JP5760972B2 - High strength bolt steel and high strength bolt with excellent delayed fracture resistance - Google Patents

Description

  The present invention relates to a steel excellent in delayed fracture resistance, for example, a high strength steel excellent in delayed fracture resistance with a tensile strength exceeding 1200 MPa, such as a steel for bolts, and a high strength bolt made of the high strength steel. It is about.

  High-strength steel used in various industrial fields such as automobiles, industrial machinery, bridges, civil engineering and construction is an alloy steel for machine structures such as SCr (chromium steel) and SCM (chromium-molybdenum steel) defined in JIS G4053. It is manufactured by quenching and tempering the steel. However, it is well known that when the above steel strength exceeds 1200 MPa, the delayed fracture resistance significantly decreases and the risk of delayed fracture due to hydrogen entering from the environment during use increases. . Therefore, for example, in steel for civil engineering and construction, the standard is a tensile strength of 1200 MPa or less, and the tensile strength is practically limited to steel of 1150 MPa class.

  With increasing strength of steel materials, various high strength steels with improved delayed fracture resistance and methods for producing the same have been proposed. For example, Patent Document 1 describes an invention in which delayed fracture resistance is improved by refining prior austenite grains. Although the method of refining prior austenite grains has been confirmed to have an effect of improving delayed fracture resistance, it has not yet improved significantly.

  Further, for example, in Patent Document 2, Patent Document 3, and Patent Document 4, delayed fracture occurs by dispersing or distributing oxides, carbides, nitrides alone or composite precipitates that trap hydrogen in steel. The invention describes improved delayed fracture resistance by increasing the critical hydrogen content. In these inventions, a technical idea of utilizing precipitates generated by quenching and tempering has been proposed as one of the mechanisms for improving delayed fracture resistance. In order to improve the delayed fracture resistance, it is essential to limit the chemical composition of the steel and the heat treatment conditions that optimally distribute and distribute the precipitate to be trapped with hydrogen.

Japanese Patent Laid-Open No. 03-243745 JP 2000-026934 A JP 2001-288539 A JP 2006-045670 A

The present invention provides a high-strength bolt steel excellent in delayed fracture resistance and a high-strength bolt capable of further suppressing hydrogen embrittlement represented by the delayed fracture resistance phenomenon that appears as the strength of steel materials increases. The purpose is to provide.

The present inventors diligently studied various factors affecting delayed fracture characteristics and found the following findings.
(A) By depositing a large amount of Fe-based carbide ε in the steel, the critical hydrogen amount at which delayed fracture occurs (hereinafter referred to as “critical diffusible hydrogen amount”) is increased, and delayed fracture resistance is improved. improves. This is because the Fe-based carbide ε traps hydrogen in the steel that has entered from the outside environment and increases the amount of critical diffusible hydrogen.
(B) By adding a large amount of Si, Fe-based carbide ε is stabilized and tempering treatment is possible at a higher temperature, so that the defect density such as dislocations in the parent phase is reduced and the embrittlement sensitivity to hydrogen is reduced. .

  The present invention has been completed based on the above findings, and the gist of the present invention is as follows.

(1) In mass%,
C: 0.20 to 1.22 %
Si: 0.50 to 5.00%
Mn: 0.10 to 3.00%,
P: 0.0005 to 0.1000%,
S: 0.0005 to 0.2000%,
N: 0.0020 to 0.0200%
The balance is composed of Fe and inevitable impurities, and the steel structure is a tempered martensite structure in which the size of the Fe-based carbide ε is dispersed at 20 nm or less and the area ratio of the Fe-based carbide ε is 1.0% or more. Oh it, hydrogen trapping amount is more than 2.5 ppm, a high strength bolts steel critical diffusible hydrogen amount is excellent in delayed fracture resistance, characterized in that at least 6.0 ppm.

( 2 ) Furthermore, in mass%,
Cr: 0.01 to 4.79 %,
Mo: 0.01 to 1.00%
The high-strength bolt steel having excellent delayed fracture resistance according to ( 1) above, comprising one or two of the above.

( 3 ) Furthermore, in mass%,
Nb: 0.01-0.10%,
V: 0.01 to 0.50%,
Ti: 0.010 to 0.300%,
Al: 0.01-0.20%,
The high-strength bolt steel excellent in delayed fracture resistance according to the above (1) or (2) , characterized by containing one or more of them.

( 4 ) A high-strength bolt comprising the high-strength bolt steel according to any one of (1) to ( 3 ) above.

