TWI642791B - Non-heat treated bolt wire, non-heat treated bolt steel wire and their manufacturing method and non-heat treated bolt - Google Patents

Abstract

A non-heat treated bolt, characterized by a compositional composition in mass %, satisfying C: 0.05 to 0.30%, Si: 1.10 to 2.5%, Mn: 0.50 to 2.5%, P: more than 0%, 0.03% or less, S: exceeding 0%, 0.03% or less, Cr: 0.03% to 0.30%, Al: 0.010 to 0.10%, and N: 0.0020 to 0.0100%, and the rest is made of iron and unavoidable impurities, and the specified carbon equivalent Ceq is 0.60 or more. Below 0.80, Ferrite has an area ratio of more than 60% in the whole organization and the area ratio of Pearlite in the whole tissue is 5% or more. In addition, the crystal of ferrite iron The granularity number is 10 or more.

Description

Non-heat treated bolt wire, non-heat treated bolt steel wire and their manufacturing method and non-heat treated bolt

The present invention relates to a wire for a non-heat treated bolt, a steel wire for a non-heat treated bolt, and a method of manufacturing the same, and a non-heat treated bolt.

Bolts of strength strengths of 70 to 90 kgf/mm ² , which have been used for various fastening parts, have been subjected to heat treatment by quenching and tempering after cold pressing to secure their specific strength.

However, in recent years, the above heat treatment, that is, non-heat treatment, has been omitted from the viewpoint of reducing CO ₂ emissions and saving energy. By omitting the non-heat treated bolt obtained by the heat treatment as described above, since the above heat treatment and the straightening after the heat treatment can be omitted, the cost can be reduced. It also contributes to the bolts. The manufacturing time of parts and parts is shortened and the working environment is improved.

The non-heat-treated bolt is formed into a bolt shape by using a steel wire having strength to match the strength of the bolt, and is specifically produced by cold pressing. Since the steel wire is formed into a bolt, a compressive force is generally applied in the axial direction to form a cold press of the head. Therefore, the above steel wire needs to be cold pressed, that is, it must have excellent cold workability.

In addition, a high-strength steel wire (wire) for non-heat-treated bolts is obtained, and there is a method of increasing the rate of reduction of the wire drawing. If a steel wire that increases the strength of the draw line and ensures the strength is used in the bolt formation, the load is reduced due to the Bauschinger effect, so that the tool life at the time of bolt formation can be extended. . However, the steel wire which is increased in strength by the above-mentioned drawing is also lowered by the Bauschinger effect, so that the obtained bolt is difficult to achieve the required high strength ratio, and as a result, the bolt is tightened. The upper limit of the axial force and the problem of the permanent elongation in the guaranteed load test determined by the mechanical properties of the steel fastening parts (JIS B1051).

In order to solve the above problem, for example, as shown in Patent Document 1, after the bolt is pressed, bluing treatment is performed by heating at about 350 ° C to remove stress and increase the fall strength. Further, as a rust-preventing surface treatment after bolt formation, Geomet (registered trademark) treatment by zinc powder burn-coating is performed at about 300 ° C or higher. However, the above-described bluing treatment or the like is also costly as the heat treatment of the quenching and tempering described above, and it is not expected to achieve the cost advantage of non-heat treatment.

The inventors of the present invention have proposed a technique in which a bolt is directly formed without heat treatment, that is, a technique for improving the high strength and the resistance to delayed fracture of the non-heat treated bolt. However, a higher strength ratio with non-heat treated bolts combined with high strength requires further review.

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open Publication No. 08-003640

[Patent Document 2] Japanese Patent Laid-Open Publication No. 2003-113444

In view of the above-described circumstances, an object of the present invention is to provide a tensile strength of 780 MPa or more and 1200 MPa or less, and an intensity ratio of 0.80 or more, even if heat treatment at a high temperature such as quenching and tempering or bluing treatment is not performed. Further, it is provided with a non-heat-treated bolt which is resistant to delayed destructiveness, and a method for producing the non-heat-treated bolt wire and steel wire for the non-heat-treated bolt, and the wire and steel wire for the non-heat-treated bolt and the non-heat-treated bolt. And below, non-heat treated bolts, non-heat treated bolt wire. Steel wires are sometimes referred to as "bolts" and "wires for bolts. Steel wires".

