JP2006070327A - High-strength low alloy steel with hydrogen intrusion suppressing effect - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide steel which has sufficient delayed fracture resistance in spite of its having a high strength of ≥1,350 MPa and is applicable to high strength parts/high strength bolts usable under severe environments, such as the seashore or the coastal region in its vicinity. <P>SOLUTION: The high-strength low alloy steel has a chemical composition consisting of, by mass, 0.35 to 0.55% C, ≤0.3% Si, ≤0.6% Mn, ≤0.025% P, ≤0.050% S, ≤0.10% Al, 0.5 to 1.5% Cr, 0.7 to 1.5% of (Mo+0.5W), ≤0.005 to 0.05% Nb, 0.005 to 0.20% of (Ti+0.5Zr), 0.15 to 0.3% V and the balance Fe with impurities and further containing, if necessary, either or both of 0.2 to 3% Ni and 0.05 to 1% Cu and/or either or both of ≤0.01% Ca and ≤0.01% Mg. Further, the composition of nonmetallic inclusions of ≥500 nm major axis contained in the steel is constituted so that it satisfies inequality (Ti+Zr)/(Ti+Zr+Mn+Ca+Mg)×100≥30(mass%). <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention can be applied to high-strength low-alloy steels, especially high-strength bolts used in automobiles, industrial machines, building structures, and other high-strength parts with excellent delayed fracture resistance with a tensile strength of 1350 MPa or more. Related to high strength alloy steel.

  With the reduction in weight of automobiles and industrial machinery and the increase in size of building structures, there is an increasing demand for high-strength bolts that can withstand high tightening forces. Conventionally, high strength low alloy steels generally used include, for example, SCM 440 having a tensile strength of 1000 MPa class defined in JIS G 4105 (1989). However, a material with a higher strength level is required today, but it is known that if the tensile strength exceeds 1200 MPa, it is easy to break the bolt, which is the biggest obstacle to increasing the strength of the bolt. It has become. This failure is called delayed fracture, and is a phenomenon in which a steel placed under static load breaks brittlely after a certain period of time, and is considered a kind of hydrogen embrittlement due to hydrogen that has penetrated into the steel due to corrosion. Yes.

  Various studies have been made on improvement of delayed fracture resistance of high strength steel having a tensile strength of 1200 MPa or more. For example, Patent Literature 1, Patent Literature 2, Patent Literature 3, Patent Literature 4, Patent Literature 5, Patent Literature 6, Patent Literature 7, and Patent Literature 8 contain Cr, Mo, and V to contain hardenability and temper softening. A steel for high-strength bolts with improved resistance is disclosed.

Patent Document 9, Patent Document 10, and Patent Document 11 disclose steels for high-strength bolts in which grain boundaries are cleaned by adding a small amount of B, and the bond strength of the grain boundaries is increased to improve delayed fracture resistance. .
In Patent Document 12, Patent Document 13, Patent Document 14, and Patent Document 15, fine Ti-based precipitates are generated by adding Ti in addition to a small amount of B, and this is used as a hydrogen trap site for delay resistance. Techniques for improving destructibility are disclosed.

All of these technologies prevent delayed fracture after hydrogen has penetrated into the steel, such as improved material structure (high-temperature tempering, grain boundary cleaning, etc.) or hydrogen trapping in finely dispersed precipitates. It is based on the idea that. The steels for high-strength bolts disclosed in these publications have improved delayed fracture resistance compared to steels that are simply strengthened from the conventional steel SCM 440 and the like. However, when high strength steel, especially steel with a tensile strength of 1350 MPa or more, is used in the severe corrosive environment found in coastal areas or coastal areas that receive salt from the sea, the amount of hydrogen intrusion due to corrosion is large. No steel has ever been able to withstand this.
Japanese Patent No. 2670937 JP 7-126799 A JP-A-8-278735 JP-A-8-120408 JP-A-8-225845 JP 2000-328191 A JP 2001-32044 A JP 2003-27186 A JP-A-5-171356 Japanese Patent Application Laid-Open No. 8-29579 Japanese Patent Laid-Open No. 9-111399 JP-A-10-17985, JP 10-36940 A JP 11-293401 A JP 2003-268495 A

  The problem to be solved by the present invention is that it has sufficient delayed fracture resistance even at a high tensile strength of 1350 MPa or more, and can be used even in a severe corrosive environment such as a coast or a coastal area close thereto. The object is to provide steel that can be applied to high-strength parts or high-strength bolts.

