JPH10196627A - Screw without head crack and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a screw which shows excellent cold pressing performance and does not cause head crack, and its manufacture. SOLUTION: A screw without head crack contains C: 0.05 to 0.15wt.%, Si: 0.20wt.% or less, Mn: 0.4 to 1.8wt.%, P: 0.015wt.% or less, S: 0.015wt.% or less, sol.Al: 0.020 to 0.080wt.%, N: 0.0060wt.% or less, the remainder iron and inevitable impurity. In addition, chemical component composition satisfies the carbon equivalent indicated by the formula: Ceq =C+Si/7+Mn/5<=0.40wt.%. Its Vicker's hardness of the surface is 550 or more, and Vicker's hardness of its core is 320 to 400. In addition, the screw contains also at least one component selected from Cr: 0.80wt.% or less, Mo: 0.30wt.% or less and B: 0.0005 to 0.0050wt.%, and/or at least one selected from Ti: 0.005 to 0.50wt.% and Nb: 0.005 to 0.50wt.%. The carbon equivalent is preferably indicated by the formula: Ceq =C+Si/7+Mn/5+Cr/9+Mo/6<=0.40wt.%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a screw which is excellent in cold forgeability and which does not cause a head jump during tightening use, and a method of manufacturing the same.

[0002]

2. Description of the Related Art Conventionally, drilling tapping screws of JIS B 1125 are used for carbon steel wires for cold heading (JIS).
G 3507) using SWRCH18A or 22A,
Threading, carburizing (carbonitriding) quenching, tempering, plating, and the like are sequentially performed on the drawn steel wire manufactured by the wire drawing, soft annealing, and finish wire drawing processes, or the softened annealing wire and the finish wire drawing process of the above wire rod. It has been manufactured by a process of performing each process of baking. Conventionally, the reason why SWRCH18A or 22A is used for a drilling tapping screw is that since it is an aluminum-killed steel, it has excellent softening and annealing properties, and has excellent cold workability due to its softness and high ductility. That is, since the deformation resistance is small, the service life of the tool is long, and the deformability is large, so that it is easy to secure the accuracy of the shape of the screw tip. In addition, the reason for the above is that since the C and Mn contents are relatively high, the carburizing curability is excellent.

[0003]

In recent years, steel houses have been manufactured in Japan. However, when a steel plate is tightened using a conventional drilling tapping screw to a drilling tapping screw used for construction of a steel house, a head jump phenomenon in which the head of the screw breaks frequently occurs in one to two days after tightening, Is in question.
This is because, in comparison with the generally known phenomenon in which delayed fracture occurs several years after bolting, it occurs in a very short time, and must be considered separately from delayed fracture. In other words, the delayed fracture of the bolt means that after the bolt is tightened, corrosion advances and hydrogen enters, and the steel causes hydrogen embrittlement and breaks, whereas in the case of drilling tapping screws, corrosion does not progress and hydrogen I can't imagine it. In addition, it is conceivable that hydrogen enters by plating, but in this case, since baking is performed at about 300 ° C. to remove hydrogen, diffusible hydrogen causing delayed fracture remains in the screw. I can't imagine that.

[0004] The head jump phenomenon of the drilling tapping screw as described above is an important problem relating to the safety of the structure of the steel house itself, and it is difficult to spread the steel house unless the head jump is prevented.

An object of the present invention is to clarify the cause of a head jump of a screw such as the above-mentioned drilling tapping screw and to provide a screw which does not cause a head jump and a manufacturing method thereof.

[0006]

Means for Solving the Problems The present inventor has conducted intensive studies on the head jump phenomenon of the drilling tapping screw. As a result, it has been found that by suppressing the growth of the crack rather than the generation of the crack which is the starting point of the head jump, the head jump of the drilling tapping screw can be prevented, and the present invention has been completed. That is, a carburized hardened layer having a thickness of about 0.1 mm is formed on the surface layer of the screw, but the screw after tightening has a small crack inside the carburized hardened layer. A screw head jumps out and breaks along the boundary. Suppressing the inside of the crack can prevent head jump. In order to suppress this crack growth, the internal hardness of the non-carburized portion of the screw must be reduced and sufficient toughness must be ensured. For this purpose, lower the C content of the screw.
In order to suppress the generation of incomplete quenched structure such as bainite and obtain a fine and complete martensitic structure, it is crucial to ensure sufficient hardenability of steel, and then it is important to strengthen the grain boundaries. Found to be effective.

