JP5072058B2 - High strength bolt with excellent hydrogen embrittlement resistance - Google Patents

Description

  The present invention relates to a high-strength bolt excellent in hydrogen embrittlement resistance, and in particular, a high-strength bolt with suppressed hydrogen embrittlement, cracking, and delayed fracture, which is a problem with bolts having a tensile strength of 1180 MPa or more. It is about.

As general steel for bolts, low alloy tough steel, especially SCM435 and SCM440, are widely used. These steel materials exhibit high strength such as tensile strength: 120 to 130 kgf / mm ² (1180 to 1270 MPa), but are tempered when further increase in strength is required.

  However, a tempered bolt obtained by tempering is prone to so-called delayed fracture such as sudden cracking after a long time has passed after tightening. In order to solve such a problem, for example, Patent Document 1 shows that an alloying element such as Cr, Mo, Ti, or V is added to basic components and the mutual ratio of these components is controlled. Has been. Patent Document 2 shows that delayed fracture resistance can be secured by making the metal structure a tempered martensite single phase and dispersing fine precipitates having a particle size of less than 10 nm in the structure. In this technique, alloy elements such as Ni, Ti, and Mo are added to obtain the precipitate.

  However, there is a limit in improving the hydrogen storage capacity only by controlling the precipitation form of the precipitate, and it is difficult to realize better hydrogen embrittlement resistance. The microalloys such as Ti and V are expensive, and there is a limit to supplying high-strength bolts excellent in delayed fracture resistance at low cost. Furthermore, when these alloy elements are contained in a large amount, there is a problem that it is difficult to recycle.

On the other hand, Patent Document 3 shows that a bolt having a tensile strength of 1300 MPa or more and an elongation of 8.8 to 12.2% can be obtained by controlling the structure without requiring the addition of the above alloy elements. Yes. Specifically, it is shown that the structure should be a bainite structure, and the ratio of the length and width of the prior austenite grains in the region from the surface layer to 300 μm or more and 630 μm or less should be 1.2 or more. However, in this technique, further improvement is required to obtain better hydrogen embrittlement resistance.
Japanese Patent No. 2614659 JP 2003-321743 A Japanese Patent No. 3494798

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a bolt having a tensile strength as high as 1180 MPa or more and significantly improved hydrogen embrittlement resistance.

The high-strength bolt according to the present invention is C: 0.20 to 0.60% (meaning mass%, the same applies to the component composition), Si: 1.0 to 3.0%, Mn: 1.0 to 3 0.5%, Al: 1.5% or less (excluding 0%), P: 0.15% or less, S: 0.02% or less, with the balance consisting of iron and inevitable impurities,
The area ratio for all tissues
-Residual austenite is 1% or more,
・ Bainitic ferrite and martensite total 80% or more,
-The total of ferrite and pearlite is 10% or less (including 0%),
The average axial ratio (major axis / minor axis) of the residual austenite crystal grains is 5 or more,
Further, there is a characteristic that the tensile strength is 1180 MPa or more (hereinafter sometimes referred to as “the bolt 1 of the present invention”).

Another high-strength bolt according to the present invention is C: 0.20 to 0.60%, Si: 1.0 to 3.0%, Mn: 1.0 to 3.5%, Al: 0.5% The following (not including 0%), P: 0.15% or less, S: 0.02% or less, the balance is made of iron and inevitable impurities,
The area ratio for all tissues
-Residual austenite is 1% or more,
・ Bainitic ferrite and martensite total 80% or more,
-The total of ferrite and pearlite is 10% or less (including 0%),
The average axial ratio (major axis / minor axis) of the residual austenite crystal grains is 5 or more,
Further, it is characterized in that the tensile strength is 1180 MPa or more (hereinafter sometimes referred to as “the bolt 2 of the present invention”).

  In addition, the high-strength bolt of the present invention further includes Nb: 0.1% or less (not including 0%) and / or Mo: 1.0% or less (not including 0%), Cu: 2% or less ( 0% is not included) and / or Ni: 5% or less (not including 0%) may be included.

  According to the present invention, a high-strength bolt having a tensile strength of 1180 MPa or more, in which hydrogen entering from the outside is rendered harmless and has improved hydrogen embrittlement resistance, can be produced with high productivity without adding an expensive element. In addition, high-strength bolts used for automobiles and the like that are extremely unlikely to cause delayed fracture can be supplied at low cost. In addition, the high-strength bolt of the present invention is excellent in recyclability because the amount of alloying elements is smaller than that of conventional products.

