JP4031068B2 - High strength steel for bolts with excellent hydrogen embrittlement resistance - Google Patents

Description

【００２３】
得られた鋼材について、化合物の数を測定するとともに、引張強さおよび遅れ破壊強度を測定し、後記の表２〜６に示す結果を得た。尚、化合物測定および、遅れ破壊強度の測定は下記の方法によって測定した。
【００２４】
＜化合物数の測定＞
▲１▼：通常の抽出レプリカ法により抽出した化合物を、走査型電子顕微鏡（ＴＥＭ）にて、加速電圧２００ＫＶ、１５万倍の写真撮影をする。
【００２５】
▲２▼：５００ｎｍ×５００ｎｍ（１５万倍で７５ｍｍ×７５ｍｍ）の視野の中で観察される５０ｎｍ以下の微細な化合物を数える。図２，図３，図４の各上側には撮影した写真の一例（一辺の長さは５００ｎｍ）を示し、各下側には、その視野及び化合物の大きさを忠実に転写し、その数（何れも図から明らかな様に５０ｎｍ以下の大きさ）を数えたところ、図２では３６個、図３では２４個、図４では１０個であった。更に図４，図５には、ＥＤＸにより組成分析した結果を併せて示す。図４は析出物組成がＭｏ−Ｔｉ−Ｖ複合の場合、図５は析出物組成がＭｏ−Ｔｉ−Ｖ−Ｃｒ−Ａｌ複合の場合を夫々示す。
【００２６】
図５の析出物の組成は主にＭｏ−Ｔｉ−Ｖ系の複合化合物であり、図６の析出物の組成は主にＭｏ−Ｔｉ−Ｖ−Ｃｒ−Ａｌ系の複合化合物である。但し、Ｃｕのピークが高いのは測定に当たってサンプルを固定する際に、Ｃｕメッシュを使用したためである。
【００２７】
▲３▼：写真撮影は１サンプルにつき、任意の５箇所とし、測定した５箇所の平均値を化合物の総数と定義し、○○個／（５００ｎｍ）² で表わす。
【００２８】
＜遅れ破壊強度の測定＞
ループ型定歪み遅れ破壊試験機を用いて、図１に示す遅れ破壊試験片を水中で応力負荷し１００時間の遅れ破壊強さを測定した。結果は表２、表３、表４に示す通りであり、微細化合物の数が多い鋼で種は、いずれも高い遅れ破壊強さを示している。
【００２９】
【表２】