[0007] The present invention has been made based on the above-mentioned findings, and the screw according to claim 1 has a C: 0.05 to
0.15 wt.%, Si: 0.20 wt.% Or less, Mn: 0.4
~ 1.8 wt.%, P: 0.015 wt.% Or less, S: 0.01
5 wt.% Or less, sol. Al: 0.020 to 0.080 wt.%,
And N: not more than 0.0060 wt.%, The balance being iron and unavoidable impurities, and carbon equivalent: C _eq = C +
It has a chemical composition in which Si / 7 + Mn / 5 is 0.40 wt.% Or less, and has a Vickers hardness HV of 55
0 or more and Vickers hardness HV of the core is 320 to 40
It is characterized by being within the range of 0.

[0008] The screw according to the second aspect, C: 0.05 ~
0.15 wt.%, Si: 0.20 wt.% Or less, Mn: 0.4
~ 1.8 wt.%, P: 0.015 wt.% Or less, S: 0.01
5 wt.% Or less, sol. Al: 0.020 to 0.080 wt.%,
And N: not more than 0.0060 wt.%,
r: 0.80 wt.% or less, Mo: 0.30 wt.% or less, and B: at least one selected from the group consisting of 0.0005 to 0.0050 wt.%, and / or Ti:
0.005 to 0.050 wt.% And Nb: 0.005
At least one selected from the group consisting of
, And the balance is composed of iron and unavoidable impurities, and carbon equivalent: C _eq = C + Si / 7 + Mn / 5
+ Cr / 9 + Mo / 6 has a chemical composition of 0.40 wt.% Or less, and the surface has a Vickers hardness HV of 5
50 or more and Vickers hardness HV of the core is 320 to 4
It is characterized by being within the range of 00.

[0009] The method for manufacturing a screw according to claim 3 is characterized in that:
0.05 to 0.15 wt.%, Si: 0.20 wt.% Or less, M
n: 0.4 to 1.8 wt.%, P: 0.015 wt.% or less,
S: 0.015 wt.% Or less, sol. Al: 0.020-0.
080 wt.% And N: 0.0060 wt.% Or less, a chemical component consisting of iron and unavoidable impurities and having a carbon equivalent: C _eq = C + Si / 7 + Mn / 5 of 0.40 wt.% Or less. A steel wire rod having a composition is softened and annealed to prepare a pre-annealed steel wire, and the pre-annealed steel wire thus prepared is subjected to thread processing to prepare a screw. It is characterized in that tempering is performed after carburizing or carbonitriding and quenching so that the HV is 550 or more and the Vickers hardness HV of the core is in the range of 320 to 400.