The non-heat-treated bolt wire material according to the embodiment of the present invention, which is characterized by the above-mentioned problem, is characterized by a composition of, in mass%, C: 0.05 to 0.30%, Si: 1.10 to 2.5%, and Mn: 0.50 to 2.5%, P. : more than 0%, 0.03% or less, S: more than 0%, 0.03% or less, Cr: 0.03 to 0.30%, Al: 0.010 to 0.10%, and N: 0.0020 to 0.0100%, and the remainder is made of iron and unavoidable impurities. The carbon equivalent Ceq expressed by the following formula (1) is 0.60 or more and 0.80 or less, and ferrite iron (Ferrite) The area ratio of the whole organization is more than 60%, and the area ratio of Pearlite in the whole organization is 5% or more. In addition, the crystal grain size of the ferrite iron is No. 10 or above, Ceq =[C]+[Si]/7+[Mn]/5+[Cr]/9...(1)

In the above formula (1), [element] represents the content of each element in the steel in mass%.

Further, in the method for producing a non-heat-treated bolt wire according to the embodiment of the present invention, the steel material satisfying the above composition is used, and the heating temperature of the rolling delay is set to 1000 ° C or more and 1200 ° C or less, and hot rolling is performed. Thereafter, the average cooling rate from 800 ° C to 500 ° C is set to 2.0 to 5.0 ° C / sec.

The steel wire for non-heat-treating bolts according to the embodiment of the present invention which solves the above-mentioned problems is characterized in that the component composition is C% 0.05% to 0.30%, Si: 1.10% to 2.5%, Mn: 0.50% to 2.5%, and P. : more than 0%, 0.03% or less, S: more than 0%, 0.03% or less, Cr: 0.03 to 0.30%, Al: 0.010 to 0.10%, and N: 0.0020 to 0.0100%, and the balance is iron and unavoidable impurities. The carbon equivalent Ceq expressed by the following formula (1) is 0.60 or more and 0.80 or less, and the area ratio of the ferrite iron to the whole organization is 60. More than %, and the area ratio of Boron iron in the whole organization is 5% or more. In addition, the grain size of the ferrite iron is No. 10 or higher, Ceq = [C] + [Si] / 7 + [Mn] /5+[Cr]/9...(1)

In the above formula (1), [element] represents the content of each element in the steel in mass%.

Further, in the method of manufacturing a non-heat-treated bolt steel wire according to the embodiment of the present invention, the wire for the non-heat-treated bolt is used, and the wire drawing is performed at a wire drawing reduction ratio of 20 to 40%.

Further, the non-heat-treated bolt according to the embodiment of the present invention which solves the above-described problems is characterized in that it has a composition of, in mass%, C: 0.05 to 0.30%, Si: 1.10 to 2.5%, and Mn: 0.50 to 2.5%, P. : more than 0%, 0.03% or less, S: more than 0%, 0.03% or less, Cr: 0.03 to 0.30%, Al: 0.010 to 0.10%, and N: 0.0020 to 0.0100%, and the rest is iron and inevitable impurities The carbon equivalent Ceq expressed by the following formula (1) is 0.60 or more and 0.80 or less. The area ratio of fertilized iron in the whole organization is more than 60%, and the area ratio of the Borne iron in the whole organization is 5% or more. In addition, the crystal grain size of the ferrite iron is above 10, Ceq= [C]+[Si]/7+[Mn]/5+[Cr]/9...(1)

In the above formula (1), [element] represents the content of each element in the steel in mass%.

The non-heat treated bolt preferably has a tensile strength of 780 MPa or more and 1200 MPa or less, and the strength ratio is 0.80 or more.

According to the embodiment of the present invention, the heat treatment at a high temperature such as quenching and tempering or bluing treatment is not performed, and the tensile strength is 780 MPa or more and 1200 MPa or less, and the strength ratio is 0.80 or more, and further, non-heat treatment resistant to delayed fracture resistance is provided. Bolts, and wires for non-heat treated bolts used in the manufacture of non-heat treated bolts. Steel wires, and their manufacturing methods.

The inventors of the present invention have repeatedly conducted active research in order to solve the aforementioned problems. As a result, it was found that the composition and structure of the non-heat-treated bolts may be as follows, that is, as long as the desired composition is required, the area ratio of the ferrite iron is 60% or more, and the area ratio of the ferrite is 5% or more. The crystal grain size of the ferrite iron is 10 or more. In addition, it has also been found that for the construction of the bolt, only the wire for the manufacture of the bolt is made. The structure of the steel wire is the same as the structure of the bolts described below, that is, as long as the wire is used. Steel wire It is only necessary to obtain a structure similar to the bolt structure described below at the stage of the wire, and thus the embodiment of the present invention is completed.

[Area area ratio of fertilized iron: 60% or more, and the area ratio of Bora iron: 5% or more]

The wire rod for bolts and bolts according to the embodiment of the present invention. The steel wire structure is a two-phase structure of ferrite iron and wave iron, which can improve the tensile strength and strength ratio in a balanced manner, and is preferably a one that improves the permanent elongation. Based on these points of view, firstly, the area ratio of ferrite iron in the whole organization needs to be more than 60%. The area ratio of the ferrite iron is preferably 62% or more, more preferably 65% or more. Further, the upper limit of the area ratio of the ferrite iron is 95% or less, preferably 94% or less, and more preferably 93% or less, in consideration of the area ratio of the following ferrite.