  The inventors consider that there is a limit to the improvement in delayed fracture resistance based on the structural modification that has been studied in the past, and in the present invention, there is a material design that suppresses hydrogen intrusion due to corrosion and prevents the occurrence of delayed fracture. We focused on whether it was possible.

   As a result of various studies, the control of sulfides is the most effective for suppressing hydrogen intrusion, and further, the use of Ni, Cu, and the use of Mo, W, V are more effective if necessary. Knowing and completing the present invention.

Here, the present invention is as follows.
(1) By mass%
C: 0.35-0.55%, Si: 0.3% or less, Mn: 0.6% or less, P: 0.025% or less, S: 0.050% or less, Al: 0.10% or less, Cr: 0.5-1.5%, Mo and W Or in total of 2 types (Mo + 0.5W): 0.7-1.5%, Nb: 0.005-0.05%, 1 or 2 types of Ti and Zr in total (Ti + 0.5Zr): 0.005-0.20%, V: 0.15-0.3 %
A high-strength low-alloy steel with a tensile strength of 1350 MPa or more, wherein the balance consists of Fe and impurities, and the composition of nonmetallic inclusions with a major axis of 500 nm or more contained in the steel satisfies the following formula: .

(Ti + Zr) / (Ti + Zr + Mn) × 100 ≧ 30 (mass%)
(2) The high-strength low-alloy steel as described in (1) above, containing 1% or 2% by mass of Ni: 0.2 to 3% and Cu: 0.05 to 1% instead of part of Fe .

  (3) By mass%, in addition to one or two of Ca: 0.01% or less and Mg: 0.01% or less, instead of a part of Fe, non-metallic inclusions having a major axis of 500 nm or more contained in steel The high-strength low-alloy steel according to any one of (1) and (2), wherein the composition satisfies the following formula.

(Ti + Zr) / (Ti + Zr + Mn + Ca + Mg) × 100 ≧ 30 (mass%)
In addition, the formula described in each item is described as (Ti + Zr) / (Ti + Zr + Mn + Ca + Mg) × 100 ≧ 30 as a general formula as described in the above item (4), and each element is not contained in the nonmetallic inclusion. Think of it as zero.

According to the present invention, it is possible to obtain a steel material with good delayed fracture resistance even at a high strength of a tensile strength of 1350 MPa or more, and could not be conventionally used as a high-strength bolt, for example.
It can also be used as a highly reliable part for applications such as automobiles, buildings and bridges.

The reason why the steel composition and the form of inclusions are defined as described above in the present invention will be described. The “%” notation indicating the steel composition and the inclusion composition is “mass%”.
First, the experimental results that led to the present invention will be described in detail.

150 kg of steel having the chemical compositions shown in Table 1 to AG was vacuum-melted and formed into a plate having a thickness of 15 mm by hot forging. Thereafter, the strength was tempered to 1500 MPa class by quenching and tempering treatment.
A specimen of 70 mm in diameter and 0.5 mm in thickness is collected from the plate material obtained in this way, and a hydrogen permeation test is conducted by two methods, an acid immersion method and a temperature / humidity control method. The characteristics were investigated.

  FIG. 1 shows a schematic diagram of a measuring apparatus using an acid immersion method (hereinafter referred to as “a method”). The disk test piece 10 was polished on both sides with 600-mm emery paper and then Ni-plated on one side 12. A cell (cathode cell) 14 into which hydrogen on the polishing surface side penetrates was filled with an aqueous solution adjusted to pH 3.5 by adding an appropriate amount of hydrochloric acid to a 1.64% sodium acetate aqueous solution at 25 ° C. The reason for using this test bath was based on the knowledge that the pH of pitting bottoms and gaps caused by atmospheric corrosion drops to about 3.5 due to hydrolysis. (Takhiro Kushida et al .: Iron and Steel, 82 (1996), 297.).

The cell (anode cell) 16 on the Ni plating surface side was filled with 1N NaOH aqueous solution, and the test piece was held at a constant potential of zero V (volt) with respect to the silver-silver chloride electrode 18 as a reference electrode. When hydrogen atoms generated on the cathode cell 14 side pass through the test piece 10 and are released to the anode cell 16 side, they are oxidized to hydrogen ions and the current value is measured as a hydrogen permeation current value (μA / cm ² ). did. The hydrogen permeation coefficient (μA / cm) indicating the hydrogen penetration characteristics is measured by multiplying the hydrogen permeation current value by the thickness of the test piece (here, 0.05 cm).