[0010] The method for manufacturing a screw according to claim 4 is characterized in that:
0.05 to 0.15 wt.%, Si: 0.20 wt.% Or less, M
n: 0.4 to 1.8 wt.%, P: 0.015 wt.% or less,
S: 0.015 wt.% Or less, sol. Al: 0.020-0.
080 wt.% And N: 0.0060 wt.% Or less, Cr: 0.80 wt.% Or less, Mo: 0.30
wt.% or less, and B: 0.0005 to 0.0050 w
at least one selected from the group consisting of t.% and / or Ti: 0.005 to 0.050 wt.%, and N
b: At least one selected from the group consisting of 0.005 to 0.050 wt.% is additionally added, and the balance consists of iron and unavoidable impurities, and carbon equivalent: C _eq = C + Si
/ 7 + Mn / 5 + Cr / 9 + Mo / 6 The steel wire having a chemical composition of 0.40 wt.% Or less is soft annealed to prepare a pre-annealed steel wire, and the pre-annealed steel wire thus prepared is The screw is processed to prepare a screw, and then the screw is carburized or carbonitrided and quenched so that the surface has a Vickers hardness HV of 550 or more and the core has a Vickers hardness HV in the range of 320 to 400. After that, tempering is performed.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the chemical composition of the screw of the present invention as described above will be described below. (1) C: 0.05 to 0.15 wt.% C is an important element for securing the strength of steel.
If it is less than 5 wt.%, Not only the desired strength cannot be obtained, but also the carburizing curability is reduced. On the other hand, if it exceeds 0.15 wt.%, The internal hardness of the screw becomes too high, the toughness of the steel is reduced, and head jump cannot be prevented. Further, the strength of the steel wire is increased, and the life of the cold working tool is reduced. Therefore, the C content is limited to the range of 0.05 to 0.15 wt.%. (2) Si: 0.20 wt.% Or less Since Si plays an important role as a deoxidizer, it must be added at the steel making stage. However, it does not have to remain until the steel material stage. However, Si increases deformation resistance and lowers cold workability. However, in the present invention, since the C content is suppressed to a low level, even if added up to 0.20 wt.%, The decrease in cold workability is small. On the other hand, Si can also enhance hardenability. Therefore, when the Si content is 0.2
0 wt.% Or less. (3) Mn: 0.4 to 1.8 wt.% Mn is important for enhancing the hardenability of steel, making the structure after quenching fine, increasing the ratio of martensite in the structure, and securing toughness. Element. For this purpose, Mn needs to be added in an amount of 0.4 wt.% Or more. On the other hand, since the screw has relatively small dimensions, sufficient hardenability can be secured by adding an upper limit of 1.8 wt.%. Therefore, the Mn content is limited to the range of 0.4 to 1.8 wt.%. (4) P: 0.015 wt.% Or less P segregates at austenite grain boundaries and weakens the grain boundary strength. In addition, it forms a solid solution in ferrite and reduces the deformability of steel. As described above, since P is an impurity element in the present invention, its content is set to 0.015 wt.% Or less. (5) S: 0.015 wt.% Or less S forms MnS and lowers the deformability of steel. Also,
MnS is a starting point of crack generation. As described above, since S is an impurity element in the present invention, its content is set to 0.0
It shall be 15 wt.% Or less. (6) sol. Al: 0.020 to 0.080 wt.% Al is not only an element necessary as a deoxidizing material, but also has the effect of fixing N segregating at grain boundaries as AlN to increase grain boundary strength. Have. In order to exert such an effect by Al, 0.020 as sol. Al (acid-soluble Al) is required.
More than wt.% is required. However, if the sol.Al exceeds 0.080 wt.%, Al ₂
Aggregates of O ₃ are formed to cause nozzle clogging, which makes the casting operation difficult. Therefore, the sol.
It is limited to the range of 20 to 0.080 wt.%. (7) N: 0.0060 wt.% Or less N also segregates at austenite grain boundaries and weakens grain boundary strength.
In addition, strain age hardening occurs at the time of threading, thereby reducing the cold workability of steel and shortening the tool life. As described above, since N is an impurity element in the present invention, its content is set to 0.0060 wt.% Or less. (8) Cr: 0.80 wt.% Or less In a low carbon steel, if the content is 0.80 wt.% Or less, the degree of strengthening of the steel is small, but the hardenability can be enhanced. Acts as a useful element in However, if it exceeds 0.80 wt.%, The softening property of the annealing is impaired, and the steel is hardened. Therefore, the Cr content is set to 0.80 wt.% Or less. (9) Mo: 0.30 wt.% Or less Mo is an element effective for preventing segregation of P to the grain boundary and increasing the grain boundary strength. However, when added in a large amount, Mo also increases the annealing softening resistance similarly to Cr. On the other hand, Mo is an expensive element. Therefore, the Mo content is 0.30 wt.%
The following is assumed. (10) B: 0.0005 to 0.0050 wt.% B has an effect of improving the hardenability by adding a small amount.
In addition, BN is formed to prevent N grain boundary segregation. By adding B, the content of Mn, Cr, and Mo can be reduced, and the cold workability of steel can be further improved. B
0.000
It is necessary to add 5 wt.% Or more. However, 0.0
If it is added in excess of 050 wt.%, Boron cementite precipitates and reduces the grain boundary strength. Therefore, the B content is 0.00
It is limited to the range of 0.05 to 0.0050 wt.%. (11) Ti: 0.005 to 0.050 wt.% Ti has an effect of refining crystal grains. However,
If the content is less than 0.005 wt.%, The effect is small, and
The effect of fixing as iN is also small. However, 0.05
Even if it is added in excess of 0 wt.%, Not only these effects are saturated, but if the Ti content is too high, a large number of hard TiN and TiC are formed, the workability is reduced, and the alloy cost is increased. Therefore, the Ti content is 0.005 to 0.050 wt.%
Within the range. (12) Nb: 0.005 to 0.050 wt.% Nb also has a crystal grain refining effect similarly to Ti. And, for the same reason as Ti, the Nb content is also 0.005 to
It is limited to the range of 0.050 wt.%.

