JPH10280036A - Wire rod for high strength bolt excellent in strength and ductility and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wire rod for a high strength bolt having >=10 T class tensile strength (>=980 N/mm<2> ), further having excellent ductility so as to regulate the reduction of area to >=60% and excellent in a balance of strength- ductility. SOLUTION: A steel contg., by mass, 0.10 to 0.30% C, <=0.80% (including zero) Si, 0.80 to 2.20% Mn and 0.50 to 1.50% Cr or further contg. one or more kinds among <=0.20% (including zero) Mo, <=0.10% (including zero) Ti, <=0.10% (including zero) Nb and 0.0050% (including zero) B, and the balance Fe with inevitable impurities is heated to the Ac3 transformation point or above, is rapidly cooled to 350 to 450 deg.C, is isothermally transformed at this temp., is cooled and is subsequently subjected to cold wire drawing at 10 to 30% working ratio.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a 10T class (tensile strength of 980-11
The present invention relates to a high-strength bolt wire having excellent strength and ductility capable of obtaining a high-strength bolt having a strength of 80 N / mm ² : 100 to 120 kgf / mm ² ) and a method for producing the same.

[0002]

2. Description of the Related Art Recently, in order to reduce processing costs and simplify manufacturing costs, there has been a trend to omit spheroidizing annealing performed before bolt forming and quenching and tempering performed after bolt forming. Yes, 8T class (tensile strength 78
5 to 980 N / mm ² : 80 to 100 kgf / mm ² )
For bolts up to now, non-tempering has been considerably advanced in order to make such a heat treatment omissible.

However, the 9T class (having a tensile strength of 88
0 to 1080 N / mm ² : 90 to 110 kgf / mm ²
Non-heat-treated bolts having a strength class higher than or equal to) have insufficient toughness and ductility compared to heat-treated bolts, and there are still few examples of practical use. There are already known methods for producing high-strength bolts excellent in strength-ductility balance which solve such problems (Japanese Patent Publication Nos. 60-406 and 64-7136). Even in the case of using these methods, if the strength becomes 10T class or more, it is difficult to ensure high ductility because ductility decreases. Due to the reduced ductility, the workability of the bolt becomes extremely poor,
Since the tool life is shortened due to high strength, conventional tempered steel is currently used.

[0004] However, recently, in order to meet the demand for reduction in processing cost and simplification of the manufacturing process, bolt forming machines and forming tools have been increased in rigidity, and it has become possible to process high-strength materials. . However, since high-strength materials generally have poor ductility, it is difficult to process them into bolts. From such a background, a wire rod for a high-strength bolt excellent in strength-ductility balance is desired.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object a 10T.
An object of the present invention is to provide a high-strength bolt wire having a tensile strength equal to or higher than a class (980 N / mm ² or more) and having an excellent ductility in which the drawing is 60% or more and having an excellent balance of strength and ductility.

[0006]

The method for producing a high-strength bolt wire excellent in strength and ductility according to the present invention, which can solve the above-mentioned problems, is described as follows. % By mass "), and C: 0.10 to 0.1%.
30%, Si: 0.80% or less (including 0%), Mn:
0.80 to 2.20%, Cr: 0.50 to 1.50%, or Mo: 0.20% or less (0%
), Ti: 0.10% or less (including 0%), N
b: 0.10% or less (including 0%), B: 0.0050
% (Including 0%), steel containing one or more kinds and the balance consisting of Fe and unavoidable impurity elements is heated above the A _C3 transformation point and then rapidly cooled to a temperature of 350 to 450 ° C. Then, after being transformed at a constant temperature at this temperature and cooled, 10 to 30%
Cold drawing at a working ratio of.

According to the above method of the present invention, 980 N / m
It is possible to obtain a high-strength non-refined bolt wire having a high strength of at least m ² (100 kgf / mm ² ) and an excellent balance between strength and ductility, such as a reduction of 60% or more.

