JP4113453B2 - Bolt Steel Formed from Bonded Film with Excellent Delayed Fracture Resistance and Bolt Manufacturing Method - Google Patents

Description

【００４３】
【表２】

【００４４】
表２の結果に表れているように、本実施例のものは何れもボルト破壊率は０％となっており、耐遅れ破壊性が良好となっているのに対し、比較例のものはボルト成形ができなかったり或いはボルト成形できたものについてもボルト破壊率が高く、耐遅れ破壊性において本実施例のものに比べ劣っている。
【００４５】
尚表１及び表２の最下段の比較例SCM440は、従来ボルト用鋼として用いられているものであり、ボルト破壊率即ち耐遅れ破壊性は良好となっているが、このものはコストの高い材料である。
【００４６】
＜実施例２＞
表１に示す化学成分の実施例Ａ，Ｂ，Ｄ，Ｆの鋼種を造塊し、各鋼を直径10.6mmの線材に、圧延仕上温度を変化させて熱間圧延した。
尚仕上温度を700℃よりも低くしたところ、圧延速度を小さくすることが必要になり、生産効率が著しく低下して線材製造コストの増加を招いた。
【００４７】
冷間伸線はボルト軸部の寸法公差を確保するために行った。
ここでは直径10.6mmの線材をボンデ皮膜処理後、直径10.0mmまで途中で焼鈍しや球状化焼鈍しを施さずに伸線した。
次いでフランジ付六角ボルトに成形し、続いて引張強度が1000〜1200MPaとなるように調質（焼戻し）した。
焼入れは加熱温度850℃，保持時間１時間とし、油冷した。
得られたボルトの引張試験結果を表３に示した。
【００４８】
遅れ破壊試験は、上記ボルトを治具に取り付けて各ボルトが弾性限界に達する降伏点にて締め付け、15質量％の塩酸水溶液に２分間浸漬した後、洗浄，乾燥し、24時間大気中に放置させるサイクルを１サイクルとし、これを14サイクルまで繰り返した時の各鋼種10本中の破損ボルトの割合で表３に示した。
【００４９】
【表３】

【００５０】
表３の結果から分るように、終止温度700〜900℃で熱間圧延を施し、更に焼戻し温度を400℃以上で行った本実施例のものは何れもボルト破壊率が０％、即ち耐遅れ破壊性が良好となっているのに対し、終止温度900℃以上で熱間圧延を施した比較例のものは何れもボルト破壊率が悪い値となっている。
【００５１】
以上本発明の実施例を詳述したがこれはあくまで一例示であり、本発明はその趣旨を逸脱しない範囲において種々変更を加えた対応で実施可能である。[0001]
BACKGROUND OF THE INVENTION
TECHNICAL FIELD The present invention relates to a bolt steel formed from a wire with a bondage film having excellent delayed fracture resistance, and a method for manufacturing the bolt.
[0002]
[Prior art and problems to be solved by the invention]
Conventionally, bolts with a tensile strength of 1000 MPa or more have been manufactured using chromium-molybdenum steel such as JIS SCM435 and SCM440.
Specifically, JIS SCM435, SCM440, and other chromium / molybdenum steel rolled wires are annealed, and after coating with a bonder film for lubrication, wire drawing and bolt forming are performed, followed by tempering treatment. It was manufactured by.
[0003]
However, these chromium-molybdenum steels have the problem that the cost of the material itself is high because of the addition of expensive elements such as Cr and Mo.
In recent years, there has been a strong demand to further reduce the manufacturing cost of bolts. In such a case, low carbon-boron steel that can secure hardenability by suppressing the addition of expensive elements and that can omit annealing. The practical application of is being studied.
[0004]
However, in the case of this kind of low-priced alloy in which the addition of expensive Cr, Mo, etc. is suppressed, the delayed fracture of the bolt, which has not been a big problem in the past SCM435, SCM440, etc., becomes a serious problem.
