WO2013145868A1 - 耐遅れ破壊性に優れたボロン添加高強度ボルト用鋼および高強度ボルト - Google Patents
耐遅れ破壊性に優れたボロン添加高強度ボルト用鋼および高強度ボルト Download PDFInfo
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- C21D1/18—Hardening; Quenching with or without subsequent tempering
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/004—Dispersions; Precipitations
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- C21D2211/005—Ferrite
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/009—Pearlite
Definitions
- Patent Document 1 proposes a steel material that has improved delayed fracture resistance by defining the contents of V, N, Si, and the like. However, it is difficult to satisfy the strength, delayed fracture resistance, and corrosion resistance at the same time only by specifying the content of the above components.
- Patent Document 2 proposes a bainite steel that does not vary in mechanical properties, but it is difficult to apply to a bolt in a bainite structure because wire drawing workability and cold forgeability deteriorate.
- the present invention has been made under such circumstances, and its purpose is to prevent delay even at a tensile strength of 1100 MPa or more without adding a large amount of expensive alloy elements such as Cr and Mo.
- An object of the present invention is to provide a boron-added high-strength bolt steel excellent in destructibility and a high-strength bolt made of such a boron-added high-strength bolt steel.
- the austenite grain size number of the bolt shaft portion after quenching and tempering is 8 or more.
- FIG. 1 is a graph showing the effect of [Si] / [C] on tensile strength and delayed fracture strength ratio.
- C is an element useful for ensuring the strength of the steel, but increasing its content deteriorates the toughness and corrosion resistance of the steel and tends to cause delayed fracture.
- Si is also an element useful for ensuring the strength of steel, but the relationship with delayed fracture was unclear. Therefore, the present inventors investigated the influence of Si on delayed fracture. As a result, it was possible to balance tensile strength and delayed fracture resistance at a high level by increasing the addition amount of Si more than the C content, thereby achieving both a tensile strength of 1100 MPa or more, toughness, and corrosion resistance. .
- the ratio of the Si content [Si] and the C content [C] is 1.0 or more for the above purpose. It is necessary to be. As a result, the amount of addition of C can be relatively reduced and the corrosion resistance can be improved as much as the strength can be secured with Si, and therefore, excellent delayed fracture resistance is exhibited.
- the value of the ratio ([Si] / [C]) is preferably 2.0 or more, and more preferably 3.0 or more. However, even if the ratio ([Si] / [C]) satisfies 1.0 or more, if the chemical component composition is out of the proper range, there is a disadvantage that the delayed fracture resistance and other characteristics deteriorate. Arise.
- the ratio ([Si] / [C]) is set to 2.0 or more, and (b) C: 0.25 to When the ratio is less than 0.29%, the ratio ([Si] / [C]) is set to 1.5 or more.
- C When C is 0.29% or more (that is, 0.29 to 0.40%). Less) and the ratio ([Si] / [C]) is preferably 1.0 or more.
- C 0.23 to less than 0.40%
- C is an element indispensable for forming carbides and securing tensile strength necessary for high-strength steel. In order to exhibit such an effect, it is necessary to contain 0.23% or more. However, when C is contained excessively, the delayed fracture resistance deteriorates due to a decrease in toughness and a deterioration in corrosion resistance. In order to avoid such an adverse effect of C, the C content needs to be less than 0.40%.
- the minimum with preferable C content is 0.25% or more, More preferably, it is good to set it as 0.27% or more.
- the upper limit with preferable C content is 0.38% or less, More preferably, it is good to set it as 0.36% or less.
- Si acts as a deoxidizer during melting and is an element necessary as a solid solution element for strengthening the matrix. By containing 0.23% or more, sufficient strength can be ensured. Further, by adding Si, carbonitrides are difficult to dissolve at the time of quenching, so that the pinning effect is increased, and the coarsening of crystal grains is suppressed. However, if Si is contained excessively exceeding 1.50%, the cold workability of the steel material is deteriorated even when spheroidizing annealing is performed, and the grain boundary oxidation in the heat treatment at the time of quenching is promoted. Deteriorating delayed fracture.
- the minimum with preferable Si content is 0.3% or more, More preferably, it is good to set it as 0.4% or more.
- the upper limit with preferable Si content is 1.0% or less, More preferably, it is good to set it as 0.8% or less.