[0008] The isothermal transformation (without M _S transformation temperature.) Temperature range between M _S transformation temperature and M _S transformation temperature + 50 ℃, the base - is preferably performed Knight transformation. By performing the isothermal transformation in this temperature range, a finer bainite structure can be obtained, and a wire for a high-strength non-refined bolt with further excellent strength-ductility balance can be obtained. The M _S transformation point may be used a value determined by the measurement, but the following equation (metal Handbook, Japan Institute of Metals, ed., 1971 issue, P725 reference) may be a value calculated in. M _S transformation point (℃) = 538-317 × C content (%) - 33
X Mn amount (%)-28 x Cr amount (%)-17 x Ni amount (%)-11 x (Si amount (%) + Mo amount (%) + W amount (%))

The high-strength bolt wire having excellent strength and ductility according to the present invention has C: 0.10 to 0.30%,
i: 0.80% or less (including 0%), Mn: 0.80
2.20%, Cr: 0.50 to 1.50%, or Mo: 0.20% or less (including 0%);
Ti: 0.10% or less (including 0%), Nb: 0.10
% Or less (including 0%), B: 0.0050% (including 0%), steel containing one or two or more kinds, the balance being Fe and unavoidable impurity elements, and having a structure of bainite. And a tensile strength of 980 N / mm ² or more and a drawing of 60% or more.

The high-strength bolt wire having excellent strength and ductility according to the present invention can be subjected to spheroidizing annealing before bolt forming, quenching and tempering after forming, which is required when conventional tempered steel is used. And the like can be omitted, and it is possible to meet the demand for reduction of the processing cost and simplification of the manufacturing process.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have conducted various studies on the causes of poor strength-ductility balance of conventional high-strength non-heat-treated bolt wires. As a result, based on the basic idea of reducing the C content in steel to improve ductility, the types of other alloying elements and their contents are strictly specified, and the low-temperature range immediately above the Ms transformation point temperature is determined. (350-450
° C) to obtain a bainite structure, and further cold-working at a wire drawing rate of 10 to 30% to obtain a high-strength bolt with excellent strength-ductility balance. Was completed.

First, the reason why the composition of the steel material is determined in the present invention will be described. C: 0.10 to 0.30% C gives a given strength to the steel material by solid solution strengthening. Further, it is an essential element indispensable for obtaining a bainite structure. However, when the amount of C increases,
It is essential to reduce the ductility to 0.30% or less because the ductility is reduced and the ductility, which is the object of the present invention, cannot be improved. In addition, the workability deteriorates as the C content increases. On the other hand, the tensile strength was 980 N / mm ² (100 kgf / mm
² ) In order to secure the above, a C content of at least 0.10% or more is required. Therefore, from the viewpoint of strength, ductility, and workability, the C content is 0.10 to 0.30%,
A more preferred C content is in the range of 0.15 to 0.25%.

Si: 0.80% or less Si is useful as a ferrite strengthening element and a deoxidizing element. However, depending on the content of other elements suitable for ferrite strengthening and deoxidizing, it is permissible to contain no Si. Can be done. However, if the amount of Si is too large, the toughness is adversely affected. The more preferred content of Si is in the range of 0.10 to 0.50%.

Mn: 0.80 to 2.20% Mn is an element having a deoxidizing and desulfurizing effect. It also improves the hardenability and contributes to a higher strength, and further reduces the A _C3 transformation point to reduce the structure. And contribute to improvement in toughness. In order to exhibit these effects effectively, it is necessary to contain 0.80% or more. However, addition of 2.20% or more adversely affects workability. Further, when the content of Mn is in the range of 0.80 to 2.20%, compared with the case where other elements, for example, C, Si or the like are contained and strengthened,
The decrease in toughness and ductility due to the inclusion of Mn is small.

Cr: 0.50 to 1.50% Cr is a promising element that improves hardenability, contributes to high strength, and can obtain excellent toughness and ductility while securing high strength. is there. The effect is effectively exhibited by containing 0.50% or more, but even if the content is too large, the effect needs to be 1.50% or less in consideration of saturation and deterioration of toughness. .