[0005]
This low carbon-boron steel has a carbon content of about 0.2 to 0.3%, and in order to obtain a tensile strength of 1000 MPa or more, it is necessary to lower the tempering temperature as compared with a chromium-molybdenum steel bolt.
However, when the tempering temperature is lowered, the susceptibility to delayed fracture starting from cracks in the prior austenite grain boundaries increases accordingly.
Here, delayed fracture in a bolt is a phenomenon in which, after being tightened with a predetermined force, hydrogen penetrates into the steel from the usage environment and becomes brittle, and suddenly breaks after a certain period of time.
For this reason, in this low carbon-boron steel, measures are taken to prevent cracking of the prior austenite grain boundaries, specifically, measures are taken to reduce the amount of impurities segregating at the austenite grain boundaries during quenching.
[0006]
By the way, when the bonded film is coated on the rolled wire as described above when manufacturing the bolt, the moldability of the subsequent bolt forming becomes good and the wire and the die can be prevented from being seized well. When the wire material coated with the bonde film is quenched after bolt forming, the P component in the bonde film penetrates along the austenite grain boundary of the bolt surface layer, causing the so-called phosphorus phenomenon, and P segregates at the grain boundary. As a result, the problem of lowering the bonding strength of the grain boundaries arises.
If hydrogen further enters the environment from the usage environment, the grain boundaries are further deteriorated in strength, and delayed fracture is more likely to occur.
[0007]
That is, P concentrated on the surface layer promotes hydrogen embrittlement, that is, delayed fracture.
Therefore, if a P-concentrated layer is formed on the bolt surface layer, delayed fracture cannot be completely prevented by merely reducing the amount of grain boundary segregation impurities in the steel.
For this reason, such steels are bolted and hardened without a bondage film . In this case, naturally, the formability during the bolt forming deteriorates.
[0008]
As a solution to the above problem, Patent Document 1 below discloses a method of avoiding the intrusion of P during quenching by using a lubricant film that does not contain P.
However, since the lubrication performance of the non-P-containing film is inferior to that of the bonder film, the mold and the wire are seized at the time of bolt forming, and the production efficiency is lowered.
[0009]
However, a method is also known in which the remaining bond film on the surface of the bolt formed from the wire with the bond film is removed by alkali cleaning.
In this case, however, the quality control of the cleaning liquid and the waste liquid processing are very expensive, which does not contribute to the reduction of the bolt manufacturing cost.
[0010]
On the other hand, Patent Document 2 below also discloses a method for preventing delayed fracture by water quenching a bolt.
Generally, oil quenching is adopted as quenching of high-strength bolts. Therefore, when water quenching is performed, it is necessary to replace a conventional oil cooling bath with a water cooling bath. In this case, replacement is required. In addition to the cost, in the case of water quenching, bolts having a complicated head shape may be cracked.
[0011]
The present invention is based on the above circumstances, and a low carbon-boron steel wire is used as a bolt steel, and in order to maintain cold workability, the bonder film is coated and wire drawing and bolt forming can be performed. In addition, a steel for bolts and a method for producing the bolts that can obtain bolts that can secure strength by oil quenching and tempering and have improved delayed fracture resistance have been created.
[0012]
[Patent Document 1]
Japanese Patent Laid-Open No. 9-104945 [Patent Document 2]
Japanese Patent Laid-Open No. 2001-62639
[Means for Solving the Problems]
The steel for bolts formed from the wire with a bond film of the present invention for solving the above-mentioned problems is, by weight, C: 0.22 to 0.32%, Si: ≦ 0.20%, Mn: 0.60 to 1.20%, P: ≦ 0.015. %, S: ≦ 0.005%, Cu: 0.07 to 0.25%, Ni: 0.03 to 0.20%, Cu + Ni: 0.10 to 0.40%, Cr: 0.05 to 0.45%, Ti: 0.01 to 0.10%, Nb: 0.01 to 0.06%, N: 0.002 to 0.010%, B: 0.0005 to 0.0030% The balance consists of inevitable impurities and Fe (Claim 1).
