effect of deoxidizing the steel. In other words, when the oxygen in the steel combines with C to form a gas, the amount of C in the steel decreases and the desired quench hardenability cannot be obtained. Therefore, it is necessary to deoxidize the steel with Al. To obtain these effects, Al needs to be contained in an amount of 0.010% or more. More preferably, it is 0.020% or more. On the other hand, if Al is present in excess, it will crystallize in large amounts as oxides that can cause nozzle clogging when combined with oxygen in the air during casting. Therefore, the upper limit of Al content is set at 0.050%. Preferably, it is 0.040% or less.

Cr: 0.65% to 0.95%

Chromium (Cr) is an element that improves the quench hardenability of steel and promotes bainitic transformation. To obtain this effect, Cr needs to be contained in an amount of 0.65% or more. On the other hand, if Cr is contained in excess above 0.95%, it increases the quench hardenability of the steel to the extent that it causes martensitic transformation, making the steel unsuitable for use in non-heat-treated bolts. Therefore, the upper limit of Cr content is set at 0.95%. More preferably, it is 0.70% or more and 0.90% or less.

Ti: 0.010% to 0.050%

Titanium (Ti) is an element that combines with N (nitrogen) to form a precipitate as a nitride, complementing the above-mentioned function of Al. Therefore, the Ti content is 0.010% or more. On the other hand, if the content exceeds 0.050%, Ti, like Al, will crystallize in large amounts as oxides that can cause nozzle clogging and so on when combined with oxygen in the air during casting. Therefore, the upper limit of Ti content is set at 0.050%. Preferably, it is 0.015% to 0.045%.

B: 0.0015% to 0.0050%

Boron (B) is an element that increases the quench hardenability of steel and promotes bainitic transformation. To obtain this effect, B needs to be contained in an amount of 0.0015% or more. On the other hand, if the content exceeds 0.0050%, the quench hardenability becomes too high and the steel inevitably has a martensitic microstructure. Therefore, the upper limit is set at 0.0050%. Preferably, it is 0.0018% or more and 0.0040% or less.

N: 0.0050% to 0.0100%

Nitrogen (N) combines with Al to form a precipitate as AlN, which suppresses the coarsening of austenite crystal grains during heating for hot rolling. To obtain this effect, the N content is 0.0050% or more. It is preferably 0.0055% or more. On the other hand, if N is present in excess in steel, it will turn into solute nitrogen to immobilize dislocations even after hot rolling, thus reducing the Bauschinger effect. Therefore, the upper limit of N content is set at 0.0100%. Preferably, it is 0.0090% or less.

As mentioned above, since the presence of N in the steel as solute nitrogen, even in small amounts, has the effect of reducing the Bauschinger effect, it is necessary to ensure that N is caused to precipitate before the end of hot rolling. To achieve this, the N content should be within the above range, and furthermore, the total content of Al and Ti, which form precipitates with N, should be greater than the N content in moles. Therefore, the following formula (2) should be satisfied:
N≤0.519Al+0.292Ti  (2),
where N, Al, and Ti represent the contents in mass % of respective elements.

The balance of the chemical composition containing the above elements includes Fe and inevitable impurities. Preferably, the balance consists of Fe and inevitable impurities. As the chemical components detected as inevitable impurities, the contents of phosphorus (P), sulfur (S), copper (Cu), and nickel (Ni) should be suppressed within the following ranges.P: 0.025% or less inclusive of 0S: 0.025% or less inclusive of 0
P and S are impurities derived from raw materials, and although efforts have been made to reduce them in the steel refining process, it is not industrially realistic to reduce their contents completely to zero. Both P and S have the effect of embrittling the steel, yet they are not harmful to the actual use of the bolts if their contents are kept as low as 0.025% or below.Cu: 0.20% or less inclusive of 0Ni: 0.30% or less inclusive of 0
Cu and Ni are impurities that are inevitably contained in the raw material when the raw material is scrap metal. If Cu is contained in the steel in excess of 0.20%, the grain boundaries on the surface of the steel become embrittled during hot rolling, causing surface defects. Therefore, it is preferable to keep the Cu content at or below 0.20%. On the other hand, Ni is an element that increases the quench hardenability of steel, and thus its concentration should be kept at or below 0.30% to avoid the formation of a martensitic microstructure. Inevitable impurities other than those mentioned above can be considered as not being added if the amount is kept below the lower limit of the analysis capability of the component analyzer.

Furthermore, the chemical composition should satisfy:
0.45≤C+Si/24+Mn/6+Ni/40+Cr/5≤0.60  (1),
where C, Si, Mn, Ni, and Cr represent the contents in mass % of respective elements.