FIG. 2 shows a schematic diagram of a measuring apparatus using a temperature / humidity control method (hereinafter referred to as b method). The test piece 10 is prepared in the same manner as in the immersion method, and the hydrogen intrusion surface is exposed to the external environment. The anode cell 16 on the Ni plating surface 12 side was filled with a 1N NaOH aqueous solution, and the test piece 10 was held at a constant potential of zero V (volt) with respect to the silver chloride electrode 18. Artificial seawater with an amount of 5 mg / cm ² is attached to the exposed surface of the external environment and dried completely, and then placed in a constant temperature and humidity chamber with various relative humidity at 40 ° C to maximize the hydrogen permeability coefficient. The value was measured. This method is closer to exposure in the actual environment than the acid immersion method.

As a result of the experiment, it was found that the following factors (1) to (3) are important for suppressing hydrogen invasion.
(1) Sulfide control Although the influence of inclusions on hydrogen penetration has not been clarified in the past, the present inventors have found that the form of sulfide greatly affects hydrogen penetration.

  Examples of measurement results by the acid immersion method are shown in FIG. Steel A, which is a comparative steel, has a maximum hydrogen permeability coefficient (hereinafter referred to as a maximum hydrogen permeability coefficient) of 0.15 μA / cm. It is known that delayed fracture of high-strength steel is caused by hydrogen penetration with a hydrogen permeation coefficient of about 0.1 μA / cm. Steel A is not sufficiently effective in suppressing hydrogen penetration and has the potential for delayed fracture. This experimental result suggests. Steel B, which is the same comparative steel with a higher S (sulfur) content than steel A, resulted in a larger hydrogen permeation coefficient.

  The reason for this is considered to be due to the fact that a large amount of Mn-based sulfide is produced in steel B with a high S content, as schematically illustrated in FIG. Mn-based sulfides are easily dissolved in neutral or acidic aqueous solutions to generate hydrogen sulfide. Since hydrogen sulfide has an effect of promoting hydrogen penetration, it is estimated that the steel B having a high S content has an extremely high hydrogen permeability coefficient. Ca, Mg, or rare earth elements (Pr, La, Nd, etc.) that may be added to improve hot workability also form sulfides, but they are neutral or acidic as with Mn sulfides. Since it dissolves easily in an aqueous solution, it is presumed to have the same effect of promoting hydrogen penetration.

  Further, in Steel C and Steel D with increased Ti content, the hydrogen permeation coefficient was reduced as compared with Steel A and Steel B. The reason for this is considered to be due to the formation of Ti-based sulfides as schematically shown in FIG. Ti has a higher ability to form sulfides than Mn, and Ti-based sulfides, unlike Mn-based sulfides, are insoluble inclusions that do not elute in neutral and acidic environments. Therefore, it was considered that Ti suppressed S (sulfur) as an insoluble sulfide, thereby suppressing the formation of Mn sulfide and suppressing hydrogen intrusion. Similar to Ti, Zr is an element that forms an insoluble sulfide.

  Further, steel E has substantially the same chemical composition as steel C, but as shown in FIG. In order to investigate this difference, the inclusion composition of Steel C and Steel E was measured. The composition of inclusions is 10 mm observation of 0.1 mm x 0.1 mm x field of view of the cross-section of the plate with SEM (scanning electron microscope), and the composition of nonmetallic inclusions with a major axis of 500 nm or more is EDX (energy dispersive X-ray diffraction) All values measured by were averaged. First, FIG. 4 shows how to determine the major axis of inclusions. Among the straight lines connecting two different points of the interface between the inclusion and the base material, the maximum diameter is defined as the long diameter here. Inclusions were apparently single composition as shown in FIG. 5 (a) and two types of inclusions were adjacent as shown in FIG. 5 (b). Was also considered as a single inclusion, and the major axis and the composition were measured. In the case of FIG. 5B, the average value of the composition was calculated by area analysis in such a manner that all of the Ti concentrated region and the Mn concentrated region were included. As elements affecting the solubility and insolubility of sulfides as non-metallic inclusions, the mass% of Ti, Zr, Mn, Ca, Mg and S (sulfur) constituting the inclusions was measured, and ( Ti + Zr) / (Ti + Zr + Mn + Ca + Mg) (%) were compared.

