JP2001294981A - High strength wire rod excellent in delayed fracture resistance and forgeability and/or under head toughness and its producing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength wire rod excellent in forgeability, under head toughness as a bolt, and a delayed fracture resistance, and having tensile strength of >=1,200 N/mm2, and to provide a useful method for producing the same high strength wire rod. SOLUTION: This wire rod is composed of steel containing 0.50 to 1.0% C, and in which the content of Si is suppressed to <0.1% as well, and, by suppressing the formation of one or more kinds of structures among pro- eutectoid ferrite, pro-eutectoid cementite, bainite and martensite, the area ratio of a pearlitic structure is controlled to >=80%, and also, by forced wire drawing, strength of >=1,200 N/mm2 and excellent delayed fracture resistance are imparted thereto.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength wire rod suitable for steel for bolts used for automobiles and various industrial machines, and a method for producing the same, and particularly has a strength (tensile strength) of 1200 N / The present invention relates to a high-strength wire rod having not less than mm ² and excellent in forgeability and under-neck toughness as well as delayed fracture resistance, and a useful method for producing such a high-strength wire rod.

[0002]

2. Description of the Related Art Generally, medium carbon alloy steels (SCM435, SCM440, SCr440, etc.) are used as high-strength bolt steel, and the required strength is obtained by quenching and tempering. However, on the other hand, when the tensile strength exceeds about 1200 N / mm ² , there is a risk of delayed fracture, which limits the use.

[0003] Delayed fracture occurs in a non-corrosive environment and in a corrosive environment, and it is difficult to identify the cause of various factors intricately intertwined. In particular, the tempering temperature,
Although the involvement of the structure, material hardness, crystal grain size, various alloying elements, etc. has been recognized for some time, means for preventing delayed fracture have not been established, and various methods have been proposed by trial and error. The fact is that it is only a matter of fact.

[0004] In order to improve the delayed fracture resistance, for example, JP-A-60-114551, JP-A-2-267243,
Techniques such as Japanese Patent Application Laid-Open No. 3-243745 have been proposed.
In these technologies, a high-strength bolt steel having excellent delayed fracture resistance even at a tensile strength of 1400 N / mm ² or more is disclosed by adjusting various main alloy elements. Are not completely eliminated, and their scope is limited.

[0005] By the way, it is not a method of drawing steel after spheroidizing annealing and bolting a relatively low-strength steel material and then obtaining the required strength by quenching and tempering. When bolting steel,
The deformation resistance at the time of bolting is increased, the tool life is greatly reduced, and productivity may be impaired. Therefore, when such a processing method is applied, it is necessary that the forging property is good as a property required for the steel for bolts.

On the other hand, when a hexagonal upset bolt with a head or a hexagonal flange bolt is manufactured using these wires, the toughness of the portion immediately below the head is smaller than that of a bolt manufactured by tempering (in the present invention, this is called " Called "under-neck toughness")
There was a risk of head jumps when using bolts. For these reasons, it is also an important requirement that the steel for high-strength bolts as described above has excellent toughness under the neck when formed into bolts.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and it is an object of the present invention to provide a material having a tensile strength of not less than 1200 N / mm ² and a forging property as well as a delayed fracture resistance. Another object of the present invention is to provide a high-strength wire excellent in strength and under-neck toughness, and a useful method for producing such a high-strength wire.

[0008]

The high-strength wire rod of the present invention, which has achieved the above object, contains 0.50 to 1.0% of C and is suppressed to less than 0.1% of Si. It is made of steel, and has an area ratio of pearlite structure of 80% or more by suppressing generation of one or more types of proeutectoid ferrite, proeutectoid cementite, bainite, and martensite, and strong wire drawing. 1200N / mm by processing
^It has a gist in that it is designed to have strength of ² or more and excellent delayed fracture resistance, and by adopting such a configuration, it is possible to obtain a high-strength wire rod with excellent delayed fracture resistance and forgeability. Become.

[0009] The object of the present invention is as follows: C: 0.50
And 1.0% or less of Al: less than 0.005%. One or two of proeutectoid ferrite, proeutectoid cementite, bainite and martensite.
The formation of more than one kind of structure is suppressed so that the area ratio of the pearlite structure is 80% or more, and high strength such as having a strength of 1200 N / mm ² or more and excellent delayed fracture resistance by strong wire drawing. This is also achieved by adopting a structure of a strength wire, and by adopting such a structure, a high-strength wire excellent in delayed fracture resistance and toughness under the neck is obtained.

Further, the above object of the present invention is as follows: C: 0.50
~ 1.0%, and consisted of steel in which Si: less than 0.1% and Al: less than 0.005%, respectively.
Formation of one or more types of microstructures of proeutectoid ferrite, proeutectoid cementite, bainite and martensite is suppressed to increase the area ratio of the pearlite structure to 80% or more, and 1200 N / It is also achieved by adopting a structure that has a strength of at least ² mm2 and excellent delayed fracture resistance. By adopting such a structure, it is excellent in both forgeability and under-neck toughness as well as delayed fracture resistance. A high strength wire rod is obtained.