In other words, in order to obtain a sufficient Bauschinger effect, the microstructure should be composed of bainite single-phase as much as possible, and the formation of a ferritic microstructure should be suppressed. This is because in the presence of a ferritic microstructure, pile-up of dislocations is concentrated in ferrite crystal grains. Therefore, the formula (1), which specifies the right balance between the components to achieve both of the above two points, needs to yield a value of 0.45 or more. The formula (1) preferably yields a value of 0.47 or more, more preferably 0.49 or more, and most preferably 0.50 or more. Note that when Ni is not contained, the value of Ni content in the formula (1) is considered to be 0 (zero).

The formula (1) is useful not only from the viewpoint of Bauschinger effect but also from the viewpoint of strength variation. That is, if the formula (1) yields a value equal to or higher than the lower limit, the microstructure becomes substantially bainite-single phase, making it possible to prevent the formation of excessively low strength portions in a part of the wire rod due to the inclusion of ferrite in the microstructure. In contrast, if martensite is mixed in with the bainite single-phase microstructure, there is a concern that excessively high strength portions may be formed. To avoid this, the formula (1), which specifies the right balance between the components, needs to yield a value of 0.60 or less. The upper limit in the formula (1) is preferably 0.59 or less, more preferably 0.58 or less, and most preferably 0.57 or less.

Optionally, the above chemical composition may further contain Nb to ensure proper quench hardenability.

Nb: 0.050% or Less

Niobium (Nb) is an element that combines with nitrogen to form a precipitate as a nitride, complementing the function of Al. In other words, in order to ensure quench hardenability by adding Nb, Nb is preferably added in an amount of 0.005% or more. On the other hand, if Nb is added in excess beyond 0.050%, nitrides will preferentially precipitate at grain boundaries of the steel, lowering the strength at the grain boundaries and causing intergranular cracking, which will leave surface cracks after casting. Therefore, the Nb content is 0.050% or less, and more preferably 0.040% or less.

Optionally, the above chemical composition may further contain Mo.

Mo: 0.70% or Less

Molybdenum (Mo) is an element that suppresses the segregation of intergranular embrittlement elements such as P and S at austenite grain boundaries during heating, and reduces the risk of cracking occurring at prior austenite grain boundaries when dislocations are piled up. To this end, Mo is preferably added in an amount of 0.05% or more. On the other hand, Mo also has the effect of increasing the quench hardenability of steel, and if added in excess, the microstructure of the steel will be martensitic instead of bainitic. Therefore, the upper limit of Mo content is preferably set at 0.70%. It is more preferably 0.60% or less.

When Mo is added, for the same reason as in the formula (1), the following formula (3) should be satisfied:
0.45≤C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4≤0.60  (3),
where C, Si, Mn, Ni, Cr, and Mo represent the contents in mass % of respective elements.

Next, it is important for the steel for bolts to have a microstructure in which bainite is present in an amount of 95% or more and that contains prior austenite grains with a grain size number of 6 or more.

Bainite: 95% or More

In order to obtain a sufficient Bauschinger effect in bolt head forming after wiredrawing, the microstructure should be composed of bainite single-phase as much as possible, as described above. From the viewpoint of suppressing strength variation, it is also preferable that the microstructure be as close to a bainite single-phase microstructure as possible. In view of the above, bainite should be present in an area ratio of at least 95% or more. The area ratio is preferably 97.5% or more, and more preferably 99% or more. Of course, it may be 100%.

The microstructure proportions of bainite and ferrite both mean the area ratios on the surface where the microstructure observation is conducted.

Grain Size Number of Prior Austenite Grains: 6 or More

Since a prior austenite grain boundary is the place where dislocations pile up when the microstructure is a bainitic microstructure, dislocations will not pile up sufficiently unless a grain size of 6 or more in terms of grain size number specified in JIS G0551 is ensured, resulting in inability to obtain a sufficient Bauschinger effect. Preferably, the grain size is 7 or more.

Strength Variation: 100 MPa or Less

Unlike the steel for heat-treated bolts, the strength of the steel for non-heat-treated bolts after work hardening by wiredrawing is directly related to the strength of the resulting bolts, and thus the strength variation of the wire rod directly affects the strength variation of the final product, the bolt. In addition, large strength variation of wire rods has a pronounced effect on the incidence of defects in the products and manufacturing equipment during the manufacturing process following the production of the wire rods, i.e., wiredrawing and bolt head forming. Taking these factors into consideration, it is desirable to keep the strength variation within 100 MPa, and more preferably within 80 MPa, in the actual manufacturing of bolts.

As mentioned above, since steel for non-heat-treated bolts is usually used in the manufacture of bolts as wire rods, the strength variation in steel for non-heat-treated bolts is directly related to the strength variation of the wire rod. The strength variation of a wire rod refers to the strength variation within a single ring of a wire rod. In the case of products shipped in coils such as steel wire rods, a wire rod is often cooled in the form of a stretched coil by stacking multiple rings with their axial centers mutually displaced in the conveying direction using a laying head or the like during the conveying process for coiling the wire rod. In this case, depending on the degree of overlap between the rings, some parts of a ring cool faster than others, and uneven cooling occurs within the same ring. This causes strength variation within the ring, and it is customary to regard this strength variation within the ring as the strength variation of the entire coil. In fact, during the outgoing inspection of a coil, several to a dozen rings are truncated from both ends of the coil immediately after rolling as the unsteady part, and then a tensile test specimen is taken from an end of the remaining steady part as appropriate to investigate the strength variation.