The essential elements of the steel material used in the present invention are as described above, and the balance is iron and inevitable impurities. However, if necessary, the following elements are included so that the properties as a wire for high-strength non-refined bolts can be obtained. It is also effective to further improve. Mo: 0.20% or less Mo is an element that enhances the hardenability and contributes to high strength. To achieve this effect, the content of Mo is 0.03%.
If the content is 0.20% or more, the effect is saturated and even if the content of Mo is increased, no further improvement effect can be obtained, so that it is economically wasteful. The preferred content of Mo is in the range of 0.03 to 0.20%.

Ti: 0.10% or less Ti fixes solid solution nitrogen at the time of austenitization,
By suppressing the generation of N, the effect of improving the hardenability by adding B can be obtained. Further, Ti is an element that generates carbides and nitrides, refines austenite crystal grains, and contributes to improvement in toughness and ductility. In order to exhibit this effect effectively, it is preferable to contain 0.03% or more.
However, even if the content exceeds 0.10%, the effect is saturated and the improvement effect cannot be obtained. The preferred content of Ti is 0.0
The range is 3 to 0.10%.

Nb: 0.10% or less Nb, like Ti, is an element effective for forming carbides and nitrides to refine austenite crystal grains, and also improves hardenability. In order to sufficiently exhibit such effects, the content must be at least 0.01%. However, even if the content exceeds 0.10%, the effect is saturated and the improvement effect cannot be obtained. The preferable content of Nb is in the range of 0.01 to 0.10%.

B: 0.0050% or less B is a promising element that, when added in a small amount, greatly improves hardenability and hardly deteriorates cold forgeability (deformation resistance and deformability). In order to sufficiently exhibit such effects, at least 0.0005% must be contained. However, when the content of B exceeds 0.0050%, the effect is only saturated. The preferable content of B is in the range of 0.0005 to 0.0050%.

Next, the reason why the heat treatment conditions for the steel material are determined in the present invention will be described. The heat treatment conditions for the steel material of the present invention are as follows:
After heating the steel material above the A _C3 transformation point, 350-450 ° C
Rapidly cooled to the temperature range of
After the steel material has a bainite structure, it is cooled.

The cooling rate at this time depends on the CCT of these steel materials.
It requires a cooling rate that does not cut the nose of the pearlite transformation start line of the curve. If the pearlite transformation occurs during cooling, the steel material has a mixed structure of pearlite and bainite, and the generated bainite structure is coarsened, so that predetermined toughness and ductility cannot be obtained. Therefore, the cooling rate is A _C3
5 ° C / sec between transformation point temperature and constant temperature transformation temperature
It is necessary to cool above. The cooling rate of the present invention is usually used in the range of 5 to 20 ° C./sec.

The constant-temperature transformation starts at the temperature immediately above the M _S transformation point.
_{The bainite} transformation is preferably performed in a temperature range between the _S transformation point and 50 ° C. By performing the isothermal transformation in this temperature range, a finer bainite structure (lower bainite structure) can be obtained, and a wire for a high-strength non-refined bolt having a further excellent strength-ductility balance can be obtained. . As the M _S transformation point, a value obtained by measurement may be used, but in the embodiment of the present invention, a value calculated by the following equation is used. M _S transformation point (℃) = 538-317 × C content (%) - 33
X Mn amount (%)-28 x Cr amount (%)-17 x Ni amount (%)-11 x (Si amount (%) + Mo amount (%) + W amount (%))

FIGS. 1 and 2 show an embodiment described later. The result which arranged the influence of isothermal transformation temperature on ductility and tensile strength is shown. FIG. 1 is a diagram showing the relationship between tensile strength and isothermal transformation temperature, and FIG. 2 is a diagram showing the relationship between drawing and isothermal transformation temperature. The constant temperature transformation temperature is indicated by the temperature difference from the above-calculated _MS point. These test materials are steel materials having the chemical components as claimed in the present invention. After cooling these test materials from the A _C3 transformation temperature to a temperature of 380 to 550 ° C. at a cooling rate of 10 ° C./sec. , At these temperatures. Thereafter, it was cooled and cold worked at a wire drawing rate of 20%. In addition, 20% of the wire drawing rate falls within the range of the optimum wire drawing rate described later.