  The results are also shown in Table 2. Comparing Steel C and Steel E, Steel E has a higher (Ti + Zr) / (Ti + Zr + Mn + Ca + Mg) value, in other words, a higher concentration of (Ti + Zr) in the sulfide. Therefore, it can be said that it is necessary to increase the rate of formation of Ti sulfide and Zr sulfide in order to more effectively exhibit the hydrogen invasion suppressing action.

(2) Cu, Ni
Steel F in Table 1 is steel containing Cu and Ni. As shown in FIG. 3, this steel has an improved hydrogen penetration inhibiting effect compared to steels A to E. Cu and Ni are elements that are difficult to oxidize. When they are contained in steel, when they are corroded due to atmospheric corrosion, they remain and accumulate on the surface of the steel. Since Cu and Ni itself have a small hydrogen permeability, the effect of suppressing hydrogen intrusion by deposits appears. Furthermore, these elements are combined with hydrogen sulfide generated with the elution of Mn sulfide, and insoluble Cu-based sulfides and insoluble Ni-based sulfides are produced, thereby inhibiting hydrogen intrusion by hydrogen sulfide. .

(3) Mo, W
Steel G in Table 1 is steel containing W. Mo and W are elements having a strong ability to generate carbides, and exist in the form of carbides when contained in steel. Hydrogen atoms generated by corrosion once adsorb on the steel surface and then penetrate into the steel, causing delayed fracture, but Mo and W carbides quickly bond hydrogen atoms adsorbed on the steel surface to hydrogen molecules. And has the effect of inhibiting hydrogen intrusion into the steel material. Similarly, when these elements are contained in combination with V, which has a strong ability to generate carbides, fine composite carbides of Mo-V, WV, and Mo-W-V are uniformly finely dispersed in the steel. Therefore, it is estimated that the effect of suppressing hydrogen penetration is further increased.

  FIG. 6 and Table 2 show the results of investigating the hydrogen penetration characteristics of the steel types shown in Table 1 by the temperature and humidity control method (method b). All the steel types showed a result that hydrogen intrusion was promoted at a relative humidity of 60 to 70%, but the magnitude of the hydrogen permeation coefficient was the same permutation as in the acid immersion method (a method).

  Next, the reason why the steel composition is defined as described above in the present invention will be described as follows. In the present specification, the ratio indicating the steel composition and the inclusion composition is expressed in mass% unless otherwise specified.

C: 0.35-0.55%
C is an element effective for enhancing the hardenability and improving the strength. If the content is less than 0.35%, it is necessary to contain 0.35% or more in order to obtain sufficient hardenability. On the other hand, the above effect is saturated even if the content exceeds 0.55%, so the upper limit was made 0.55%. A desirable range is 0.38 to 0.52%.

Si: 0.3% or less
Si is effective in improving deoxidation, hardenability and strength. In order to obtain these effects, 0.05% or more is desirable. On the other hand, even if the content exceeds 0.3%, these effects are saturated, and the formability to bolts and parts particularly when cold forging is significantly reduced. Therefore, the upper limit was made 0.3%.

Mn: 0.6% or less, 0.02Mn ≦ (Ti + 0.5Zr)
Mn segregates at the grain boundary and promotes delayed fracture of the grain boundary cracking type. It also forms Mn-based sulfides and promotes hydrogen penetration. Since these influences become remarkable when it exceeds 0.6%, the upper limit was made 0.6%. Further, in the preferred embodiment of the present invention, it is preferable to satisfy 0.02Mn ≦ (Ti + 0.5Zr) in order to prevent the formation of Mn sulfide and form a Ti—Zr-based insoluble sulfide.

P: 0.025% or less P segregates at the grain boundaries and lowers toughness and delayed fracture resistance. If the content exceeds 0.025%, the effect becomes significant, so the upper limit was made 0.025%. The content of P is preferably as low as possible.

S: 0.050% or less, 2S ≦ (Ti + 0.5Zr)
S is usually present in steel as Mn sulfide and dissolves in neutral and acidic aqueous solutions to promote hydrogen penetration and reduce delayed fracture resistance. However, S is effective in improving the machinability and may be intentionally included. However, if the S content exceeds 0.050%, S cannot be stably fixed as a Ti-Zr sulfide, so the upper limit was made 0.050%. In a preferred embodiment of the present invention, it is preferable to satisfy 2S ≦ (Ti + 0.5Zr) in order to prevent formation of Mn-based sulfides and to generate Ti-Zr-based insoluble sulfides.