- Mn is an element that improves hardenability, and is an important element for achieving high strength. The effect can be exhibited by containing 0.30% or more of Mn. However, if the Mn content is excessive, segregation to the grain boundary is promoted and the grain boundary strength is lowered, and the delayed fracture resistance is lowered, so 1.45% was made the upper limit.
- the minimum with preferable Mn content is 0.4% or more, It is good to set it as 0.6% or more more preferably.
- the upper limit with preferable Mn content is 1.3% or less, More preferably, it is good to set it as 1.1% or less.
- P 0.03% or less (excluding 0%)
- P is contained as an impurity, but if it is present in an excessive amount, it causes segregation at the grain boundary, lowers the grain boundary strength, and deteriorates delayed fracture characteristics. Therefore, the upper limit of the P content is 0.03%.
- the upper limit with preferable P content is 0.01% or less, More preferably, it is good to set it as 0.005% or less.
- the upper limit of the S content is set to 0.03%.
- the upper limit with preferable S content is 0.01% or less, More preferably, it is good to set it as 0.006% or less.
- Cr 0.05 to 1.5%
- Cr is an element for improving corrosion resistance, and exhibits an effect by adding 0.05% or more. However, if it is contained in a large amount, the steel material cost increases, so the upper limit is made 1.5%.
- the minimum with preferable Cr content is 0.10% or more, More preferably, it is 0.13% or more.
- the upper limit with preferable Cr content is 1.0% or less, More preferably, it is 0.70% or less.
- V is a carbon / nitride-forming element. It contains 0.02% or more, and by adding Si, V charcoal / nitride is difficult to dissolve at the time of quenching. To do. However, if contained in a large amount, coarse charcoal / nitride is formed and cold forgeability is lowered, so the upper limit is made 0.30%.
- the minimum with preferable V content is 0.03% or more, More preferably, it is 0.04% or more.
- the upper limit with preferable V content is 0.15% or less, More preferably, it is 0.11% or less.
- Ti 0.02 to 0.1%
- Ti is an element that forms charcoal / nitride, and by adding 0.02% or more, crystal grains are refined and toughness is improved. Moreover, since free B increases by fixing N in steel as TiN, hardenability can be improved. However, if the Ti content is excessive and exceeds 0.1%, the workability is reduced.
- the minimum with preferable Ti content is 0.03% or more, More preferably, it is good to set it as 0.045% or more.
- the upper limit with preferable Ti content is 0.08% or less, More preferably, it is good to set it as 0.065% or less.
- B is an element effective in improving the hardenability of steel, and in order to exhibit the effect, it is necessary to contain 0.0003% or more and to add Ti in combination. However, if the B content becomes excessive and exceeds 0.0050%, the toughness is lowered instead.
- the minimum with preferable B content is 0.0005% or more, More preferably, it is good to set it as 0.001% or more.
- the upper limit with preferable B content is 0.004% or less, More preferably, it is good to set it as 0.003% or less.
- Al 0.01 to 0.10%
- AlN austenite grains can be prevented from becoming coarse. Further, by fixing N, the free B increases, so that the hardenability is improved.
- the Al content needs to be 0.01% or more. However, even if the Al content exceeds 0.10% and becomes excessive, the effect is saturated.
- the minimum with preferable Al content is 0.02% or more, More preferably, it is good to set it as 0.03% or more.
- the upper limit with preferable Al content is 0.08% or less, More preferably, it is good to set it as 0.05% or less.
- Mo 0.10% or less
- Mo is an element that improves hardenability and has high resistance to temper softening, and is therefore an effective element for securing strength. However, if it is contained in a large amount, the production cost increases, so the content is made 0.10% or less.
- the minimum with preferable Mo content is 0.03% or more, More preferably, it is 0.04% or more.
- the upper limit with preferable Mo content is 0.07% or less, More preferably, it is 0.06% or less.
- the boron-added high-strength bolt steel having the above chemical composition is heated to 950 ° C. or higher at the time of billet reheating before rolling, and finished into a wire or bar shape in the temperature range of 800 to 1000 ° C.
- the structure after rolling basically becomes a mixed structure of ferrite and pearlite (sometimes referred to as “ferrite / pearlite”).
- the finish rolling temperature is preferably 1000 ° C. or lower.
- the finish rolling temperature is higher than 1000 ° C., Ti and V charcoal / nitrides are difficult to precipitate, and the effect of grain refinement during quenching is reduced.
- the finish rolling temperature is too low, there is an increase in rolling load and an increase in the occurrence of surface flaws, which is unrealistic.