In addition, in order to particularly increase the strength ratio, the area ratio of Borneol in the whole organization is 5% or more. The area ratio of the Boron iron is preferably 6% or more, more preferably 7% or more. The upper limit of the area ratio of the ferrite is 40% or less, preferably 38% or less, and more preferably 35% or less, in consideration of the area ratio of the ferrite iron.

The wire rod for bolts and bolts according to the embodiment of the present invention. Steel wire, in addition to the structure is made of ferrite iron and Borne iron, in the above-mentioned fertiliser iron and Borne iron, it also has an area ratio of 5% or less in the whole organization, which is inevitable in the manufacturing process. The situation of organizations such as Bainite and Martensite. In addition, when a structure such as toughened iron is contained, when the area ratio of the structure and the area ratio of the ferrite iron are discriminated, the area ratio of the Borne iron is subtracted from the area of the toughened iron or the like from 100%. The rate and the area ratio of the ferrite iron are obtained.

[Ferrous iron crystal size: No. 10 or above]

In order to improve the strength ratio, the ferrite iron crystal grain size number is set to be No. 10. The ferrite iron crystal grain size number is preferably 10.3 or more, more preferably 10.5 or more. Since the finer grain size of the ferrite iron crystal can improve the mechanical properties, the upper limit of the crystal grain size of the ferrite iron is not particularly limited. However, in consideration of the component composition and the production method of the embodiment of the present invention, the upper limit of the grain size of the ferrite iron crystal is preferably about 12 or so.

Next, the wire for non-heat treated bolts and non-heat treated bolts according to the embodiment of the present invention is specified. The reasons for the composition of the steel wire are described below.

[C: 0.05~0.30%]

C is an essential element to ensure steel strength. In order to achieve the target bolt tensile strength: 780 MPa or more, the lower limit of the C amount is set to 0.05%. The amount of C is preferably 0.10% or more, more preferably 0.15% or more. On the other hand, when the C content is excessive, there is a large amount of solid solution carbide remaining during heating before hot rolling, and as a result, sufficient ferrite iron cannot be secured, and a desired strength ratio cannot be obtained. In addition, there is also a tensile strength of the bolt exceeding the upper limit of 1200 MPa. Therefore, the upper limit of the amount of C is set to 0.30%. The amount of C is preferably 0.25% or less, more preferably 0.20% or less.

[Si: 1.10~2.5%]

Si acts as a deoxidizer and is also necessary to ensure steel strength element. In addition, Si also suppresses the precipitation of coarse snow carbon (Cementite) and enhances the resistance to delayed destructive elements. In addition, Si can also reduce the permanent elongation at a certain addition amount. In order to obtain such effects, the lower limit of the amount of Si is set to 1.10%. The amount of Si is preferably 1.20% or more, more preferably 1.30% or more, still more preferably 1.50% or more. According to the embodiment of the present invention, the amount of Si added is further increased from the viewpoint of ensuring the strength of the steel while reducing the permanent elongation, compared to the conventional non-heat treated bolt. On the other hand, if the Si content is excessive, since the tensile strength of the bolt exceeds the upper limit of 1200 MPa, the upper limit of the amount of Si is 2.5%. The amount of Si is preferably 2.20% or less, more preferably 2.00% or less.

[Mn: 0.50~2.5%]

Mn is used to ensure the strength of steel and has the effect of being a deoxidizer. In order to obtain these effects, the lower limit of the amount of Mn is set to 0.50%. The amount of Mn is preferably 0.75% or more, more preferably 1.00% or more. On the other hand, when the Mn content is excessive, Mn segregation occurs, and the supercooled structure of the granulated iron is deteriorated to deteriorate the wire drawability. Therefore, the upper limit of the amount of Mn is set to 2.5%. The amount of Mn is preferably 2.20% or less, more preferably 2.00% or less.

[P: more than 0%, 0.03% or less]

The P system segregates the grain boundaries and causes an element which can deteriorate the cold forgeability. It is also an element that causes a decrease in resistance to delayed damage. Therefore, the amount of P is suppressed to 0.03% or less. The amount of P is preferably 0.015% or less, more preferably 0.010% or less. Although the amount of P is preferably as small as possible, it is difficult to achieve 0% in production, and usually it is at least about 0.003%.

[S: more than 0%, 0.03% or less]

S becomes thick at the grain boundary to form MnS, and this becomes a stress concentration portion, and cracks are generated to cause deterioration in resistance to delayed fracture. Therefore, the amount of S is suppressed to 0.03% or less. The amount of S is preferably 0.015% or less, more preferably 0.010% or less. Although the amount of S is preferably as small as possible, it is difficult to achieve 0% in production, and usually it is at least about 0.003%.