Next, a method of manufacturing a steel for bolts will be described in detail.

It is important to finish hot rolling of a steel billet having the above chemical composition at a hot-rolling finish temperature of 800° C. to 950° C., and then cool them at a cooling rate of 2° C./s or higher and 12° C./s or lower in a temperature range from the hot-rolling finish temperature to 500° C. In order to maximize the Bauschinger effect, it is necessary to cause bainitic transformation while suppressing ferrite precipitation during cooling after hot rolling of the steel. When the hot-rolling finish temperature exceeds 950° C., it becomes industrially difficult to ensure a cooling rate of at least 2° C./s in a temperature range down to 500° C., and ferrite precipitation occurs. Even if ferrite precipitation could be suppressed, austenite grains would be coarsened, and prior austenite grains in the resulting microstructure would have a grain size number of less than 6. The hot-rolling finish temperature is more preferably 925° C. or lower.

On the other hand, when the hot-rolling finish temperature is lower than 800° C., recovery of dislocations introduced during the hot rolling and recrystallization are inhibited, and ferrite precipitation occurs using the dislocations as precipitation nuclei. Therefore, the hot-rolling finish temperature is 800° C. or higher. More preferably, it is 825° C. or higher. In order to cause bainitic transformation in a steel with the component proportions balanced as in the formula (1) or (3), it is necessary to cool the steel at a cooling rate of 2° C.//s or higher after hot rolling. It is preferably 3° C./s or higher, more preferably 4° C./s or higher, and most preferably 5° C./s or higher. On the other hand, if the cooling rate is too fast than 12° C./s, a martensitic microstructure will be formed. Therefore, the cooling rate is 12° C./s or lower. It is preferably 11° C./s or lower, and more preferably 10° C./s or lower.

The above steel for bolts after hot rolling is generally made as a coiled wire rod, and the roundness of the cross-sectional shape of the wire rod is low. In addition, the surface of the wire rod is covered with an oxide film formed during cooling after hot rolling. Thus, it is not desirable to use it as is for bolts. Therefore, after removing the oxide film from the above wire rod by pickling, the wire rod is drawn to make a steel wire for bolts with high roundness. The steel wire obtained by the wiredrawing process preferably has a critical compression ratio of 40% or more. As used herein, the critical compression ratio refers to a critical setting ratio determined by the cold setting test established by the Cold Forging Subcommittee of the Japan Society for Technology of Plasticity (see, “Journal of Plasticity and Machining”,1981, Vol. 22, No. 241, p. 139, published by the Material Research Group of Cold Forging Subcommittee).

Example 1

The present disclosure will be described below based on examples. However, it is not limited to the examples disclosed herein. Note that P, S, Cu, and Ni are the components derived from raw materials. P and S are impurities that are difficult to remove completely. Cu and Ni are concentrated in the steel at concentrations that are orders of magnitude higher when scrap is used as the raw material than when iron ore is used as the raw material. Accordingly, these components were intentionally added to each steel specimen to match the actual conditions.

Steel specimens with the chemical compositions listed in Table 1 were smelted in a vacuum melting furnace, and a 50 kg steel ingot was cast. In this case, Steel Nos. 52 and 56 were abandoned because a large amount of Si oxides, Al oxides, or Ti oxides were precipitated during casting, the hot ductility decreased, many cracks occurred in the ingot, and these specimens were unusable for subsequent rolling.

Each steel specimen thus obtained was heated to 1050° C. or higher and drawn to a wire rod of 16.0 mmϕ by applying hot rolling. At that time, the hot-rolling finish temperature was adjusted as listed in Table 2. Then, the wire rods after hot rolling were cooled at various cooling rates listed in Table 2 to build up microstructures presented in Table 2. A cylindrical specimen for measuring the deformation resistance was processed from each wire rod thus obtained. Each cylindrical specimen was sized 10 mmϕ×15 mm. The deformation resistance measurement method was as proposed by Osakada et al. in Ann. CIRP in 1981 based on the above-described cold setting test method. The stress at a strain of 0.50 in the stress-strain curve obtained in the compression test according to this method was used as the deformation resistance. The compression speed during the compression test was set at 5 mm/min.

