JP2002069565A - High strength steel having excellent delayed fracture resistance and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide high strength steel in which high strength and delayed fracture resistance are compatible. SOLUTION: In steel having tensile strength of >=1,200 N/mm2, the center line average roughness Ra in the surface roughness of the steel is controlled to <=1.0 μm, and PPI50 to <=175. In the steel, when immersed into an aqueous solution of 5 weight % NaCl having pH 3 by 24 hours, the amount of diffusible hydrogen after the immersion may be <=0.5 ppm (mass%). The components in the steel, e.g. contains 0.05 to 0.8% C, <=1.5% Si, <=2% Mn, <=0.03% P and <=0.04% S. Further, the steel may contain Mo, V, Ti, Nb, Cu or the like.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to steel useful for materials or parts for mechanical structures such as structures and transportation equipment, and more particularly to a steel having a tensile strength of not less than 1200 N / mm ² and a resistance to steel. The present invention relates to a high-strength steel excellent in delayed fracture.

[0002]

2. Description of the Related Art In recent years, a large structure has been required to reduce its own weight, and transportation equipment has been required to have improved fuel efficiency and ultra-high speed. Therefore, in these large structures and transportation equipment,
High weight reduction needs. Therefore, in various structural materials such as bolts, reduction in thickness by increasing strength is being pursued. In the automobile field in particular, there is an extremely high demand for reducing the weight of a vehicle body in order to improve fuel efficiency. Since weight reduction is possible, there is a strong need for high strength.

[0003] The biggest problem to be solved by these high-strength steels is to improve delayed fracture resistance. For example, bolt steel
While the strength can be easily increased by adjusting the composition and controlling the heat treatment conditions, in the case of high-strength steel for bolts (for example, high-strength bolt steel with a tensile strength of 1200 N / mm ² or more), the bolts break brittlely during use. Time-dependent "delay failure" is likely to occur. For this reason, the bolt steel currently used is often restricted in strength to a certain level or less, although the strength can be increased.

In view of the above, various high-strength steel materials having improved delayed fracture resistance by improving the composition and structure and processed products thereof have been proposed. For example, (1) JP-A-7-7069
No. 5, JP-A-8-60291, JP-A-7-1
No. 12236 discloses Mo, V, Nb, Ni, C
A method for improving delayed fracture resistance by suppressing the amount of elements such as u is disclosed. (2) Japanese Patent Application Laid-Open No. 11-
JP-A-229075, JP-A-2000-26934, JP-A-11-270531, and the like disclose methods for improving delayed fracture resistance by controlling a heat treatment and the like to control a steel structure. These methods mainly focus on the suppression of processes from hydrogen enrichment to embrittlement fracture among the processes leading to delayed fracture such as hydrogen intrusion, hydrogen enrichment, and embrittlement fracture by hydrogen. However, delayed fracture is thought to be triggered by a small amount of diffusible hydrogen generated or penetrated by a corrosion reaction, and despite the fact that suppression of hydrogen intrusion can be an important issue, there is little research on such suppression technology. It is.

[0005] Techniques that mention suppression of hydrogen intrusion include:
(3) JP-A-7-292412, JP-A-7-29
According to these publications, it is described that, by shot peening, the residual compressive stress and hardness of the surface are controlled to improve the delayed fracture resistance and hydrogen penetration resistance of steel. I have. However, these methods are not only insufficient in suppressing hydrogen intrusion, but according to the study of the present inventors, hydrogen intrusion is rather accelerated depending on the degree of shot peening treatment, and delayed fracture resistance is deteriorated. It turns out that there are times when you do.

As described above, in the conventional method, research on hydrogen intrusion which triggers the delayed fracture phenomenon is insufficient, and the improvement of the delayed fracture resistance is not sufficient even if the component structure is simply improved. On the other hand, it is expected that demand for high-strength steel will increase in the future, so new technologies are required.

[0007]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above circumstances, and has found that the amount of hydrogen penetration largely depends on the surface condition of a steel material, especially the physical condition. It is an object of the present invention to provide a high-strength steel capable of achieving both high strength and delayed fracture resistance as a basis, and a method for producing the same.