【００３０】
【表３】

【００３１】
【表４】

【００３２】
【表５】

【００３３】
【表６】

【００３４】
表２，３，４に示す様に、微細化合物は５０ｎｍ以下の化合物が特に有効であるが、特に表２に示す様に、３０ｎｍ以下、更に１０ｎｍ以下と小さくなるほど耐遅れ破壊特性の改善に有効であることが分かる。一方５０ｎｍ以上の大きさの化合物については、５０〜３００ｎｍの大きさの化合物は表６に示す様に耐遅れ破壊特性の良否との間に良好な相関を見出すことはできず、３００ｎｍ以上化合物については、表５に示す様にその数を減少させることが耐遅れ破壊特性の向上に有効で有ることが分かる。この様に化合物の大きさ（及び数）と耐遅れ破壊特性との間の相関については、極めて特異な様相を示すものであることが本発明によって明らかとなった。
一方５０ｎｍ以下の化合物を多く微細析出させる手段としては、表４に示す様に、凝固速度を制御することが有効であることが分かった。
【００３５】
【発明の効果】
本発明は以上の様に構成されており、１２００Ｎ／ｍｍ² レベル以上で且つ耐水素脆化特性に優れた高強度ボルト鋼が提供されることとなった。
【図面の簡単な説明】
【図１】 遅れ破壊強度測定に用いた試験片の寸法、形状を示す図である。
【図２】 化合物の大きさ及び数の測定例を示す顕微鏡写真（ＴＥＭ像）及び当該写真から読み取ることのできる析出物の表示図である［析出物は３６個／（５００ｎｍ）² ］。
【図３】 化合物の大きさ及び数の測定例を示す顕微鏡写真（ＴＥＭ像）及び当該写真から読み取ることのできる析出物の表示図である［析出物は２４個／（５００ｎｍ）² ］。
【図４】 化合物の大きさ及び数の測定例を示す顕微鏡写真（ＴＥＭ像）及び当該写真から読み取ることのできる析出物の表示図である［析出物は１０個／（５００ｎｍ）² ］。
【図５】 図２中の１について、ＥＤＸによる化合物組成を分析して得たスペクトル像の一例を示す写真である（析出物組成：Ｍｏ−Ｔｉ−Ｖ複合）。
【図６】 図２中の２について、ＥＤＸによる化合物組成を分析して得たスペクトル像の一例を示す写真である（析出物組成：Ｍｏ−Ｔｉ−Ｖ−Ｃｒ−Ａｌ複合）。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength steel having improved delayed fracture resistance due to excellent hydrogen embrittlement resistance and a method for producing the same.
[0002]
[Background Art and Problems to be Solved by the Invention]
Regarding the cause of delayed fracture that occurs after a certain period of time has passed since the stress is applied to the steel material, it is considered that various factors are intertwined in a complicated manner, so it is difficult to identify the cause. However, in general, there is a common recognition that hydrogen embrittlement is involved. On the other hand, as factors affecting the hydrogen embrittlement phenomenon, the effects of tempering temperature, structure, material hardness, crystal grain size, various alloy elements, etc. have been recognized for some time, but measures to prevent hydrogen embrittlement have been established. In fact, various methods have been proposed only by trial and error.
[0003]
Further, JP-A-60-114551, JP-A-2-267243, JP-A-3-243745 and the like disclose delayed fracture resistance even when the tensile strength is 140 kgf / mm ² or more by adjusting various main alloy elements. It is disclosed that it is excellent in steel for high-strength bolts. However, it does not completely eliminate the risk of delayed fracture, and their scope of application is limited.
[0004]
[Means for Solving the Problems]
Therefore, the present inventors investigated in detail the relationship between various compounds in the steel and the hydrogen embrittlement resistance of the conventional high-strength steel. As a result, there is a good correlation between the amount of the fine compound and the delayed fracture characteristics, and the fact that the number of fine compounds is focused on and the number of the fine compounds is prescribed, the occurrence of delayed fracture can be truly prevented. I found it. In other words, one of the causes for the deterioration of delayed fracture characteristics is that hydrogen (diffusible hydrogen) moving around in steel has an adverse effect. To reduce this diffusible hydrogen, It is effective to actively precipitate the compound and trap hydrogen with the compound. At this time, it seems that any size compound can exert the hydrogen trapping ability, but according to the study by the present inventors, it has been clarified that the compound having a small hydrogen trapping ability is superior. . In other words, fine compounds of 50 nm or less, which have not been attracting attention until now, are extremely effective in improving delayed fracture resistance as compared with those having a size of more than 50 nm. The prevention effect becomes remarkable. Particularly in the case of quenching and tempering of alloy steel, high temperature tempering after quenching causes many fine alloy-based compounds to precipitate, which is effective in improving delayed fracture.
[0005]
That is, according to the present invention, in the steel containing C: 0.20 to 0.60% (particularly 0.30 to 0.60%), the total number of fine compounds having a size of 50 nm or less is 20 / (500 nm). ^It was set as ² or more. These compounds include Mo compounds , V compounds, or composite compounds containing two or more metals selected from Mo, Ti, and V (particularly, composite compounds are shown), and these fine compounds. Due to the presence of this, it becomes a high strength steel having better hydrogen embrittlement resistance.
[0006]
As a component of the high-strength steel of the present invention that satisfies the above requirements,
Mo: 2.00% or less (excluding 0%)
Ti: 0.20% or less (excluding 0%)
V: 0.20% or less (excluding 0%)
It is recommended to contain one or more selected from the group consisting of, more preferably as other components,
Cr: 2.00% or less (excluding 0%)
Al: 0.05% or less (excluding 0%)
Nb: 0.20% or less (excluding 0%)
W: 0.20% or less (excluding 0%)
B: 0.003% or less (excluding 0%)