  In the case of tempered martensite steel or martensite + ferritic steel that has been generally adopted as a high-strength steel material, delayed fracture due to hydrogen is caused by accumulation of hydrogen at the prior austenite grain boundaries, etc. It is thought that this occurs as a starting point, and in order to reduce the susceptibility to delayed fracture, as mentioned in the prior art, disperse the concentration of diffusible hydrogen by uniformly and finely dispersing carbides as hydrogen trap sites. Has been adopted as a general solution. However, even if a large number of carbides or the like are dispersed as hydrogen trap sites in this way, there is a limit to the trapping capability, so that delayed fracture due to hydrogen cannot be sufficiently suppressed.

  Accordingly, the present inventors have reexamined specific means in order to realize higher hydrogen embrittlement resistance (delayed fracture resistance) that fully considers the usage environment of bolts.

  As a result, in order to increase the hydrogen embrittlement resistance by reducing the starting point of grain boundary fracture, the parent phase of the steel constituting the bolt is changed to a martensite single phase structure generally adopted for high-strength steel materials. Rather than conclude, it was concluded that it is optimal to use a bainitic ferrite-based “two-phase structure of bainitic ferrite and martensite”. In the case of the above-described martensite single phase structure, carbides (for example, film-like cementite) precipitate at the grain boundaries and easily cause grain boundary fracture, whereas bainitic ferrite is the main component of “bainitic ferrite and martensite. “Site two-phase structure” means that the bainitic ferrite is a plate-like ferrite, unlike ordinary (polygonal) ferrite, and has a high dislocation density, making it easy to increase the strength of the entire structure as in the case of a single martensite phase. Furthermore, since a large amount of hydrogen is trapped on this dislocation, the resistance to hydrogen embrittlement can be improved. In addition, the presence of the bainitic ferrite and residual austenite described later also has an advantage that it is possible to prevent the formation of carbides that are the starting point of grain boundary fracture.

  It has also been found that the formation of lath-like retained austenite is very effective in improving the hydrogen embrittlement resistance by improving the hydrogen trapping capability and detoxifying hydrogen. Conventionally, retained austenite has been considered to adversely affect hydrogen embrittlement resistance and fatigue. However, as a result of investigations by the present inventors, conventional retained austenite is a lump of micron order, and this form of retained austenite adversely affects hydrogen embrittlement resistance and fatigue, but the form of retained austenite is submicron order. It was found that by controlling to a lath shape, the hydrogen storage ability inherent in retained austenite is exhibited, hydrogen can be stored and trapped in a large amount, and the hydrogen embrittlement resistance can be greatly improved.

  Hereinafter, the reason why each organization is defined in the present invention will be described in detail.

<Bainitic ferrite (BF) + martensite (M): 80% or more>
In the present invention, a two-phase structure of bainitic ferrite and martensite (mainly bainitic ferrite) is used. As described above, the bainitic ferrite structure is hard and high strength is easily obtained. In addition, since the dislocation density of the parent phase is high and many hydrogens are trapped on the dislocations, there is an advantage that a large amount of hydrogen can be occluded as compared with other TRIP steels. Furthermore, there is an advantage that lath-like retained austenite as defined in the present invention is easily generated at the boundary of the lath-like bainitic ferrite, and extremely excellent elongation can be obtained. In order to effectively exhibit such an action, the total area ratio of bainitic ferrite and martensite is 80% or more, preferably 85% or more, and more preferably 90% or more. . In addition, the upper limit can be determined by balance with other structures (residual austenite), and when the structure other than retained austenite (ferrite or the like) is not contained, the upper limit is controlled to 99%.

  The bainitic ferrite is a plate-like ferrite and means a substructure having a high dislocation density. Polygonal ferrite having a substructure with no or very few dislocations is the following by SEM observation. The streets are clearly distinguished.

  The area ratio of the bainitic ferrite structure is obtained as follows. That is, the sample (cylindrical shape) is cut out so that a cross section at a radius of 1/2 can be observed, and then corroded with nital, and a measurement region (approximately 50 × 50 μm) at an arbitrary position on the plane is scanned with a scanning electron microscope (SEM). , A scanning electron microscope) observation (magnification: 1500 times).