In addition to the above elements, such as Cu, Sn, etc.
In general, it may contain elements that are unavoidably mixed. (13) C_eq: 0.40 wt.% Or less Carbon equivalent of steel (C_eq) Exceeds 0.40 wt.%, Annealing
Later strength is high and the life of the cold working tool is shortened. Ma
In addition, the deformability of steel decreases, and the sharpness of the screw tip is poor.
C_eqIs 0.40 wt.% Or less. here,
C_eq= C + Si / 7 + Mn / 5 + Cr / 9 + Mo / 6
And C, Si, Mn, Cr and Mo are contained in steel of each element.
Rate (wt.%). However, Cr and / or
Or when not containing Mo, it contains Cr and / or Mo.
Assuming that the percentage (wt.%) Is 0 (zero), C _eqIs calculated. (14) Vickers hardness HV of the screw surface: 550 or more Vickers hardness of the screw surface, especially the hardness of the screw tip
If the HV is less than 550, the tip is missing when screwing into the iron plate
It breaks and becomes impossible to screw in. Also screw
It takes a long time to penetrate. Therefore, the hardness of the screw surface
HV: 550 or more. (15) Vickers hardness HV of the screw core: 320 to 40
0 Screw head jump is closely related to screw core hardness,
If the HV exceeds 400, the head jumps easily occur.
It must be 400 or less. But HV: less than 320
The strength of the screw is not enough,
Easy to rub. Therefore, the strength of the screw core is set to HV: 320.
It must be within the range of ~ 400.

[0013]

Next, the present invention will be described in more detail with reference to examples. Using steels with various chemical composition,
Drilling tapping screws having various values of surface hardness and core hardness were manufactured. FIG. 1 shows the relationship between the surface hardness and the core hardness of the drilling tapping screw manufactured in this way, and whether or not the head jumps after the tightening.
In addition, the above-mentioned screw shape is countersunk, nominal diameter 5 mm, length 35
mm. As can be seen from the figure, the occurrence of head jump of the screw does not depend on the hardness of the screw surface but depends on the hardness of the screw core. That is, since the conventional screw has been manufactured with a core hardness in the range of HV: 400 to 450, a head jump occurs at a rate of half or more. On the other hand, the core hardness is HV: 320 to
In the case of 400, the occurrence of head jump was completely eliminated.

FIG. 2 shows that the C content is 0.05 to 0.24 watts.
The relationship between the C content of drilling tapping screws (screw shape: hexagon with a rim, nominal diameter 5 mm, length 35 mm) manufactured from steel with sufficient hardenability and t. Show. The core hardness is determined by the C content, and in order to obtain a core hardness of HV: 320 to 400, the C content needs to be in the range of 0.05 to 0.15 wt.%. Recognize.

Next, tapping screws falling within the scope of the present invention (Examples 1 to 8) and tapping screws falling outside the scope of the present invention (Comparative Examples 1 to 10) were produced as follows. Note that Comparative Examples 9 and 10 are each a carbon steel wire for cold heading (JIS G).
3507) is a conventional tapping screw using SWRCHs 18A and 22A. Table 1 shows the chemical component compositions and C equivalents of the respective examples and comparative examples.

[0016]

[Table 1]

A 5.5 mm diameter wire having the chemical composition shown in Table 1 was drawn to a diameter of 4.8 mm, and then subjected to spheroidizing annealing. Wire was manufactured. From the steel wire thus manufactured, a drilling tapping screw having a cutting edge and a nominal diameter of 4.8 mm and a length of 35 mm was manufactured by cold forging from a cross-headed countersunk head, and the screw workability was evaluated. Next, carburizing or carbonitriding and quenching were performed, tempering was performed at 250 ° C., and zinc plating was performed with a thickness of 0.20 mm, and baking was performed to remove hydrogen.

The Vickers surface hardness and the core hardness of ten screws were measured for each of the examples and comparative examples thus manufactured. On the other hand, a penetration test of the screw was performed as follows.