The strength variation was also investigated in each wire rod after hot rolling. Each specimen was in the form of a coil of the corresponding wire rod after hot rolling as described above. After truncating 10 rings from both ends of the coil of each wire rod as the unsteady part, a wire rod of 3 m long was cut from an end of the remaining steady part. Then, each 3 m-long wire rod was further divided into 12 sections, each of which sections was used as a No. 2 test piece as specified in JIS Z2241 and examined for tensile strength. The reason why the length was set to 3 m is that since the inner diameter of the coil of each wire rod at the time of the investigation was 1 m, the present inventors multiplied the inner diameter by the circumference factor to obtain a ring equivalent to about 3 m, and decided to divide each 3 m-long wire rod into 12 sections. The speed of the tensile test was set at 10 mm/min. The strength of each wire rod is the maximum stress attained during the tensile test, and the strength variation is the difference between the specimen that showed the highest attained maximum stress and the lowest among the 12 specimens.

In addition, the above hot-rolled wire rods were drawn by cold wiredrawing into 12.7 mmϕ or, for some, 14.7 mmϕ (Sample No. 79 in Table 2) and 10.4 mmϕ (Sample No. 80) steel wires. Each steel wire obtained after the wiredrawing was processed into test pieces for measuring the deformation resistance and tensile test pieces in the same way as described above. The test specimens and test method for determining the deformation resistance were the same as above. The tensile test specimens were No. 2 test specimens as specified in JIS Z2241. The tensile speed was set at 10 mm/min. The strength of each steel wire was the maximum stress attained during the tensile test, and the drawability was determined by comparing the diameter of the fractured part of each specimen after application of tension with the diameter of the specimen before application of tension.

From each drawn steel wire, a grooved cylindrical specimen was also machined to measure the critical compression ratio. The specimen for measuring the critical compression ratio was a 10 mmϕ×15 mm cylindrical specimen with a single groove extending in the axial direction (opening angle: 30°±5°, depth: 0.8 mm±0.05 mm, radius of the groove bottom: 0.15 mm±0.05 mm) machined at an arbitrary position on its circumference. The test method for the critical compression ratio was also based on the method established by the Cold Forging Subcommittee of the Japan Society for Technology of Plasticity. The compression speed of the compression test to measure the critical compression ratio was also set to 5 mm/min. Note that in the actual manufacture of bolts in general, when the critical compression ratio of the steel wire is 40% or higher, the incidence of cracks during bolt head forming is reduced, which improves the process capability and leads to improved efficiency in spot-checking and inspection of the product, which in turn reduces the risk of outflow of defective products.

The test results are listed in Table 2.

Note that Comparative Examples of Sample Nos. 57 and 63 contained a large amount of Nb and Cu, respectively, beyond the amounts specified in this disclosure, which caused a large number of surface defects in the wire rods after hot rolling and made it impossible to practically perform wiredrawing. Thus, items including the prior austenite grain size are shown as blank.

The Bauschinger effect was evaluated as “good” when the deformation resistance of the steel wire after wiredrawing was not greater than the value obtained by multiplying the deformation resistance of the wire rod after hot rolling by 1.05, and as “poor” when the deformation resistance exceeded the value. As for the strength, if the strength of 800 MPa or more, which is required for bolts with a strength classification of 8.8 or higher, was obtained in the steel wire that had undergone the above process, the specimen passed the test, whereas if the strength was less than 800 MPa, the specimen failed the test. In addition, if a drawability of 52% or more, which is required for bolts with a strength classification of 8.8 or higher, was achieved, the specimen passed the test, whereas if the drawability was less than 52%, the specimen failed the test.