[Cr: 0.03~0.30%]

The Cr system is an element which ensures strength, and is precipitated again during the baking treatment by the plating treatment, and has an effect of suppressing the decrease in strength. In order to obtain these effects, the lower limit of the amount of Cr is set to 0.03%. The amount of Cr is preferably 0.05% or more, more preferably 0.07% or more, still more preferably 0.10% or more. On the other hand, if the Cr content is excessive, since the effect tends to be saturated and the cost is also increased, the upper limit of the amount of Cr is set to 0.30%. The amount of Cr is preferably 0.25% or less, more preferably 0.20% or less.

[Al: 0.010~0.10%]

Al is an element used as a deoxidizer. Further, it is also possible to form a nitride and prevent the coarsening of the old Austenite crystal grains by the pinning effect, and at the same time, the effective element of the strength is ensured by the grain refinement. In order to obtain these effects, the lower limit of the amount of Al is set to 0.010%. The amount of Al is preferably 0.015% or more, more preferably 0.020% or more. On the other hand, if Al is contained in excess, it will form a coarse nitride. Since the fatigue characteristics were deteriorated, the upper limit of the amount of Al was set to 0.10%. The amount of Al is preferably 0.08% or less, more preferably 0.06% or less.

[N: 0.0020~0.0100%]

The constituent elements of the carbonitride precipitated in the N-based ferrite iron, and the coarsening of the old Worthite iron crystal grains is prevented by the pinning effect. In order to obtain these effects, the lower limit of the amount of N is set to 0.0020%. The amount of N is preferably 0.0030% or more, more preferably 0.0035% or more. On the other hand, if N is excessively contained, a coarse nitride is formed and the fatigue characteristics are deteriorated. Therefore, the upper limit of the amount of N is set to 0.0100%. The amount of N is preferably 0.0080% or less, more preferably 0.0060% or less.

The wire rod for bolts and bolts according to the embodiment of the present invention. The composition of the steel wire is as described above, and the rest is iron and unavoidable impurities. The aforementioned unavoidable impurities may be mixed in the raw materials and manufacturing process, and may also be used as wires for bolts and bolts. The characteristics of the steel of the steel wire are not harmful. For example, Ni may be contained in an amount of 0.03% or less, preferably 0.02% or less, and Cu is 0.03% or less, preferably 0.02% or less. Further, V may be 0.02% or less, Nb may be 0.02% or less, and Ti may be 0.02% or less as an unavoidable impurity. When these V, Nb, and Ti are actively used, the form of the precipitate must be difficult to control in the production stage. Therefore, in the embodiment of the present invention, the degree of unavoidable impurities as described above is suppressed. In the embodiment of the present invention, it is different from Patent Document 2 in that it is not actively added with V or the like and is required to be precipitated and strengthened.

[The carbon equivalent Ceq represented by the following formula (1) is 0.60 or more and 0.80 or less]

Ceq=[C]+[Si]/7+[Mn]/5+[Cr]/9...(1)

In the above formula (1), [element] represents the content of each element in the steel in mass%.

In order to make the bolt strength set to 8.8~10.9 grade according to the strength defined by JIS B1051 (2012) or JIS B1051 (2015), and the strength ratio is 0.80 or more, the bolt and bolt wire according to the embodiment of the present invention. The carbon equivalent Ceq expressed by the above formula (1) is set to be 0.60 or more and 0.80 or less. The carbon equivalent Ceq is preferably 0.65 or more, more preferably 0.70 or more, and is preferably 0.78 or less.

[non-heat treated bolt characteristics] (Bolt tensile strength: 780~1200MPa)

As a bolt of 8.8 to 10.9 in the strength defined in JIS B1051 (2012) or JIS B1051 (2015), the tensile strength of the bolt is 780 MPa or more and 1200 MPa or less. The tensile strength of the bolt is preferably 800 MPa or more, more preferably 850 MPa or more, still more preferably 900 MPa or more; preferably 1100 MPa or less, more preferably 1000 MPa or less.

(0.2% strength of bolt: 870MPa or more, 0.01% strength of bolt: 660MPa or more)

In order to ensure the high bolt axial force of the fastening part, reduce the number of bolts used, and lighten the bolt fastening parts, it depends on the tensile strength, but The 0.2% strength of the bolt is preferably 870 MPa or more, more preferably 880 MPa or more, and the 0.01% strength of the bolt is preferably 660 MPa or more, more preferably 670 MPa or more. The higher the strength, the better. However, considering the composition and manufacturing method of the embodiment of the present invention, the upper limit of the 0.2% strength of the bolt is about 900 MPa, and the upper limit of the 0.01% strength of the bolt is about 690 MPa.