- the finish rolling temperature is the average surface temperature that can be measured with a radiation thermometer before the final rolling pass or before the rolling roll group.
- the heating at the time of quenching In the heating at the time of quenching, heating at 850 ° C. or higher is necessary for stable austenitization treatment. However, when heated to a high temperature exceeding 920 ° C., the V charcoal / nitride dissolves, so that the pinning effect is reduced, the crystal grains become coarse, and the delayed fracture characteristics are deteriorated. Therefore, in order to prevent coarsening of crystal grains, it is useful to quench by heating at 920 ° C. or lower.
- the preferable upper limit of the heating temperature at the time of hardening is 900 degrees C or less, More preferably, it is 890 degrees C or less.
- the minimum with the preferable heating temperature at the time of hardening is 860 degreeC or more, More preferably, it is 870 degreeC or more.
- the boron-added high-strength bolt steel of the present invention suppresses dissolution of V-based precipitates at the time of quenching by adding V and Si in a composite manner, and increases the pinning effect to refine the crystal grains. . Therefore, V-type precipitates (V-containing carbide, V-containing nitride, V-containing carbonitride) remain in the bolt after quenching or quenching and tempering, and the precipitate (precipitate of 0.1 ⁇ m or more) ) Is preferably 10% or more of the V content of the steel material (VI value defined by the following formula (1) is 10% or more). By satisfying these requirements, the crystal grains can be further refined, and the delayed fracture resistance is further improved by the hydrogen trap effect. This VI value is more preferably 15% or more, and still more preferably 20% or more.
- VI value (%) (V content contained in precipitates of 0.1 ⁇ m or more / V content of steel material) ⁇ 100 (1)
- the as-quenched bolts have low toughness and ductility and do not become bolt products as they are, so they need to be tempered. For this purpose, it is effective to perform a tempering treatment at a temperature of at least 350 ° C. or higher. However, when the tempering temperature exceeds 550 ° C., the steel material having the above chemical composition cannot secure a tensile strength of 1100 MPa or more.
- the austenite crystal grains (former austenite crystal grains) in the shaft portion are preferable because the delayed fracture resistance is improved as the size is reduced. From such a viewpoint, it is preferable that the austenite crystal grains in the bolt shaft portion be 8 or more in crystal grain size number (JIS G 0551).
- the grain size number is more preferably 9 or more, and still more preferably 10 or more.
- Steel materials (steel types A to Y) having the chemical composition shown in Table 1 below were melted and then rolled (billet reheating temperature: 1000 ° C., finish rolling temperature: 800 ° C.) to obtain a wire rod having a diameter of 14 mm ⁇ . .
- Table 1 shows the structure of each wire after rolling. The rolled material was subjected to wire drawing and spheroidizing annealing after descaling / coating treatment, and finishing wire drawing after further descaling / coating treatment. In Table 1, the part represented by “ ⁇ ” means no addition.
- the structure was observed by observing the D / 4 position with an SEM after filling the cross section of the rolled material with resin.
- Table 1 when the structure after rolling is “ferrite / pearlite”, the structure other than ferrite and pearlite is 10 area% or less. In the case where the structure after rolling is “many bainite”, bainite is more than 10 area%. In steel type S, bainite reached about 20%.
- a flange bolt of M12 x 1.25P, length 100mmL is produced by cold heading, and bolt formability (cold heading) is determined by the presence or absence of cracks in the flange part. Evaluation was made (in Table 3 below, the case where the flange portion was cracked was indicated as “x”, and the case where the flange portion was not cracked was indicated as “bolt formability“ ⁇ ”). Thereafter, quenching and tempering were performed under the conditions shown in Table 2 below.
- quenching heating time 20 minutes
- quenching furnace atmosphere air
- quenching cooling condition oil cooling (70 ° C.)
- tempering heating time 30 minutes
- tempering furnace atmosphere Air and tempering cooling conditions: Oil cooling (25 ° C.).
- the bolts subjected to quenching and tempering were evaluated for VI value, crystal grain size, tensile strength, corrosion resistance and delayed fracture resistance in the following manner.
- the tensile strength of the bolt was determined by conducting a tensile test in accordance with JIS B1051, and those having a tensile strength (tensile strength) of 1100 MPa or more were regarded as acceptable.
- Corrosion resistance was evaluated by corrosion weight loss before and after immersion when a bolt was immersed in a 15% HCl aqueous solution for 30 minutes.