As shown in FIG. 1, although the tensile strength varies greatly, as a whole, as the temperature difference from the Ms transformation point temperature increases, that is, as the isothermal transformation temperature increases, the tensile strength decreases. A tendency is observed. All the test materials having a constant temperature transformation temperature of 350 to 450 ° C. of the present invention (indicated by white circles and white triangles in the figure) show high strength of tensile strength of 980 N / mm ² or more, whereas 450 ° C. Specimens (black circles and black triangles in the figure) with a tensile strength of 980 N / mm
There are also those that do not reach to ^two or more. The white ○ and black ○ marks are
Steel that satisfies the chemical composition of claim 1 of the present invention, and white and black symbols are steel that satisfies the chemical composition of claim 2 of the present invention.

Next, the aperture will be described with reference to FIG.
As shown in FIG. 2, in a temperature range between M _S transformation temperature and M _S transformation temperature + 50 ℃ of the present invention, the diaphragm is to indicate more than 60%, M _S transformation temperature + 60 ℃ or higher At this temperature, the aperture becomes 50% or less. This M _S transformation temperature +
Between 50 ° C. and M _S transformation temperature + 60 ° C., it was found that the diaphragm is rapidly decreased. The reason for this is that the specimens (white circles and white triangles in the figure) which were subjected to constant temperature transformation in the temperature range between the M _S transformation point temperature and the M _S transformation point temperature + 50 ° C. had a temperature just above the Ms transformation temperature. At 350 ° C. to 350 ° C., and the structure of these test materials is a finer lower bainite structure, which is considered to have exhibited excellent ductility.

In the present invention, high ductility can be obtained by making the steel material have a finer bainite structure (lower bainite structure) by making the isothermal transformation of bainite as low as possible. . on the other hand,
When the constant temperature transformation temperature is lower than the Ms transformation point temperature, martensite is formed, and the ductility of the steel material is reduced. Therefore, the constant temperature transformation temperature does not include the Ms transformation point temperature.

In the present invention, the Ms transformation point temperature is obtained by calculation. However, the transformation point temperature previously measured by a formastar or the like can be used. Since the Ms transformation point temperature also changes depending on the austenitizing conditions and the cooling rate, the measured Ms under these austenitizing conditions and the cooling rate.
When the transformation point temperature is used, the bainite isothermal transformation can be controlled more accurately.

Further, the reason why the working conditions for the cold drawing of a steel material are determined in the present invention will be explained. The processing conditions for cold drawing of the steel material of the present invention are to perform cold drawing at a working ratio of 10 to 30%. The test material No. of Table 1 of the example of the present invention.
, 16, 17 as shown in FIG.
% As a result of performing cold working, the wire drawing rate 20
%, It was confirmed that a high-strength bolt excellent in strength-ductility balance with a reduction of 60% or more can be obtained by performing cold working at%. Note that, in Table 1, No. The circles in the column indicate test materials having the chemical components of the present invention. Table 1
In the description of No., o is omitted (for example, No. is No. 1).
And ).

According to the present invention, the isothermal transformation temperature for converting a steel material having the claimed chemical composition into a bainite structure is a temperature that does not become lower than the Ms transformation point temperature, and uses a temperature as low as possible. By setting the cooling rate from the _C3 transformation point temperature to the isothermal transformation temperature to 5 ° C./sec or more, the tensile strength is 980 Mpa or more and the drawing is 60% or more. A wire for use is obtained.