[0008]

The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, the surface of the conventional high-strength steel has an uneven appearance and strength due to unevenness in the manufacturing process. After examining the details of the presence of fine irregularities at a level that does not cause a problem, and examining the substance of the fine irregularities in more detail, among the various indices that can express the degree of irregularities, the center line average roughness of the JIS standard and later detailed When irregularities are arranged in accordance with the concept of the PPI described above, the amount of hydrogen penetrating into steel changes according to the degree of the irregularities (there is a proportional relationship between the two).
When the surface roughness is reduced to a specific level, the penetration of hydrogen into the steel can be reduced to a substantially harmless amount. The inventors have found that the destructibility can be drastically improved, and completed the present invention.

That is, the high-strength steel according to the present invention has 12
It has a tensile strength of at least 00 N / mm ² , has a center line average roughness Ra of 1.0 μm or less among the surface roughness of steel,
PI50 is 175 or less. Such steel has excellent delayed fracture resistance. Preferred steels include steels having a diffusible hydrogen content of 0.5 ppm or less (by mass) after immersion in a 5% by mass NaCl aqueous solution of pH 3 for 24 hours. The steel is, for example, C: 0.05 to 0.5%.
(Mean% by mass, hereinafter the same), Si: 1% or less, M
n: 0.3 to 1.5%, P: 0.010% or less, S:
In addition to 0.010% or less, Mo, V, Ti, Nb, Cu or the like may be contained, and a film may be formed on the surface. The high-strength steel can be manufactured by mechanically polishing a steel surface with an abrasive.

[0010] In the present specification, "steel" means not only steel materials and wires before product processing but also steel products such as bolts.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The high strength steel of the present invention has a strength of 12
The components in the steel are not particularly limited as long as they can be adjusted to 00 N / mm ² or more, but usually C: 0.05 to 0.8%, Si:
1.5% or less, Mn: 2% or less, P: 0.03% or less,
S: 0.04% or less, Cr: 1.5% or less, Al: 0.03% or less. The balance is substantially Fe, but may contain other components and unavoidable impurities as long as the effects of the present invention are not impaired.
Other components include components capable of improving delayed fracture resistance, for example, Mo (content is about 0.5% or less, preferably 0.0% or less).
V (content is about 0.5% or less, preferably about 0.1 to 0.5%), Ti (content is about 0.1%).
% Or less, preferably about 0.01 to 0.1%), N
b (content is about 0.1% or less, preferably 0.01 to
0.1%), Cu (content is about 1.0% or less, preferably about 0.1 to 1.0%) and the like. The components capable of improving the delayed fracture resistance may be used alone or in combination of two or more.

Hereinafter, the reason why the chemical composition of the steel material is determined in the present invention will be clarified.

C: 0.05-0.8% C is a component essentially contained in steel, and is useful for enhancing the hardenability of the steel and ensuring the strength. C
Is usually at least an effective amount, preferably 0.05%
It is more preferably at least 0.2%, most preferably at least 0.3%. On the other hand, if the amount of C is too large,
In some cases, toughness is deteriorated, and delayed fracture resistance and cold workability are reduced. The content of C is usually 0.8% or less, preferably 0.6% or less, more preferably 0.5% or less.

Si: 1.5% or less (excluding 0%) Si is useful as a deoxidizing element. On the other hand, if the Si content is too high, the cold forgeability may decrease. In addition, in some cases, grain boundary oxidation during heat treatment such as quenching is promoted to reduce delayed fracture resistance. The Si content is, for example, 1.5% or less, preferably 1.2% or less, and more preferably 1.0% or less.

Mn: 2% or less (excluding 0%) Mn is useful as a hardenability improving element, and an appropriate amount of Mn
This makes it easy to secure high strength. The Mn content is usually at least an effective amount, for example, at least 0.1%, preferably at least 0.2%, more preferably at least 0.3%.
On the other hand, when the content of Mn is too large, segregation at the grain boundary becomes remarkable, the grain boundary strength decreases, and the delayed fracture resistance may decrease. The content of Mn is, for example, 2% or less, preferably 1.7% or less, and more preferably 1.5% or less.