As one or more selected from the group consisting of :
N: 0.001 to 0.010%
O: 0.005% or less (excluding 0%)
S: 0.025% or less (excluding 0%)
It is recommended to contain at least one selected from the group consisting of:
[0007]
DETAILED DESCRIPTION OF THE INVENTION
Next, the configuration of the present invention will be described in more detail.
(1) The requirement that the total number of fine compounds having a size of 50 nm or less is 20 / (500 nm) ² or more:
As described above, the reason why the delayed fracture resistance is deteriorated is that hydrogen (diffusible hydrogen) moving around in steel has an adverse effect. In order to reduce this diffusible hydrogen, it is necessary to increase the hydrogen trapping ability by precipitating many fine compounds in the steel. Furthermore, these compounds have the effect of increasing the toughness of the steel material itself and increasing the resistance to hydrogen. It has been found that particularly when the total number of fine compounds (size is 50 nm or less) is 20 / (500 nm) ² or more, the effect of improving hydrogen embrittlement resistance is remarkably exhibited. Further, it is preferably 30 / (500 nm) ² or more.
[0008]
The compounds larger than these are as follows.
(1) When a softening process such as spheroidizing annealing is included before the compound quenching and tempering treatment with a size of 300 nm or more, if the quenching heating is insufficient, a coarse compound with a spheroidized structure is formed. May remain. The size of these compounds is 300 nm or more, and when these compounds are large, the toughness of the steel decreases and the delayed fracture strength tends to decrease as shown in Table 5 below. It is desirable to reduce these size compounds as much as possible.
[0009]
(1) Compound having a size of 50 to 300 nm Precipitates of these sizes are classified into Fe ₃ C (cementite) -based and alloy-based precipitates (Mo ₂ C, Cr ₇ C _3, etc.). ₃ is not large difference in hydrogen trapping effect in C-based and precipitates of the alloy system, the vast majority towards Fe ₃ C compound as the amount of the compound. However Fe ₃ C based compound, a plate etc. grain boundary In some cases, the grain boundary strength may be reduced and delayed fracture strength may be reduced.
[0010]
For a compound of this level, the correlation with delayed fracture strength is unclear as shown in Table 6 below, and even if these amounts are specified, hydrogen embrittlement cannot be prevented.
[0011]
In addition, the measuring method of the magnitude | size of a compound is demonstrated in an Example.
(2) Compound types:
Examples of the compound that efficiently precipitates as a fine compound in steel and can exhibit a high hydrogen trapping ability include alloy compounds, and in particular, Mo compounds, Ti compounds, V compounds, and Mo, Ti, In the case of a composite compound containing two or more kinds of metals selected from V, it was found that a particularly excellent effect was exhibited in improving the hydrogen embrittlement resistance. Furthermore, these compounds are effective for preventing coarsening of austenite crystal grains and are effective for improving the strength and toughness of steel.
[0012]
(3) Important chemical components constituting the steel of the present invention:
(3) -1 One or more selected from the group consisting of Mo: 2.00% or less, Ti: 0.20% or less, and V: 0.20% or less. These elements contain fine compounds in the steel. It is an element effective for efficient precipitation, and is necessary for precipitating Mo compounds, Ti compounds, V compounds, and complex compounds containing two or more metals selected from Mo, Ti, and V. Element. Furthermore, these elements are effective elements for improving hardenability, and are effective in improving the strength and toughness of steel materials.
[0013]
The effect of Mo not only saturates at about 2.00%, but addition beyond that leads to a reduction in the tool life due to increased deformation resistance. If Ti or V is added in an amount exceeding 0.20%, huge nitrides and carbides are generated, and the toughness is lowered. Therefore, it was determined that Mo: 2.00% or less, Ti: 0.20% or less, and V: 0.20% or less. In addition, each preferable upper limit content is Mo: 1.5% or less, Ti: 0.15% or less, V: 0.15% or less, More preferably, Mo: 1.05% or less, Ti: 0.1% or less, V: 0.1% or less. On the other hand, the preferred lower limit contents are Mo: 0.3% or more, Ti: 0.01% or more, V: 0.15% or more, and more preferably Mo: 0.6% or more, Ti: 0. 0.02% or more, V: 0.02% or more.
[0014]