  Bainitic ferrite is dark gray in the SEM photograph (in the case of SEM, bainitic ferrite may not be distinguished from retained austenite and martensite in some cases), but polygonal ferrite is black in the SEM photograph. It has a square shape and does not contain retained austenite or martensite.

  The SEM used in the present invention is a “high resolution type FE-SEM (EB30, made by Philips, XL30S-FEG) equipped with an EBSP (Electron Back Scattering Pattern) detector”. There is an advantage that the region can be simultaneously analyzed on the spot by the EBSP detector. The EBSP method will be briefly described here. The EBSP determines the crystal orientation of the electron beam incident position by making an electron beam incident on the sample surface and analyzing the Kikuchi pattern obtained from the reflected electrons generated at this time. If the electron beam is scanned two-dimensionally on the sample surface and the crystal orientation is measured at every predetermined pitch, the orientation distribution on the sample surface can be measured. According to this EBSP observation, there is an advantage that textures in the plate thickness direction having different crystal orientation differences, which are determined to be the same in normal microscope observation, can be identified by the color tone difference.

<Residual austenite (residual γ, γ _R ): 1% or more>
Residual austenite is not only useful for improving the total elongation as is conventionally known, but also a structure that greatly contributes to the improvement of the resistance to hydrogen embrittlement. Preferably it is 2% or more, more preferably 3% or more. If a large amount of the above retained austenite is present, the desired high strength cannot be ensured. Therefore, it is recommended that the upper limit be 20%. More preferably, it is 15% or less.

  As described above, when the retained austenite is made into a lath shape, the hydrogen trapping capability is overwhelmingly greater than that of carbide, and particularly when the shape is 5 or more in terms of the average axial ratio (long axis / short axis), so-called atmospheric air It has been found that the hydrogen intrusion resistance can be remarkably improved by detoxifying hydrogen that invades due to corrosion. The average axial ratio of the retained austenite is preferably 10 or more, more preferably 15 or more.

From the viewpoint of the stability of the retained austenite, it is recommended that the C concentration (C _γR ) in the retained austenite is 0.8% or more. Further, if this _CγR is controlled to 0.8% or more, the elongation and the like can be effectively increased. Preferably it is 1.0% or more, More preferably, it is 1.2% or more. Although the higher C _γR is more preferable, it is considered that the upper limit that can be adjusted is about 1.6% in actual operation.

The said retained austenite means the area | region observed as a FCC phase (face centered cubic lattice) by the FE-SEM / EBSP method mentioned above. As a specific example of the measurement by EBSP, as in the case of the observation of bainitic ferrite and martensite, a measurement region (about 50) in an arbitrary position in the cross section at the radius 1/2 position of the sample (cylindrical shape). X50 μm) and measuring at a measurement interval of 0.1 μm. Incidentally, when polishing up to the measurement surface, electrolytic polishing is performed in order to prevent transformation of retained austenite. Next, using the “FE-SEM equipped with the EBSP detector”, an electron beam is irradiated onto the sample set in the SEM column. Take an EBSP image projected on the screen with a high-sensitivity camera (VE-1000-SIT, manufactured by Dage-MTI Inc.) and capture it as an image on a computer. Then, image analysis is performed by a computer, and an FCC phase determined by comparison with a simulation pattern using a known crystal system [in the case of retained austenite, FCC phase (face-centered cubic lattice)] is color-mapped. The area ratio of the region mapped in this manner is obtained, and this is defined as “area ratio of residual austenite”. Note that the OIM (Orientation Imaging Microscopy ^™ ) system of TexSEM Laboratories Inc. was used as hardware and software for the above analysis.

  The average axis ratio is measured with a TEM (Transmission Electron Microscope) (magnification: 15,000 times), and the major axis and minor axis of the remaining austenite crystal grains are measured in three arbitrarily selected visual fields. The axial ratio was determined, and the average value was taken as the average axial ratio.

<Ferrite (F) + Pearlite (P): 10% or less (including 0%)>
The bolt of the present invention may be composed of only the above structure (that is, a mixed structure of bainitic ferrite + martensite and retained austenite). However, as long as the effect of the present invention is not impaired, It may have ferrite (herein, “ferrite” means polygonal ferrite, ie, ferrite having no or very low dislocation density) and pearlite. These are structures that can inevitably remain in the production process of the present invention, but the smaller the number is, the more preferable it is in the present invention. Preferably it is less than 5%, more preferably less than 3%.

