JPH0248607B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a method for producing a cold-drawn ferrite/pearlite steel bar for a main metal fitting of a spark plug, which is mainly of a non-heat refining type and has high breaking torque and excellent hot and cold caulking properties. Conventional technology Steel materials for the main metal fittings of spark plugs that are screwed onto the cylinder head of internal combustion engines are conventionally used.
Low carbon steels for mechanical structures such as S10C to S20C steels are mainly used. However, in recent years, with the demand for lower fuel consumption, lighter weight, and higher output in automobile engines, the installation space for spark plugs has become narrower, and spark plugs with smaller diameter M10 installation screws have become smaller.
Attempts have been made to use the following small spark plugs. In such a small spark plug, as shown in FIG. 1, the thickness of the screw neck 13 between the large-diameter body 11 of the metal shell 10 and the mounting threaded portion 12 is necessarily thin, for example, M10. About 1.14mm for spark plugs, 0.86mm for M8 spark plugs
It's just a matter of degree. Therefore, the above-mentioned S10C and S20C
When low carbon steel such as steel is used, breakage is likely to occur at the screw neck 13 when the spark plug is tightened to the cylinder head. On the other hand, reducing the diameters of the insulator 20 and the center electrode 30 fitted into the cavity of the metal shell 10 reduces the structural strength and withstand voltage performance of the insulator, and also reduces the axial direction of the center electrode. Since the abrasion resistance and heat resistance deteriorate due to the decrease in thermal conductivity of It is also difficult in practice to prevent this. Furthermore, even if a steel material with a high carbon content such as S30C or S35C steel is used for the purpose of improving the breaking strength of the threaded portion of the metal shell, other problems cannot be avoided. That is, as shown in FIG. 1, in the spark plug, the insulator 20 holding the center electrode 30 is inserted into the inner cavity of the metal shell 10, and the stepped portion 22 connected to the large diameter portion 21 of the insulator 20 is inserted into the inner cavity of the metal shell 10. is secured to the inwardly protruding step portion 14 of the metal shell 10 via the gasket 40, while the insulator 2
A sealing material 50 and a packing 60 are fitted between the upper small diameter portion 23 of the metal shell 10 and the inner cavity of the metal shell 10, and then the body portion 11 of the metal shell and the fastening portion 15 such as a hexagonal ring in the shape of a bolt head are connected. The insulator 20 is fixed by so-called heat caulking, in which the thin heating portion 16 between the two is heated by passing an electric current or by high-frequency induction heating, and the upper peripheral edge 17 of the metal shell is caulked. In this way, the insulator 20 usually
When using S30C or S35C steel, which has a large amount of C as mentioned above, the heating part 16 is rapidly cooled during this heat caulking, resulting in a martensitic structure, and the heating part Cracks and cracks will occur, and the product will lose its value. On the other hand, in addition to the hot caulking described above, cold caulking, which does not apply heat, is also known as a caulking method, and instead of hot caulking, cold caulking can also be used to fix the metal shell to the insulator. . S10C mentioned above
and S20C steel can be cold caulked without any problems, but as mentioned above, breakage tends to occur at the screw neck. When using high carbon steels such as S30C and S35C steels, they do not have sufficient toughness and the screw necks are thin, which causes cracks and cracks during cold caulking. As another method, applying heat treatment to the screw neck portion and caulking portion after forming the metal shell is difficult because each portion is narrow, and also increases manufacturing costs. As mentioned above, especially in the manufacture of metal shells for small spark plugs, the steel material used has high breaking torque and excellent hot and cold caulking properties without post-heat treatment after manufacturing the metal shell. However, there has been no known steel material for the main metal fitting of a spark plug that has such novel properties. Problems to be Solved by the Invention The present invention has been made in order to solve the above-mentioned problems in manufacturing small spark plugs with a small mounting thread diameter of M10 or less. The object of the present invention is to provide a method for producing a cold-drawn ferrite/pearlite steel bar for a main metal fitting of a spark plug, which is mainly of a non-heat refining type and has both hot caulkability and cold caulkability. Means for Solving the Problems The method for manufacturing a cold-drawn steel bar for a spark plug main metal fitting according to the present invention includes (a) C 0.03 to 0.20%, Si 0.35% or less, Mn 0.1 to 2.0%, P 0.025 in weight %; (b) Zr 0.005-0.25%, Nb 0.005-0.10%, V 0.03-0.25%, Ti 0.005-0.25%, Cr 0.05-0.05%. 0.50%, and at least one selected from the group consisting of Ni 0.05~0.50%, with the balance consisting of iron and unavoidable impurities.