P: 0.03% or less (including 0%) P may cause grain boundary segregation to deteriorate delayed fracture resistance. The P content is preferably as low as possible, for example, 0.03% or less, preferably 0.025% or less, more preferably 0.02% or less, and most preferably 0.01% or less.

S: 0.04% or less (including 0%) S forms MnS in the steel and acts as a stress concentration point when stress is applied, and thus causes an increase in delayed fracture. . The content of S is preferably as small as possible, for example, 0.04% or less, preferably 0.03% or less, more preferably 0.02% or less, and most preferably 0.1% or less.
01% or less.

The steel of the present invention has not only high strength, but also
Very high surface uniformity. For example, in the steel of the present invention, the height difference between the peaks and valleys of the fine unevenness on the surface is small, and the number of peaks is small. When such steel is used, intrusion of hydrogen from the steel surface (particularly, valleys of fine irregularities on the steel surface) can be prevented, and the delayed fracture resistance of the steel can be improved.

More specifically, the strength and surface uniformity (roughness) of the steel are defined by an index (a maximum height Rmax, a ten-point average roughness Rz, a center line average roughness) indicating the tensile strength and the surface roughness. Ra, PP
I50). From these,
If the surface roughness is specified by the center line average roughness Ra and the PPI 50, the relationship between the surface roughness and the hydrogen penetration or delayed fracture resistance can be unified and advantageously. The PPI
50 means that a positive and negative reference level is provided at a height displaced by 50 μ inch (1.27 μm) in both the positive and negative directions from the average line of the roughness curve of the steel surface, and the curve is lower than the negative reference level. Later, the number of peaks per inch (2.54 cm) when counting as one peak when exceeding the positive reference level is shown.

The tensile strength of steel is, for example, 1200 N / mm
² or more, preferably 1250 N / mm ² or more, more preferably 1300 N / mm ² or more. The tensile strength is usually 1600 N / mm ² or less, preferably 1500 N / mm ² or less.
N / mm ² or less, more preferably 1400 N / mm ² or less.

The center line average roughness Ra is, for example, 1.0 μm.
m or less (about 0 to 1.0 μm), preferably 0 to 0.5
It is about μm, more preferably about 0 to 0.1 μm.

The PPI 50 is, for example, 175 or less (0 to
About 175), preferably about 0 to 100, and more preferably about 0 to 50.

The surface roughness of the steel is calculated from the center line average roughness Ra and P
When represented by PI50, it is important that Ra and PPI50 both fall within the range of the predetermined value. That is, when both Ra and PPI 50 are out of the range of the predetermined value, the delayed fracture resistance of the steel decreases. For example, components that improve delayed fracture resistance (Mo, V, Nb, Ni, Cu
) Is insufficient in delayed fracture resistance. Further, even if one of the values of Ra and PPI 50 is out of the predetermined range, delayed fracture resistance is poor. For example, even when Ra is small (for example, about 0.5 μm), when PPI50 exceeds 175, or when PPI5
Even if 0 is small (for example, about 130), Ra is 1.0 μm
If it exceeds, it is inferior in delayed fracture resistance.

Preferred steels include those having an Ra value of 1.0 μm or less and a PPI50 value of 175 or less, and more preferably those having an Ra value of 0.5 μm or less and a PPI50 value of 1 or less.
Steels having a Ra value of 0.1 μm or less and a PPI50 value of 50 or less are included.

It is to be noted that a film may be formed on the surface of steel (steel material, wire material, steel processed product, etc.). Even in this case, as long as the surface of the base material steel (such as the interface between the steel and the coating) is uniform, the penetration of hydrogen can be prevented, and the delayed fracture resistance of the steel can be improved. That is, even when a film is formed,
It is not the roughness of the coating surface but the roughness of the steel surface that is important for preventing hydrogen intrusion and improving delayed fracture resistance.
The properties (type, film thickness, defect degree, etc.) of the coating are not particularly limited, but depending on the properties of the coating, penetration of hydrogen into steel can be prevented, and delayed fracture resistance can be improved.