[0014]
Further, the second aspect of the present invention is characterized in that, in the first aspect, one or two of Mo: 0.03 to 0.10% and V: 0.05 to 0.20% are further contained.
[0015]
Claim 3 relates to a method for producing a bolt. Hot rolling of a wire is performed at the end temperature of 700 to 900 ° C. using the steel for bolt according to any one of claims 1 and 2 as a raw material, and thereafter a coating treatment of a bonder film is performed. Then, after drawing, the wire is drawn and bolted, and after that, after oil quenching, it is tempered at a temperature of 400 ° C. or higher.
[0016]
[Operation and effect of the invention]
The steel for bolts of the present invention as defined in claim 1 eliminates as much as possible high-priced alloy components such as Cr and Mo as much as possible to reduce the cost, while lowering the hardenability by reducing them In order to ensure the hardenability by solute B, N, which is an inhibitory factor, is fixed by adding Ti, and the Nb carbonitride pinning effect by adding Nb makes the grains finer. The main point is to contain a predetermined amount of Cu and Ni and to regulate the total amount of them, in addition to making them into a proper shape.
[0017]
In the present invention, it is confirmed that the delayed fracture resistance of the bolt can be effectively improved while suppressing the formation of surface defects during hot rolling of the wire, particularly by adding Cu and Ni and regulating their total amount. However, the specific reason has not been clearly confirmed.
[0018]
However, it is speculated that Cu is segregated at the grain boundary during the austenitizing heating by adding Cu, and prevents diffusion of P from the bonder film to the grain boundary.
In addition, it is considered that Cu forms precipitates such as intermetallic compounds during tempering, and this prevents hydrogen from entering the grain boundary from the environment of use and also acts to trap hydrogen.
However, when Cu is contained in a large amount exceeding a certain amount, the generation frequency of surface defects increases during hot rolling of the wire. The combined addition of Ni serves to suppress the formation of surface defects.
[0019]
According to the present invention, it is possible to perform wire drawing and bolt forming after coating a bonder film using a low-cost material, and even in such a case, delayed fracture is promoted. Thus, a high-strength bolt excellent in delayed fracture resistance can be obtained.
That is, according to the present invention, a bolt can be manufactured at a low cost, and a bolt having excellent delayed fracture resistance can be obtained.
[0020]
In the present invention, one or two of Mo: 0.03 to 0.10% and V: 0.05 to 0.20% can be added as required (Claim 2).
[0021]
Claim 3 relates to a method of manufacturing a bolt, and this manufacturing method particularly performs hot rolling so that its end temperature is in the range of 700 to 900 ° C., and after the coating treatment of the bonder film, A wire and bolt are formed, followed by oil quenching which is conventionally performed at the time of quenching, and subsequent tempering is performed at a temperature of 400 ° C. or higher.
[0022]
In this case, hot rolling is performed at a low temperature of 700 to 900 ° C. for the following reason.
In other words, when the wire is hot rolled, a rolling scale is formed on the surface, but Cu is oxidized during hot rolling and discharged to the outside, that is, into the rolling scale, and as a result, the Cu concentration of the surface of the wire, that is, the bolt surface is dilute. Thus, the trapping effect of intrusion hydrogen by Cu described above, that is, the effect of improving delayed fracture resistance is lowered.
Therefore, in the present invention, hot rolling is performed so that the end temperature is 900 ° C. or less in order to suppress Cu oxidation during rolling.
[0023]
On the other hand, the reason why tempering is performed at a temperature of 400 ° C. or higher is that Cu is effectively precipitated by aging by such tempering, and the effect of improving delayed fracture resistance by Cu is sufficiently brought out.
[0024]
Next, the reasons for limiting each chemical component and the like in the present invention will be described in detail below.
C: 0.22 to 0.32%
C is an effective element for obtaining a required strength by heat treatment, and it is necessary to contain 0.22% or more.
