JPH05192736A - Manufacture of hexagon socket head cap screw - Google Patents

Abstract

PURPOSE:To prevent the generation of a crack at the time of working a hexagon socket and to prolong the service life of a working tool. CONSTITUTION:At the time of continuously casting molten steel adjusted to a prescribed composition, in the vicinity of a crater end for which the molten steel in the inner part of a cast billet completes the solidification, preventing treatment for concentrating the components, in which the ratio C/C0 of C content (C) at a core part in the cast billet to the C content (C0) of the molten steel in a ladle becomes <=0.90, is applied. Successively, after the billet is formed to a wire rod or a bar steel by hot-rolling and cut to a prescribed length, the base stock is formed to a bolt shape by cold-forging and further, after executing a hexagon socket forming work to a bolt top part by the same cold forging, a screw is formed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an advantageous method of manufacturing a hexagon socket head cap bolt.

[0002]

2. Description of the Related Art Hexagon socket head cap bolts are produced by cutting a material made into wire rod or steel bar by hot rolling into a predetermined length, and then cold forging to adjust the outer shape to a bolt shape, and then cold forging as well. It is manufactured by drilling a hexagonal hole in the bolt head and then threading the bolt shank. By the way, during the cold forging described above, especially during the hexagonal hole forging, cracks often occur at each corner of the hexagon. Such cold forging cracks are due to segregation at the center of the slab that is generated during solidification during continuous casting.

As a measure for preventing such center segregation, for example, electromagnetic stirring in the secondary cooling zone has been tried.
Even the semi-micro segregation has not been reduced, and the effect was not sufficient.

Also, the application of a so-called in-line reduction method {iron and steel 60th year (1974) No. 7 pp. 875-884} in which a large reduction is performed using a pair of rolls at the final stage of solidification of a cast piece has been attempted. However, this method has a problem that cracking occurs at the solidification interface when the reduction in the cast piece region having a large unsolidified layer is insufficient. In addition, JP-A-49-12173
No. 8 discloses a method in which light rolling is performed by a pair of rolls in the vicinity of the solidification tip of a slab to compensate for the amount of solidification shrinkage of the part by rolling, and Japanese Patent Laid-Open No. 52-54623 discloses forging. Methods have been proposed in which a die is used to largely reduce the vicinity of the solidification completion point of a slab.

However, in the case of light reduction by rolls, even if a plurality of pairs of rolls are applied to reduce the pressure by several mm / m, it is not possible to sufficiently prevent solidification shrinkage and bulging that occur between roll pitches. If the reduction position is not appropriate, there is a disadvantage that the center segregation deteriorates. On the other hand,
The method of greatly reducing the vicinity of the solidification completion point of the slab using a forging die is less likely to crack the solidification interface than large reduction by a roll like the in-line reduction method, and it is possible to avoid negative segregation as much as possible. Although it has been clarified that even semi-macro segregation can be improved, cracking occurs at the solidification interface if the reduction in the cast area of the large unsolidified layer is still insufficient, and even if the area of the small unsolidified layer is reduced. Since the effect cannot be obtained, it is the current situation to seek the optimum rolling reduction condition. Therefore, the center segregation generated in the slab has not been dramatically improved, and the target value of the C concentration of the molten steel is lowered to reduce the concentrations of C, P, S, etc. in the segregated portion, and P, S, etc. However, it is the actual situation that it is dealt with by reducing diffusion as much as possible, and depending on the type of steel, by performing diffusion annealing or the like at the stage of slab, a significant increase in cost cannot be avoided.

[0006]

SUMMARY OF THE INVENTION The present invention advantageously solves the above-mentioned problems. Even when a continuous casting method is used, the formation of center segregation is reduced as much as possible, so that during drilling. It is an object of the present invention to propose an advantageous method for producing a hexagon socket head cap bolt that effectively prevents the occurrence of cracks in the bolt.