[0011] The high-strength wire of the present invention contains Cr: 0.5% or less (excluding 0%) and / or Co: 0.5% or less (excluding 0%) as necessary. It is also effective.

On the other hand, in producing the high-strength wire of the present invention as described above, the steel material is subjected to hot rolling or forging so that the end temperature of rolling or forging becomes 800 ° C. or higher, and then the average cooling rate V (° C./sec) is continuously cooled to 400 ° C. so as to satisfy the following (1), and then allowed to cool to room temperature, whereby one or more structures of proeutectoid ferrite, bainite and martensite are formed. The area ratio of the pearlite structure may be reduced to 80% or more by suppressing generation, and then the strength may be increased to 1200 N / mm ² or more by strong drawing. 166 × (wire diameter: mm) ^-1.4 ≦ V ≦ 288 × (wire diameter: mm) ^-1.4 (1)

Further, the high-strength wire of the present invention comprises
After heating to 0 ° C. or higher, it is rapidly cooled to a room temperature of 500 to 650 ° C., and is kept at that temperature to form one or more types of microstructures of proeutectoid ferrite, proeutectoid cementite, bainite, and martensite. It can also be manufactured by suppressing the area ratio of the pearlite structure to 80% or more by suppressing it, and then increasing the strength to 1200 N / mm ² or more by strong wire drawing.

Further, the high-strength wire of the present invention is subjected to hot rolling or hot forging so that the end temperature of rolling or forging of the steel is 800 ° C. or more, and then at a cooling rate of 520 to 520 ° C./sec or more. Cool to a temperature of 750 ° C., from which temperature 1.
By cooling at a cooling rate of 0 ° C./sec or less and holding for 200 seconds or more, and then allowing to cool, one of proeutectoid ferrite, proeutectoid cementite, bainite and martensite
It can also be produced by suppressing the generation of seeds or two or more kinds of structures to increase the area ratio of the pearlite structure to 80% or more, and thereafter to make the strength of 1200 N / mm ² or more by strong drawing.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have studied from various angles the cause of the inferior delayed fracture resistance of conventional high-strength steel for bolts. As a result, in the conventional improvement method, the structure was tempered martensite, and the delayed fracture resistance was compensated by avoiding the tempered brittle zone, reducing the elements of grain boundary segregation, and refining the crystal grains. Turns out there are limitations.

The inventors of the present invention have conducted intensive studies to further improve the delayed fracture resistance, and as a result, the structure was changed to a pearlite structure having certain restrictions, and the structure was subjected to a strong drawing process.
It has been found that when the strength is not less than 00 N / mm ² , the delayed fracture resistance is improved.

Further, when forging or forging a wire rod having the above pearlite structure with a strength of 1200 N / mm ² or more by strong wire drawing, if the Si content in the steel material is reduced to less than a predetermined amount, the deformation resistance is reduced. It was also found that the tool life was improved due to the reduction of oxides in the steel, the deformability was improved, and the occurrence of cracks could be suppressed, that is, excellent forgeability could be exhibited.

Further, when the above pearlite structure is formed into a bolt with a head by using a wire rod having a strength of 1200 N / mm ² or more by strong drawing, the Al content in the steel material is reduced to less than a predetermined amount. By doing so, it was also found that oxides in the steel could be reduced and breakage (head jump) under the neck when using the bolt could be suppressed, that is, excellent under-neck toughness could be exhibited.

In the high-strength wire rod of the present invention, excellent forgeability can be exhibited by reducing the Si content in the steel material to less than the predetermined amount as described above, and the Al content in the steel material is reduced to the predetermined amount. Excellent toughness under the neck was able to be exhibited by reducing to less than, but by reducing both of these Si and Al, together with the above-mentioned delayed fracture resistance, both the forgeability and the toughness under the neck were excellent. It can also be.

The high-strength wire rod according to the present invention suppresses the formation of one or more structures of proeutectoid ferrite, proeutectoid cementite, bainite and martensite as described above, and reduces the area ratio of the pearlite structure to 80% or more. It is necessary to When a large amount of pro-eutectoid ferrite and pro-eutectoid cementite is generated in the above structure, a longitudinal crack occurs at the time of wire drawing, and the wire cannot be drawn, and a strength of 1200 N / mm ² or more cannot be obtained by strong wire drawing. In addition, proeutectoid cementite and martensite cause disconnection during wire drawing, and thus need to be reduced. Further, bainite has a smaller amount of work hardening than pearlite, so that it is not possible to expect an increase in strength due to strong wire drawing.