After heating to 1150℃, hot rolling is performed at an average cooling rate of 0.1 to 10℃/sec from a temperature in the range of 800 to 950℃.
It is cooled to a temperature in the range of 500°C to room temperature to form a steel with a ferrite-pearlite structure, and then
This is then cold drawn to a tensile strength of 70Kgf/
It is characterized by being formed into a steel bar of mm ² or more. First, the reason for limiting the chemical composition of the steel used in the method of the present invention will be explained. C must be added in an amount of at least 0.03% to provide the required strength to the steel through solid solution strengthening. When the amount of C is less than 0.03%, not only will the steel strength be insufficient, but the zinc phosphate lubricant will often not adhere to the steel material sufficiently, and in such cases, cold forging During molding into the spark plug main metal fitting, seizure occurs between the tool and the steel material, significantly shortening the tool life. However, when adding excessive amounts exceeding 0.2%,
When a spark plug main metal fitting is manufactured from such steel material and hot caulked to an insulator as described above,
A martensitic structure with a Bitkers hardness (Hv) of 450 or more is formed in that part, and cracks occur in the heat caulked part. Also, when adding excessive amounts exceeding 0.2%,
This causes a decrease in the toughness of the steel material itself, so when a spark plug main metal fitting is manufactured from such a steel material and cold caulked to an insulator as described above, cracks will occur in the caulked portion. Preferably, the amount of C is in the range of 0.05-0.20%. Generally, Si dissolves in steel to increase its strength, but in the steel of the present invention, when it exceeds 0.35%, it deteriorates the toughness of the hot caulked part or the toughness of the steel material itself. Since it deteriorates cold caulking properties, the upper limit is set at 0.35%. Mn fixes the impurity element S, which is the main cause of cracking during hot rolling and cold forging, as MnS,
It has the effect of making it harmless, and is an effective element for imparting both a predetermined strength and excellent toughness to relatively low-carbon steel, as in the present invention. In order to effectively obtain such effects, it is necessary to add 0.1% or more;
However, when added in excess, it deteriorates the workability during molding of the spark plug metal shell and also deteriorates the toughness of the metal shell. Furthermore, since it increases the hardenability of the steel and deteriorates both hot caulking and cold caulking, the upper limit of the amount added is set at 2.0%. Preferably, the amount added is in the range of 0.5-2.0%. P promotes work hardening and deteriorates cold caulking properties, as well as deteriorating the toughness of hot caulked parts, so the lower the content, the better, but lowering the content too much may impair the economic efficiency of steel manufacturing. Therefore, in the present invention, it is sufficient to set it to 0.025% or less. S forms sulfide-based inclusions, which become the starting point for cracks during hot caulking, and also deteriorates the deformability during cold forging when manufacturing the metal shell, so its content should be 0.025% or less. It is sufficient.
However, depending on the required characteristics such as the required hot caulking degree, cold caulking degree, cold working degree, machinability, etc.
It can be set even lower, preferably with an upper limit of 0.015%. In addition to its effect as a deoxidizing agent during steel manufacturing, Al has the effect of refining crystal grains during hot caulking, thereby improving the toughness of the hot caulked portion. Furthermore, Al has the effect of fixing solid solution N by combining with N and precipitating as AlN, and suppressing strain aging and blue brittleness.
In order to effectively obtain these effects, at least
It is necessary to add 0.005% or more. However, when adding too much, it increases the number of B-based inclusions and causes cold forging cracks, hot caulking cracks, or cold caulking cracks during molding of the metal shell. Should be 0.080% or less. Furthermore, in addition to the above-mentioned elements, the steel used in the method of the present invention contains Zr, Nb, V, Ti, Cr, and Ni.