(3) -2 Cr: 2.00% or less, Al: 0.05% or less, Nb: 0.20% or less, W: 0.20% or less, B: 0.003% or less One or more of these elements are also effective elements for efficiently precipitating fine compounds in steel, and the compounds containing these elements also have an effect of preventing coarsening of austenite crystal grains. Therefore, it is effective in improving the strength and toughness of steel. Further, Cr contributes to the improvement of corrosion resistance and exhibits an effect of increasing hydrogen embrittlement resistance, and B is concentrated at the grain boundaries to enhance the hardenability of the steel.
[0015]
The effect of Cr is saturated at about 2.0%, and when Al, Nb, W, and B are added in a large amount, huge nitrides and carbides are formed and the toughness is lowered. Therefore, Cr: 2.00% or less, Al: 0.05% or less, Nb: 0.20% or less, W: 0.20% or less, B: 0.003% or less. In addition, each preferable upper limit content is Cr: 1.5% or less, Al: 0.045% or less, Nb: 0.15% or less, W: 0.15% or less, B: 0.0025% or less. More preferably, Cr: 1.05% or less, Al: 0.04% or less, Nb: 0.1% or less, W: 0.1% or less, B: 0.0020% or less. On the other hand, preferable lower limit contents are Cr: 0.3% or more, Al: 0.01% or more, Nb: 0.01% or more, W: 0.01% or more, B: 0.0005% or more, and more Preferably, Cr: 0.5% or more, Al: 0.02% or more, Nb: 0.02% or more, W: 0.02% or more, B: 0.001% or more.
[0016]
(3) -3 C: 0.10 to 0.60%, N: 0.001 to 0.010%, O: 0.005% or less, S: 0.025% or less These elements are compounds in steel. Is an element necessary for precipitating, and forms carbides, nitrides, oxides, and sulfides.
C is an element indispensable for forming carbides and securing the tensile strength necessary for high-strength steel, and needs to be contained in an amount of 0.10% or more. On the other hand, C exceeding 0.60% invites coarsening of the carbide and also causes a decrease in toughness and deteriorates delayed fracture resistance. Therefore, the C content is determined to be 0.10 to 0.60%. A preferable lower limit C amount is 0.20%, and a more preferable lower limit C amount is 0.30%. On the other hand, a preferable upper limit C amount is 0.45%, and a more preferable upper limit C amount is 0.40%.
[0017]
N forms nitrides and has a positive influence on the refinement of crystal grains and thus on the improvement of delayed fracture resistance. To obtain these effects, 0.001% or more must be added. On the other hand, when the N content exceeds 0.010%, the solid solution N content increases, which is harmful to delayed fracture resistance. Therefore, the N content is determined to be 0.001 to 0.010%. A preferable lower limit N amount is 0.002%, and a more preferable lower limit N amount is 0.004%. A preferable upper limit N amount is 0.007%, and a more preferable upper limit N amount is 0.006%.
[0018]
O forms an oxide and is finely dispersed in the steel. However, if the amount of O exceeds 0.005%, a coarse oxide precipitates, causing a decrease in toughness and deteriorating delayed fracture resistance. Therefore, the content of O is set to 0.005% or less. In addition, a preferable amount of O is 0.003% or less, and a more preferable amount of O is 0.001% or less.
[0019]
S forms sulfides and is finely dispersed in the steel. However, if the amount of S exceeds 0.025%, coarse MnS or the like is formed, resulting in a stress-concentrated portion, which deteriorates delayed fracture resistance. Therefore, the content of S is determined to be 0.025% or less. A preferable amount of S is 0.010% or less, and a more preferable amount of O is 0.005% or less.
In addition, although inevitable impurities may be included in the steel of the present invention in production, they are allowed as long as the effects of the present invention are not impaired.
Moreover, if the manufacturing conditions of the steel material are adjusted as described below, the useful compounds can be efficiently finely precipitated, and a high-strength steel excellent in hydrogen embrittlement resistance can be easily obtained.
[0020]
That is, it is preferable to define the cooling rate in the solidification process when manufacturing the steel material. That is, in the solidification process (during cooling from 1500 ° C. to 1300 ° C.), by cooling at a rate of 10 ° C./min or more, precipitation of coarse compounds is suppressed, and many fine compounds are precipitated. A more preferable cooling rate is 20 ° C./min or more, and a more preferable cooling rate is 30 ° C./min or more.
Next, examples of the present invention will be described.
[0021]
【Example】
Steel materials having the composition shown in Table 1 were melted in a 150 kg vacuum melting furnace, cast into a 150 kg ingot, and cooled. Further, in order to change the cooling rate of the solidification process in part, it was cast into an ingot of 50 to 150 kg and cooled while keeping part of the temperature. Thereafter, forging to 25 mmφ was performed, and after a 1200 ° C. × 30 minute volumeification treatment, normalization treatment was performed, and final heat treatment was performed so that the tensile strength was about 1000 to 2000 N / mm ² .
[0022]
[Table 1]