However, if the content exceeds 0.32%, the cold workability deteriorates, so 0.32% or less is necessary.
[0025]
Si: ≤0.20%
Si is an element that promotes grain boundary oxidation by high-temperature heating during austenitization, and is the starting point of delayed fracture due to separation of the oxide and grain boundaries.
Moreover, the hardness of the wire after rolling is raised and cold workability is deteriorated.
For this reason, it is desirable that the Si content is low, and the upper limit is made 0.20%.
Since Si increases the steelmaking and refining costs, it is desirable to make it 0.01% or more.
[0026]
Mn: 0.60 to 1.20%
Since Mn is an element that is effective as a deoxidizer during melting and contributes to improvement in hardenability, it needs to be 0.60% or more.
However, Mn, together with Si, promotes grain boundary oxidation during quenching and may degrade delayed fracture resistance.
Moreover, since the hardness of the wire after rolling may be increased and the cold workability may be deteriorated, the upper limit is made 1.20%.
[0027]
P: ≤ 0.015%
P segregates at the austenite grain boundaries during austenitizing heating and degrades delayed fracture resistance, so it was made 0.015% or less.
[0028]
S: ≤ 0.005%
S, like P, segregates at the austenite grain boundaries during austenitizing heating, and forms MnS and becomes the starting point of delayed fracture, so it was made 0.005% or less.
[0029]
Cu: 0.07 to 0.25%
Ni: 0.03-0.20%
Cu + Ni: 0.10 to 0.40%
It was found that P penetrates along the austenite grain boundary during quenching from the bond film and the delayed fracture susceptibility increases, but delayed fracture becomes difficult when an appropriate amount of Cu is added.
This is presumably because Cu segregates at the grain boundaries during the austenitizing heating and prevents the diffusion of P from the coating.
This effect increases with the amount of Cu added, but the yield of surface damage may increase and the yield may decrease during hot rolling of the wire.
As a result of earnest investigation of the element for preventing the formation of surface flaws, the combined addition of Ni was effective.
However, when both elements were added in large quantities, the hardness of the wire became higher and the cold workability deteriorated, so the amounts of Cu and Ni were regulated.
[0030]
Cr: 0.05-0.45%
Since Cr is an element that contributes to the improvement of hardenability, the amount of addition should be adjusted according to the dimensions of the bolts and the like.
However, adding too much Cr promotes grain boundary oxidation like Si and Mn and degrades delayed fracture resistance.
Moreover, the hardness of the wire after rolling is raised and cold workability is deteriorated. For this reason, the upper limit was made 0.45%.
[0031]
Ti: 0.01-0.10%
Boron steel uses solid solution B to ensure hardenability.
If the solid solution N in the steel is high, this may react with the solid solution B to form BN precipitates, which may consume the solid solution B that contributes to hardenability.
Therefore, it is necessary to add a small amount of Ti and fix solid solution N as TiN.
For this purpose, the Ti amount needs to be 0.01% or more. However, if Ti is added excessively, coarse TiN is formed, and the bolt formability deteriorates. Therefore, the amount of Ti was regulated to 0.10% or less.
[0032]
Nb: 0.01-0.06%
In boron steel, N becomes TiN and is fixed. This TiN has a size of submicron (μm) to several μm and cannot be expected to have a pinning effect at the grain boundaries during quenching.
Therefore, it is necessary to add Nb to prevent crystal grain coarsening due to Nb carbonitride. For this reason, 0.01% or more is necessary. However, if excessively added, coarse Nb carbonitrides are produced and the grain boundary pinning effect is lost. For this reason, the Nb content was regulated to 0.06% or less.
[0033]
N: 0.002 to 0.010%
It is necessary to form Nb carbonitrides that contribute to grain refinement.
If added excessively, coarse Nb carbonitrides are formed and the grain boundary pinning effect is lost, and delayed fracture resistance and bolt formability deteriorate, so the content is restricted to 0.002 to 0.010%.