- Delayed fracture resistance is obtained by immersing a bolt in a 15% HCl aqueous solution for 30 minutes, washing with water and drying, then applying a constant load, and comparing the load that does not break for more than 100 hours. Carried out. At this time, a value obtained by dividing the load that does not break for 100 hours or more after acid immersion by the maximum load when the tensile test is performed without acid immersion is defined as a delayed fracture strength ratio, and this value (delayed fracture strength ratio) is 0.70. The above was judged as acceptable.
- Test No. Examples 1 to 13 are examples (invention examples) that satisfy the requirements [chemical composition and ratio ([Si] / [C]), structure] defined in the present invention, and have high strength and excellent delay resistance. It can be seen that it is destructive. Among these, test No. From 1 to 3, 6 to 8, the influence of the VI value can be seen. It can be seen that the larger the VI value, the finer the crystal grains, and the delayed fracture resistance is improved.
- test no. Nos. 14 to 30 do not satisfy any of the requirements defined in the present invention, and any of the characteristics is deteriorated. That is, test no. No. 14 is an example using a steel type (steel type I) with a low C content, and high strength cannot be achieved by ordinary heat treatment.
- No. No. 15 is an example using a steel type with excessive C content (steel type J), and delayed fracture resistance deteriorated due to a decrease in toughness.
- Test No. No. 16 is an example using a steel type with low Si content (steel type K) (ratio of [Si] / [C] is also less than 1.0), and high strength can be achieved by ordinary heat treatment. In addition, the crystal grains were not sufficiently refined. Test No. In Nos. 17 to 20, although the content of each additive element is satisfactory (steel types L, M, N, and O), the ratio of [Si] / [C] is less than 1.0, so the corrosion resistance is deteriorated. However, the delayed fracture strength ratio decreased.
- Test No. No. 21 is an example using a steel type (steel type P) having a low Mn content, and high strength could not be achieved due to a decrease in hardenability (other evaluation was not performed).
- Test No. No. 22 is an example using a steel type (steel type Q) having an excessive Mn content, and the grain boundary strength is reduced due to segregation, resulting in poor delayed fracture resistance.
- Test No. No. 23 is an example using a steel type with excessive P content (steel type R). P caused grain boundary segregation, resulting in a decrease in grain boundary strength and a deterioration in delayed fracture resistance.
- Test No. No. 24 is an example using a steel type with excessive S content (steel type S), and the segregation of sulfides to the crystal grain boundaries reduced the grain boundary strength and deteriorated delayed fracture resistance.
- Test No. No. 25 is an example using a steel type to which Cr is not added (steel type T), the corrosion resistance is deteriorated, and the delayed fracture resistance is low.
- Test No. No. 26 is an example using a steel type with less V (steel type U), and since the crystal grains were not sufficiently refined, the toughness deteriorates and the delayed fracture resistance is low.
- Test No. No. 27 is an example using a steel type with excessive V content (steel type V), and because of the formation of coarse charcoal / nitride, the cold heading (bolt formability) was reduced (other evaluations were made) Absent).
- Test No. No. 28 is an example using a steel type to which Ti is not added (steel type W). The formation of BN deteriorated the hardenability and lowered the delayed fracture resistance.
- Test No. No. 29 is an example using a steel type with excessive Ti content (steel type X), and because of the formation of coarse charcoal / nitride, the cold heading (bolt formability) was reduced (other evaluations were made) Absent).
- Test No. No. 30 is an example in which a rolled wire rod containing a large amount of bainite in the structure is obtained because the cooling rate after rolling is higher than 3 ° C./second, and the hardness is sufficiently lowered even when spheroidizing annealing is performed. As a result, the cold forgeability deteriorated.
- the evaluation results are collectively shown in Table 3 below (“Good” when good, “No” when not deteriorated, “ ⁇ ” not evaluated).
- Figure 1 shows test no. 1 to 13 (invention examples) and test no. For 16 to 20 (comparative examples), the effect of [Si] / [C] on the tensile strength (tensile strength) and delayed fracture strength ratio is shown. As is clear from this result, it is useful to appropriately control [Si] / [C] in order to make delayed fracture resistance excellent even at a tensile strength of 1100 MPa or more. I understand.