According to the present invention, it is possible to provide a high-strength bolt steel having a tensile strength exceeding 1200 MPa excellent in delayed fracture resistance by selecting the above-described steel component range, heat treatment conditions, and microstructure form. . Such high strength bolt steel can be expected applied to the required bolt materials of delayed fracture resistance, the effect on the industry is extremely remarkable.

  The present invention will be described in detail below. First, the reason for limiting the steel component range described above will be described.

C: 0.20 to 1.22 %
C is an important element that determines the strength of steel. In order to obtain sufficient strength, the lower limit is made 0.20%. Compared to other alloy elements, the alloy cost is low. If a large amount of C can be added, the alloy cost of the steel material can be reduced. However, since the increasing embrittlement susceptibility to hydrogen when adding a large amount and C, the upper limit is 1.50%, but is limited to 1.22% from the examples.

Si: 0.50 to 5.00%
Si is an important element that not only greatly contributes to temper softening resistance by stabilizing the Fe-based carbide ε and delaying the transition to the Fe-based carbide θ, but also improves delayed fracture resistance. In order to obtain these effects, the lower limit is made 0.20% or more. Preferably it is 0.30% or more, more preferably 0.50% or more. However, if added in a large amount, remarkable decarburization occurs in the surface layer of the steel material, so the upper limit is made 5.00%.

Mn: 0.10 to 3.00%
Mn is an element effective for improving the hardenability of steel and has an effect of preventing hot brittleness by fixing S in the steel as MnS. In order to obtain these effects, the lower limit is made 0.10% or more. However, the addition exceeding 3.00% increases the embrittlement susceptibility to hydrogen and lowers the delayed fracture characteristics. Therefore, the upper limit is made 3.00%.

P: 0.0005 to 0.1000%
Since P contains 0.0005% or more as an inevitable impurity in steel, the lower limit is made 0.0005%. P in steel lowers the delayed fracture resistance, so the upper limit is made 0.1000%.

S: 0.0005 to 0.2000%
S, like P, usually contains 0.0005% or more as an unavoidable impurity in steel, and when present in steel, it causes embrittlement of the steel. In the case of S, it is desirable that the amount added is small although the influence is minimized by fixing as MnS partially. Therefore, the content of S is set to 0.0005 to 0.2000%.

N: 0.0020 to 0.0200%
N forms nitrides and has the effect of refining prior austenite grains. In order to obtain this effect, the lower limit of N is set to 0.0020%. However, even if added in a large amount, this effect is saturated, so the upper limit of N is 0.0200%.

Cr: 0.01 to 4.79 %
Cr is an element effective for improving the hardenability of steel and an element contributing to temper softening resistance. In order to obtain these effects, the lower limit is made 0.01%. However, Cr needs to be heated at a high temperature before the quenching treatment in order to obtain a solid solution and stabilize in the Fe-based carbide θ and to stabilize it and to add a large amount to obtain effects of improving hardenability and temper softening resistance. As a result, the prior austenite grains become coarse, and the delayed fracture resistance deteriorates. Therefore, the upper limit is set to 5.00%, but is limited to 4.79% from the examples.

Mo: 0.01 to 1.00%
Mo is an element effective for improving the hardenability of steel and an element contributing to temper softening resistance. In order to obtain these effects, the lower limit is made 0.01%. However, these effects are saturated when added over 1.00%. Therefore, the upper limit is made 1.00%.

One or two of Nb: 0.01 to 0.10%, V: 0.01 to 0.50%, Ti: 0.010 to 0.300%, Al: 0.01 to 0.20% Contains more than seeds.
Nb, V, Ti, and Al have the effect of forming nitrides and refining prior austenite grains. In order to obtain this effect, the lower limit of Nb, V, Ti, and Al is 0.01%. However, even if added in a large amount, this effect is saturated. Therefore, the upper limit of Nb is 0.10%, the upper limit of V is 0.50%, the upper limit of Ti is 0.300%, and the upper limit of Al is 0.00%. 20%.

  Next, the reason for limiting the steel structure described above will be described.

  The precipitates in the steel structure are defined as Fe-based carbides ε, because coarse Fe-based carbides θ cannot trap hydrogen in steel that has entered from the external environment, whereas fine Fe-based carbides ε. This is because hydrogen in steel can be trapped and the amount of critical diffusible hydrogen can be increased. The reason why the steel structure is defined as the tempered martensite structure is that quenching and tempering treatment is effective for finely dispersing a large amount of Fe-based carbide ε.