[0034]
B: 0.0005-0.0030%
If the amount of alloying elements that contribute to the hardenability of Mn, Cr, Si, etc. is reduced in order to improve the cold workability of the wire, the hardenability of the material will be insufficient and the incompletely hardened structure during bolt quenching Produce.
B compensates for this decrease in hardenability and also has the effect of reducing segregation of P and S by preferentially segregating at the austenite grain boundaries during quenching.
In order to obtain these effects, it is necessary to contain 0.0005% or more, but even if it is contained in a large amount, the effect is saturated, and conversely, coarse boron carbide precipitates at the grain boundaries, and both hardenability and delayed fracture resistance deteriorate. Therefore, the upper limit is made 0.0030%.
[0035]
Mo: 0.03-0.10%
V: 0.05-0.20%
Primary carbides crystallize when the molten metal solidifies, but gradually dissolve again and disappear during hot rolling.
At this time, Mo and V reduce the re-solution rate and contribute to the remaining primary carbide after rolling.
It is considered that the primary carbide remaining in the quenched and tempered bolt functions as a hydrogen trap site and improves delayed fracture resistance.
However, when a large amount is added, a huge primary carbide crystallizes out, and an upper limit is set in order to deteriorate the cold workability.
If the upper limit of Mo is exceeded, bainite is generated in the wire after rolling, resulting in deterioration of cold workability.
[0036]
Hot rolling end temperature of wire: 700-900 ° C
In order to efficiently increase delayed fracture resistance by adding Cu, it is important that the Cu content does not decrease in the bolt surface layer before quenching and tempering.
However, Cu is oxidized during hot rolling and discharged to the outside, so that the Cu concentration in the surface of the wire, and in turn, the surface of the formed bolt becomes dilute.
In order to suppress oxidation during hot rolling and prevent external diffusion from the surface layer of Cu, it is necessary to control the end temperature of hot rolling to 900 ° C. or lower.
However, low temperature rolling lowers rolling efficiency and increases costs, so the end temperature is set to 700 ° C. or higher.
[0037]
Tempering temperature: 400 ℃ or more
Cu and Ni-free materials decrease in hardness with increasing tempering temperature and monotonously improve delayed fracture resistance.
A similar tendency was seen with Cu and Ni composite additives, but the improvement effect was even greater when tempering at 400 ° C or higher.
The reason for this is not clear, but it is thought that both elements concentrated in the vicinity of the surface layer formed intermetallic compounds at 400 ° C. or higher, which became hydrogen trap sites.
[0038]
【Example】
Next, examples of the present invention will be described in detail below.
<Example 1>
Examples A to F and Comparative Examples G to X of the present invention having chemical components shown in Table 1 and Comparative Examples G to X and SCM440 were melted and then ingot, and each steel was rolled into a wire having a diameter of 10.6 mm. At that time, the rolling end temperature was 830 ± 50 ° C.
The conventional JIS SCM440 was held at 760 ° C. for 10 hours after rolling and then annealed to perform spheroidizing annealing.
Cold drawing was performed in order to ensure the dimensional tolerance of the bolt shaft. Here, a wire having a diameter of 10.6 mm was drawn without being annealed or spheroidized to a diameter of 10.0 mm after the bond film treatment.
[0039]
Next, the hexagon bolts with flanges were molded, and cracks in the bolt heads at that time were also shown in Table 2 as x marks.
Subsequently, it was tempered (tempered) so that the tensile strength was 1000 to 1200 MPa. Quenching was performed at a heating temperature of 850 ° C. and a holding time of 1 hour, and cooling was oil cooling.
Table 2 shows the tensile test results of the obtained bolts.
[0040]
In the delayed fracture test, the bolts are attached to a jig, tightened at the yield point where each bolt reaches the elastic limit, immersed in a 15% by mass hydrochloric acid solution for 2 minutes, washed, dried, and left in the atmosphere for 24 hours. Table 2 shows the ratio of broken bolts in 10 steel types when this cycle was repeated up to 14 cycles.