On the other hand, the pearlite structure has the effect of trapping hydrogen at the interface between cementite and ferrite and reducing the amount of hydrogen accumulated at the grain boundaries, and needs to be as large as possible. Therefore, the formation of at least one type of microstructure such as proeutectoid ferrite, proeutectoid cementite, bainite, and martensite is suppressed (ie,
Less than 20%), and the area ratio of the pearlite structure is 80%.
It is necessary to do above. That is, at least one of the microstructures such as proeutectoid ferrite, proeutectoid cementite, bainite and martensite is reduced as much as possible, so that the total area ratio is less than 20% and the area ratio of the pearlite structure is 80% or more. There is a need to. The area ratio of the pearlite structure is preferably 90% or more, and more preferably 100% pearlite structure.

In the high-strength wire rod of the present invention, if the pearlite nodule size and pearlite lamellar interval of the pearlite structure satisfy the following conditions, the delayed fracture resistance is further improved and the drawability is also improved. preferable.

First, the pearlite nodule size is represented by the particle size number. It is desirable that the number be 7 or more. When the pearlite nodule size is reduced, the stress applied to the grain boundaries is reduced, and the grain boundary strength is increased. As a result, grain boundary destruction observed during delayed destruction is suppressed and delayed destruction resistance is improved. Further, by reducing the pearlite nodule size, ductility and toughness are improved, and from this viewpoint, delayed fracture resistance is also improved. The pearlite nodule size is No. in the particle size number. No. 8 or more, more preferably No. It is good to be 10 or more.

On the other hand, the pearlite lamellar interval of the pearlite structure is preferably 200 nm or less. Refinement of the pearlite lamellar spacing is effective in increasing the strength of steel, increasing the interface between cementite and ferrite,
Promotes the hydrogen trapping effect. In order to sufficiently exhibit these effects, the pearlite lamellar interval is set to 200 nm.
It is better to do the following. Further preferred pearlite lamellar spacing is 150 nm or less, more preferably 100 n.
m, and more preferably 75 nm or less.

In the high-strength wire rod of the present invention, necessary dimensional accuracy cannot be obtained as it is as rolled or forged, and it becomes difficult to obtain a strength of 1200 N / mm ² or more. Required. In addition, cementite in some pearlite is finely dispersed by strong wire drawing to improve the hydrogen trapping ability, and the structure is arranged along the wire drawing direction to reduce the crack propagation resistance (the crack propagation direction is Perpendicular to the drawing direction).

The high-strength wire of the present invention has a C of 0.50 to 0.50.
Medium carbon steel containing 1.0% is assumed, but the range limitation of the C content is as follows.

C: 0.50 to 1.0% C is a necessary and economical element for securing the strength of steel.
The strength increases as the C content increases. In order to secure the target strength, C must be contained at 0.50% or more. However, when the C content exceeds 1.0%, the precipitation amount of proeutectoid cementite increases, so that the toughness and ductility are significantly reduced and the wire drawing workability is deteriorated. A preferred lower limit of the C content is 0.65%, and more preferably 0.7%. The preferred upper limit of the C content is 0.1.
9%, more preferably 0.85%. Most preferably, eutectoid steel is used.

The high-strength wire rod of the present invention can exhibit excellent forgeability and toughness under the neck by reducing at least one of the Si content and the Al content in the steel material to less than a predetermined amount as described above. However, in order to exert these effects, it is necessary to reduce the content of at least one of Si and Al to the above range, but if at least one satisfies the above range, the other one The content may be increased to some extent. The reasons for limiting the ranges of the contents of Si and Al in consideration of these allowable amounts are as follows. As described above, both the forging property and the under-neck toughness can be improved by reducing the content of both Si and Al to less than a predetermined amount.

Si: 2.0% or less (including 0%) Si has the effect of improving the hardenability of the steel wire and suppressing the precipitation of proeutectoid cementite. In addition, it is expected to act as a deoxidizing agent, and exhibits a remarkable solid solution strengthening effect by being dissolved in ferrite. These effects increase as the content increases, but when the Si content is excessive, the ductility of the steel material after drawing is reduced, and the cold forging property is significantly reduced. And

However, as described above, in order to exhibit excellent forgeability, it is necessary to suppress the Si content to less than 0.1%. That is, as the content of Si increases, the cold workability tends to decrease, and a large amount of oxide-based inclusions are generated, which serves as a starting point of delayed fracture and deteriorates delayed fracture resistance. Also. From such a viewpoint, when exhibiting excellent forgeability together with delayed fracture resistance, Si
The content should be less than 0.1%. From such a viewpoint, a preferable range of the Si content is 0.05% or less, and a more preferable range is 0.03% or less.

Al: 0.05% or less (including 0%) Al captures N in steel to form AlN and refines crystal grains, thereby contributing to the improvement of delayed fracture resistance. However, if the Al content is excessive and exceeds 0.05%, nitrides and oxide-based inclusions are generated, and the drawability is reduced. Therefore, the Al content is preferably set to 0.05% or less.