(Bolt strength ratio: 0.80 or more)

In the embodiment of the present invention, by setting the intensity ratio to 0.80 or more, the permanent elongation in the above-described guaranteed load test can be suppressed. The strength is preferably 0.85 or more, more preferably 0.90 or more.

[Wires for non-heat treated bolts. Manufacturing methods for steel wire and non-heat treated bolts]

The wire for non-heat treated bolts of the embodiment of the present invention. Steel wire and non-heat treated bolts can be manufactured by the following steps.

(heating delay heating temperature: 1000 ° C ~ 1200 ° C)

In order to solidify the carbides present in the steel, a uniform Worthite iron structure is formed, and the heating temperature of the hot press delay is set to 1000 ° C or higher. The heating temperature is preferably 1010 ° C or higher, more preferably 10 15 ° C or higher. On the other hand, when the heating temperature is too high, the crystal grains are coarsened, and the mechanical properties of the bolt are deteriorated. Therefore, the upper limit is 1200 °C. The heating temperature is preferably 1190 ° C or lower, more preferably 1185 ° C or lower.

After heating at the above heating temperature, hot rolling is performed. The rolling conditions of the hot rolling are not particularly limited, and general conditions can be employed. After hot rolling Cool down as follows.

(Average cooling rate from 800 ° C to 500 ° C after hot rolling: 2.0 ~ 5.0 ° C / sec)

If the cooling rate after hot rolling is too fast, a hard structure such as toughened iron or granulated iron is formed, and the cold workability is lowered. In the embodiment of the present invention, in order to obtain a two-phase structure of the ferrite-grained iron and the ferritic iron which satisfy the area ratio of the ferrite-grained iron of 60% or more and the area ratio of the ferritic iron of 5% or more, in the above temperature range The average cooling rate was set to 5.0 ° C / sec or less. The average cooling rate is preferably 4.8 ° C / sec or less, more preferably 4.5 ° C / sec or less, still more preferably 4.2 ° C / sec or less. On the other hand, the lower limit of the above average cooling rate is set to 2.0 ° C /sec or more from the viewpoint of ensuring strength. The average cooling rate is preferably 2.3 ° C / sec or more, more preferably 2.5 ° C / sec or more, still more preferably 2.7 ° C / sec or more. Further, the above cooling from 500 ° C to room temperature is not particularly limited, and for example, cooling can be performed.

The steel wire for non-heat-treated bolts according to the embodiment of the present invention can be obtained by wire drawing under the following conditions using a wire manufactured under the above conditions.

(Drawing line reduction rate: 20~40%)

In order to ensure strength and increase resistance to delayed destructiveness, the lower limit of the drawdown rate is set to 20%. The drawdown rate of the draw line is preferably 20.5% or more, more preferably 21.0% or more. On the other hand, if the drawdown rate becomes too high, the strength of the steel wire is excessively increased to deteriorate the cold forgeability. Therefore, the upper limit of the drawdown rate is set to 40%. The reduction rate of the draw line is preferably 39.0% or less, more preferably 38.0% or less. Further, the drawdown rate is a value calculated by the following formula (2).

Draw line reduction rate = 100 × [(wire diameter) ² - (steel wire diameter) ² ] / (wire diameter) ² ... (2)

The tensile strength of the steel wire obtained by the above-mentioned drawing is about 750 to 990 MPa. The method of manufacturing the bolt using the steel wire is not particularly limited, and the bolt may be formed by cold pressing to form a bolt, and then the screw portion may be converted to manufacture a bolt. However, as described above, the embodiment of the present invention does not include a heat treatment at a high temperature of about 300 ° C or higher, such as quenching and tempering or bluening treatment.

When it is used for applications requiring high corrosion resistance, it is preferred to apply a zinc-based surface treatment, for example, a zinc plating treatment. When zinc plating is applied, hydrogen is absorbed during electrolysis. Therefore, in order to dehydrogenate after zinc plating, the heating temperature is preferably 100 ° C and less than 300 ° C, and more preferably 200 ° C or less. Heating for 300 minutes. If the heating temperature is too high, the cost advantage of non-heat treatment cannot be expected, and zinc is also melted.

If the person skilled in the art of manufacturing the steel wire for non-heat-treated bolts according to the embodiment of the present invention described above is known, it is possible to obtain the non-invention of the present invention by a manufacturing method different from the above-described manufacturing method by attempting an erroneous method. The possibility of heat treating steel wire for bolts.

The non-heat treated bolt according to the embodiment of the present invention can be preferably used for automobiles, various industrial machines, and the like. When the steel for non-heat-treating bolts according to the embodiment of the present invention is used, as described above, it is not necessary to perform heat treatment such as quenching and tempering or bluing treatment, and it is also inexpensive and easy to manufacture to exhibit particularly high tensile strength. And high strength ratio non-heat treated bolts.