Examples of the film include a phosphate-treated film and a lubricating film (such as a lime-treated film) useful for enhancing forgeability. In addition, as a film of a steel processed product, an oxide film (such as an oxide film formed in a quenching and tempering process),
Chemical conversion coating (coating formed by water glass treatment to enhance corrosion resistance and design, black iron oxide coating, etc.), metal coating (plating coating such as zinc alloy plating), non-metal coating (paint coating, rubber or plastic) Lining, ceramic coating, etc.).

The method for measuring the surface roughness of steel is not particularly limited, but it can be generally measured as follows depending on whether or not a film is formed. That is, when the surface of the steel is exposed, or when a thin film is formed on the surface (for example, the increase or decrease in the surface roughness due to the film is negligible), the contact type or laser type surface roughness is used. The roughness of the surface can be measured directly using a thickness meter. If a film is formed on the surface, remove the film so that the roughness of the steel surface (that is, the interface between the steel and the film) does not change, and use a contact or laser type surface roughness meter. The surface roughness may be measured after the film is removed, or the roughness may be measured simply by cutting out a cross section, performing precision polishing, and measuring the profile of the interface.

The method for evaluating the delayed fracture resistance of steel is also not particularly limited. Preferably, a sample (steel) processed into the shape of a tensile test piece is subjected to an SSRT test (pH 3 to 7) in an aqueous solution containing Cl ^- ions. Slow Strain Rate Technique (low strain rate test)
After T (Cyclic Corrosion Test) test, S
It can be evaluated by performing an SRT test. In addition, SSR
The crosshead speed in the T test is 2 μm / min or less.

According to such a method, the delayed fracture resistance can be evaluated more accurately than the conventional delayed fracture resistance test. In other words, strong acids (about pH 1), which are often used in the past
In the immersion test, the dissolution reaction on the surface is intense, and the environment is completely different from the actual use condition of the steel material, and the delayed fracture may not be evaluated correctly. In addition, in the conventional evaluation method,
In many cases, delayed fracture testing is performed after a large amount of hydrogen is forcibly penetrated into steel by hydrogen charging, but it is difficult to accurately grasp the hydrogen penetration resistance, and it may not be possible to correctly evaluate delayed fracture. There is. On the other hand, according to the above-described preferred evaluation method, the test conditions and the use conditions can be approximated because the sample is immersed in an aqueous solution containing a neutral (pH 3 to 7) Cl ^- ion from a weak acid closer to the actual environment, and Delayed fracture resistance under an actual use environment can be correctly evaluated.

In the method for evaluating delayed fracture resistance, the load may be applied in accordance with a constant load test, but it takes time for the measurement, and not only may the presence of hydrogen in the material be different, but also Delayed fracture resistance may not be evaluated depending on the material, stress, and environment. On the other hand, SSRT
If a load is applied according to the test, the effect of mild environment on delayed fracture [Effect of trace corrosion (trace hydrogen), etc.]
The effect of slight differences in surface properties such as roughness and roughness can be quickly evaluated with high sensitivity.

In the present invention, the roughness (Ra, PPI50
In other words, by reducing the amount of hydrogen, it is possible to suppress the intrusion of hydrogen into the steel and improve the delayed fracture resistance. The lower the roughness, the lower the surface of the steel (the actual surface, the interface between the steel and the coating, etc.) This is considered to be because the surface area or interface irregularities (particularly, concave portions) can be reduced and the corrosion reaction can be suppressed in addition to the fact that the area can be reduced. For that reason, the smaller the amount of hydrogen entering the steel during the immersion test, the smaller the amount of hydrogen entering the steel even in an actual use environment, and the delayed fracture resistance is improved. That is, preferred steels in the present invention not only have the above-mentioned predetermined strength and roughness, but also have an amount of diffusible hydrogen in the steel after being immersed in a 5% by mass NaCl aqueous solution adjusted to pH 3 for 24 hours. 5 ppm (by mass) or less, preferably 0.3 ppm (by mass) or less, more preferably 0.1 ppm (by mass).
Steel containing less than 2 ppm (by mass) is included. Use of such steel further improves delayed fracture resistance.