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Abstract
Description
VI値(%)=(0.1μm以上の析出物中に含まれるV量/鋼材のV含有量)×100 …(1)
Cは、炭化物を形成すると共に、高強度鋼として必要な引張強さ確保する上で欠くことができない元素である。こうした効果を発揮させるためには、0.23%以上含有させる必要がある。しかし、Cを過剰に含有させると、靭性低下や耐食性悪化を招いて耐遅れ破壊性が劣化する。このようなCの悪影響を避けるためには、C含有量は0.40%未満とする必要がある。尚、C含有量の好ましい下限は0.25%以上であり、より好ましくは0.27%以上とするのが良い。また、C含有量の好ましい上限は0.38%以下であり、より好ましくは0.36%以下とするのが良い。
Siは、溶製時の脱酸剤として作用すると共に、マトリクスを強化する固溶元素として必要な元素であり、0.23%以上含有させることによって十分な強度を確保できる。また、Siを添加することにより焼入れ時に炭窒化物が固溶しにくくなるため、ピンニング効果が増加することにより結晶粒の粗大化が抑制される。しかしながら、1.50%を超えてSiを過剰に含有させると、球状化焼鈍を実施しても鋼材の冷間加工性が低下すると共に、焼入れ時の熱処理での粒界酸化を助長して耐遅れ破壊性を劣化させる。尚、Si含有量の好ましい下限は0.3%以上であり、より好ましくは0.4%以上とするのが良い。また、Si含有量の好ましい上限は1.0%以下であり、より好ましくは0.8%以下とするのが良い。
Mnは焼入れ性向上元素であり、高強度化を達成する上で重要な元素である。Mnは0.30%以上含有させることで、その効果を発揮させることができる。しかしながら、Mn含有量が過剰になると、粒界への偏析を助長して粒界強度が低下し、耐遅れ破壊性が却って低下するため、1.45%を上限とした。尚、Mn含有量の好ましい下限は0.4%以上であり、より好ましくは0.6%以上とするのが良い。また、Mn含有量の好ましい上限は1.3%以下であり、より好ましくは1.1%以下とするのが良い。
Pは不純物として含有するが、過剰に存在すると粒界偏析を起こして粒界強度を低下させて、遅れ破壊特性を悪化させる。そのため、P含有量の上限は0.03%とした。尚、P含有量の好ましい上限は0.01%以下であり、より好ましくは0.005%以下とするのが良い。
Sが過剰に存在すると、硫化物が結晶粒界に偏析し、粒界強度の低下を招いて耐遅れ破壊性が低下する。そのため、S含有量の上限を0.03%とした。尚、S含有量の好ましい上限は0.01%以下であり、より好ましくは0.006%以下とするのが良い。
Crは耐食性向上元素であり、0.05%以上添加することで効果を発揮する。しかしながら、多量に含有させると鋼材コストの増大を招くため、上限は1.5%とする。尚、Cr含有量の好ましい下限は0.10%以上であり、より好ましくは0.13%以上である。また、Cr含有量の好ましい上限は1.0%以下であり、より好ましくは0.70%以下である。
Vは炭・窒化物形成元素であり、0.02%以上含有し、且つSiを複合添加することにより焼入れ時にV炭・窒化物が固溶しにくくなるため、結晶粒微細化の効果を発揮する。しかしながら、多量に含有させると粗大な炭・窒化物を形成して冷間鍛造性の低下を招くため、上限は0.30%とする。尚、V含有量の好ましい下限は0.03%以上であり、より好ましくは0.04%以上である。また、V含有量の好ましい上限は0.15%以下であり、より好ましくは0.11%以下である。
Tiは、炭・窒化物を形成する元素であり、0.02%以上添加することで結晶粒が微細化し、靭性が向上する。また、鋼中のNをTiNとして固着することにより、フリーBが増加するため、焼入れ性を向上することができる。しかしながら、Ti含有量が過剰になって0.1%を超えると、加工性の低下を招くことになる。尚、Ti含有量の好ましい下限は0.03%以上であり、より好ましくは0.045%以上とするのが良い。また、Ti含有量の好ましい上限は0.08%以下であり、より好ましくは0.065%以下とするのが良い。
Bは、鋼の焼入れ性を向上させる上で有効な元素であり、その効果を発揮させるためには0.0003%以上含有し、且つTiを複合添加する必要がある。しかしながら、B含有量が過剰になって0.0050%を超えると靭性が却って低下する。尚、B含有量の好ましい下限は0.0005%以上であり、より好ましくは0.001%以上とするのが良い。また、B含有量の好ましい上限は0.004%以下であり、より好ましくは0.003%以下とするのが良い。