TABLE 1-1SteelChemical compositionFormulaSatisfy orsample(mass %)(ppm by mass)(mass %)(1)′ ornot satisfyNo.CSiMnPSCuNiCrAlTiBNMoNb(3)′formula (2)Remarks10.180.120.610.0100.0250.200.150.880.0490.01016100——0.47satisfyExample20.200.210.990.0150.0100.050.080.710.0110.0165045——0.52satisfyExample30.220.140.660.0120.0210.150.300.790.0390.0321969——0.50satisfyExample40.240.190.940.0130.0050.190.220.940.0220.0463951——0.60satisfyExample50.190.100.710.0080.0150.060.140.890.0300.0482679——0.49satisfyExample60.230.130.890.0140.0240.140.090.680.0480.0121781——0.52satisfyExample70.210.160.860.0250.0110.180.290.720.0120.0174899——0.51satisfyExample80.180.200.620.0120.0200.070.210.800.0380.0332046——0.46satisfyExample90.210.220.980.0100.0060.130.130.930.0230.0473868——0.57satisfyExample100.240.150.600.0240.0160.170.100.880.0310.0132752——0.52satisfyExample110.230.180.650.0060.0230.080.280.690.0470.0181878——0.49satisfyExample120.190.160.700.0200.0120.120.200.730.0130.0344782——0.46satisfyExample130.200.170.850.0050.0190.160.120.810.0370.0392198——0.51satisfyExample140.180.120.900.0110.0070.090.110.920.0240.0143747——0.52satisfyExample150.240.190.950.0160.0170.100.270.870.0330.0192867——0.59satisfyExample160.210.161.000.0220.0220.200.230.860.0140.0351953——0.56satisfyExample170.240.170.670.0140.0130.150.160.850.0100.0384677——0.53satisfyExample180.200.210.930.0120.0180.180.170.840.0210.0152283——0.54satisfyExample190.200.190.720.0240.0080.080.180.740.0290.0203697——0.48satisfyExample200.230.200.880.0180.0240.090.150.760.0400.0362948——0.54satisfyExample210.210.160.870.0050.0140.080.300.750.0500.0374566——0.52satisfyExample220.190.160.630.0130.0090.050.290.770.0250.0162454——0.46satisfyExample230.240.140.970.0070.0250.140.100.660.0460.0212376——0.54satisfyExample240.200.160.640.0210.0050.080.110.700.0260.0294184——0.46satisfyExample250.220.180.610.0060.0200.160.160.780.0350.0174296——0.49satisfyExample260.210.190.890.0250.0120.180.080.900.0340.0222649——0.55satisfyExample270.230.210.600.0210.0130.170.090.950.0190.0281965——0.53satisfyExample280.240.161.000.0110.0240.150.130.890.0330.0114555——0.59satisfyExample290.200.190.880.0230.0100.140.120.690.0280.0154875——0.50satisfyExample300.230.220.970.0130.0050.120.230.860.0390.0312885——0.58satisfyExample310.230.190.990.0240.0240.200.180.740.0270.0454595——0.56satisfyExample320.240.200.710.0050.0160.050.300.660.0350.0493351——0.51satisfyExample330.190.140.620.0100.0070.140.100.950.0100.0181564——0.49satisfyExample340.210.190.650.0050.0180.130.300.680.0400.0231856——0.47satisfyExample350.220.100.950.0130.0240.100.290.880.0260.0272574——0.57satisfyExample360.180.200.720.0240.0250.080.200.920.0410.0194086——0.50satisfyExample370.240.170.970.0140.0210.160.160.840.0480.0244994——0.58satisfyExample380.200.100.990.0240.0110.140.300.670.0390.0461699—0.0500.51satisfyExample390.230.160.710.0250.0060.150.160.710.0120.0332052—0.0400.50satisfyExample400.190.190.980.0130.0070.060.130.800.0470.0144767—0.0050.52satisfyExample410.200.161.000.0240.0140.130.150.810.0210.0202248—0.0100.54satisfyExample420.180.160.660.0140.0250.100.220.660.0260.02824760.70—0.60satisfyExample430.190.220.600.0110.0200.180.210.700.0390.03119650.60—0.59satisfyExample440.200.100.650.0070.0100.050.280.710.0480.01925860.50—0.59satisfyExample450.220.140.650.0240.0130.140.200.740.0260.0314867——0.49satisfyExample* For Mo-free steel, formula (1)′: C + Si/24 + Mn/6 + Ni/40 + Cr/5, for Mo-containing steel, formula (3)′: C + Si/24 + Mn/6 + Ni/40 + Cr/5 + Mo/4.

TABLE 1-2SteelChemical compositionsample(mass %)No.CSiMnPSCuNiCrAlTi460.190.210.950.0220.0230.150.100.820.0260.043470.180.220.620.0220.0210.110.110.660.0290.022480.360.121.950.0190.0150.150.030.310.0200.031490.240.130.220.0150.0180.140.080.950.0160.019500.220.152.500.0230.0120.110.040.820.0390.047510.180.200.850.0130.0140.190.170.650.0060.015520.180.200.990.0140.0120.060.150.880.0620.038530.250.190.890.0120.0120.110.110.670.0240.025540.250.200.680.0180.0130.140.190.880.0110.012550.180.170.650.0130.0160.080.221.500.0420.045560.190.140.960.0240.0240.170.110.910.0170.056570.200.190.880.0180.0230.130.050.700.0230.022580.200.210.930.0120.0160.070.130.810.0330.048590.180.110.620.0130.0160.170.110.660.0410.042600.150.200.890.0120.0190.130.030.910.0170.036610.180.110.790.0320.0160.130.220.660.0160.012620.210.160.800.0160.0310.150.080.810.0300.043630.190.200.690.0190.0140.320.220.910.0400.041640.180.220.990.0150.0210.170.340.660.0400.021650.220.150.880.0120.0130.090.050.920.0110.005660.180.180.750.0150.0090.130.100.760.0050.042670.220.100.990.0090.0090.100.100.270.0220.031680.180.100.600.0110.0140.100.050.650.0320.029690.240.191.000.0210.0110.100.020.950.0250.041700.200.110.600.0150.0220.090.130.650.0260.035710.210.200.680.0240.0080.080.180.660.0290.020720.210.300.890.0130.0050.120.230.880.0270.023730.180.341.210.0100.0200.090.051.330.0270.045740.170.321.180.0090.0110.060.051.390.0250.044SteelChemical compositionFormulaSatisfy orsample(ppm by mass)(mass %)(1)′ ornot satisfyNo.BNMoNb(3)′formula (2)Remarks461366——0.52satisfyComparative Example474861——0.43satisfyComparative Example482576——0.75satisfyComparative Example493647——0.47satisfyComparative Example503163——0.81satisfyComparative Example511977——0.46not satisfyComparative Example522246——0.53satisfyComparative Example5327122——0.54satisfyComparative Example542497——0.55not satisfyComparative Example554789——0.60satisfyComparative Example56565——0.54satisfyComparative Example572757—0.0720.50satisfyComparative Example586666——0.53satisfyComparative Example5934630.71—0.60satisfyComparative Example601642——0.49satisfyComparative Example612767——0.45satisfyComparative Example622462——0.51satisfyComparative Example632477——0.50satisfyComparative Example642485——0.49satisfyComparative Example652265——0.56satisfyComparative Example662984——0.47satisfyComparative Example672095——0.45satisfyComparative Example68157——0.42satisfyComparative Example694499——0.61satisfyComparative Example7019870.69—0.61satisfyComparative Example713647——0.47satisfyComparative Example722848——0.55satisfyComparative Example732255——0.66satisfyComparative Example742165—0.0340.66satisfyComparative Example* For Mo-free steel, formula (1)′: C + Si/24 + Mn/6 + Ni/40 + Cr/5, for Mo-containing steel, formula (3)′: C + Si/24 + Mn/6 + Ni/40 + Cr/5 + Mo/4.