[0030]

EXAMPLES Next, examples of the present invention will be described. However, the present invention is not limited to the following examples, and the present invention may be practiced with appropriate modifications within a range that can be adapted to the purpose described above. Of course, they are possible, and all of them are included in the technical scope of the present invention.

EXAMPLE After steel having the chemical composition shown in Table 1 was melted, 15 steels were forged by forging.
Each billet of 5 mm was manufactured and a wire rod having a desired diameter was manufactured by hot rolling. After reheating the wire after hot rolling at 965 ° C., 350-55 mm to change the isothermal transformation temperature.
These were quenched to these temperatures (350-550 ° C) by immersion in a lead bath furnace maintained at the desired temperature of 0 ° C. The average cooling rate at this time was 10 ° C./sec. The wire rod was kept in the lead bath furnace for 20 minutes to perform a constant temperature transformation treatment of the wire. Subsequently, wire drawing was performed at a processing rate of 5 to 40% to produce a wire having a diameter of 7 mm, and these were used as test materials.
About the obtained test material, the tensile strength and the drawing (both are J
IS Z 2241). Table 1 shows the tensile strength and drawing of the obtained test materials. In addition, No. 1 to
No. 24 is an invention steel having a chemical composition according to the present invention.
Nos. 1 to 15 are examples of the present invention. Nos. 16 to 24 are comparative examples in which heat treatment conditions or cold drawing conditions are out of the range of the present invention. In addition, No. Reference numerals 25 to 32 denote wires whose chemical components according to the present invention are not specified.

[0032]

[Table 1]

The steel rod of the present invention manufactured by the method of the present invention was manufactured by the method of No. As shown in 15, a tensile strength of ^{1000~1400N / mm 2 (980N / mm} 2
Above), and a diaphragm with an aperture of 60% or more was obtained. Even if the steel of the present invention is not according to the present invention, an excellent strength-ductility balance cannot be obtained, and a steel having a high tensile strength has a low drawing, while a steel having a high drawing has a low tensile strength. there were. In addition, even if the comparative steel was manufactured by the method of the present invention, if it was out of the range of the chemical composition, the predetermined characteristics could not be obtained.

Regarding the influence of the wire drawing rate, the test material No.
This will be further described with reference to 1, 16, and 17. The chemical components and heat treatment conditions of these test materials are almost the same. For the test material (No. 16) with a wire drawing rate of 5%, the drawing was 66%.
, The effect of work hardening was small, and the tensile strength was 903 N / mm ² . On the other hand, the test material (No. 17) having a wire drawing rate of 40% had a tensile strength of 1069.
Although a high value of N / mm ² was obtained, the aperture was as low as 46%. The test material of the present invention having a wire drawing rate of 20% (N
o. 1) has a tensile strength of 1118 N / mm and a drawing of 6
8%, and a high-strength bolt excellent in strength-ductility balance can be obtained.

Sample No. Reference numerals 18 to 24 denote chemical components according to the present invention, and cold drawing conditions are also within the scope of the present invention. However, since the heat treatment conditions are out of the range of the present invention, none of the test materials has a reduction of 60%, and some have high tensile strength, while others have low tensile strength. ,
The variation in tensile strength was large. Therefore, the object of the present invention, tensile strength of 10T class or more (980 N /
mm ² or more) has, and the diaphragm could not obtain a high strength bolt wire becomes 60% or more.

The test material No. 25 to 32 are wires for high-strength bolts outside the specified range of chemical components according to the present invention;
None of them satisfied both the tensile strength of 980 N / mm ² or more and the drawing of 60% or more. Higher tensile strength results in lower draw, and higher draw results in lower tensile strength.

Sample No. 1 and the test material No. 25 and 26 will be further described. These test materials are
Only the C content is different, and other chemical components, heat treatment conditions and cold drawing conditions are almost the same, and are within the scope of the present invention. The specimen (No. 25) having a low C content of 0.05% had a drawing of 62.
% Excellent ductility was obtained, but the tensile strength was 852 N / m.
m ² . On the other hand, the specimen (No. 26) having a high C content of 0.33% had a tensile strength of 1234 N / mm.
^Although it increased to ² , the aperture was as low as 39%.
The test material (No. 1) of the present invention has a tensile strength of 1118N.
/ Mm, the drawing was 68%, and a high-strength bolt excellent in strength-ductility balance was obtained.