Contains at least one selected from the group consisting of: These elements are effective in improving the strength and toughness of steel. Zr, Nb, V and Ti are
During rolling or during the cooling process of the steel after rolling, strength is improved by grain refinement and precipitation effects due to the precipitation of fine carbonitrides. In addition, the refinement of grains improves the workability during cold forging, and when hot staking, it improves the toughness of the part, and when cold staking, it improves the toughness of the part. Cold caulking properties can be obtained. In order to effectively obtain these effects, the above-mentioned elements are added in the following ranges. That is,
Zr0.005~0.25%, Nb0.005~0.10%, V0.03~0.25
%, and Ti ranges from 0.005 to 0.25%. Even if excessive amounts of each element are added exceeding the above upper limit values, the effects will be saturated and this is not preferable from the economic point of view of steel production. Cr improves hardenability and increases strength.
In order to effectively obtain this effect, it is necessary to add 0.05% or more. However, when adding too much, the hardenability becomes excessively large and the heat caulkability deteriorates, and the cold caulkability deteriorates as the toughness of the steel material itself decreases, so 0.50%
It is added in the following range. Ni improves strength and cold workability, and
It is necessary to add 0.05% or more in order to improve the toughness of the hot caulked part and also improve the cold caulking property, but even if it is added in excess, the effect will be saturated and the steel manufacturing process will be affected. Since it impairs economic efficiency, the upper limit of the amount added is set at 0.50%. The ferrite/pearlite cold-drawn steel bar for the main metal fitting of a spark plug according to the present invention is produced by heating steel having the above-mentioned chemical composition to 950 to 1150°C, then hot rolling, and then averaging cooling from a temperature in the range of 800 to 950°C. The steel is cooled at a rate of 0.1 to 10°C/sec to a temperature in the range of 500°C to room temperature to produce a steel with a ferrite/pearlite structure, which is then cold drawn to a tensile strength of 70 Kgf/mm ² or more. Manufactured by forming into a steel bar. By cutting such a steel bar into a predetermined shape and threading it, it can be used as a main metal fitting of a spark plug without any post-heat treatment. In the method of the present invention, the steel heating temperature is set such that after the precipitates are dissolved in the matrix, fine carbonitrides are precipitated to refine the crystal grains and harden the precipitation, or to reduce the initial crystal grains. In order to achieve fineness, a temperature of 1150°C or lower is sufficient, but if the temperature is lower than 950°C, deformation resistance becomes high during hot rolling, leading to a decrease in productivity. Therefore, in the method of the present invention,
Steel heating temperature shall be in the range of 950-1150℃. Next, after hot rolling, water cooling is performed to set the cooling start temperature to a range of 800 to 950°C, and from this temperature to an average cooling rate of 0.1 to 10°C/sec, preferably,
Cool to a temperature in the range of 500°C to room temperature at an average cooling rate of 0.1°C/sec or more but less than 5°C/sec. If the cooling start temperature exceeds 950°C, the crystal grains become coarse, which deteriorates toughness and ductility, and deteriorates workability such as cold forging, thread rolling, and cold caulking. On the other hand, the cooling start temperature is
When the temperature is lower than 800°C, the quality of the obtained products varies greatly, making it difficult to maintain constant quality. According to the method of the present invention, by cooling at a predetermined cooling rate from the cooling start temperature to a predetermined temperature range, a dense ferrite-pearlite structure is obtained, and the required strength and toughness are obtained. , prevents strain aging during cold forging and suppresses shortening of tool life. For this purpose, an average cooling rate of 0.1° C./sec or more is required, but normally cooling is sufficient. However, when the average cooling rate exceeds 10°C/sec, bainite structure will be mixed, resulting in excessively high strength or large variations in strength, and furthermore, toughness and ductility will decrease. , the threads are likely to fold during thread rolling, and the cold caulking properties are reduced. Furthermore, tool life is also deteriorated. According to the present invention, the ferrite-pearlite steel thus obtained is subjected to cold drawing at a reduction in area of 20 to 30%, for example, to achieve a tensile strength of 70%.