Al: 0.10% or less
Al is an element effective for deoxidation of steel. In order to sufficiently secure this effect, the content is preferably 0.005% or more. On the other hand, even if the content exceeds 0.10%, the effect is saturated, so the upper limit was made 0.10%. The Al content of the present invention refers to acid-soluble Al (so-called “sol.Al”).

Cr: 0.5-1.5%
Cr is an element effective for enhancing the hardenability of steel, and in order to obtain this effect, 0.5% or more is contained. However, even if the content exceeds 1.5%, the effect is saturated, so the upper limit was made 1.5%. A desirable range is 1.0 to 1.5%.

Mo + 0.5W: 0.7-1.5%
At least one of Mo and W is blended.
Mo and W increase the hardenability of the steel and form fine carbides during tempering, and the fine carbides exhibit an effect of suppressing hydrogen intrusion. In order to obtain this effect, (Mo + 0.5 W) is set to 0.7% or more. Moreover, even if (Mo + 0.5W) exceeds 1.5%, the effect is saturated, so the upper limit was made 1.5%. A desirable range is 0.7 to 1.0%.

Nb: 0.005-0.05%
Nb has the effect of improving the resistance to delayed fracture by forming fine carbides and refining the structure. In order to acquire this effect, it contains 0.005% or more. On the other hand, even if the content exceeds 0.05%, the above effect is saturated, so the upper limit was made 0.05%. A desirable range is 0.01 to 0.04%.

V: 0.15-0.3%
V precipitates together with Mo as fine Mo-V carbide during tempering, and is finely dispersed in the carbide, which is effective in suppressing hydrogen intrusion. In order to acquire this effect, it contains 0.15% or more. On the other hand, even if the content exceeds 0.3%, the effect is saturated, so the upper limit was made 0.3%. A preferred range is 0.2-0.25%.

Ti + 0.5Zr: 0.005-0.20%
Ti and Zr are important elements in the present invention, and at least one of them is contained in total (Ti + 0.5Zr): 0.005 to 0.20%. These elements combine with sulfur in the steel to form insoluble Ti-based sulfides, Zr-based sulfides, and Ti-Zr-based sulfides, and exert an effect of suppressing hydrogen penetration. In order to obtain this effect, (Ti + 0.5Zr) is made 0.005% or more. On the other hand, if it is excessively contained, problems such as a decrease in toughness occur. Particularly, when (Ti + 0.5Zr) exceeds 0.20%, this effect becomes significant, so the upper limit was made 0.20%. Desirably, it is 0.10% or less. A more desirable content is (Ti + 0.5Zr) 0.01 to 0.05%.

Ni: 0.2-3%, Cu: 0.05-1%
Ni and Cu do not need to be included, but if they are included, they will accumulate in the corrosion products, or combine with S in the steel to generate insoluble Ni-based sulfides and Cu-based sulfides to prevent hydrogen penetration. To prevent. In order to obtain this effect, it is necessary to contain 0.2% or more of Ni and 0.05% or more of Cu. On the other hand, even if Ni is contained in 3% and Cu is contained in excess of 1%, the effect is saturated, so the upper limit was made 3% for Ni and 1% for Cu. Desirable ranges are Ni: 0.2-1% and Cu: 0.05-0.5%.

B: Less than 0.0005% B may rarely be mixed into the steel material as an impurity, but even in such a case, according to the knowledge of the present inventors, coarse carbon borides are precipitated at the grain boundaries. Reduces delayed fracture resistance. Therefore, in the present invention, it is desirable to reduce the content of B mixed as an impurity to less than 0.0005%, desirably less than 0.0003%.

Ca: 0.01% or less, Mg: 0.01% or less
Ca and Mg may not be contained, but if at least one of them is contained, it combines with S in the steel to form a sulfide, thereby improving the hot workability of the steel. In order to obtain this effect, it is preferable to contain at least one 0.0003% or more of each. On the other hand, even if the content exceeds 0.01%, the effect is saturated, and coarse Ca inclusions and Mg inclusions are generated, which promotes hydrogen penetration by dissolving in acidic and neutral aqueous solutions. The upper limit was made 0.01%. A desirable range is 0.0003 to 0.003% respectively.