[0038]

According to the present invention, the composition of the steel material is specified, and its structure is made bainite by heat treatment.
By giving work hardening by proper wire drawing, the tensile strength is 980 N / m higher than that of conventional non-heat treated bolt steel wire.
m ² or more, the diaphragm 60% strength - became the ductility balance may provide excellent high strength bolt wire. With the recent increase in rigidity of bolt forming machines and improvement of forming tools by using cemented carbide, it has become possible to process high-strength materials. With the emergence of high-strength bolt wire with excellent ductility,
In the production of not only 10T class bolts but also 12T class bolts, spheroidizing annealing before forming the bolts and quenching and tempering after forming the bolts can be omitted, and a large cost reduction can be achieved.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between tensile strength and isothermal transformation temperature.

FIG. 2 is a diagram illustrating a relationship between a throttle and a constant temperature transformation temperature.

Claims (6)

[Claims] 1. A mass% of C: 0.10 to 0.30%,
Si: 0.80% or less (including 0%), Mn: 0.80
２2.20%, Cr: 0.50 to 1.50%,
After heating the steel consisting of Fe and unavoidable impurity elements to the A _C3 transformation point or higher, the steel is rapidly cooled to a temperature of 350 to 450 ° C., transformed at a constant temperature at this temperature, and cooled.
A method for producing a high-strength bolt wire excellent in strength and ductility, wherein cold drawing is performed at a working ratio of up to 30%. 2. C: 0.10 to 0.30% by mass%
Si: 0.80% or less (including 0%), Mn: 0.80
Mo: 0.20% or less (including 0%), Ti: 0.10
% Or less (including 0%), Nb: 0.10% or less (including 0%), B: 0.0050% (including 0%), one or more of the following, with the balance being Fe After heating the steel comprising the unavoidable impurity elements to a temperature above the A _C3 transformation point, the steel is rapidly cooled to a temperature of 350 to 450 ° C., transformed at a constant temperature at this temperature, and cooled.
A method for producing a high-strength bolt wire excellent in strength and ductility, wherein cold drawing is performed at a working ratio of up to 30%. 3. A temperature range (M _S transformation point temperature between M _S transformation point temperature and M _S transformation point temperature + 50 ° C.)
Does not include _S transformation point temperature. 3. The method for producing a high-strength bolt wire according to claim 1 or 2, which has excellent strength and ductility. 4. The method for producing a high-strength bolt wire according to claim 1, wherein the tensile strength is 980 N / mm ² or more and the drawing is 60% or more. 5. The steel composition has a C content of 0.10 to 0.1% by mass.
0.30%, Si: 0.80% or less (including 0%), M
n: 0.80 to 2.20%, Cr: 0.50 to 1.50
%, The balance consists of Fe and unavoidable impurity elements, the structure consists of bainite, and the tensile strength is 980 N / mm
A high-strength bolt wire excellent in strength and ductility, characterized in that it has a drawing of at least ² and a drawing of at least 60%. 6. The steel composition has a C content of 0.10 to 0.1% by mass.
0.30%, Si: 0.80% or less (including 0%), M
n: 0.80 to 2.20%, Cr: 0.50 to 1.50
Mo: 0.20% or less (including 0%), Ti: 0.10
% Or less (including 0%), Nb: 0.10% or less (including 0%), B: 0.0050% (including 0%), one or more of the following, with the balance being Fe And unavoidable impurity elements, the structure is made of bainite, and the tensile strength is 980 N / mm
A high-strength bolt wire excellent in strength and ductility, characterized in that it has a drawing of at least ² and a drawing of at least 60%.