  As described above, the present invention is particularly characterized in that the metal structure is controlled. In order to easily improve the hydrogen embrittlement resistance and high strength by forming the structure, the component composition of the bolt is as follows. Need to satisfy the street.

<C: 0.20 to 0.60%>
C is an element necessary for securing high strength of 1180 MPa or more and securing retained austenite. Specifically, it is an important element for allowing a desired austenite phase to remain at room temperature by containing a sufficient amount of C in the austenite phase, and it is necessary to contain 0.20% or more. Preferably it is 0.25% or more. However, if the amount of C is excessive, the hydrogen embrittlement resistance tends to decrease due to a decrease in toughness, so it is suppressed to 0.60% or less. Preferably it is 0.5% or less.

<Si: 1.0-3.0%>
Si is an important element that effectively suppresses the generation of carbides by decomposition of retained austenite, and is also a substitutional solid solution strengthening element that is effective for hardening the material. In order to effectively express such an action, it is necessary to contain 1.0% or more. Preferably it is 1.2% or more, More preferably, it is 1.5% or more. However, if the amount of Si is excessive, the hydrogen embrittlement resistance tends to be lowered due to a decrease in toughness, so the content is suppressed to 3.0% or less. Preferably it is 2.7% or less, more preferably 2.5% or less.

<Mn: 1.0 to 3.5%>
Mn is an element necessary for stabilizing austenite and obtaining desired retained austenite. In order to exhibit such an action effectively, it is desirable to contain 1.0% or more. Preferably it is 1.2% or more, More preferably, it is 1.5% or more. On the other hand, when the amount of Mn is excessive, segregation becomes prominent and the workability tends to deteriorate, so 3.5% is made the upper limit. Preferably it is 3.2% or less, More preferably, it is 3.0% or less.

<Al: 1.5% or less (excluding 0%)> (In the case of the bolt 1 of the present invention)
<Al: 0.5% or less (not including 0%)> (In the case of the bolt 2 of the present invention)
Al may be added in an amount of 0.01% or more for deoxidation. Further, Al is an element having not only a deoxidizing action but also a corrosion resistance improving action and a hydrogen embrittlement resistance improving action.

  As a mechanism of the above-mentioned corrosion resistance improving action, specifically, the effect of the corrosion resistance improvement of the base metal itself and the generated rust caused by the atmospheric corrosion can be considered, but it is estimated that the effect of the latter generated rust is particularly large. The reason is that the generated rust is denser and more protective than ordinary iron rust, so that atmospheric corrosion is suppressed, and as a result, the amount of hydrogen generated by the atmospheric corrosion is reduced, resulting in hydrogen embrittlement, that is, delay. It is thought that destruction is effectively suppressed.

  In addition, the details of the mechanism for improving the hydrogen embrittlement resistance of Al are unclear, but it is difficult for hydrogen to penetrate into steel due to the concentration of Al on the bolt surface, and the hydrogen in steel. It is presumed that the diffusion rate of hydrogen decreases, hydrogen migration becomes difficult, and hydrogen embrittlement hardly occurs. Furthermore, it is considered that the addition of Al increases the stability of the lath-like retained austenite, which contributes to the improvement of hydrogen embrittlement resistance.

  In order to effectively exhibit such an effect of improving the corrosion resistance and hydrogen embrittlement resistance of Al, the Al content is 0.02% or more, preferably 0.2% or more, more preferably 0.5% or more. It is good to do.

However, while suppressing the increase and enlarging of inclusions such as alumina to ensure workability, ensuring the production of fine retained austenite, further suppressing corrosion starting from Al-containing inclusions, and manufacturing costs In order to suppress the increase, it is necessary to suppress the Al amount to 1.5% or less. From a manufacturing point of view, it is preferable to adjust so that A ₃ point is 1000 ° C. or less.

  On the other hand, when the Al content is increased as described above, inclusions such as alumina are increased and the delayed fracture characteristics are deteriorated. Therefore, inclusions such as alumina are sufficiently suppressed, and a bolt having better delayed fracture characteristics is obtained. Therefore, the amount of Al is suppressed to 0.5% or less. Preferably it is 0.3% or less, More preferably, it is 0.1% or less.

