JP2952382B2 - Manufacturing method of hot pipe making tool - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a hot pipe making tool used as an inner rolling tool for piercing and elongation rolling when manufacturing a seamless pipe by the Mannesmann method. In this specification,% represents% by weight unless otherwise specified.

[0002]

2. Description of the Related Art In manufacturing a seamless pipe by the Mannesmann method, a billet is pierced and rolled into a hollow shell by a rolling mill such as a piercer, and then the hollow shell is stretched and rolled by a rolling mill such as an elongator. For these rollings, internal rolling tools such as piercer plugs and mandrels are used.

[0003] These tools are usually used by forming a highly lubricious oxide scale on the surface of a steel substrate manufactured by cutting into a tool shape after sand casting. As a material of the steel base, for example, in the case of a piercer plug, a 3% Cr-1% Ni-based steel (hereinafter referred to as a 3Cr-1Ni steel) developed for ordinary steel is generally used.

[0004]

When S45C is pierced and rolled using a piercer plug made of 3Ni-1Cr steel, the average rolling time is about 100 to 500 times, depending on the rolling conditions of the mill used. Disables the piercer plug. The cause is as follows.

[0005] Since the high temperature strength of the steel substrate is not sufficient, under severe use conditions, plastic flow such as surface wrinkles occurs as the number of uses increases, and the steel base becomes unusable. When the thermal load such as rolling using a large tool or continuous rolling for a long time is large, the high-temperature strength is insufficient even by the coarsening of the structure due to rolling, and the surface cannot be used due to wrinkles. Because the toughness of the steel substrate is not sufficient, fatigue fracture progresses due to repeated piercing rolling stress, and fracture such as thermal cracking progresses from the surface layer due to the heat cycle of the surface layer due to repeated hot piercing rolling and water cooling, In some cases, it may result in breakage.

In order to extend the life of the piercer plug, improvements in the composition of the steel base and heat treatment for forming an oxide scale on the surface of the steel base have been conventionally devised. However, recently the quality of seamless pipes has been further improved,
While the rolling conditions have become very severe, the demands for reducing the cost of pipe production have become more stringent than before. For this reason, it is not possible to sufficiently respond to recent advanced requirements only by improving the composition of the components and devising the heat treatment, and new countermeasures are required from a viewpoint different from conventional ones.

An object of the present invention is to provide a method for manufacturing a hot pipe making tool which can significantly extend the tool life by taking measures from aspects other than the composition of the components and heat treatment.

[0008]

As described above, the service life of the piercer plug made of 3Ni-1Cr steel is not due to the wear of the oxide scale, but rather to the occurrence of surface wrinkles or the like due to insufficient high-temperature strength of the steel base. It is determined. Therefore, the present inventors sought one of the causes of the lack of the high-temperature strength of the steel base in the manufacturing method of the base, and conducted various experimental studies.As a result, the following facts were found, and the present invention was completed. Was.

A conventional steel substrate is manufactured by sand casting. Sand casting is much lower cost than mold casting, so the demand for cost reduction is so severe that it is not possible to raise the cost of fabricating substrates from the current situation. Was considered difficult.

However, according to the investigation by the present inventors, when the production method of the substrate is changed from sand casting to die casting, the mechanical properties of the substrate are increased, and the oxide scale on the surface is modified. Extending the service life by more than 1.5 times,
In addition, the excellent mechanical properties are imparted not only to the surface part of the substrate but also to the inner layer, so that the substrate can be used repeatedly by re-cutting and re-scaling. It was found that the basic unit greatly improved.

The method for manufacturing a hot pipe making tool according to the present invention is directed to a method for manufacturing an oxide scale type hot pipe making tool which uses an oxide scale formed on the surface of a steel base. It is produced by die casting.

[0012]

[Effect] The mold has a higher cooling rate during casting than the sand mold. Therefore, the solidification structure becomes fine, and macro-micro segregation of main components and impurities is reduced. As a result, the steel substrate produced by die casting has a higher high-temperature strength than the steel substrate produced by sand casting, and has excellent ductility and toughness in a temperature range from room temperature to high temperature. Due to its small amount, it has high resistance to surface wrinkles and generation and propagation of thermal cracks, and also high resistance to surface damage due to plastic flow. Further, the oxide scale formed on the surface of the substrate becomes denser as the base structure becomes finer, and the peeling resistance and the seizure resistance are improved. As a result, the tool life is 1.5
More than double.