However, as described above, in order to exhibit excellent under-neck toughness, it is necessary to suppress the Al content to less than 0.005%. That is, when an oxide is formed and processed into a headed bolt as the content of Al increases, the head is easily broken under the head and neck. The Al content when exhibiting lower toughness needs to be less than 0.005%. From such a viewpoint, a preferable range of the Al content is 0.003.
%, More preferably 0.001% or less.

The high-strength wire rod of the present invention can be prepared by adding various elements (Cr, Co, Mn, Cu, Ni, Mo, Ti,
Nb, V, W, B, N, etc.) may of course be contained, but in particular, containing a predetermined amount of Cr and / or Co is effective in suppressing the precipitation of proeutectoid cementite. It is. The reasons for limiting each element to be added as necessary are as follows.

Cr: 0.5% or less (excluding 0%)
And / or Co: 0.5% or less (excluding 0%) Cr and Co have the effect of suppressing the precipitation of pro-eutectoid cementite like Si, and the high strength of the present invention for reducing pro-eutectoid cementite It is particularly effective as an additive component in a wire. All of these effects increase as the content increases, but if the content exceeds 0.5%, the effect is saturated and uneconomical. Therefore, the upper limit is set to 0.5%. The preferred range of these elements is 0.05 to
0.3%, more preferably 0.1 to 0.2%.

Mn: 0.2 to 1.0% Mn has an effect as a deoxidizing agent and an effect of improving the hardenability of the steel wire and increasing the uniformity of the sectional area structure of the steel wire. These effects are effectively exhibited by containing 0.2% or more. However, when the amount of Mn becomes excessive, a supercooled structure such as martensite or bainite is formed in the segregated portion of Mn to deteriorate wire drawing workability. Therefore, the upper limit of the amount of Mn is set to 1.0%. Incidentally, a preferred lower limit of the Mn content is:
It is 0.40%, more preferably 0.45%. The preferable upper limit of the Mn content is 0.7%, and more preferably 0.55%.

Cu: 0.5% (excluding 0%) Cu is an element that contributes to increasing the strength of a steel wire by a precipitation hardening action. However, an excessive addition causes grain boundary embrittlement and deteriorates delayed fracture resistance.
Is the upper limit. The preferred lower limit of the Cu content is 0.1.
It is preferably 0.05%, more preferably 0.1%. The upper limit of the Cu content is preferably 0.3%, and more preferably 0.2%.

Ni: 1.0% or less (excluding 0%) Ni does not contribute much to the increase in the strength of the steel wire, but has the effect of increasing the toughness of the drawn wire. However, if the amount of Ni is excessive, the transformation end time becomes too long, resulting in an increase in the size of the equipment and a decrease in productivity. Therefore, the upper limit is 1.0%. The lower limit of the Ni content is preferably 0.05%, and more preferably 0.1%. Also Ni
A preferred upper limit of the content is 0.5%, and more preferably 0.3%.

Group consisting of Mo, Ti, Nb, V and W
At least one element selected from the group consisting of: 0.01 to 0.5% in total These elements all contribute to the improvement of delayed fracture resistance by forming fine carbon and nitride. These nitrides and carbides are effective for miniaturization of crystal grains. In order to exert such effects, it is necessary that the total content be 0.01% or more, but if it is excessively contained, the delayed fracture resistance and toughness are impaired. is there. Note that a preferable lower limit of the content of these elements is 0.02% in total, and more preferably 0.03%.
% Is good. A preferable upper limit is 0.3 in total.
%, More preferably 0.1%.

B: 0.0005% to 0.003% B is added for improving the hardenability of steel, but it is necessary to contain 0.0005% or more in order to exert its effect. However, if the content exceeds 0.003%, the toughness is rather hindered. Note that a preferable lower limit of the B content is 0.0010%, and a preferable upper limit is 0.0.
025%.

N: 0.015% (excluding 0%) N has a favorable effect on the refinement of crystal grains and the improvement in delayed fracture resistance due to the formation of nitrides of AlN and TiN. However, if it is contained excessively, not only does the nitride excessively increase, adversely affecting drawability, but also solute N promotes aging during drawing, so it must be 0.015% or less. . The upper limit of the N content is preferably 0.007%, and more preferably 0.005% or less.

The chemical composition of the high-strength wire of the present invention is as described above, and the balance is substantially composed of iron.
Here, “substantially iron” means that the high-strength wire rod of the present invention is F
In addition to e, it may contain trace components (permissible components) to the extent that its properties are not impaired. Examples of such permissible components include Ca, Zr, Pb, Bi, Te, As, S
Elements such as n and Sb are listed. From the viewpoint of further improving the characteristics, it is preferable to suppress the impurities of P, S, and O as described below.

P: 0.03% or less (including 0%) P is an element that causes grain boundary segregation and deteriorates delayed fracture resistance. Therefore, by setting the P content to 0.03% or less, delayed fracture resistance can be improved. The P content is preferably reduced to 0.015% or less, more preferably 0.01% or less, and still more preferably 0.005%.
It is better to do the following.