<P: 0.15% or less>
P is an element that promotes grain boundary fracture due to grain boundary segregation, so a lower value is desirable, and its upper limit is 0.15%. Preferably it is 0.1% or less, more preferably 0.05% or less.

<S: 0.02% or less>
Since S is an element that promotes hydrogen absorption of steel in a corrosive environment, a lower value is desirable, and its upper limit is 0.02%. Preferably it is 0.01% or less.

  The contained elements specified in the present invention are as described above, and the remaining component is substantially Fe, but as an inevitable impurity brought into the steel depending on the situation of raw materials, materials, manufacturing equipment, etc., 0.01% or less As a matter of course, it is allowed to contain other elements as described below within a range that does not adversely affect the operation of the present invention. It is.

<Nb: 0.1% or less (not including 0%) and / or Mo: 1.0% or less (not including 0%)>
Nb is an element that is very effective for increasing the strength and refining of bolts, and the effect is sufficiently exerted particularly by the combined addition with Mo described below. In order to exert such an effect, it is recommended to contain Nb in an amount of 0.005% or more (more preferably 0.01% or more). However, even if Nb is contained excessively, these effects are saturated and economically useless, so the content is suppressed to 0.1% or less.

  Mo stabilizes austenite, secures retained austenite, has the effect of suppressing hydrogen intrusion and improving the resistance to hydrogen embrittlement, and the effect of improving the hardenability of the bolt. Furthermore, it has the effect of strengthening the grain boundaries and suppressing the occurrence of hydrogen embrittlement. In order to effectively exhibit such an action, it is recommended to contain Mo in an amount of 0.005% or more (more preferably 0.01% or more). However, even if the amount of Mo is excessive, the above effect is saturated and is economically wasteful, so it is suppressed to 1.0% or less.

<Cu: 2% or less (not including 0%), and / or Ni: 5% or less (not including 0%)>
By containing Cu and / or Ni, generation of hydrogen that causes hydrogen embrittlement can be sufficiently suppressed, and penetration of generated hydrogen into the bolt can be suppressed. As a result, the diffusible hydrogen concentration in the bolt can be sufficiently reduced to a harmless level due to a synergistic effect with the improvement of the hydrogen trap capability by the structure control.

  Specifically, Cu and Ni have the effect of improving the corrosion resistance of the steel material itself and sufficiently suppressing the generation of hydrogen due to bolt corrosion. These elements also have the effect of promoting the generation of iron oxide: α-FeOOH, which is said to be thermodynamically stable and protective among rust generated in the atmosphere. By promoting, penetration of the generated hydrogen into the bolt can be suppressed, and the hydrogen embrittlement resistance can be sufficiently enhanced in a severe corrosive environment. This effect is easily manifested particularly by the coexistence of Cu and Ni.

  In order to exhibit the said effect, when making it contain Cu, it is preferable to set it as 0.03% or more. More preferably, it is 0.1% or more. Moreover, when it contains Ni, it is preferable to set it as 0.03% or more, More preferably, it is 0.1% or more.

  If both elements are contained excessively, the workability deteriorates, so in the case of Cu, 2% or less (more preferably 1.5% or less), in the case of Ni, 5% or less (more preferably 3% or less). ) Is preferable.

<Cr: 2% or less (excluding 0%)>
Cr is a useful element that can easily achieve high strength by increasing hardenability with almost no loss of deformability. In order to exert such an effect sufficiently, it is preferable to contain 0.1% or more, but if it is contained excessively, cementite is likely to be generated and residual austenite is difficult to remain, so it is added in a range of 2% or less. It is preferable.

<Ti and / or V: 0.003 to 1.0% in total>
Ti, like Cu and Ni, has an effect of promoting the generation of protective rust. The protective rust has a very beneficial effect of suppressing the production of β-FeOOH which is generated particularly in a chloride environment and adversely affects the corrosion resistance (resulting in hydrogen embrittlement resistance). The formation of such a protective rust is promoted particularly by the combined addition of Ti and V (or Zr). Ti is an element that imparts very excellent corrosion resistance, and also has the advantage of cleaning steel.

  V is an element that is effective in improving the hydrogen embrittlement resistance by coexisting with Ti as described above, and is effective in increasing the strength and grain size of the bolt.