  Next, the reasons for limiting the size and area ratio of the Fe-based carbide ε will be described.

  The reason why the size of the Fe-based carbide ε is defined to be 1 to 20 nm is that when the thickness exceeds 20 nm, the consistency between the interface of the parent phase and the precipitate is not maintained, and the strain for trapping hydrogen is not formed. This is because a thickness of 1 nm or more is necessary to obtain. Preferably they are 2 nm or more and 10 nm or less. Also, the area ratio of Fe-based carbide ε is defined to be 1.0% or more. Even if Fe-based carbide ε precipitates and traps hydrogen, if the amount is small, the delayed fracture resistance cannot be greatly improved. Because. The area ratio is naturally saturated by the amount of C in steel.

  Next, the reasons for limiting the hydrogen trap amount and the limit diffusible hydrogen amount will be described.

  If the hydrogen trap amount is less than 2.5 ppm, or the limit diffusible hydrogen amount is less than 6.0 ppm, delayed fracture occurs due to hydrogen entering the steel from the external environment. The amount of reactive hydrogen was limited to 6.0 ppm or more.

  Next, the reason for limiting the tempering temperature described above will be described.

In order to impart predetermined strength, toughness and ductility to the steel, it is necessary to perform tempering after quenching. Tempering is generally performed in a temperature range from 150 ° C. to AC ₁ point, but in the present invention, it is necessary to limit the temperature range to 250 to 525 ° C. The reason is that when the tempering temperature exceeds 525 ° C., even if 5% of Si is added to stabilize the Fe-based carbide ε, the transition to Fe-based carbide θ occurs and the critical diffusible hydrogen amount decreases. However, since the temperature at which the Fe-based carbide ε transitions to θ greatly depends on the amount of Si, even if the temperature is within the specified tempering temperature range, the Fe-based carbide θ is precipitated by high-temperature tempering if the Si amount is small.

Therefore, the relationship between the tempering conditions for precipitation of Fe-based carbide ε and the amount of Si was examined. C amount: 0.60% (mass%, the same applies hereinafter), Si amount: 0.20 to 5.00%, Mn amount: 0.75%, P amount: 0.005 to 0.009%, S amount: 0.006 to 0.0010%, N amount: 0.0070%, the balance is prepared with a steel material composed of Fe and inevitable impurities, and after heating these steel materials to 1050 ° C, a tempering temperature of 150 to 550 ° C and a holding time of 1 ~ 18000 sec. It heat-processed in the range. Precipitates in the steel were observed using a transmission electron microscope, and Fe-based carbides were identified from the limited field diffraction pattern. As a result of the study, relational expressions (a) and (b) between the tempering treatment conditions in which Fe-based carbide ε precipitates and the amount of Si in the steel were obtained.
When 0.20 ≦ [Si] <2.00 (273 + T) × (log (t / 3600) +20)
-2067 [Si] -9467> 0 (a)
When 2.00 ≦ [Si] ≦ 5.00 (273 + T) × (log (t / 3600) +20)
-733 [Si] -12133> 0 (b)
Here, [Si] is the Si content (% by mass) in the steel, T is the tempering temperature (° C.), and t is the retention time (seconds). Therefore, when the values of the formulas (a) and (b) are 0 or less, Fe-based carbide ε is precipitated. When the tempering temperature exceeds 525 ° C., Fe-based carbide θ precipitates regardless of the amount of Si. Therefore, the application range in which Fe-based carbide ε precipitates from the equations (a) and (b) is tempering temperature 150 to 525 ° C. It is.

  However, when the tempering temperature is less than 250 ° C., the Fe-based carbide ε is precipitated, but the defect density such as dislocations in the matrix phase is high and the embrittlement sensitivity to hydrogen is high. Therefore, in the present invention, the lower tempering temperature is as described above. The temperature was 250 ° C. If the Fe-based carbide ε is stably present in the steel, it is desirable to temper at a high temperature in order to further reduce the susceptibility to hydrogen embrittlement. Preferably, the tempering temperature is 350 ° C. or higher.

  The invention is described in detail below by means of examples. These examples are for explaining the technical significance and effects of the present invention, and do not limit the scope of the present invention.