S: 0.03% (including 0%) S forms MnS in the steel, and when a stress is applied, the MnS becomes a stress concentration point. Therefore, in order to improve the delayed fracture resistance, it is necessary to reduce the S content as much as possible. From such a viewpoint, the S content is preferably set to 0.03% or less. The S content is preferably reduced to 0.015% or less, more preferably 0.01% or less, and further preferably 0.005% or less.

O: 0.005% or less (including 0%) O hardly forms a solid solution in steel at normal temperature, exists as hard oxide-based inclusions, and causes a disconnection of copper during wire drawing. Therefore, the O content should be as low as possible, and should be suppressed to at least 0.005% or less.
Note that the O content is preferably reduced to 0.003% or less, more preferably to 0.002% or less.

The high-strength wire rod of the present invention can be manufactured by each of the above-mentioned manufacturing methods, and the operation of each method is as follows. First, using a steel material having the above chemical composition, hot rolling or forging is performed so that the rolling or forging end temperature of the steel material is 800 ° C. or higher, and then the average cooling rate V (° C./sec) is reduced. Continuously cool to 400 ° C. so as to satisfy the following (1), and then cool to room temperature. 166 × (wire diameter: mm) ^-1.4 ≦ V ≦ 288 × (wire diameter: mm) ^-1.4 (1)

By this step, a more uniform pearlite structure can be obtained than in a normal rolled material, and the strength before drawing can be increased. If the end temperature of rolling or forging is too low, austenitization becomes insufficient and a homogeneous pearlite structure cannot be obtained, so the end temperature needs to be 800 ° C. or higher. The preferred range of this temperature is about 850 to 950 ° C, and more preferably 850 to 900 ° C.

The average cooling rate V is 166 × (wire diameter: m
m) If it is less than ^-1.4, not only a uniform pearlite structure cannot be obtained, but also proeutectoid ferrite or proeutectoid cementite tends to form. The average cooling rate V is 28
When it is larger than 8 × (wire diameter: mm) ^-1.4 , bainite and martensite are easily generated. The cooling end temperature when cooling at such an average cooling rate V is 400 ° C.
The reason is that the temperature is at the temperature at which the structural transformation is sufficiently completed.

The high-strength wire of the present invention uses a steel having the above-mentioned chemical composition. The steel is heated to 800 ° C. or higher, rapidly cooled, rapidly cooled to 500 to 650 ° C. By holding (patenting treatment), a more uniform pearlite structure can be obtained than a normal rolled material,
The strength before drawing can be increased.

In this method, the specified range of the steel material heating temperature must be 800 ° C. or higher for the same reason as the above-mentioned rolling or forging end temperature. The preferred range of the heating temperature is the same as described above. In the patenting process, it is desirable to use a salt bath, lead, a fluidized bed, or the like to rapidly cool the heated wire at a cooling rate as fast as possible. Further, in order to obtain a homogeneous pearlite structure, 500 to
It is necessary to carry out constant temperature transformation at 650 ° C. The preferable temperature range of the constant temperature is 550 to 600 ° C.
The most preferred constant temperature is a temperature near the pearlite nose in the TTT diagram.

On the other hand, when the rolling or forging end temperature of the steel material is 8
After hot rolling or hot forging so as to be at least 00 ° C,
By cooling to a temperature of 520 to 750 ° C. at an average cooling rate of 5 ° C./sec or more, holding at that temperature at an average cooling rate of 1.0 ° C./sec or less for 200 seconds or more, and then allowing to cool down, A more uniform pearlite structure can be obtained than the rolled material, and the strength before drawing can be increased. The operation in each step when such a method is adopted is as follows.

First, the specified range of the rolling or forging end temperature was set to 800 ° C. or higher for the same reason as the steel material heating temperature. The preferred range of the heating temperature is the same as described above. If the cooling rate after hot rolling or hot forging is too slow, ferrite transformation may occur during cooling, so it is preferable to cool at a cooling rate as high as possible. Therefore, the cooling rate at this time is specified to be 5 ° C./sec or more. The preferred range of the cooling rate is 10 ° C.
/ Sec or more, more preferably 30 ° C / sec or more. It is necessary to cool down to 520 to 750 ° C. by this cooling.
If the temperature exceeds 50 ° C., a structure other than pearlite is likely to be generated by subsequent slow cooling.