[0023]
About the obtained steel materials, while measuring the number of compounds, the tensile strength and delayed fracture strength were measured, and the results shown in Tables 2 to 6 below were obtained. The compound and the delayed fracture strength were measured by the following methods.
[0024]
<Measurement of the number of compounds>
(1): A compound extracted by a normal extraction replica method is photographed with a scanning electron microscope (TEM) at an acceleration voltage of 200 KV and 150,000 times.
[0025]
{Circle around (2)} Count fine compounds of 50 nm or less observed in a field of view of 500 nm × 500 nm (150,000 × 75 mm × 75 mm). Each of the upper side of FIGS. 2, 3 and 4 shows an example of a photograph taken (the length of one side is 500 nm). Each lower side faithfully transfers the field of view and the size of the compound. (As is apparent from the figure, the size was 50 nm or less), the number was 36 in FIG. 2, 24 in FIG. 3, and 10 in FIG. Further, FIGS. 4 and 5 also show the results of composition analysis by EDX. FIG. 4 shows the case where the precipitate composition is a Mo—Ti—V composite, and FIG. 5 shows the case where the precipitate composition is a Mo—Ti—V—Cr—Al composite.
[0026]
The composition of the precipitate in FIG. 5 is mainly a Mo—Ti—V composite compound, and the composition of the precipitate in FIG. 6 is mainly a Mo—Ti—V—Cr—Al composite compound. However, the reason why the peak of Cu is high is that a Cu mesh was used when fixing the sample during measurement.
[0027]
{Circle around (3)} Photographing was performed at an arbitrary five points per sample, and the average value of the five points measured was defined as the total number of compounds, and expressed as ◯ / (500 nm) ² .
[0028]
<Measurement of delayed fracture strength>
Using a loop type constant strain delayed fracture tester, the delayed fracture test piece shown in FIG. 1 was stressed in water and the delayed fracture strength for 100 hours was measured. The results are as shown in Table 2, Table 3, and Table 4, and the steel is a steel with a large number of fine compounds, and the seeds show high delayed fracture strength.
[0029]
[Table 2]