  A steel bar having a diameter of 15 mm was produced by hot rolling the steel having chemical components shown in Table 1 in a vacuum melting furnace. Then, according to the steel component composition, the temperature at which solution of Fe-based carbides and austenite of the structure can be selected is selected from 850 to 1050 ° C., heated, quenched in oil at 60 ° C., and then tempered as shown in Table 2. Tempering was performed for 60 minutes at a temperature. JIS Z 2201 No. 14 tensile test specimens were collected from these heat treated materials and evaluated for tensile strength.

In order to identify precipitates in the steel, they were observed using a transmission electron microscope and judged from the limited-field diffraction pattern. The size and area ratio of the precipitates were determined as an average value by image analysis by observing 20 visual fields with a transmission electron microscope with an area of 1 visual field of 50000 nm ² .

The amount of the hydrogen trap was determined by immersing the test piece in an aqueous solution obtained by adding 3 g / l of NaCl to a 3% ammonium thiocyanate solution and performing electrolytic hydrogen charging at a current density of 0.2 mA / cm ² for 42 hours, and then at room temperature for 96 hours. After leaving it to stand, it was measured by a temperature rising hydrogen analysis method using a gas chromatograph. The temperature rising rate of the gas chromatograph is 100 ° C./hour, and the amount of hydrogen released from the test piece from room temperature to 400 ° C. is defined as the hydrogen trap amount. As a test piece for evaluating the amount of hydrogen trap, a 7 mmφ round bar was prepared. The amount of critical diffusible hydrogen is iron and steel, Vol. 83 (1997), p. Measured by the method described in 454. Those having a limit diffusible hydrogen content of less than 6.0 ppm were judged to be inferior in delayed fracture characteristics.

  As can be seen from Table 2, no. In all of Examples 1 to 16 of the present invention, the steel composition and the steel structure are within the specified ranges, so that the delayed fracture property is excellent despite the high strength of 1600 MPa or more. In addition, Invention Example No. 4 and 15, no. As can be seen by comparing 11 and 16, the higher the tempering temperature, the lower the susceptibility to hydrogen embrittlement and the better the delayed fracture characteristics. In contrast, Comparative Example No. In Nos. 17 and 20, although the steel structure is within the specified range and has the ability to trap hydrogen, the contents of C and Mn are large, the embrittlement susceptibility to hydrogen is increased, and the delayed fracture characteristics are inferior. Comparative Example No. Nos. 18 and 19 are inferior in delayed fracture characteristics because the Si content is small and Fe-based carbides θ are precipitated. Comparative Example No. No. 21, although the steel structure is within the specified range and has the ability to trap hydrogen, the Cr content is large, and in order to obtain the effects of improved hardenability and temper softening resistance, the heating temperature before the quenching treatment should be adjusted. It is necessary to heat to a high temperature of 1200 ° C. Thereby, the prior austenite grains become coarse and inferior in delayed fracture characteristics. Comparative Example No. No. 22 has a high tempering temperature of 550 ° C., and the Fe-based carbide θ is precipitated. Comparative Example No. In No. 23, although the steel composition and the tempering temperature are within the specified ranges, the deviation from the relationship between the tempering conditions in which the Fe-based carbide ε precipitates and the amount of Si results in the precipitation of Fe-based carbide θ, which is inferior in delayed fracture characteristics.

Claims (4)

% By mass
C: 0.20 to 1.22 %
Si: 0.50 to 5.00%
Mn: 0.10 to 3.00%,
P: 0.0005 to 0.1000%,
S: 0.0005 to 0.2000%,
N: 0.0020 to 0.0200%
The balance is composed of Fe and inevitable impurities, and the steel structure is a tempered martensite structure in which the size of the Fe-based carbide ε is dispersed at 20 nm or less and the area ratio of the Fe-based carbide ε is 1.0% or more. Oh it, hydrogen trapping amount is more than 2.5 ppm, a high strength bolts steel critical diffusible hydrogen amount is excellent in delayed fracture resistance, characterized in that at least 6.0 ppm. Furthermore, in mass%,
Cr: 0.01 to 4.79 %,
Mo: 0.01 to 1.00%
The high-strength bolt steel having excellent delayed fracture resistance according to claim 1, wherein one or two of them are contained. Furthermore, in mass%,
Nb: 0.01-0.10%,
V: 0.01 to 0.50%,
Ti: 0.010 to 0.300%,
Al: 0.01-0.20%,
The high-strength bolt steel excellent in delayed fracture resistance according to claim 1 or 2 , wherein one or more of them are contained. A high-strength bolt comprising the high-strength bolt steel according to any one of claims 1 to 3 .