[Examples]

The embodiments of the present invention are specifically described below, but the present invention is not limited to the following embodiments, and may be modified and implemented within the scope of the scope of the present invention. The technical scope of the present invention.

The steel which satisfies the composition of the components shown in Table 1 below (the rest is iron and unavoidable impurities, and the blank column indicates that it is not added) is dissolved. Then, the steel material is heated at a heating temperature of 1030 to 1050 ° C, and then hot rolled, and then calendered by cooling at an average cooling rate of 800 ° C to 500 ° C: 3.0 to 4.0 ° C / sec. material. Further, it was allowed to cool to room temperature from the aforementioned 500 °C. Thereafter, it was pickled under the conditions shown below, and then subjected to lead impregnation to obtain a wire. Further, after the pickling and the lubricating film treatment were carried out under the conditions shown below, the wire was subjected to the drawing of the wire drawing reduction rate shown in Table 2, and the steel wire of No. 1 to 12 having a diameter of 9.06 mm was obtained. A steel wire having a diameter of 7.04 mm of No. 13 to 16 was obtained.

(pickling conditions)

Solution: hydrochloric acid

Concentration: 20%

Liquid temperature: 50 ° C

Dipping time: 4~8 minutes

(Lubricating film processing conditions)

Film type: phosphate film

Metal soap (Bonderite, lubricant (Bonder-Lube) treatment) immersion time of the coating layer: 3 to 15 minutes

(1) Organization observation of wire and steel wire

The structure of the wire and the steel wire is cut in such a manner that the circular cross section perpendicular to the rolling direction of the lead wire after the lead is immersed is cut, and then measured in the same manner as the structure evaluation method of the bolt described later. As a result, in either case, the structure of the wire and the steel wire is a two-phase structure of the ferrite iron and the wave iron satisfying the predetermined area ratio. Moreover, for example, the wire rod and the steel wire of No. 3 in Table 2 are the ferrite grain area ratio of 64.7%, the brite iron area ratio is 35.2%, and the fertilizer grain crystal size number is 12.1, and the wire is used. The bolt structure obtained from the steel wire is almost the same. In any other example, the wire is the same as the above No. 3. The organization of the steel wire has been confirmed to be almost identical to the structure of the bolt. Further, in Table 2 and Table 3 to be described later, F represents ferrite iron, P represents bunde iron, and F particle size indicates ferrite iron crystal grain size number.

(2) Tensile strength of steel wire

Using the above steel wire, a tensile test was carried out under the conditions of a clamp jig interval: 100 mm, and a crosshead speed: 5 mm/min, and the tensile strength was measured. The results are shown in Table 2.

(3) Cold pressability of steel wire

The steel wire was cold-pressed under the following conditions, and No. 1 to 12 were obtained, and M10 and bolts having a bolt diameter of 9.06 mm were obtained. Further, in No. 13 to 16, a bolt of M8 and a bolt diameter of 7.04 mm was obtained. Further, the obtained bolt rim portion was visually observed to confirm the presence or absence of cracks. Therefore, when there is no crack, it is evaluated as "excellent cold workability", and when cracks occur, it is evaluated as "poor press workability". The results are shown in Table 2.

(Bolt pressing conditions)

Pressing device: NBP550 manufactured by Sakamura Machinery Manufacturing Co., Ltd.

Number of sections: The first section is the first molding of the head, the second section is the second molding of the head, and the third section is the molding of the head three times, totaling 3 segments.

Pressing speed: 1 / sec

For the obtained bolts, No. 2, 4, 6, and 8, as shown in Table 3, were subjected to zinc plating treatment as a surface treatment, followed by baking treatment under the conditions shown below; No. 10, No. 14 and No. 16, and baking treatment was performed without performing zinc plating treatment. No. 1 and No. 2, No. 3 and No. 4, No. 5 and No. 6, No. 7 and No. 8, No. 9 and No. 10, No. 13 and No. 14, and No. The groups of .15 and No.16 are used in the manufacturing steps, respectively, using steels A, B, C, D, E, G and H, which are manufactured under the same conditions before bolting, and after plating with bolts - There are different examples of baking treatment.

(baking conditions)

Processing temperature: 195 ° C

Processing time: 4 hours

Number of times: 1 time

No. 1, 3, 5, 7, 9, 11, 12, 13 and 15 are bolts which are only cold pressed, and No. 2, 4, 6, 8, 10, 14 and 16 are subjected to the above plating treatment. And/or the bolt after the baking treatment, the structure was observed by the following method, and the mechanical characteristics and the delayed destructive property were evaluated.