[0007]

That is, the present invention includes C: 0.10 to 0.50 wt% (hereinafter simply referred to as%), Si: 0.01 to 0.50%, Mn: 0.20 to 2.00%, and Cr: 0.50 to 2.00%, Mo: 0.05-2.00% Al: 0.002-0.100%, Ti: 0.002-0.050%, Nb: 0.005-0.050%, V: 0.005-0.050% and B: 0.0002-0.0030% At least one kind selected from Molten steel having the composition of Fe and unavoidable impurities in the balance is continuously cast, in which case the C content of the molten steel in the ladle (C ₀ ) Of the C content (C) in the axial center of the cast slab to a component concentration preventing treatment such that the ratio C / C ₀ is 0.90 or less, and then hot-rolled into a wire rod or steel bar, and then a predetermined length After cutting it into a bolt shape by cold forging, and then hexagonally drilling a hole in the bolt head by cold forging as well. After giving the process, the production method of the hexagon socket head bolt comprises forming a thread (first
Invention).

The present invention is also the method for producing a hexagon socket head cap bolt according to the above-mentioned first invention, further containing Sb: 0.0005 to 0.0500% as a material component (second invention).
Is.

[0009]

First, the reason why the composition of molten steel is limited to the above range in the present invention will be explained. C: 0.10 to 0.50% From the viewpoint of guaranteeing the strength of the bolt after forging, the amount of C is
The lower limit was 0.10%. On the other hand, the higher the amount of C, the higher the strength can be obtained, but on the other hand, higher C makes the material brittle and increases the frequency of cracking during forging, so the upper limit is 0.50%.
Stipulated in.

Si: 0.01 to 0.50% Si is an element useful not only as a deoxidizing agent but also for strengthening the matrix, and at least 0.01% is required. On the other hand, Si has the effect of increasing the activity of C, and especially
When the content exceeds 50%, the decarburized layer is generated, the surface hardness becomes insufficient, a step of removing the decarburized layer is required, and the yield is reduced. For this reason, the Si content is set to the range of 0.01 to 0.50%.

Mn: 0.20-2.00% Mn, like Si, acts not only as a deoxidizing agent but also fixes S that causes embrittlement of steel, and further improves hardenability to improve strength and ductility. Although it is a useful element in increasing the content of steel, if its content is less than 0.20%, its addition effect is poor, while if it exceeds 2.00%, it not only becomes expensive but also in controlled cooling after hot rolling or in the heat treatment step during processing. Since it promotes the formation of a micro martensite structure and impairs the cold forgeability, it was added in the range of 0.20 to 2.00%.

Cr: 0.50 to 2.00%, Mo: 0.05 to 2.00% Cr and Mo are equal as components for enhancing hardenability and strength. However, in either case, if the content is too small, there is no effect of the addition, while if it is too large, the cost is increased and the cost is increased.

Al: 0.002 to 0.100% Al is a strong deoxidizing agent and at the same time makes the crystal grain size fine,
It has the function of controlling hardenability. However, if the content is
If less than 0.002%, the effect of addition is poor, while 0.10%
If it exceeds 0%, not only the effect reaches saturation, but it also leads to an increase in alumina-based nonmetal oxides, so 0.002 to 0.
The content was set to be 100%.

Ti: 0.002 to 0.050% Ti, like Al, is a strong deoxidizing agent and, at the same time, has the function of making the crystal grain size fine and controlling the hardenability. However, if the content is less than 0.002%, the addition effect is poor,
On the other hand, if it exceeds 0.050%, the effect reaches saturation, so
It was made to contain in 0.002 to 0.050% of range.

Nb: 0.005 to 0.050%, V: 0.005 to 0.05
Each of 0% Nb and V effectively contributes to the improvement of strength and has the effect of making the crystal grain size fine. However, if the content is less than 0.005%, the effect of addition is poor, while if it exceeds 0.050%, the effect reaches saturation, so the content was made 0.005 to 0.050% respectively.