  In order to fully exhibit the effects of Ti and / or V, it is preferable to contain 0.003% or more (more preferably 0.01% or more) in total. In particular, from the viewpoint of improving the hydrogen embrittlement resistance, it is preferable to add more than 0.03% of Ti, more preferably 0.05% or more of Ti. On the other hand, even if Ti is added excessively, the effect is saturated, which is economically undesirable. If V is added excessively, the precipitation of carbonitride increases and the workability and resistance to hydrogen embrittlement deteriorate. Invite. Therefore, it is recommended to add Ti and / or V within a total range of 1.0% or less. More preferably, it is 0.8% or less in total.

<Zr: 0.003-1.0%>
Zr is an element effective for increasing the strength and refining of bolts and has the effect of coexisting with Ti and improving the resistance to hydrogen embrittlement. In order to exhibit such an effect effectively, it is preferable to contain 0.003% or more of Zr. On the other hand, if Zr is contained excessively, the precipitation of carbonitrides becomes excessive and the workability and hydrogen embrittlement resistance deteriorate, so it is preferable to add within 1.0% or less.

<B: 0.0002 to 0.01%>
B is an element effective for increasing the strength of the bolt, and is preferably contained in an amount of 0.0002% or more (more preferably 0.0005% or more). On the other hand, if B is contained excessively, the hot workability deteriorates, so it is preferable to suppress it to 0.01% or less (more preferably 0.005% or less).

The present invention does not stipulate the manufacturing conditions, and uses steel materials satisfying the above composition, and after forging, both ends are threaded by thread rolling, or a bolt head is formed at one end thereof by warm forging. A stud bolt or the like can be manufactured by forming and screwing the other end portion by thread rolling or cutting before or after warm forging. In that case, in the formation of the tissue can improve the hydrogen embrittlement resistance and strength at the same time, it is recommended to perform the finishing temperature at A ₃ points or more in the rolling. This is because if the finishing temperature is lower than the temperature, the diffusion of C becomes insufficient, and a predetermined bainitic ferrite structure or residual austenite structure cannot be obtained.

On the other hand, when the finishing temperature is too high, leading to grain growth of the austenite, since it is no longer possible to create the minute residual austenite not more than (A ₃ point + 100 ° C.) preferred. More preferably, it is (A ₃ points + 50 ° C.) or less.

  Then, it is cooled. In the present invention, it is recommended to cool to a temperature not lower than (Ms point −50 ° C.) and not higher than Bs point at an average cooling rate of 3 ° C./s or higher and hold by heating for 60 to 3600 seconds in this temperature range. Is done.

  As described above, the average cooling rate of 3 ° C./s or more is to secure a desired bainitic ferrite structure and to avoid formation of a pearlite structure that is not preferable for the present invention. The larger the average cooling rate, the better. It is recommended that the average cooling rate be 10 ° C./s or more (more preferably 20 ° C./s or more).

  Next, a predetermined structure | tissue can be introduce | transduced by carrying out an isothermal transformation after rapidly cooling to the temperature below (Ms point-50 degreeC) and below Bs point. When the heating and holding temperature exceeds the Bs point, a large amount of pearlite that is undesirable for the present invention is generated, and a predetermined bainitic ferrite structure cannot be secured. On the other hand, when the heating and holding temperature is lower than (Ms point−50 ° C.), the area ratio of retained austenite decreases.

  On the other hand, if the heating and holding time exceeds 3600 seconds, the retained austenite decomposes and cementite is generated, and the desired characteristics cannot be exhibited. On the other hand, when the heating and holding time is less than 60 seconds, the retained austenite is not formed because C is not sufficiently diffused, and also in this case, desired characteristics cannot be obtained. The heating and holding time is preferably 100 seconds or more and 3000 seconds or less, more preferably 180 seconds or more and 2400 seconds or less.

  Examples of the high-strength bolts of the present invention include high tension bolts, torcia-type bolts, hot-dip galvanized high-strength bolts, rust-proof high-strength bolts, refractory steel high-strength bolts, and the like in the automotive field, construction field, industrial machine field. It is optimal as a bolt with high strength and excellent hydrogen embrittlement resistance used in

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.