[0041]
In the embodiment of the present invention, annealing or spheroidizing annealing is not performed for cold drawing, but the bolt shape is complicated, and if the material requires excellent cold forming ability, before forming, Various softening heat treatments may be performed.
[0042]
[Table 1]

[0043]
[Table 2]

[0044]
As shown in the results of Table 2, all of the examples have a bolt failure rate of 0%, and the delayed fracture resistance is good, whereas the comparative example has a bolt failure rate. Even those that could not be molded or that could be bolted had a high bolt fracture rate and were inferior to those of the present example in delayed fracture resistance.
[0045]
The comparative example SCM440 at the bottom of Table 1 and Table 2 is conventionally used as a steel for bolts, and has a good bolt fracture rate, that is, delayed fracture resistance, but this is expensive. Material.
[0046]
<Example 2>
The steel types of Examples A, B, D, and F having chemical components shown in Table 1 were ingoted, and each steel was hot-rolled into a wire having a diameter of 10.6 mm while changing the rolling finishing temperature.
When the finishing temperature was lower than 700 ° C., it was necessary to reduce the rolling speed, resulting in a significant reduction in production efficiency and an increase in wire manufacturing cost.
[0047]
Cold drawing was performed in order to ensure the dimensional tolerance of the bolt shaft.
Here, a wire with a diameter of 10.6 mm was subjected to a bond film treatment, and then drawn to a diameter of 10.0 mm without being annealed or spheroidized.
Next, it was molded into a hexagon bolt with a flange, and subsequently tempered (tempered) so that the tensile strength was 1000 to 1200 MPa.
Quenching was performed at a heating temperature of 850 ° C. and a holding time of 1 hour, and then oil-cooled.
Table 3 shows the tensile test results of the obtained bolts.
[0048]
In the delayed fracture test, the bolts are attached to a jig, tightened at the yield point where each bolt reaches the elastic limit, immersed in a 15% by mass hydrochloric acid solution for 2 minutes, washed, dried, and left in the atmosphere for 24 hours. Table 3 shows the ratio of broken bolts in 10 steel types when this cycle was repeated up to 14 cycles.
[0049]
[Table 3]

[0050]
As can be seen from the results in Table 3, all of the examples in which hot rolling was performed at an end temperature of 700 to 900 ° C. and tempering temperature was 400 ° C. or higher had a bolt fracture rate of 0%, While the delayed fracture property is good, all the comparative examples subjected to hot rolling at an end temperature of 900 ° C. or higher have a bad bolt fracture rate.
[0051]
Although the embodiment of the present invention has been described in detail above, this is merely an example, and the present invention can be implemented with various modifications without departing from the spirit of the present invention.

Claims (3)

% By weight
C: 0.22 to 0.32%
Si: ≤0.20%
Mn: 0.60 to 1.20%
P: ≤ 0.015%
S: ≦ 0.005%
Cu: 0.07 to 0.25%
Ni: 0.03-0.20%
Cu + Ni: 0.10 to 0.40%
Cr: 0.05-0.45%
Ti: 0.01-0.10%
Nb: 0.01-0.06%
N: 0.002 to 0.010%
B: 0.0005-0.0030%
A steel for bolts formed from a wire with a bondage coating with a tensile strength of 1000 MPa or more and excellent in delayed fracture resistance, characterized by consisting of the balance inevitable impurities and Fe. % By weight
Mo: 0.03-0.10%
V: 0.05-0.20%
The steel for bolts formed from the wire with a bondage film excellent in delayed fracture resistance according to claim 1, further comprising one or two of the following.   The steel for bolts according to any one of claims 1 and 2 is subjected to hot rolling of the wire at a final temperature of 700 to 900 ° C, and then subjected to a coating treatment of the bonder film, followed by wire drawing and bolt forming, After that, after quenching with oil and tempering at a temperature of 400 ° C or higher, a method for producing a bolt having a tensile strength of 1000 MPa or more and excellent delayed fracture resistance.