Steel bars and wire rods such as hexagonal steel bars and round steel bars with a weight of Kgf/mm ² or more, usually 80 to 100 Kgf/mm ² can be obtained. The steel used in the present invention is melted, for example, in an LD converter, but the melting method is not limited at all. In order to manufacture a spark plug main metal fitting using the above-mentioned steel material according to the present invention, for example, the above-mentioned cold-drawn steel wire is cut, cold-forged, or a combination thereof is processed to form the desired shape. After forming, thread cutting may be performed by cutting, rolling, etc. Effects of the Invention As described above, the ferrite/pearlite steel bar or wire rod obtained by the method of the present invention not only has a predetermined chemical composition and excellent workability, but also has a tensile strength of 70 Kgf/mm ² In addition to having a high breaking torque and excellent heat caulking properties and excellent cold caulking properties, the spark plug main metal fitting manufactured from such a steel material does not require post-heat treatment. , has higher tensile strength than conventional materials,
Therefore, when hot caulking the metal shell to the insulator, there is no cracking in the hot caulking part, and when cold caulking, there is no cracking in the thin wall part, and the thread properties are good. It is. Furthermore,
Since there is no breakage of the screw neck of the metal shell when the spark plug is screwed into the cylinder head, it can be particularly suitably used in manufacturing metal shells for small spark plugs. EXAMPLES The present invention will be explained below with reference to Examples, but the present invention is not limited to these Examples in any way. Steel having the chemical composition shown in Table 1 is melted in a vacuum melting furnace, hot rolled under the rolling conditions shown in Table 1, and then cold drawn with an area reduction of 20 to 30%. Steel wire was manufactured using Next, a spark plug main metal fitting with a mounting screw diameter of M10 was formed by cutting from this steel wire, and the breaking torque at the screw neck was measured. The results are shown in Table 1, along with the structure of the rolled material, the mechanical properties of the drawn steel wire, and the tensile strength of the threaded portion of the metal shell. In Table 1, steel numbers 1 to 3 are examples of commercially available low carbon steels for mechanical structures. In addition, when measuring the breaking torque of the screw neck of the metal shell, the breaking torque is significantly affected by the surface condition of the jig and the metal shell, that is, the degree of friction, so apply lubricating oil to both the metal shell and the jig. The friction coefficient was then measured at a constant value (0.15), and a relative comparison was made with the conventional material S17C as 100. As is clear from the results shown in Table 1, the steel material obtained by the method of the present invention is manufactured by cutting.

【table】

[Table] All of the shaped spark plug main metal fittings have sufficient strength, and the tensile strength of the threaded part is
It was ^over 70Kgf/mm2. In addition, the breaking torque of the screw neck is lower than that of the metal shell made of S17C material.
The value is about 50% higher. Therefore, the steel material according to the present invention can be particularly suitably used for manufacturing a small spark plug main metal fitting. Next, after heating the thin-walled heating part of the metal shell with electricity,
As a result of hot caulking and then examining the occurrence of cracks in that part, as shown in Table 1, no cracks were observed in the spark plug main metal fittings molded and manufactured from the steel material according to the present invention. Ta. Furthermore, there was no folding of the threads, and the thread properties were good. Instead of the heat caulking described above, the insulator was fixed to the metal shell by cold caulking without applying heat. Even in the case of this cold caulking, as shown in Table 1, no cracks occurred in the thin walled portion of the metal shell (corresponding to the heating section 16 in FIG. 1).

[Brief explanation of drawings]

FIG. 1 is a partial sectional view showing a spark plug. DESCRIPTION OF SYMBOLS 10...Spark plug main metal fitting, 11...Large diameter body, 12...Mounting threaded part, 13...Screw neck of main metal fitting, 14...Step part, 15...Fascinating part, 16...
Heating part, 17... Upper edge of metal shell, 20... Insulator, 21... Insulator large diameter part, 22... Step part, 3
0... Center electrode, 40... Packing, 50... Seal material, 60... Packing.

Claims (1)

[Claims] Contains (a) C 0.03 to 0.20%, Si 0.35% or less, Mn 0.1 to 2.0%, P 0.025% or less, S 0.025% or less, and Al 0.005 to 0.080% in 1% by weight; Furthermore, (b) at least one member selected from the group consisting of Zr 0.005-0.25%, Nb 0.005-0.10%, V 0.03-0.25%, Ti 0.005-0.25%, Cr 0.05-0.50%, and Ni 0.05-0.50%. 950~ steel with the balance consisting of iron and unavoidable impurities
After heating to 1150°C, hot rolling is performed at a temperature of 800 to 950°C at a rate of 0.1°C/sec or more,
The ferrite-pearlite transformation is completed by cooling to a temperature in the range of 500℃ to room temperature at an average cooling rate of less than ℃/sec, which is then cold drawn to a tensile strength of 70Kgf/mm ² A method for manufacturing a cold-drawn steel bar for a spark plug main metal fitting, characterized by forming the steel bar as described above.