  Here, according to the present invention, the composition of the non-metallic inclusions having a major axis of 500 mm or more contained in the steel is defined so that the value of the above formula is 30% or more, which is Ti-based and Zr-based It is intended to suppress hydrogen intrusion by containing a large amount of inclusions. Preferably, (Ti + Zr) / (Ti + Zr + Mn + Ca + Mg) × 100 ≧ 50% is preferable.

  In order to produce inclusions with a major axis of 500 mm or more and such composition, adjust the cooling rate during solidification, increase the area reduction ratio during processing, and further adjust the steel composition in advance. Can be realized. As a preferred composition example, the steel composition may be adjusted so as to satisfy 0.02Mn ≦ (Ti + 0.5Zr) and further 2S ≦ (Ti + 0.5Zr) as described above.

   Next, the effects of the present invention will be described more specifically with reference to examples.

  Steels having the chemical composition shown in Table 3 were melted, billets of various dimensions were cast, and hot working and annealing were performed to obtain a wire rod having an outer diameter of 30 mm. The billet cooling rate and degree of processing were varied in this case. Bolts having a size of M22 were made from the wire by cold rolling. Thereafter, the tensile strength was tempered to 1500 MPa class by quenching and tempering treatment.

  Using the bolts thus produced, the hydrogen penetration characteristics and delayed fracture resistance were investigated by the following tests. First, a disk specimen having a diameter of 20 mm and a thickness of 0.5 mm was taken from the bolt, and the hydrogen penetration characteristics were evaluated by the acid immersion method (a method) and the temperature and humidity control method (b method) described above. Moreover, the thing which fastened the bolt to the board | plate material with 85% yield stress was used for the test piece, it was immersed in the same bath as the a method for 200 hours, and delayed fracture resistance was evaluated by the presence or absence of a fracture | rupture. Table 4 shows the chemical composition, production conditions, inclusion composition, maximum hydrogen permeability coefficient, and delayed fracture test results of steel. In the table, the result of the delayed fracture test indicates that “◯” indicates that no fracture was observed, and “×” indicates that the fracture was observed.

  Steels H-17 had a maximum hydrogen permeation coefficient of 0.1 μA / cm or less for both the method a and the method b, and it was confirmed that the steel H-17 had a sufficient hydrogen penetration inhibiting effect. Moreover, it was confirmed that it did not break even in the delayed fracture test and had good delayed fracture resistance.

  When the inclusion composition of the steels H to 17 is investigated, all of the values shown in the following formula (1) are 30% or more, insoluble Ti-Zr sulfide is formed, and the effect of suppressing hydrogen penetration is exhibited. It was confirmed. In particular, steels U to Z have a high value of 50% or more in the following formula (1), insoluble Ti-Zr sulfide is stably formed, and the maximum hydrogen permeation coefficient is 0.04 to 0.06 μA for both the a method and the b method. It was extremely low. Here, in addition to the alloy elements and S shown below, the inclusions may contain C, N, O (oxygen), Cr, Mo, W, V, Nb, and the like.

(Ti + Zr) / (Ti + Zr + Mn + Ca + Mg) × 100 (1)
Steels 1 to 10 are steels containing Cu and / or Ni, Steels 11 to 13 are steels containing W, Steels 14 to 17 are steels containing Cu, Ni and W, and steels H to T In comparison, the effect of suppressing hydrogen intrusion was further improved.

On the other hand, Steels 18 to 34 broke in the delayed fracture test, and the delayed fracture resistance was unsatisfactory.
Steels 18 to 25 have almost the same chemical composition as steels H to Z, but the (Ti + Zr) / (Ti + Zr + Mn + Ca + Mg) value in inclusions is less than 30%, which is lower than steels H to Z, and Ti-Zr sulfide is formed. As a result, a soluble Mn-based sulfide was formed, and the effect of suppressing hydrogen penetration was not exhibited. Table 3 also shows the billet production conditions. Here, the cooling rate indicates a cooling rate (° C./min) between 1000 ° C. immediately after the billet is solidified. Further, the area reduction ratio refers to a value represented by the following formula, where A is the cross-sectional area of the billet immediately after solidification and B is the cross-sectional area of the wire after the final wire drawing, which is the material of the bolt, and affects the material. It described as a standard aiming at the total processing degree.