(1) Organization observation of bolts

As the structure of the bolt, the area ratio of the ferrite iron, the area ratio of the Boron iron, and the grain size of the ferrite iron crystal were measured as described below. The results are shown in Table 3.

(area ratio of fertilized iron, grain size of ferrite grain)

After being cut in the axial direction of the vertical bolt shaft portion, the screw portion of the bolt was etched with a nital alcohol solution, and the D/4 position (field of view: 0.08 mm ² ) was observed by an optical microscope at a magnification of 400 times. The area ratio of the ferrite iron was measured by image analysis. In the image analysis described above, Analytical Software Particle Analysis III (manufactured by Sumitomo Metal Technology Co., Ltd.) was used. In addition, the ferrite iron crystal grain size number is obtained by comparison with a standard sample according to a comparison method.

(the area ratio of the Bora)

The area ratio of the Bora iron is obtained by subtracting the area ratio of the ferrite iron from 100%.

The manufacturing method of the embodiment of the present invention, after lead immersion No longer heated to the Worthfield iron area, so the area ratio of the ferrite iron, the grain size of the ferrite grain, and the area ratio of the Borne iron are almost equal in the wire, steel wire and bolt after lead impregnation. In addition, the results of the construction of the bolts No. 1, 13, 15, and 10 in Table 3, and the grain size of the ferrite iron crystal of No. 9 are all "-", and the results are considered to be respectively No. 1, No. No.2, No.14 and No.16 obtained by plating and/or baking treatment of each of the bolts of .13 and No.15, No.9 before the baking treatment of the bolt of No.10, and The No. 10 obtained by baking the No. 9 bolt was almost the same.

(2) Mechanical properties of bolts

The tensile test of the bolt was carried out under the conditions of the clearance thread length: 1.5 × bolt shaft diameter d and crosshead speed: 5 mm/min, and the tensile strength and strength were measured, and the strength ratio was determined. The results are shown in Table 3.

(3) Determination of permanent elongation

Further, in the invention examples of the plating-baking treatment, that is, in Nos. 2, 4, 6, and 8, the permanent stretching of the bolt was measured as follows. That is, the bolt load after the above treatments was guaranteed to be loaded (9.8 grade M8 bolt: 35500 N, grade 8.8 M10 bolt: 21200 N) for 15 seconds, and the elongation after the load was removed was measured. Further, the conventional examples of baking and the inventive examples of baking, namely Nos. 10, 14, and 16, were also measured for permanent elongation.

As a result, No. 2 was 10.7 μm, No. 4 was 6.7 μm, No. 6 was 3.5 μm, No. 8 was 3.2 μm, No. 14 was 6.5 μm, No. 16 was 3.3 μm, and No. 16 was 3.3 μm. 10 is 8.7 μm. Meet JIS In the case of the guaranteed load specified in B1051 (2014), it is preferable that the permanent elongation at the time of the guaranteed load test is 12.5 μm or less, but the above examples satisfy this range. The permanent elongation is preferably 7 μm or less, more preferably 5 μm or less. Further, from these examples, No. 4, 6, 8, 14, and 16 were subjected to baking treatment at a lower temperature than the blue treatment, but the permanent elongation was sufficiently suppressed.

(4) Bolt resistance to delayed damage

For some of the bolts, the evaluation was resistant to delayed damage. The details were as follows: after immersing in a 36% hydrochloric acid aqueous solution for 30 minutes for hydrogen charging, the micro-speed tensile test was carried out under the following conditions to determine the elongation at break ratio, that is, the crack elongation of the bolt after hydrogen permeation. Degree] / [rupture elongation without hydrogen permeation]. Then, No. 1 to 12 having a bolt diameter of 9.06 mm was evaluated as "the bolt has the necessary resistance to delayed fracture" when the elongation at break ratio is 0.80 or more, and the bolt elongation is less than 0.80. Have the necessary resistance to delay damage." On the other hand, No. 13 to 16 having a bolt diameter of 7.04 mm was evaluated as "the bolt has the necessary anti-delay resistance" when the elongation at break ratio is 0.70 or more, and the elongation at break ratio is less than 0.70. Bolts do not have the necessary resistance to delay damage." As a result, No. 2 was 0.95, No. 4 was 0.98, No. 6 was 1.00, No. 8 was 0.94, No. 14 was 0.72, No. 15 was 0.87, No. 16 was 0.89, and each bolt was necessary. It is resistant to delayed damage.

(Micro speed tensile test conditions)

Crosshead speed: 0.01mm/min

Thread length: 1.5d

These results can be explored as follows. No. 1 to 8 and No. 13 to 16 are examples of the invention which satisfy the requirements of the embodiment of the present invention, and the steel wire according to the embodiment of the present invention exhibits excellent cold pressability, and it is known from the obtained bolt that even if it is not performed The high-temperature heat treatment such as bluing treatment also shows a high strength and strength ratio similar to No. 10 of the conventional example which has to undergo blue treatment, and also has anti-delay resistance. In particular, No. 14 and No. 16 are excellent non-heat-treated bolts having both desired strength and reduced permanent elongation.