Test steel No. 1 composed of the components shown in Table 1 Using A to Q, No. 1-14 and 16-21 are (A ₃ points | pieces +30 degreeC), No.1. No. 15 was heated at 780 ° C. for 60 to 1800 seconds, then rapidly cooled to To ° C. in Table 2, held at the temperature (To ° C.) for t seconds as shown in Table 2, and then allowed to cool. In this example, a sample was prepared with the pattern. Alternatively, after rolling at (A ₃ points + 30 ° C.), the sample was cooled to room temperature, and then again (A ₃ points + 30 ° C.). Until it is heated and held for 60 to 1800 seconds, rapidly cooled to To ° C. in Table 2, held at the temperature (To ° C.) for t seconds, and then allowed to cool.

  The metal structure, tensile strength (TS), elongation [total elongation (El)], and hydrogen embrittlement resistance of the sample thus obtained were examined in the following manner.

[Observation of metal structure]
Bainitic ferrite was obtained by observing and photographing a measurement area (approximately 50 μm × 50 μm, measurement interval 0.1 μm) at an arbitrary position in the cross section of the obtained sample (diameter 10 mm) at a radius of 1/2. The area ratio of (BF) and martensite (M) and the area ratio of retained austenite (residual γ) were measured according to the methods described above. And it measured similarly in the 2 visual fields selected arbitrarily, and calculated | required the average value. The other tissues were obtained by subtracting the area ratio occupied by these tissues. Furthermore, the average axial ratio of residual austenite crystal grains was measured according to the method described above.

[Measurement of tensile strength]
A tensile test piece having a diameter of 8 mm was manufactured from each sample by machining, and a tensile test was performed using the test piece to measure the tensile strength (TS).

[Evaluation of hydrogen embrittlement resistance]
Next, the hydrogen embrittlement resistance was evaluated for those having a tensile strength of 1180 MPa or more. Specifically, first, a delayed fracture test piece with a circular notch notch (a diameter of a parallel part of 8 mm and a diameter of a notch of 6 mm) was produced from each sample by machining. And after hydrogen-charging to the said test piece on the conditions of a current density of 1.0 mA / cm < ² > in the dilute sulfuric acid (liquid temperature of 30 degreeC) of pH3.0, 30-80% of said TS is carried out in the unit of 10%. Each was loaded and the time to break was measured. From the relationship between the load (load stress) and the rupture time, the load stress when the rupture time was 200 hours was determined. The value obtained by dividing the “load stress at 200 hours of breakage of a sample charged with hydrogen” by “load stress at 200 hours of breakage of a sample without hydrogen charge” is defined as “crack fracture strength ratio”. It was used as an index of hydrogen embrittlement characteristics. The delayed fracture strength ratio is about 0.5 at maximum in SCM435, which is widely used as a bolt with a tensile strength of 1000 MPa. Therefore, those with a delayed fracture strength ratio of 0.5 or more have excellent hydrogen embrittlement resistance. It was evaluated.

Furthermore, a hydrogen charge 4-point bending test was also performed on some steel types. Specifically, a strip test piece of 65 mm × 8 mm cut out from each of the above steel materials is immersed in a (0.5 mol / H ₂ SO ₄ +0.01 mol / KSCN) solution to perform cathodic hydrogen charging, and does not break for 1 hour. The stress was measured as critical breaking stress (DFL). Then, in Experiment No. 2 of Table 2, The ratio of 1 (steel type symbol A) to DFL (DFL ratio) was determined.

  These results are also shown in Table 2.

  The following can be considered from Tables 1 and 2 (the following No. indicates the experiment No. in Table 2).

  No. satisfying the requirements defined in the present invention. 1-8 and 16-20 show high strength of 1180 MPa or more and are excellent in hydrogen embrittlement resistance even under harsh environments. In particular, no. It turns out that 16-20 has shown the more excellent hydrogen embrittlement resistance.

  On the other hand, No. which does not satisfy the provisions of the present invention. 9 to 15 and 21 have the following problems, respectively.

  That is, no. No. 9 has an insufficient amount of C, so the strength specified in the present invention cannot be secured.

  No. No. 10 was inferior in hydrogen embrittlement resistance because the amount of Si was insufficient and the defined residual γ could not be secured.

  No. No. 11 is an example using steel type K having an excessive amount of C, but is inferior in hydrogen embrittlement resistance due to precipitation of carbides.

  No. Although 12-15 used the steel materials which satisfy | fill the component composition prescribed | regulated by this invention, since it was not manufactured on the conditions recommended, each malfunction occurred.