B: 0.0002 to 0.0030% B is an element useful for improving the hardenability, but its content is 0.
If it is less than 0002%, its effect is poor, while 0.0030
Even if it exceeds%, the effect is saturated, and no further effect can be expected, so the content was made 0.0002 to 0.0030%.

Sb: 0.0005 to 0.0500% In the bolt targeted by the present invention, decarburization of the surface layer progresses by annealing treatment such as spheroidization, but Sb effectively contributes to the suppression of such decarburization. However, if it is less than 0.0005%, the effect is poor, while if it exceeds 0.0500%, the effect reaches saturation, so Sb was made to be contained in the range of 0.0005 to 0.0500%. Sb is also effective in preventing the coarsening of crystal grains in the surface layer.

According to the present invention, in the continuous casting of molten steel having the above-described preferable component composition, the ladle is subjected to the component concentration preventing treatment near the crater end where the molten steel inside the slab completes the solidification. The ratio C / C ₀ of the C content (C) in the slab axial center to the C content (C ₀ ) of the medium molten steel is 0.
Control to 90 or less (preferably 0.50 or more). Forging treatment is particularly advantageously applied here as the component concentration preventing treatment, but the present invention is not limited to this, and if the C / C ₀ ratio can be controlled to 0.90 or less, other It may be a means.

The reason why the C / C ₀ ratio can be controlled by the above forging process will be described below. That is, at the final stage of solidification of the internal molten steel, molten steel with a high concentration of C exists near the crater end, so if it solidifies as it is, central segregation will occur, but if forging processing is performed before solidification, As a result of the C-rich molten steel being extruded upward, the C concentration in the central portion does not rise so much. Therefore, by adjusting the timing of forging according to the degree of enrichment of C, the C content in the slab axial center can be adjusted.

In FIG. 1, the wire rod or steel bar obtained by subjecting to a forging process so that the C / C ₀ ratio becomes various values is formed into a bolt shape and then C / C is obtained when the hole is forged. _The results obtained by examining the relationship between the ₀ ratio and the crack occurrence rate are shown. As is clear from the figure, when the C / C ₀ ratio is 0.90 or less, the occurrence of cracks is sharply reduced. The hole forgeability becomes better as the C / C ₀ ratio becomes smaller, but if the negative segregation degree is increased too much, the corners of the hexagonal hole will become bent, so the C / C ₀ ratio should be 0.50 or more. Preferably.

Therefore, according to the present invention, C in the axial center of the slab to be controlled by the component concentration preventing treatment such as forging.
The / C ₀ ratio is limited to the range of 0.90 or less (preferably 0.50 or more). As a preferable forging method, the continuous forging method previously disclosed by the inventors in JP-A-60-82257 is advantageously suitable.

[0022]

EXAMPLES Molten steel having the chemical composition shown in Table 1 (steel types A to
F) is continuously cast with a 400 × 560 mm mold, and the C / C ₀ ratio of the slab axial center is 0.70 with respect to the slab being drawn, near the crater end where the molten steel inside the slab completes solidification. Forging is continuously performed with the goal of, and the C / C ₀ ratio is 0.50 to ₀ .
Bloom was produced by controlling the range of 90. After that, a billet having a size of 150 × 150 mm was formed by a slab and a billet mill, and then hot rolled into a wire rod having a diameter of 7 mm by a bar mill. Then, following the steps described below, after forming and forging into a bolt shape, a hexagonal hole having a diameter of 5 mm and a depth of 3 mm was also formed by cold forging, and then thread cutting was performed to obtain a product. Azroll wire → pickling / water washing → spheroidizing annealing → surface treatment → wire drawing → forming forging → punching forging → pre-drawing → rolling → spheroidizing annealing. In addition, FIG. 2 shows the transition of the bolt shape in the above-mentioned forming forging-forging-forging-preliminary-rolling process, and FIG.
Illustrates the hole forging procedure.