After the above cooling, the temperature (520 to 750 ° C.) is used from the viewpoint of obtaining a homogeneous pearlite structure.
(Temperature: slow cooling start temperature), it is necessary to hold for 200 seconds or more while cooling (slowly cooling) at an average cooling rate of 1.0 ° C./sec or less. At this time, if the average cooling rate is higher than 1.0 ° C./sec or the holding time is less than 200 seconds, the pearlite structure is left to cool before transformation, and bainite and martensite are easily generated. The preferred range of the cooling rate is 0.5 ° C./sec or less, and more preferably 0.2 ° C./sec or less. The preferable range of the holding temperature is 300 seconds or more, and more preferably, 600 seconds or more. It is most desirable to keep the temperature near the pearlite nose in the TTT diagram for a long time.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and any modification of the design based on the gist of the preceding and following examples will be described. Included in the technical scope.

[0054]

Example 1 A sample steel having a chemical composition shown in Table 1 below was hot-rolled to a wire diameter of 8 to 14 mm so that the rolling end temperature was about 940 ° C., and then an average cooling rate was obtained. 4.1 to 1
The blast cooling was performed in the range of 2.5 (Table 2 below). Thereafter, the wire was drawn to a wire diameter of 7.06 mmφ (drawing ratio: 22 to 75).
%).

[0055]

[Table 1]

Using the obtained various wire rods, M8 shown in FIG.
× P1.25 stud bolts were prepared and subjected to a delayed fracture test. In the delayed fracture test, the bolt was immersed in acid (1
(5% HCl × 30 minutes), washed and dried, subjected to a stress load in air (load stress is 90% of tensile strength), and evaluated by the presence or absence of breakage after 100 hours. In addition, pro-eutectoid ferrite, pro-eutectoid cementite, bainite, and martensite or pearlite structure were classified by the following method, and the area ratio of each tissue was obtained. Further, the pearlite nodule size and the pearlite lamellar interval were measured by the following methods. At this time, for the purpose of comparison, a delayed fracture test was also performed on a part of the steel which had been quenched and tempered to have a 100% tempered martensite structure.

Using the drawn wire rod, the compression characteristics were also evaluated at normal temperature by an end face constraint compression test.
At this time, the deformation resistance was compared with the value when the rolling reduction was 50%, and the deformability was the lowest rolling reduction that did not cause cracking.

(Classification method of each tissue) After embedding the cross section of the wire and polishing, 15 to 3%
After corrosion for 0 seconds, the structure of a D / 4 (D is a diameter) portion was observed with a scanning electron microscope (SEM). 1000-30
After photographing 5 to 10 visual fields at a magnification of 00 to determine the pearlite tissue portion, the area ratio of each tissue was determined by an image analyzer. As for the bainite structure and the proeutectoid ferrite structure which are hard to distinguish from the pearlite structure, the structure shown in FIG.
The structure shown in FIG. 3 (micrograph of a microstructure instead of a drawing) was determined to be a pro-eutectoid ferrite structure. As a tendency of these structures, pro-eutectoid ferrite and pro-eutectoid cementite were precipitated in the form of needles along the prior austenite crystal grain boundaries, and martensite was precipitated in a lump.

(Method of measuring perlite nodule size) After embedding the cross section of the wire, polishing and immersing it in a 1-2% nital solution for 2 to 10 seconds, a D / 4 (D is a diameter) portion was observed with an optical microscope. The tissue was observed. The particle size number of perlite nodule is JIS G0551 or JIS
It was defined in the same unit (grain number) as the austenite grain size or ferrite grain size of SG0552.

(Measurement method of pearlite lamellar interval) After embedding the cross section of the wire, polishing and immersing it in a 5% picric acid alcohol solution for 15 to 30 seconds to corrode it, the D was measured by a scanning electron microscope (SEM). / 4 (D is the diameter) was observed for its structure. In the pearlite structure near D / 4 part, the part considered to have the narrowest lamellar interval was 5,000 to 10
Ten visual fields were photographed at a magnification of 0000, and the length of a line perpendicular to each lamellar interval was determined to measure the lamellar interval.
The average value of 10 visual fields was defined as the average pearlite lamellar interval.

The structure of each wire is shown in Table 2 below together with the average cooling rate V, and the results of the delayed fracture test and the compression properties are shown in Table 3 below together with the drawing conditions and mechanical properties. The appropriate range of the average cooling rate V is [the range satisfying the above formula (1)].
Is 4.12 ≦ V ≦ 7.1 when the wire diameter is 14.0 mm.
6. (° C./sec), 5.78 ≦ when the wire diameter is 11.0 mm
V ≦ 10.03 (° C./second) and 9.03 ≦ V ≦ 15.67 (° C./second) when the wire diameter is 8.0 mm.

[0062]

[Table 2]

[0063]

[Table 3]

Example 2 The test steel C shown in Table 1 was used, and the wire diameter was 11.0 mmφ.
After hot-rolling until the rolling end temperature is about 940 ° C., the steel sheet is rapidly cooled and then subjected to a patenting treatment (heating temperature: 750 to 940 ° C., constant temperature transformation: 495 to 665 ° C.) shown in Table 4 below.
4 minutes). Thereafter, the wire was drawn to a wire diameter of 7.06 mmφ (drawing ratio: 59%).