TABLE 2-1Hot(1) Defor-Area(2) Defor-rollingStrengthmationreductionmationfinishBainitePriorvariationresistancerate ofresistanceSteeltemper-microstructureausteniteof wireof wirewireof steelSamplesampleatureCoolingproportiongrainrodroddrawingwireNo.No.(° C.)rate(%)size(MPa)(MPa)(%)(MPa)119075.19678996837999228635.0100855970996338555.695993987976448143.79698499210115589510.8100752970970669234.71001059984978779115.799665971980888305.0987699861001998515.89896298697610108626.31001160974100811118274.0100951969100812128155.29679296996213138463.99976398198814148364.295999968100415158863.596782972101216168455.910065597099417179195.697108898698618189403.6100757961101119198205.09668797799420208087.1100759993980212190711.399106295799422228753.79986497597423238295.91006569751000242486610.8987739883798125259014.796794966100326268275.71008589671016272781410.197677973101828288953.010095296099129298456.395109899999030309234.310095395199631318995.29897196899532329174.19579996998033338303.910075497898734348095.096789968100835358623.59777397096536368365.997979973100037379165.695119396197838388083.6971176984103139398355.396781987102040408368.810075997899641419056.399766986101842428684.399667952980434382910.1100105896297644448153.09777599299145458363.696108397099146468662.9887109102137111947479323.662711181691148488476.2martensite10951189128949498656.0798135926102050508603.8martensite8811141127951519204.79946396110595252———————53538905.796778976106854549173.697472970104755559434.4martensite799122013095656———————57578294.6many————surfacedefects58588933.4martensite9831171127959598936.1martensite8861222132260608757.7728129926102061618745.39977196796162628033.69577797597963638514.6many————surfacedefects64648845.6martensite791119937128965658705.097866992105866668805.3717122943105467679005.3818126990113268688624.0898131984104169698295.7798114996106270708628.86971201002106971718884.799463961105972729038.8979771013119573739110.49591191222135674748880.69791131174142075198511.67881199541049761986313.5martensite8791208131377199772.79051079621082781977911.07291339511044791984510.81007739921699380198745.79776995458994TensileEvaluationstrengthCriticalSteelofafter wirecompressionSamplesampleBauschingerdrawingDrawabilityratioNo.No.(2)/(1)effect(MPa)(%)(%)Remarks111.03good8727762.0Example221.03good8806252.1Example330.99good8715455.9Example441.02good8886645.5Example551.00good9457258.5Example660.99good9106157.1Example771.01good9126954.4Example881.02good9516044.5Example990.99good9135548.3Example10101.04good8385466.6Example11111.04good9476148.9Example12120.99good8745959.1Example13131.01good8967256.3Example14141.04good8095757.1Example15151.04good8907458.8Example16161.03good8777554.0Example17171.00good9266459.5Example18181.05good8997345.2Example19191.02good9225449.2Example20200.99good9707262.0Example21211.04good8795757.2Example22221.00good8907057.9Example23231.03good8466354.0Example24240.99good8396549.4Example25251.04good9745959.1Example26261.05good8847655.8Example27271.05good8065466.3Example28281.03good8996559.4Example29290.99good9287252.1Example30301.05good9556359.8Example31311.03good8667758.5Example32321.01good8727358.3Example33331.01good8806663.2Example34341.04good9517254.1Example35351.00good8696148.3Example36361.03good8886263.3Example37371.02good9456045.0Example38381.05good9105447.6Example39391.03good8297660.1Example40401.02good9126655.9Example41411.03good8066552.7Example42421.03good8997257.4Example43431.01good9286062.5Example44441.00good8905657.2Example45451.02good8807655.9Example46461.10poor9135138.4Comparative Example47471.12poor8057739.2Comparative Example48481.08poor10055039.1Comparative Example49491.10poor8887437.7Comparative Example50501.12poor10135137.6Comparative Example51511.10poor8466637.3Comparative Example5252—————Comparative Example53531.09poor9056255.1Comparative Example54541.08poor8796339.1Comparative Example55551.07poor11034919.1Comparative Example5656—————Comparative Example5757—————Comparative Example58581.09poor9994430.9Comparative Example59591.08poor10484930.8Comparative Example60601.10poor8646037.7Comparative Example61610.99good8704934.4Comparative Example62621.00good9004738.9Comparative Example6363—————Comparative Example64641.08poor10294834.6Comparative Example65651.07poor9844838.0Comparative Example66661.12poor8925337.6Comparative Example67671.14poor9735538.7Comparative Example68681.06poor7637134.6Comparative Example69691.07poor11594839.1Comparative Example70701.07poor10874832.2Comparative Example71711.10poor8466637.7Comparative Example72721.18poor9755344.5Comparative Example73731.11poor12034433.3Comparative Example74741.21poor11663929.8Comparative Example75191.10poor8165938.3Comparative Example76191.09poor10413832.2Comparative Example77191.12poor8496237.6Comparative Example78191.10poor8067039.1Comparative Example79191.00good8017459.1Example80191.04good9485543.2Example