Alは、鋼の脱酸に有効な元素であり、且つAlNを形成することによって、オーステナイト粒の粗大化を防止することができる。またNを固着することで、フリーBが増加するため、焼入れ性が向上する。こうした効果を発揮させるためには、Al含有量は0.01%以上とする必要がある。しかしながら、Al含有量が0.10%を超えて過剰になっても、その効果が飽和する。尚、Al含有量の好ましい下限は0.02%以上であり、より好ましくは0.03%以上とするのが良い。また、Al含有量の好ましい上限は0.08%以下であり、より好ましくは0.05%以下とするのが良い。
Nは、溶製後の凝固段階で、TiやVと結合して窒化物(TiN,VN)を形成し、結晶粒の微細化を図って耐遅れ破壊性を向上させる。こうした効果は、Nの含有量が0.002%以上で有効に発揮される。しかしながら、TiNやVNが多量に形成されると、1300℃程度の加熱では溶解せず、Ti炭化物の形成を阻害する。また過剰のNは、遅れ破壊特性に対し却って有害となり、特に含有量が0.010%を超えて過剰になると、遅れ破壊特性を著しく低下させる。尚、N含有量の好ましい下限は0.003%以上であり、より好ましくは0.004%以上とするのが良い。また、N含有量の好ましい上限は0.008%以下であり、より好ましくは0.006%以下とするのが良い。
Moは、焼入れ性を向上する元素であり、焼戻し軟化抵抗も高いため、強度確保に有効な元素である。しかしながら、多量に含有させると製造コストが増大するため、0.10%以下とする。尚、Mo含有量の好ましい下限は0.03%以上であり、より好ましくは0.04%以上である。また、Mo含有量の好ましい上限は0.07%以下であり、より好ましくは0.06%以下である。
ビレット再加熱では、結晶粒微細化に有効なTiやVの炭・窒化物を、オーステナイト域に固溶させる必要があり、そのためにはビレットの再加熱温度を950℃以上にすることが好ましい。この温度が950℃未満では炭・窒化物の固溶量が不十分となり、後の熱間圧延で微細なTiやVの炭・窒化物が生成しにくくなるため、焼入れ時の結晶粒微細化の効果が減少する。この温度は、より好ましくは1000℃以上である。
圧延では、ビレット再加熱時に固溶させたTiやVを微細な炭・窒化物として鋼中に析出させる必要があり、そのためには仕上げ圧延温度を1000℃以下にすることが好ましい。仕上げ圧延温度が1000℃よりも高くなるとTiやVの炭・窒化物が析出しにくくなるため、焼入れ時の結晶粒微細化の効果が減少する。一方、仕上げ圧延温度が低くなりすぎると、圧延荷重の増加や表面疵の発生増大があり、非現実的となるためその下限を800℃以上とした。ここで、仕上げ圧延温度は、最終圧延パス前または圧延ロール群前の放射温度計で測定可能な表面の平均温度とした。
圧延後の冷却では、後のボルト加工での成形性を向上させるため、組織をフェライト・パーライト組織にすることが重要であり、そのためには圧延後の平均冷却速度を3℃/秒以下にすることが好ましい。平均冷却速度が3℃/秒より速くなると、ベイナイトやマルテンサイトが生成するため、ボルト成形性が大幅に悪化する。平均冷却速度は、より好ましくは2℃/以下とすることが望ましい。
ボルト中に含まれる0.1μm以上の析出物中のV量は、抽出残渣法を用いて測定した。このとき、表2に示したような焼戻し条件では、析出物中のV量は焼入れ後(焼戻し前)と焼戻し焼入れ後とでは析出物中のV量はほとんど変わらないため、焼入れ後(焼戻し前)のボルトを対象として析出物中のV量を測定した。焼入れ後のボルトについて10%のアセチルアセトン溶液を用いた電解抽出残渣測定を行い、析出物を0.1μmの隙間を有するメッシュで回収した後、IPC発光分光分析法を用いて析出物中に含まれるV量を測定した。得られたV量を鋼材のV含有量(鋼材全体のトータルV量)で除し、100を掛けることによって[前記(1)式]、VI値を求めた。
ボルトの軸部を横断面(ボルトの軸に対して垂直な断面。以下同じ)で切断後、D/4位置(Dは軸部の直径)の任意の0.039mm2の領域を、光学顕微鏡で観察し(倍率:400倍)、JIS G0551に従って結晶粒度番号を測定した。測定は4視野について行い、これらの平均値をオーステナイト結晶粒度とし、結晶粒度番号が8以上のものを合格(「○」)とした。
ボルトの引張強さは、JIS B1051に従って引張試験を行って求め、引張強さ(引張強度)が1100MPa以上のものを合格とした。
耐食性は、15%HCl水溶液にボルトを30分浸漬した際の浸漬前後の腐食減量によって評価した。
耐遅れ破壊性は、15%HCl水溶液にボルトを30分浸漬し、水洗および乾燥した後、一定荷重を負荷し、100時間以上破断しない荷重を比較することで実施した。このとき、酸浸漬後に100時間以上破断しない荷重を、酸浸漬なしで引張試験した際の最大荷重で除した値を遅れ破壊強度比として定義し、この値(遅れ破壊強度比)が0.70以上のものを合格と判断した。
Claims (5)
- C :0.23~0.40%未満(質量%の意味、以下同じ)、
Si:0.23~1.50%、
Mn:0.30~1.45%、
P :0.03%以下(0%を含まない)、
S :0.03%以下(0%を含まない)、