The amount of diffusible hydrogen in the steel before the immersion test is usually about 0 to 0.01 ppm (by mass).

The amount of diffusible hydrogen in steel is determined by Kobe Steel R & D Technical Report Vol. 1, the method described on pages 24 to 27 (1997), for example, removing the film and rust on the steel surface, and increasing the temperature from room temperature to 350 ° C. at a heating rate of 6 to 12 ° C./min by continuous heating; A measuring device [preferably an atmospheric pressure ionization mass spectrometer (API-
MS), vacuum mass spectrometer (TDS), etc.].

Steels having a small amount of diffusible hydrogen in the steel even after the immersion test include steels having a hydrogen penetration resistance component (for example, steels containing Mo, V, Nb, Ni, Cu, etc.),
Examples include steel on which a film is formed. When such steel is used, even if the roughness is the same, the amount of diffusible hydrogen entering during the erosion test can be further suppressed, and the delayed fracture resistance can be further improved.

The high-strength steel of the present invention is advantageous for steel products requiring high delayed fracture resistance (such as steel products for machine structural use), for example, bolts, nuts, bolt-attached parts, gears, and especially bolts. Available to

The high-strength steel (steel, wire, steel product, etc.) of the present invention can be produced by using a conventional method for producing high-strength steel. For example, steel material, steel smelting process, hot rolling process,
It can be manufactured by a winding step, a cold rolling step, an annealing step (such as a spherical annealing step). The wire rod is a steel melting process,
It can be manufactured by a casting step, a hot rolling step, a wire rod processing step, a quenching and tempering step, and the like. A steel product, for example, a bolt, can be manufactured by a wire cutting process, a bolt head working process, a thread rolling process, a quenching and tempering process, and the like. Also,
The bolts may then be pickled, plated or painted as needed.

According to the production method of the present invention, (1) the production steps are stabilized (for example, the pickling step of the steel material is suppressed, the temperature unevenness is suppressed in the spherical annealing step, and the surface flaws are suppressed in the processing). Etc.) or (2) a polishing step of polishing the steel is added after any of the manufacturing steps to make the surface of the steel uniform. That is, in the case of conventional high-strength steel materials or steel processed products (such as bolts), minute irregularities are formed on the surface due to slight unevenness in the manufacturing process, slight surface flaws during processing, and the like. Of the degrees, Ra is 0.7 to 2.5
The μm and the PPI 50 were about 180 to 250.
In general, even with the roughness as described above, the surface of the steel is sufficiently uniform and there is no problem in appearance or product performance (strength reduction due to large cracks, flaws, etc.). There was no polishing beyond the roughness. On the other hand, in the present invention, since the surface of the steel is made more uniform by the stabilization of the manufacturing process and the polishing treatment, intrusion of hydrogen can be prevented, and the delayed fracture resistance of the steel can be improved.

A preferred production method includes a method including a polishing step (polishing treatment). The polishing treatment includes tumbling treatment (barrel polishing and the like), wet or dry mechanical polishing (buff polishing, polishing with a portable grinder or sander, etc.), electrolytic polishing, chemical polishing and the like.

Preferred examples of the polishing treatment include a method of mechanically (or physically) polishing with an abrasive (a tumbling method such as barrel polishing, mechanical polishing, particularly barrel polishing). According to the barrel polishing, not only the polishing can be simply performed, but also the processed product having a complicated shape can be effectively polished. The barrel polishing may be any of a horizontal type, an inclined type, and a vertical type.

As abrasives (such as barrel polishing media), ceramic powders such as Al ₂ O ₃ , Si ₃ N ₄ , SiC and glass beads; hard materials such as diamond and steel powder can be used.

The shape of the abrasive is not limited to a spherical shape (S type), but may be a diamond shape (D type), a triangle shape (T type), a granular shape (P type), a wedge shape (N type), a cylindrical shape (RP type), or the like. Any of these may be used.