In Tables 1 and 2, sample Nos. 1 to 45 are our examples having steel components within the scope of the present disclosure.

In a comparative example of sample No. 46, the B content was less than the lower limit of the present disclosure and sufficient quench hardenability could not be obtained, and the fraction of bainite microstructure was less than the lower limit of the present disclosure, and instead the fraction of ferrite was increased, resulting in low-strength parts being mixed in, and the strength variation exceeded 100 MPa. In addition, the Bauschinger effect and critical compression ratio were insufficient.

In contrast, sample No. 47 is a comparative example in which the alloy composition range was within the specified range of the present disclosure, but the value yielded in the formula (1) was less than 0.45 and ferrite was mixed in with the bainite microstructure, resulting in large strength variation and an insufficient Bauschinger effect. Since the ferrite fraction was high in this comparative steel, the drawability was in the acceptable range.

Comparative examples of sample Nos. 48, 50, 55, 58, 59, and 64 were not only unable to obtain a sufficient Bauschinger effect because the microstructure became martensite single phase, but also the drawability was not more than 52%, making the steel unsuitable for use in bolts.

Sample No. 49 is a comparative example in which the Mn content was less than the lower limit of the present disclosure and the fraction of bainite microstructure was less than the lower limit of the present disclosure, resulting in large strength variation, an insufficient Bauschinger effect, and a low critical compression ratio. Since the ferrite fraction was high in this comparative steel, the drawability was in the acceptable range.

In a comparative example of sample No. 51, the Al content was outside the range of the present disclosure and did not satisfy the formula (2), resulting in coarsening of prior austenite crystal grains and inability to obtain a sufficient Bauschinger effect.

In the comparative example of Sample No. 53, the N content exceeded the upper limit of the present disclosure, and thus the strain aging did not produce a sufficient Bauschinger effect.

In a comparative example of sample No. 54, the content of each alloying component was within the specified range of the present disclosure, but the concentrations of Al and Ti did not satisfy the formula (2), resulting in coarsening of prior austenite crystal grains during heating of the steel prior to hot rolling and inability to obtain a sufficient Bauschinger effect.

Sample No. 60 is a comparative example in which the C content was less than the lower limit of the present disclosure and the fraction of bainite microstructure was less than the lower limit of the present disclosure, resulting in large strength variation, an insufficient Bauschinger effect, and a low critical compression ratio. Since the ferrite fraction was high in this sample No. 60, the drawability was in the acceptable range.

In a comparative example of sample No. 61, the P content exceeded 0.025%, resulting in embrittlement of the steel and inability to obtain a sufficiently high critical compression ratio after being drawn into a steel wire.

In a comparative example of sample No. 62, the S content exceeded 0.025%, resulting in embrittlement of the steel and inability to obtain a sufficiently high critical compression ratio after being drawn into a steel wire.

In a comparative example of sample No. 65, the toughness of the steel decreased due to insufficient addition of Ti, resulting in inability to obtain a sufficiently high drawability and critical compression ratio.