On the other hand, Nos. 9, 11, and 12 are examples in which any of the requirements specified in the embodiment of the present invention is not satisfied, and the characteristics are deteriorated. In other words, in No. 9, since the amount of C is excessive, a large amount of solid solution-free carbide remains during heating before hot rolling, and sufficient ferrite is not secured, and the strength ratio is small. No. 11 is an example in which V and Nb which are not actively used in the embodiment of the present invention are added. Such as When V and Nb are added, although the characteristics can be ensured, it is not easy to control the form of precipitates in the production stage. Further, the composition and organization of the No. 12-based rigid material were outside the predetermined range, and a comparative example in which the desired strength ratio could not be obtained by heat treatment such as bluening treatment.

This application also claims Japanese Patent Application No. 2016-071429, which is filed on March 31, 2016, and Japanese Patent Application No. 2017-033373, which is filed on February 24, 2017. Priority of the basic application. Japanese Patent Application No. 2016-071429 and Japanese Patent Application No. 2017-033373 are hereby incorporated by reference.

Claims (6)

A wire for non-heat-treated bolts, characterized by a component composition in mass %, satisfying C: 0.05 to 0.30%, Si: 1.10 to 2.5%, Mn: 0.50 to 2.5%, P: more than 0%, 0.03% or less, S : more than 0%, 0.03% or less, Cr: 0.03% to 0.30%, Al: 0.010 to 0.10%, and N: 0.0020 to 0.0100%, and the rest is formed by iron and unavoidable impurities, by the following formula (1) The carbon equivalent Ceq is 0.60 or more and 0.80 or less, the area ratio of ferrite iron in the whole structure is 60% or more, and the area ratio of pearlite in the whole organization is 5% or more. In addition, the grain size number of the ferrite iron is 10 or more, Ceq=[C]+[Si]/7+[Mn]/5+[Cr]/9 (1) in the above formula (1) [Element] represents the content of each element in the steel in mass%. A method for manufacturing a non-heat-treated bolt wire according to claim 1, characterized in that the heating temperature of the press delay is set to 1000 ° C or more and 1200 ° C or less, and after the hot rolling, the average cooling rate from 800 ° C to 500 ° C is set. for 2.0~5.0°C/sec. A steel wire for non-heat-treated bolts, characterized in that the composition of the components is in mass%, satisfying C: 0.05 to 0.30%, Si: 1.10 to 2.5%, Mn: 0.50 to 2.5%, P: more than 0%, and 0.03% or less, S: more than 0%, 0.03% or less, Cr: 0.03% to 0.30%, Al: 0.010 to 0.10%, and N: 0.0020 to 0.0100%, and the rest is formed by iron and unavoidable impurities, by the following formula (1) The carbon equivalent Ceq is 0.60 or more and 0.80 or less, and the area ratio of ferrite iron in the whole tissue is 60% or more, and the area ratio of the Borne iron in the whole tissue is 5% or more. The grain size of the granular iron is No. 10 or higher, Ceq = [C] + [Si] / 7 + [Mn] / 5 + [Cr] / 9 (1) In the above formula (1), [element] Indicates the content of each element in the steel in mass %. The invention relates to a method for manufacturing a steel wire for non-heat-treated bolts, which is a method for manufacturing a steel wire for non-heat-treated bolts according to claim 3, which is characterized in that the wire for non-heat-treated bolts according to claim 1 is used, and the surface reduction rate is 20 ~40% Line drawing. A non-heat treated bolt, characterized by a compositional composition in mass %, satisfying C: 0.05 to 0.30%, Si: 1.10 to 2.5%, Mn: 0.50 to 2.5%, P: more than 0%, 0.03% or less, S: exceeding 0%, 0.03% or less, Cr: 0.03% to 0.30%, Al: 0.010 to 0.10%, and N: 0.0020 to 0.0100%, and the rest is formed by iron and unavoidable impurities, and is represented by the following formula (1). The carbon equivalent Ceq is 0.60 or more and 0.80 or less, and the area ratio of the ferrite iron in the whole structure is 60% or more, and the area ratio of the ferrite to the whole structure is 5% or more. The particle size number is 10 or more, Ceq=[C]+[Si]/7+[Mn]/5+[Cr]/9 (1) In the above formula (1), [element] represents each of the steels. The content of the element in mass%. The non-heat treated bolt of claim 5, wherein the tensile strength is 780 MPa or more and 1200 MPa or less, and the strength ratio is 0.80 or more.