  That is, no. No. 12 had a remarkably high austempering temperature, so that bainitic ferrite, martensite and residual γ could not be secured, and high strength could not be secured.

  No. No. 13 is too long because the austempering time is too long. No. 14 has an austempering time that is too short. In No. 15, since heating was performed in a two-phase region (780 ° C.), the residual γ became a polygonal state, resulting in poor hydrogen embrittlement resistance.

  No. No. 21 exceeds the amount of Al defined as the bolt 1 of the present invention, so that a predetermined amount of retained austenite can be secured, but the retained austenite does not satisfy the average axial ratio defined by the present invention, and the desired mother Since it does not become a phase and further includes inclusions such as AlN, it is inferior in hydrogen embrittlement resistance.

Claims (8)

% By mass
C: 0.20-0.60%,
Si: 1.0-3.0%,
Mn: 1.0 to 3.5%
Al: 1.5% or less (excluding 0%),
P: 0.15% or less,
S: satisfying 0.02% or less, the balance is made of iron and inevitable impurities,
The metal structure is the area ratio relative to the whole structure.
Bainitic ferrite and martensite in total 80% or more (mainly bainitic ferrite)
1% or more of retained austenite,
While ferrite and pearlite are 10% or less in total (including 0%),
The average axial ratio (major axis / minor axis) of the residual austenite crystal grains is 5 or more,
A high-strength bolt excellent in hydrogen embrittlement resistance characterized by a tensile strength of 1180 MPa or more. % By mass
C: 0.20-0.60%,
Si: 1.0-3.0%,
Mn: 1.0 to 3.5%
Al: 1.5% or less (excluding 0%),
P: 0.15% or less,
S: satisfying 0.02% or less, the balance is made of iron and inevitable impurities,
The metal structure includes at least bainitic ferrite, martensite, and retained austenite,
The area ratio for all tissues
Bainitic ferrite and martensite total 80% or more,
1% or more of retained austenite,
While ferrite and pearlite are 10% or less in total (including 0%),
The average axial ratio of the upper Symbol residual austenite crystal grains (major axis / minor axis) is 5 or more,
High-strength bolt having excellent hydrogen embrittlement resistance, characterized in that the further tensile strength is not less than 1180 MPa. % By mass
C: 0.20-0.60%,
Si: 1.0-3.0%,
Mn: 1.0 to 3.5%
Al: 0.5% or less (excluding 0%),
P: 0.15% or less,
S: Satisfies 0.02% or less, the balance is made of iron and inevitable impurities,
The metal structure is the area ratio relative to the whole structure.
Bainitic ferrite and martensite in total 80% or more (mainly bainitic ferrite)
1% or more of retained austenite,
While ferrite and pearlite are 10% or less in total (including 0%),
The average axial ratio (major axis / minor axis) of the residual austenite crystal grains is 5 or more,
A high-strength bolt excellent in hydrogen embrittlement resistance characterized by a tensile strength of 1180 MPa or more. % By mass
C: 0.20-0.60%,
Si: 1.0-3.0%,
Mn: 1.0 to 3.5%
Al: 0.5% or less (excluding 0%),
P: 0.15% or less,
S: Satisfies 0.02% or less, the balance is made of iron and inevitable impurities,
The metal structure includes at least bainitic ferrite, martensite, and retained austenite,
The area ratio for all tissues
Bainitic ferrite and martensite total 80% or more,
1% or more of retained austenite,
While ferrite and pearlite are 10% or less in total (including 0%),
The average axial ratio of the upper Symbol residual austenite crystal grains (major axis / minor axis) is 5 or more,
High-strength bolt having excellent hydrogen embrittlement resistance, characterized in that the further tensile strength is not less than 1180 MPa. Furthermore, in mass%,
Nb: 0.1% or less (not including 0%) and / or Mo: 1.0% or less (not including 0%)
The high-strength bolt in any one of Claims 1-4 containing. Furthermore, in mass%,
Cu: 2% or less (not including 0%) and / or Ni: 5% or less (not including 0%)
A high-strength bolt according to any one of claims 1 to 5.   Furthermore, the high intensity | strength volt | bolt in any one of Claims 1-6 containing Cr: 2% or less (0% is not included) by the mass%.   Furthermore, the high intensity | strength volt | bolt in any one of Claims 1-7 containing Ti: 0.003-1.0% by mass%.