In a comparative example of sample No. 66, a sufficiently high quench hardenability and bainite fraction could not be obtained because the oxygen in the steel was combined with carbon due to the low Al content, resulting in inability to obtain a sufficient Bauschinger effect and critical compression ratio.

Sample No. 67 is a comparative example in which the Cr content was less than the lower limit of the present disclosure and a sufficient bainite microstructure could not be obtained, resulting in an insufficient Bauschinger effect and a low critical compression ratio. Since the ferrite fraction was high in this comparative steel, the drawability was in the acceptable range.

Sample No. 68 is a comparative example in which the content of each alloying component was within the specified range of the present disclosure, but the value yielded in the formula (1) was less than 0.45, resulting in large strength variation as a result of ferrite being mixed in with the bainite microstructure and an insufficient Bauschinger effect, for which the strength was judged as failed. Since the ferrite fraction was high in this comparative steel, the drawability was in the acceptable range.

Sample No. 69 is a comparative example in which the content of each alloying component was within the specified range of the present disclosure, but the value yielded in the formula (1) exceeded 0.60, resulting in large strength variation as a result of martensite being mixed in with the bainite microstructure and an insufficient Bauschinger effect, for which the strength was judged as failed.

Sample No. 70 is a comparative example in which the content of each alloying component was within the specified range of the present disclosure, but the value yielded in the formula (1) exceeded 0.60, resulting in large strength variation as a result of martensite being mixed in with the bainite microstructure and an insufficient Bauschinger effect, for which the strength was judged as failed.

In a comparative example of sample No. 71, the N content was less than the lower limit of the present disclosure, resulting in coarsening of prior austenite crystal grains and inability to obtain a sufficient Bauschinger effect.

In a comparative example of sample No. 72, the Si content was more than the upper limit of the present disclosure, resulting in a large amount of work hardening during wiredrawing and an insufficient Bauschinger effect.

A comparative example of sample No. 73 is a steel sample in which the Mn and Cr contents exceeded the specified ranges of the present disclosure and the left-hand side of the formula (1) exceeded the upper limit, as in sample Nos. 50 and 55. In order to obtain a bainite microstructure within the scope of the present disclosure, the cooling rate was intentionally lowered below the rate specified in the present disclosure. As a result, the microstructure itself became a bainite single phase, which was, however, a mixture of bainite microstructures with deviations in strength. Thus, the strength variation was outside the scope of the present disclosure, and the Bauschinger effect was not sufficient because of the excessive addition of alloys. In addition, the drawability and the critical compression ratio were low.

A comparative example of sample No. 74 is a steel sample in which the Mn and Cr contents exceeded the specified ranges of the present disclosure and the left-hand side of the formula (1) exceeded the upper limit, as in sample Nos. 50 and 55. In order to obtain a bainite microstructure within the scope of the present disclosure, the cooling rate was intentionally lowered below the rate specified in the present disclosure. As a result, the microstructure itself became a bainite single phase, which was, however, a mixture of bainite microstructures with deviations in strength. Thus, the strength variation was outside the scope of the present disclosure, and the Bauschinger effect was not sufficient because of the excessive addition of alloys. In addition, the drawability and the critical compression ratio were low.

A comparative example of sample No. 75 is a steel sample with the same composition as No. 19 in Table 1. However, since the cooling rate after hot rolling was lower than 2° C./s, a bainite-dominated microstructure could not be obtained, and since the microstructure proportion was outside the specified range of the present disclosure, a sufficient Bauschinger effect could not be obtained.

A comparative example of sample No. 76 is a steel sample with the same composition as No. 19 in Table 1. However, the cooling rate after hot rolling was higher than 12° C./s, resulting in a martensitic single-phase microstructure. As a result, not only was the Bauschinger effect insufficient, but also the drawability was not more than 52%, making the steel unsuitable for use in bolts.

A comparative example of sample No. 77 is a steel sample with the same composition as No. 19 in Table 1. However, since the hot-rolling finish temperature was higher than 950° C., ferrite was precipitated in excess of 5% and prior austenite grains were coarsened, resulting in an insufficient Bauschinger effect.

A comparative example of sample No. 78 is a steel sample with the same composition as No. 19 in Table 1. However, the hot-rolling finish temperature was lower than 800° C., resulting in a higher ferrite fraction and an insufficient Bauschinger effect.

Samples No. 79 and 80 are steel wires obtained by wiredrawing at an area reduction rate of 16% and 58%, respectively, from wire rods formed under the conditions according to the present disclosure in terms of the hot-rolling finish temperature and the subsequent cooling rate. Since the steel microstructure was a bainite single phase or had a bainite fraction of 95% or more and a ferrite fraction of less than 5%, a sufficient Bauschinger effect was achieved and good results were obtained for both drawability and critical compression ratio. Note that in a general manufacturing process of bolts, the area reduction rate for wiredrawing ranges from 15% to 60%.