FIG. 3 is a schematic perspective view of a screwing test device used in the screw penetration test. Screw test piece 4 is attached to chuck 3 below electric screwdriver 2 vertically supported by support column 1.
Is fixed vertically. On the other hand, a cold-rolled steel sheet having a thickness of 1.6 mm (JIS G 3141 S
(PCC) is attached to the fixing jig 6 and fixed horizontally. The screw test piece 4 is made of a screw test steel plate 5 according to the weight of the weight 7.
The rotation speed is 3000 by the operation of the electric screwdriver 2 while applying a load of 180 N / mm ² at right angles to the surface of
Screwed in at rpm. The starting point and the end point when the screw test piece 4 is screwed from the screw tip to the first complete thread on the surface of the screw-in test steel plate 5 are detected by the detecting device 8, and the time when the start point and the end point are reached is transmitted to the timer 9, The time required for screwing (screw penetration time) is measured by the timer 9. By the method described above, ten screw test pieces 4 were tested for each of the examples and comparative examples, and the average value of the screw penetration time was determined. On the other hand, in the head jump test after tightening the screws, 20 screw test pieces 4 of each of Examples and Comparative Examples were completely screwed into the screw test steel plate 5, and one week later, the presence or absence of head jump was observed. Table 1 shows the results of the above tests.

Table 1 shows the following results. In Examples 1 to 8, which are tapping screws of the present invention, the workability of the screw is good, that is, the life of the forging tool and the shape of the screw head dish are good, and the hardness of the screw surface is HV: 550 or more. The screw core hardness is between HV: 320 to 400, and the thread penetration time is 3.5 seconds or less. In addition, no head jump occurred after the screw was tightened, which was satisfactory as an excellent tapping screw with no head jump which is the target of the present invention.

On the other hand, in Comparative Example 1, the C content was low outside the range of the present invention, and the core hardness was HV: 32.
Below 0, the strength of the screw is insufficient, and the screw penetration time is very long at 8.9 seconds. Further, the Mn content was high outside the range of the present invention, the carbon equivalent was as high as 0.43, and the tool life was short.

In Comparative Example 2, since the C content was high outside the range of the present invention, and the core hardness exceeded HV: 400, head jump occurred. In Comparative Example 3, since the Si content was high outside the range of the present invention, the screw workability was poor, and the P content was high outside the range of the present invention. As a result, the screwing time is long. In addition, a head jump occurred.

In Comparative Example 4, since the S content was high outside the range of the present invention, and the sol.Al content was too low and the N content was too high, the core toughness was poor and the screwing time was long. , And a head jump also occurred.

In Comparative Example 5, since the Cr content was high outside the range of the present invention, the softening was delayed in the spheroidizing annealing, and the workability of the screw was slightly poor. Also, since the sol.Al content was too high, there were many nonmetallic inclusions, the screwing time was long, and head jump occurred.

In Comparative Example 6, the Mo content was high outside the range of the present invention, the softening was delayed similarly to Cr, and since the Ti content was too high, many hard TiN and TiC were present. Forging cracks occurred on the head and screw workability was poor. In addition, the tip of the screw was chipped starting from the hard inclusion, so that the screwing time was long and the head jumped.

In Comparative Example 7, since the B content was high outside the range of the present invention and a large number of boron-containing cementites were present, the screwing time was long and head jump occurred. Comparative Example 8 has a high P content outside the range of the present invention and a high N content.
Since the b content was too high, the screw workability was somewhat poor, the screwing time was long, and the head jumped.

Comparative Example 9 is a conventional SWRCH18A, but the hardness of the core is too high because the C content is high outside the range of the present invention. In addition, since the S and N contents were too high, the grain boundary strength was weak, and a head jump occurred.

Similarly, in Comparative Example 10, the conventional SWRCH2
Although the content was 2A, the hardness of the core was high because the C content was high outside the range of the present invention, and the head jump occurred because the P content was high outside the range of the present invention.

Although the present invention has been described with reference to the embodiment of the tapping screw, the present invention is not limited to the tapping screw and the method of manufacturing the tapping screw. For example, a drywall used after carburizing or carbonitriding and quenching and tempering is used. The present invention can be applied to screws in which flying becomes a problem.

[0030]

As described above, according to the present invention,
It is possible to provide a screw which is excellent in screw workability and thread penetrability and can completely prevent the occurrence of head jump after tightening, and a method for manufacturing the screw, and has an industrially useful effect.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the surface hardness and core hardness of a drilling tapping screw and the occurrence of head jump after tightening.