On the other hand, as a comparative material, as in the conventional process, after continuous casting, forging was not carried out to form a steel bar, and thereafter a hexagon socket head cap bolt was similarly prepared. The molten steel heating degree at the time of tapping was cast in the range of 25 to 30 ° C. Further, the hot rolling temperatures from the slab rolling to the steel bar rolling were considered to have the same temperature history for both the steel material obtained according to the present invention and the comparative material.

Table 2 shows the results of investigations on the crack occurrence rate during the hexagonal hole drilling, tool life, and decarburization amount during spheroidizing annealing.

[0025]

[Table 1]

[0026]

[Table 2]

As can be seen from the table, the one subjected to the forging process according to the present invention has extremely few cracks during the forging process of the hexagonal head and has a long tool life. On the other hand, in all of the comparative materials, many cracks occur during the forging process of the hexagonal head, and the tool life is short. In the test steel according to the second invention, the decarburization amount after the spheroidizing annealing was extremely small.

[0028]

As described above, according to the present invention, cracking during hexagonal hole drilling can be achieved by continuously imparting a component concentration preventing treatment such as forging during continuous casting to control C / C ₀ of the slab core. Can be effectively prevented, and the tool life can be extended.

[Brief description of drawings]

FIG. 1 is a graph showing a relationship between a C / C ₀ ratio and a crack occurrence rate.

FIG. 2 is a diagram showing a transition of a bolt shape in a forming forging-forging-forging-preliminary-rolling process.

FIG. 3 is a view showing a procedure for forging a hexagonal head.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display location C22C 38/04

Claims (2)

[Claims] 1. C: 0.10 to 0.50 wt%, Si: 0.01 to 0.50 wt%, Mn: 0.20 to 2.00 wt%, Cr: 0.50 to 2.00 wt%, Mo: 0.05 to 2.00 wt% Al: 0.002 ~ 0.100 wt%, Ti: 0.002-0.050 wt%, Nb: 0.005-0.050 wt%, V: 0.005-0.050 wt% and B: 0.0002-0.0030 wt%, and the balance Fe. And molten steel having the composition of unavoidable impurities are continuously casted, and at that time, near the crater end where the molten steel inside the slab completes solidification, the slab axial center portion relative to the C content (C ₀ ) of the molten steel in the ladle After the component concentration prevention treatment that makes the ratio C / C ₀ of C content (C) in 0.90 to 0.90 or less, and then hot-rolling it into a wire rod or bar steel, it is cut into a predetermined length and then cold forged. It is characterized in that it is formed into a bolt shape by, and then a hexagonal hole is drilled in the bolt head by cold forging, and then a thread is formed. Hexagon socket head cap bolt manufacturing method. 2. C: 0.10 to 0.50 wt%, Si: 0.01 to 0.50 wt%, Mn: 0.20 to 2.00 wt%, Cr: 0.50 to 2.00 wt%, Mo: 0.05 to 2.00 wt% Al: 0.002 -0.100 wt%, Ti: 0.002-0.050 wt%, Nb: 0.005-0.050 wt%, V: 0.005-0.050 wt% and B: 0.0002-0.0030 wt%, and at least one selected from Sb: Molten steel containing 0.0005 to 0.0500 wt% and the balance of Fe and unavoidable impurities is continuously cast. At that time, C of molten steel in the ladle near the crater end where the molten steel inside the slab completes solidification Content (C
The ratio of C content in the slab axis portion with respect to ₀₎ (C) C /
After subjecting to component thickening prevention treatment to make C ₀ 0.90 or less, and then hot-rolling it into a wire rod or steel bar, after cutting it to a prescribed length, it is formed into a bolt shape by cold forging, and then cold-rolled as well. A method for manufacturing a hexagon socket head cap screw, which comprises forming a screw thread after performing hexagonal hole drilling on the bolt head by forging.