(A / B) × 100 (2)
Comparing the production methods of Steel H-17 and Steel 18-25, Steel 18-25 has a lower cooling rate or a smaller area reduction rate. That is, it is suggested here that not only the chemical composition of steel but also the production conditions need to be optimized in order to stably produce insoluble Ti—Zr-based sulfides. When the cooling rate is low, it is estimated that coarse Mn sulfide grew during the cooling process and promoted hydrogen intrusion. It is also presumed that when the area reduction rate is small, coarse Mn sulfide remains without being crushed and promotes hydrogen intrusion.

Steel 26 had a low C content, so that sufficient hardenability was not obtained and delayed fracture resistance was unsatisfactory.
In Steels 27 to 28, the Mn content was higher than that of Ti and Zr, so a large amount of Mn sulfide was generated and the amount of hydrogen intrusion was extremely large. That is, it is preferable to contain a sufficient amount of Ti and Zr compared to Mn in order to stably generate Ti-Zr sulfide and suppress hydrogen intrusion, and the critical value at this time is expressed by the following formula ( 3).

Ti + 0.5Zr ≧ 0.02Mn (3)
Steels 29 to 30 had a larger S content than Ti + 0.5Zr, and a large amount of Mn sulfide was produced and the amount of hydrogen intrusion was extremely large as in Steels 27 to 28. That is, it is preferable to contain Ti and Zr in a sufficient amount as compared with the S amount in order to stably generate Ti-Zr sulfide and suppress hydrogen intrusion. 4).

Ti + 0.5Zr ≧ 2S (4)
Steel 31 had a low Cr content, insufficient hardenability, and poor delayed fracture resistance. Steel 32 had a low Mo content, an insufficient hydrogen penetration inhibiting effect, and poor delayed fracture resistance. Steel 33 had a low V content, an insufficient precipitation amount of fine carbides that exhibited a hydrogen penetration inhibiting effect, and the delayed fracture resistance was unsatisfactory. Steel 34 is a steel containing B, and a coarse carbon boride containing B was precipitated at the grain boundaries, and the delayed fracture resistance was unsatisfactory.

  7 and 8 are graphs showing the relationship between the value of the formula (1), the cooling rate, and the area reduction rate in this example, respectively, and the inclusion composition can be changed by controlling the respective factors. I understand.

It is typical explanatory drawing which shows the outline | summary of a hydrogen permeation test (acid immersion method). It is typical explanatory drawing which shows the outline | summary of a hydrogen permeation test (temperature humidity control method). It is a graph which shows the hydrogen permeability coefficient measured by the acid immersion method. It is the typical explanatory view showing the difference in the action of Mn system sulfide and Ti system sulfide. It is typical explanatory drawing which showed the measuring method of the dimension of an inclusion. It is a graph which shows the maximum hydrogen permeability coefficient measured by the temperature / humidity control method. It is a graph which shows the correlation of the cooling rate of a casting billet, and an inclusion composition. It is a graph which shows the correlation of the area reduction rate of a casting billet, and an inclusion composition.

Claims (3)

% By mass
C: 0.35-0.55%, Si: 0.3% or less, Mn: 0.6% or less, P: 0.025% or less, S: 0.050% or less, Al: 0.10% or less, Cr: 0.5-1.5%, Mo and W Or in total of 2 types (Mo + 0.5W): 0.7 to 1.5%, Nb: 0.005 to 0.05%, 1 or 2 types of Ti and Zr in total (Ti + 0.5Zr): 0.005 to 0.20%, V: 0.15 to 0.3 %
A high-strength low-alloy steel with a tensile strength of 1350 MPa or more, wherein the balance consists of Fe and impurities, and the composition of nonmetallic inclusions with a major axis of 500 nm or more contained in the steel satisfies the following formula: .
(Ti + Zr) / (Ti + Zr + Mn) × 100 ≧ 30 (mass%)   The high-strength low-alloy steel according to claim 1, further comprising, in mass%, one or two of Ni: 0.2 to 3% and Cu: 0.05 to 1% instead of part of Fe. The composition of non-metallic inclusions having a major axis of 500 nm or more contained in steel is included in the steel, including one or two of Ca: 0.01% or less and Mg: 0.01% or less instead of part of Fe. The high-strength low-alloy steel according to claim 1 or 2, satisfying the formula.
(Ti + Zr) / (Ti + Zr + Mn + Ca + Mg) × 100 ≧ 30 (mass%)

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