The polishing step is preferably carried out before the formation of the finishing film of the product, for example, before the formation of an oxide film, a plating film, a coating film, a chemical conversion film or the like in the quenching and tempering process. For example, in the case of a high-strength steel bolt, (1)
It is preferable to perform a polishing treatment after drawing and before cutting, or (2) after bolt head processing and thread rolling, before quenching and tempering.

When a non-finish film such as a thin oxide scale or a lubricating film for processing is formed, a polishing treatment may be performed while also removing the film. In this case, the polishing process may be divided into a plurality of stages, and the polishing process may be performed while reducing the particle size of the abrasive. For example, in the first polishing step, polishing is performed using an abrasive for removing a film (for example, abrasive particles having a high hardness and a large particle diameter), and then polishing is performed in a multi-step manner while reducing the particle diameter of the abrasive particles. In the process, polishing may be performed using a finishing abrasive (a fine abrasive particle or the like). By the final polishing step, the surface roughness of the processed product can be adjusted.

[0044]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and may be appropriately performed within a range that can conform to the purpose of the preceding and following descriptions. Of course, the present invention can be embodied with modifications, all of which are included in the technical scope of the present invention.

Examples 1 to 16 and Comparative Examples 1 to 8 [Production of test pieces] A drawn wire of test steels (test steels A to F) containing the chemical components (mass%) shown in Table 1 below was used. After quenching and tempering so that the tensile strength became 1300 to 1400 N / mm ² , processing was performed to produce delayed fracture test pieces (test pieces A to F) having the dimensions and shape shown in FIG. 1. The test piece had a dumbbell shape, a total length of 140 mm, and a distance between marked lines of 10 mm. The grips at both ends have a circular cross section with a diameter of 8 mm, and the thin portion at the center has a circular cross section with a diameter of 4 mm. Also, a screw portion having a length of about 15 mm is formed at both ends of the test piece. Thereafter, the portions other than the threaded portions of this test piece were polished by mechanical polishing and electrolytic polishing, and the surface roughness (Ra: 0.08 to
2.82 μm, PPI50: 0 to 280).
In addition, the test piece formed with the test steel D (Examples 12-13,
In Comparative Example 7), after that, a phosphate treatment was performed using a commercially available solution, and the surface was coated with a zinc phosphate film of 3 μm and a metal soap film of 1 μm.
m, and a three-layer coating consisting of a soap coating of 1 μm.
Further, a test piece formed of the test steel E (Examples 14 to 16,
In Comparative Example 8), a lime treatment was performed using a commercially available liquid to form a soap film 3 μm on the surface.

The surface roughness, delayed fracture resistance, and diffusible hydrogen content of the obtained test pieces were examined as follows.

[Measurement of Surface Roughness] With respect to the test pieces having no surface coating (Examples 1 to 11 and Comparative Examples 1 to 6), the surface of the test piece was directly measured by a laser type surface roughness meter. The test pieces (Examples 12 to 16 and Comparative Examples 7 to 8) provided with the surface film were cut after the SSRT test in the atmosphere described later, and the cross section was polished, and the roughness was measured from the profile of the interface.

[Delayed Resistance to Resistance] A test piece was immersed in a 5 mass% NaCl aqueous solution whose pH was adjusted to 3.0 with HCl and NaOH, and subjected to an SSRT test (crosshead speed: 2 ×
(10 ⁻³ mm / min), the elongation E ₁ of the test piece was measured. In addition, an SSRT test (crosshead speed: 2 × 1) was performed in the air using a test piece separately subjected to the same surface roughness and surface treatment.
0 ^-3 mm / min), the elongation E ₀ of the test piece was measured. Delayed fracture (DF) sensitivity was calculated by the following formula, and delayed fracture resistance was evaluated according to the following criteria. Delayed fracture susceptibility = 100 × (1−E ₁ / E ₀ ) (where E ₁ is the elongation of the test piece in a pH 3 aqueous solution, E ₂
Indicates the elongation of the test piece in the atmosphere) □: Delayed fracture sensitivity is 0.5 or more. Poor in delayed fracture resistance Δ: Delayed fracture sensitivity is 0.3 to 0.5. Excellent delayed fracture resistance ○: Delayed fracture sensitivity is less than 0.3. Excellent in delayed fracture resistance

[Amount of Diffusible Hydrogen] After the SSRT test was performed in the atmosphere, the test piece was adjusted to pH 3 with 5 mass% NaC.
After being immersed in an aqueous solution for 24 hours, rust was removed. AP
The amount of diffusible hydrogen in the test piece was measured using I-MS.