[0065]

[Table 4]

Using the obtained various wires, M8 × P1.25 stud bolts shown in FIG. 1 were produced, and a delayed fracture test was performed in the same manner as in Example 1. Also, a compression characteristic test was performed in the same manner as in Example 1. The structure of each wire is shown in Table 4 above, and the results of the delayed fracture test and the compression properties are shown in Table 5 below together with the drawing conditions and mechanical properties.

[0067]

[Table 5]

Example 3 Using the test steel C shown in Table 1 above, hot rolling was performed to a wire diameter of 11 mmφ under the rolling conditions shown in Table 6 below. afterwards,
Wire diameter: drawn to 7.06 mmφ (drawing ratio: 59
%).

[0069]

[Table 6]

Using the obtained various wire rods, an M8 × P1.25 stud bolt shown in FIG. 1 was produced, and a delayed fracture test was performed in the same manner as in Example 1. Further, a compression characteristic test was performed in the same manner as in Example 1. The structure of each wire is shown in Table 7 below, and the delayed fracture test results and compression properties are shown in Table 8 below together with the drawing conditions and mechanical properties.

[0071]

[Table 7]

[0072]

[Table 8]

As is apparent from these results, when a bolt is manufactured using a high-strength wire material that satisfies the requirements of the steel of the present invention, the bolt can be obtained with excellent forgeability and can be obtained. It can be seen that the bolt has excellent delayed fracture characteristics even when the tensile strength is 1200 N / mm ² or more.

Example 4 A sample steel having a chemical composition shown in Table 9 below was hot-rolled to a wire diameter of 8 to 14 mm so that the rolling end temperature was about 940 ° C., and the average cooling rate was reduced. 4.0-1
The blast cooling was performed in the range of 2.6 (Table 2 below). Thereafter, the wire was drawn to a wire diameter of 7.06 mmφ (drawing ratio: 22 to 75).
%).

[0075]

[Table 9]

Using the obtained various wire rods, M8 × P1.25 stud bolts shown in FIG. 1 were produced, and a delayed fracture test was performed in the same manner as in Example 1. Further, classification of pro-eutectoid ferrite, pro-eutectoid cementite, bainite, martensite or pearlite structure was performed by the method described above, and the area ratio of each tissue was obtained. Further, the pearlite nodule size and the pearlite lamellar interval were measured by the methods described above. At this time, for the purpose of comparison, a delayed fracture test was also performed on a part of the steel which had been quenched and tempered to have a 100% tempered martensite structure.

Regarding the toughness under the neck, JIS B150
1 was evaluated by a "wedge tensile test". First, using the wire drawn above, a hexagonal headed bolt with a R part under the neck of 0.1 mm is prepared by cold heading, a wedge having an angle of 6 ° is inserted using the bolt, and wedge tension is applied. A test was performed to confirm the state of breakage at the lower part of the neck. And
The test was performed for each steel material, 100 pieces at a time, and the fracture probability of the lower part of the neck was obtained.

The structure of each wire is shown in Table 10 below together with the average cooling rate, and the results of the delayed fracture test and the wedge tensile test are shown in Table 11 below together with the drawing conditions and mechanical properties. The appropriate range of the average cooling rate V [the range that satisfies the expression (1)] is the same as the above.

[0079]

[Table 10]

[0080]

[Table 11]

Example 5 Using the test steel L shown in Table 9 above, a wire diameter of 11.0 mmφ
After hot rolling until the rolling end temperature becomes about 940 ° C., the steel sheet is rapidly cooled and then subjected to a patenting treatment (heating temperature: 750 to 940 ° C., constant temperature transformation: 495 to 665 ° C.) shown in Table 12 below.
× 4 minutes). Thereafter, the wire was drawn to a wire diameter of 7.06 mmφ (drawing ratio: 59%).

[0082]

[Table 12]

Using the obtained various wire rods, M8 × P1.25 stud bolts shown in FIG. 1 were produced, and a delayed fracture test was performed in the same manner as in Example 1. In addition, a wedge tensile test was performed in the same manner as in Example 4. Table 12 shows the structure of each wire rod, and the results of the delayed fracture test and the result of the wedge tensile test are shown in Table 13 below together with the drawing conditions and mechanical properties.

[0084]

[Table 13]

Example 6 The test steel L shown in Table 9 was hot-rolled to a wire diameter of 11 mmφ under the rolling conditions shown in Table 14 below. Thereafter, the wire was drawn to a wire diameter of 7.06 mmφ (drawing ratio: 59).
%).

[0086]

[Table 14]

Using the obtained various wire rods, M8 × P1.25 stud bolts shown in FIG. 1 were produced, and a delayed fracture test was performed in the same manner as in Example 1. A wedge tensile test was performed in the same manner as in Example 4. The structure of each wire is shown in Table 15 below, and the results of the delayed fracture test and the wedge tensile test are shown in Table 16 below together with the drawing conditions and mechanical properties.