FIG. 2 is a graph showing the relationship between the C content of a drilling tapping screw and the hardness of a screw core after heat treatment.

FIG. 3 is a schematic perspective view of a screwability test apparatus.

[Explanation of symbols]

 Reference Signs List 1 support 2 electric screwdriver 3 chuck 4 screw test piece 5 screw-in test steel plate 6 fixing jig 7 weight 8 detecting device 9 timer 10 operation control panel 11 power cord

Claims (4)

[Claims] 1. C: 0.05 to 0.15 wt.%, Si: 0.20 wt.% Or less, Mn: 0.4 to 1.8 wt.%, P: 0.015 wt.% Or less, S: 0 0.015 wt.% Or less, sol. Al: 0.020 to 0.080 wt.% And N: 0.0060 wt.% Or less, the balance being iron and unavoidable impurities, and carbon equivalent: C _eq = C + S
i / 7 + Mn / 5 is 0.40 wt.% or less, and the Vickers hardness of the surface is 550 or more, and the Vickers hardness of the core is in the range of 320 to 400. Characteristic, headless screw. 2. C: 0.05 to 0.15 wt.%, Si: 0.20 wt.% Or less, Mn: 0.4 to 1.8 wt.%, P: 0.015 wt.% Or less, S: 0 0.015 wt.% Or less, sol.Al: 0.020 to 0.080 wt.%, And N: 0.0060 wt.% Or less, Cr: 0.80 wt.% Or less, Mo: 0. 30 wt.% Or less, and B: at least one selected from the group consisting of 0.0005 to 0.0050 wt.%, And / or Ti: 0.005 to 0.050 wt.%, And Nb: At least one selected from the group consisting of 0.005 to 0.050 wt.%, The balance being iron and unavoidable impurities, and carbon equivalent: C _eq = C + Si / 7 +
Mn / 5 + Cr / 9 + Mo / 6 has a chemical composition of 0.40 wt.% Or less, and the surface has a Vickers hardness of 550 or more, and the core has a Vickers hardness of 320-4.
A screw having no head jump, which is within the range of 00. 3. C: 0.05 to 0.15 wt.%, Si: 0.20 wt.% Or less, Mn: 0.4 to 1.8 wt.%, P: 0.015 wt.% Or less, S: 0 .015 wt.% Or less, sol. Al: 0.020 to 0.080 wt.%, And N: 0.0060 wt.% Or less, the balance being iron and unavoidable impurities, and carbon equivalent: C _eq = C + Si
/ 7 + Mn / 5 is 0.40 wt.% Or less. A steel wire having a chemical composition of 0.40 wt.% Or less is softened and annealed to prepare a pre-annealed steel wire, and the pre-annealed steel wire thus prepared is subjected to thread processing. A screw having a Vickers hardness of 550 or more on the surface and a Vickers hardness of 3
A method for producing a screw without head jumps, comprising performing carburizing or carbonitriding and quenching so as to fall within the range of 20 to 400 and then tempering. 4. C: 0.05 to 0.15 wt.%, Si: 0.20 wt.% Or less, Mn: 0.4 to 1.8 wt.%, P: 0.015 wt.% Or less, S: 0 0.015 wt.% Or less, sol. Al: 0.020 to 0.080 wt.%, And N: 0.0060 wt.% Or less, Cr: 0.80 wt.% Or less, Mo: 0.30 wt. % Or less, and B: at least one selected from the group consisting of 0.0005 to 0.0050 wt.%, And / or Ti: 0.005 to 0.050 wt.%, And Nb: 0. 0.0005 to 0.050 wt.%, The balance being iron and unavoidable impurities, and carbon equivalent: C _eq = C + Si / 7
+ Mn / 5 + Cr / 9 + Mo / 6 having a chemical composition of 0.40 wt.% Or less is softened and annealed to prepare a pre-annealed steel wire, and a screw is attached to the pre-annealed steel wire thus prepared. A screw is prepared by working, and then the screw is carburized or carbonitrided and quenched so that the Vickers hardness of the surface is 550 or more and the Vickers hardness of the core is in the range of 320 to 400. And a method of manufacturing a screw which does not cause head jumps.