Cr:0.05~1.5%、
V :0.02~0.30%、
Ti:0.02~0.1%、
B :0.0003~0.0050%、
Al:0.01~0.10%、および
N :0.002~0.010%、
を夫々含有し、残部が鉄および不可避不純物からなり、
且つSiの含有量[Si]とCの含有量[C]の比([Si]/[C])が1.0以上であると共に、フェライトとパーライトの混合組織であることを特徴とする耐遅れ破壊性に優れたボロン添加高強度ボルト用鋼。 - 更に、Mo:0.10%以下(0%を含まない)を含有するものである請求項1に記載のボロン添加高強度ボルト用鋼。
- 請求項1または2に記載の高強度ボルト用鋼を使用し、ボルト形状に成形加工した後、850℃以上、920℃以下で加熱して焼入れ処理を行い、その後、焼戻し処理を行ったものである耐遅れ破壊性に優れた高強度ボルト。
- 請求項1または2に記載の高強度ボルト用鋼を使用し、ボルト形状に成形加工した後、焼入れ処理を行い、その後、焼戻し処理を行った高強度ボルトであり、0.1μm以上の析出物中に含まれるV量と、鋼材のV含有量とで下記(1)式で規定されるVI値が10%以上である耐遅れ破壊性に優れた高強度ボルト。
VI値(%)=(0.1μm以上の析出物中に含まれるV量/鋼材のV含有量)×100 …(1) - 焼入れおよび焼戻し後のボルト軸部のオーステナイト結晶粒度番号が8以上である請求項3または4に記載の耐遅れ破壊性に優れた高強度ボルト。
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- 2012-09-24 JP JP2012209869A patent/JP6034632B2/ja not_active Expired - Fee Related
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2013
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- 2013-02-05 MX MX2014011470A patent/MX2014011470A/es unknown
- 2013-02-05 KR KR1020147026545A patent/KR20140123111A/ko not_active Application Discontinuation
- 2013-02-05 US US14/388,361 patent/US9845519B2/en not_active Expired - Fee Related
- 2013-02-05 EP EP13767370.3A patent/EP2832875A4/en not_active Withdrawn
- 2013-02-05 CN CN201380015695.4A patent/CN104204254B/zh not_active Expired - Fee Related
- 2013-02-05 WO PCT/JP2013/052613 patent/WO2013145868A1/ja active Application Filing
- 2013-03-07 TW TW102108046A patent/TWI484045B/zh not_active IP Right Cessation
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Also Published As
Publication number | Publication date |
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EP2832875A1 (en) | 2015-02-04 |
JP2013227647A (ja) | 2013-11-07 |
EP2832875A4 (en) | 2016-04-06 |
MX2014011470A (es) | 2014-12-08 |
US20150053315A1 (en) | 2015-02-26 |
CA2864453C (en) | 2015-11-03 |
CN104204254A (zh) | 2014-12-10 |
TWI484045B (zh) | 2015-05-11 |
TW201348460A (zh) | 2013-12-01 |
CA2864453A1 (en) | 2013-10-03 |
CN104204254B (zh) | 2016-06-22 |
US9845519B2 (en) | 2017-12-19 |
KR20140123111A (ko) | 2014-10-21 |
JP6034632B2 (ja) | 2016-11-30 |
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