[0088]

[Table 15]

[0089]

[Table 16]

As is clear from these results, the bolts manufactured using the high-strength wires satisfying the requirements of the steel of the present invention have excellent delayed fracture characteristics even if the tensile strength is 1200 N / mm ² or more. It can be seen that they have excellent toughness under the neck.

[0091]

According to the present invention, there is provided a high-strength wire rod having a tensile strength of not less than 1200 N / mm ² and excellent forging property and toughness under the neck as well as delayed fracture resistance.
And a useful method for producing such a high-strength wire rod has been realized.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view showing a shape of a bolt subjected to a delayed fracture test in an example.

FIG. 2 is a drawing-substituting micrograph showing a bainite structure.

FIG. 3 is a drawing-substituting micrograph showing a proeutectoid ferrite structure.

Continued on the front page F term (reference) 4K032 AA01 AA05 AA06 AA09 AA11 AA16 AA21 AA26 AA27 AA29 AA31 BA02 CC03 CC04 CD03 4K043 AA02 AB01 AB04 AB05 AB15 AB20 AB25 AB26 AB27 BA03 BA04 BB04 BB06 BB07

Claims (7)

[Claims] 1. A steel containing C: 0.50 to 1.0% (mean% by mass, the same applies hereinafter) and having a Si content of less than 0.1%, comprising proeutectoid ferrite and proeutectoid cementite. Or one of bainite and martensite
The formation of more than one kind of structure is suppressed so that the area ratio of the pearlite structure is 80% or more, and the strength of 1200 N / mm ² or more and excellent delayed fracture resistance are obtained by strong drawing. High strength wire with excellent delayed fracture resistance and excellent forgeability. 2. A steel containing C: 0.50 to 1.0% and Al: less than 0.005%, and is one of proeutectoid ferrite, proeutectoid cementite, bainite and martensite or The formation of two or more types of structures is suppressed so that the area ratio of the pearlite structure is 80% or more, and the strength is 1200 N / mm ² or more and excellent delayed fracture resistance by strong wire drawing. A high-strength wire excellent in delayed fracture resistance and under-neck toughness characterized by having been made. 3. A steel containing C: 0.50 to 1.0% and containing Si: less than 0.1% and Al: less than 0.005%, respectively, comprising proeutectoid ferrite and proeutectoid cementite. , An area ratio of a pearlite structure of 80% or more by suppressing the formation of one or more structures of bainite and martensite, and an excellent strength of 1200 N / mm ² or more by strong wire drawing. A high-strength wire excellent in delayed fracture resistance, forgeability and toughness under the neck characterized by having delayed fracture resistance. 4. Cr: 0.5% or less (excluding 0%)
And / or Co: 0.5% or less (excluding 0%)
The high-strength wire according to any one of claims 1 to 3, which comprises: 5. The method for producing a high-strength wire according to claim 1, wherein hot rolling or forging is performed so that the end temperature of rolling or forging of the steel is 800 ° C. or higher, and then the average is obtained. By continuously cooling to 400 ° C. so that the cooling rate V (° C./second) satisfies the following (1), and then allowing to cool to room temperature, one or more of proeutectoid ferrite, bainite and martensite The formation rate of pearlite structure is suppressed to 80% or more by suppressing the formation of the structure, and then the strength is increased to 1200 N / mm ² or more by strong wire drawing.
Or a method of manufacturing a high-strength wire rod having excellent under-neck toughness. 166 × (wire diameter: mm) ^-1.4 ≦ V ≦ 288 × (wire diameter: mm) ^-1.4 (1) 6. High strength according to claim 1.
Heats steel to 800 ° C or more when manufacturing wire
After that, it is rapidly cooled to a room temperature of 500 to 650 ° C.
By keeping the temperature, proeutectoid ferrite and proeutectoid cementer
One or two of the following: lite, bainite and martensite
Suppresses the generation of more than one kind of structure and the area ratio of pearlite structure
80% or more, and then 1200% by strong wire drawing.
N / mm ^TwoDelay resistance characterized by having the above strength
High strength wire with excellent breakability, forgeability and / or toughness under the neck
The method of manufacturing the material. 7. In producing the high-strength wire according to any one of claims 1 to 4, after hot rolling or hot forging such that the end temperature of rolling or forging of the steel is 800 ° C. or more, 520-750 ° C at an average cooling rate of at least ° C / sec
At a cooling rate of 1.0 ° C./sec or less over 200 seconds, and then allowed to cool to obtain one of proeutectoid ferrite, proeutectoid cementite, bainite and martensite. Alternatively, the area ratio of the pearlite structure is reduced to 80% or more by suppressing the formation of two or more types of structures, and thereafter, 1200 N / mm ² is formed by strong drawing.
A method for producing a high-strength wire excellent in delayed fracture resistance, forgeability and / or toughness under the neck characterized by having the above strength.