[0030]
[Table 3]

[0031]
[Table 4]

[0032]
[Table 5]

[0033]
[Table 6]

[0034]
As shown in Tables 2, 3, and 4, fine compounds with 50 nm or less are particularly effective. However, as shown in Table 2, 30 nm or less, and further with 10 nm or less, the more effective in improving delayed fracture resistance. It turns out that it is. On the other hand, with respect to compounds having a size of 50 nm or more, compounds having a size of 50 to 300 nm cannot find a good correlation with the quality of delayed fracture resistance as shown in Table 6. As shown in Table 5, it can be seen that reducing the number is effective in improving the delayed fracture resistance. As described above, the present invention has revealed that the correlation between the size (and number) of the compound and the delayed fracture resistance shows a very specific aspect.
On the other hand, as shown in Table 4, it was found that controlling the coagulation rate is effective as a means for finely depositing a large amount of compounds of 50 nm or less.
[0035]
【The invention's effect】
The present invention is configured as described above, and a high-strength bolt steel having a level of 1200 N / mm ² or more and excellent hydrogen embrittlement resistance is provided.
[Brief description of the drawings]
FIG. 1 is a diagram showing the size and shape of a test piece used for delayed fracture strength measurement.
FIG. 2 is a micrograph (TEM image) showing measurement examples of the size and number of compounds and a display diagram of precipitates that can be read from the photograph [36 precipitates / (500 nm) ² ].
FIG. 3 is a micrograph (TEM image) showing measurement examples of the size and number of compounds and a display diagram of precipitates that can be read from the photograph [24 precipitates / (500 nm) ² ].
FIG. 4 is a micrograph (TEM image) showing measurement examples of the size and number of compounds and a display diagram of precipitates that can be read from the photograph [10 precipitates / (500 nm) ² ].
FIG. 5 is a photograph showing an example of a spectrum image obtained by analyzing the compound composition by EDX for 1 in FIG. 2 (precipitate composition: Mo—Ti—V composite).
6 is a photograph showing an example of a spectrum image obtained by analyzing the compound composition by EDX for 2 in FIG. 2 (precipitate composition: Mo—Ti—V—Cr—Al composite).

Claims (4)

Containing C, Si, Mn, N, O, S, Cr, Al,
C: 0.30 to 0.60% (meaning mass%, the same shall apply hereinafter)
N: 0.001 to 0.010%,
O: 0.005% or less (excluding 0%),
S: 0.025% or less (excluding 0%),
Cr: 0.14 to 2.00 %,
Al: 0.05% or less (excluding 0%);
Contains normal amounts of Mn and Si as bolt steel,
Furthermore,
Mo: 2.00% or less (excluding 0%),
Containing two or more selected from the group consisting of Ti: 0.20% or less (not including 0%) and V: 0.20% or less (not including 0%);
The balance is steel composed of Fe and inevitable impurities,
A composite compound containing two or more metals selected from Mo, Ti, and V and having a size of 50 nm or less is present in steel at 20 pieces / (500 nm) ² or more. Steel for high-strength bolts with excellent hydrogen embrittlement resistance with a strength of 1200 N / mm ² or more. Containing C, Si, Mn, N, O, S, Cr, Al,
C: 0.30 to 0.60% (except when C is 0.33%),
N: 0.001 to 0.010%,
O: 0.005% or less (excluding 0%),
S: 0.025% or less (excluding 0%),
Cr: 0.14 to 2.00 %,
Al: 0.05% or less (excluding 0%);
Contains normal amounts of Mn and Si as bolt steel,
Furthermore,
Containing one or more selected from the group consisting of Mo: 2.00% or less (excluding 0%) and V: 0.20% or less (not including 0%),
The balance is steel composed of Fe and inevitable impurities,
The size of either a Mo-based compound or a V-based compound: a compound having a size of 50 nm or less is present in steel at 20 pieces / (500 nm) ² or more, and has a tensile strength of 1200 N / mm ² or more. High strength steel for bolts with excellent hydrogen embrittlement characteristics. The steel for high-strength bolts according to claim 2, wherein the steel contains Ti: 0.20% or less (not including 0%). Steel is still another component,
Nb: 0.20% or less (excluding 0%)
W: 0.20% or less (excluding 0%)
The steel for high-strength bolts according to any one of claims 1 to 3, which contains at least one selected from the group consisting of B: 0.003% or less (excluding 0%).

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