The composition of the test steel is shown in Table 1, and the results are shown in Table 2 and FIGS. FIG. 2 is a graph in which the relationship between roughness (Ra, PPI50) and delayed fracture susceptibility is arranged based on the results shown in Table 2. Symbols □, △, ○ in FIG.
Indicates the evaluation of delayed fracture resistance. FIG. 3 shows samples of Table 2 having relatively close roughness (Examples 2, 11, 1).
2, 14; Ra = 0.8 to 0.82; PPI50 = 12
4 is a graph in which the relationship between the amount of diffusible hydrogen and the delayed fracture susceptibility is arranged using data of 0 to 140).

[0051]

[Table 1]

[0052]

[Table 2]

As is clear from Table 2 and FIG. 2, in the test piece of the embodiment, the roughness is controlled to a specific range.
Regardless of the type of material used, regardless of the presence or absence of surface treatment, delayed fracture sensitivity is small and delayed fracture resistance is excellent. Further, as is clear from Table 2, in the test piece of the example, the amount of diffusible hydrogen is small because the roughness is controlled to a specific range. Furthermore, from FIG. 3, when the roughness is approximate,
It can be seen that the smaller the amount of diffusible hydrogen, the lower the delayed fracture sensitivity and the better the delayed fracture resistance. On the other hand, in the test piece of the comparative example, since the roughness is out of the predetermined range, the amount of diffusible hydrogen is large and the delayed susceptibility is large. As is evident from the above results, by adjusting the roughness to a specific range, the amount of diffusible hydrogen penetrating into the steel can be suppressed, and a high-strength steel excellent in delayed fracture resistance can be obtained.

[0054]

According to the present invention, since the roughness of the steel surface is reduced, the penetration of hydrogen into the steel can be suppressed even for a high-strength steel, and a high-strength steel with excellent delayed fracture resistance can be obtained. You can get steel. For this reason, the delayed fracture resistance of a high-strength structural material such as a bolt can be improved.

[Brief description of the drawings]

FIG. 1 is a schematic front view of a test piece used in Examples and Comparative Examples.

FIG. 2 is a graph showing the relationship between roughness (Ra, PPI50) and delayed fracture susceptibility.

FIG. 3 is a graph showing a relationship between the amount of diffusible hydrogen and delayed fracture sensitivity.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yuichi Namimura 2nd Nadahama Higashicho, Nada-ku, Kobe Inside Kobe Steel, Ltd. In Kobe Works

Claims (6)

[Claims] 1. A steel having a tensile strength of 1200 N / mm ² or more, wherein a center line average roughness Ra of the steel surface roughness is 1.0%.
A high-strength steel excellent in delayed fracture resistance, which is not more than μm and PPI50 is not more than 175. 2. A 24 mass% NaCl aqueous solution having a pH of 3
When immersed for a period of time, the amount of diffusible hydrogen after immersion is 0.5 pp
The high-strength steel according to claim 1, which is not more than m (by mass). 3. C: 0.05 to 0.8% (meaning mass%, the same applies hereinafter), Si: 1.5% or less, Mn: 2% or less, P: 0.03% or less, S: The high-strength steel according to claim 1, comprising 0.04% or less. 4. Further, Mo, V, Ti, Nb and Cu
The high-strength steel according to claim 3, comprising at least one selected from the group consisting of: 5. The method according to claim 1, wherein a film is formed on the surface.
4. The high-strength steel according to any one of 4. 6. A method for producing a high-strength steel according to claim 1, wherein the steel surface is mechanically polished with an abrasive.