In addition, these excellent properties are maintained not only on the surface layer of the substrate but also on the inner layer, and the deterioration is hardly recognized by external cutting, so that it can be used repeatedly by cutting again and scaling. Becomes

It is generally said that the cost of manufacturing a mold casting material is twice as large as the cost of producing a sand casting material even if repeated use of the mold is taken into consideration. The service life is 1.5 times or more, and its repetitive use is possible, so that the high cost of casting can be offset to show excellent economic efficiency.

In the method of the present invention, usually, a raw steel is die-cast into a shape slightly larger than a tool, and the cast material is cut into a tool shape to produce a steel base, which is then subjected to a heat treatment for scaling. Do.

The material steel is 3Cr-1Ni steel, for example, M
It is possible to use Mo-W added steel or the like in which o and W are added in a total amount of 1.5% or more to increase the high-temperature strength, and the steel type is not particularly limited. In the present invention, the service life of the tool can be extended 1.5 times or more regardless of the type of steel. The main components of the material steel applicable to the present invention are as follows.

C: 0.05 to 0.5% by weight, Si: 0.0
5 to 1.0%, Mn: 0.1 to 2.0%, Cr: 5.0% or less,
Ni: 4.0% or less, W and / or Mo: 10 in total
% Or less, and, if necessary, one or two of Co: 3.0% or less, and Ti and / or Nb: 0.5% or less in total.

C is effective for improving the high-temperature strength. C <0.
If the content is 05%, sufficient high-temperature strength cannot be obtained, and if C> 0.5%, the hardness of the portion where the surface is burned after the tube is formed becomes too high, and quenching cracks are easily generated by water cooling after the tube is formed.

Si is effective for deoxidation, raising the Ac ₁ point, and densifying the oxide scale on the surface of the substrate, but Si <0.05%
However, if Si is more than 1.0%, the toughness deteriorates and the lubricity deteriorates due to the thinning of the scale layer.

When a large amount of Mo or W is added, Mn has an effect of securing an austenite single phase at a high temperature. Mn <
At 0.1%, the effect is not sufficiently obtained, and at Mn> 2.0%, Mn in the scale lowers the compactness of the scale and raises the melting point of the scale to deteriorate the lubricity.

Cr is an effective component for securing high-temperature strength and improving the adhesion of the oxide scale layer by densification. However, when Cr> 5.0%, the oxidation resistance is excessively improved and the oxide scale is excessively improved. The layer does not thicken.

Ni has the effect of improving the toughness of the quenched and hardened layer formed on the surface of the substrate after pipe production, and of suppressing the precipitation of the δ-ferrite phase. Further, Ni is finely dispersed without being oxidized in the scale and greatly improves the peel resistance of the scale. However, when Ni> 4.0%, the formation of scale is suppressed, and the lubricity is rather deteriorated.

Co has the same effect as Ni and can be added up to 3%.

W and Mo are effective elements for securing high-temperature strength. For tools used for rolling high-alloy steel such as stainless steel, it is desirable to add 1.5% or more in total. Also, W + Mo ≧ 1.5% and 7W (%) + 8
By satisfying Ni (%) ≦ 56, the oxide scale structure in the vicinity of the base material changes from the grain boundary oxidation type to the intragranular oxidation type, and the peeling resistance of the oxide scale is particularly improved. However, when the total amount exceeds 10%, the ferrite remains even at high temperatures, and conversely, the high-temperature strength decreases and the toughness also deteriorates.

Ti and Nb are effective elements for refining the structure and improving the strength. However, if Ti + Ni> 0.5%, it precipitates as inclusions and lowers toughness.

As for impurities, N is used during melting (at the time of solidification).
0.05% or less, P and S are 0.035% or less and 0.05% or less, respectively, from the viewpoint of toughness and crack prevention during casting.
It is desirable to limit it to 030% or less.

The main components of the 3Cr-1Ni steel-based plug material are, by weight%, 0.20 to 0.40% C and 0.3% Si.
11.0%, Mn: 0.3 to 1.0%, Cr: 1.0 to 4.0%,
Ni: 0.5 to 4.0%, W and Mo are added in a total amount of less than 1.5% to increase the high-temperature strength, and Ti and B are added to refine the structure and improve the strength. May be added.

In the casting, as shown in FIG. 1, the mold 1 having an inner shape slightly larger than the outer shape of the tool base and having a gas vent hole 8 in the upper part, and the ceramic 7 in a portion corresponding to a pouring port are provided. Place it on the embedded flat mold 2 (platen),
Material steel 4 is cast therein through a runner made of refractory material 6. Gray cast iron (FC15 or the like in JIS) or cast steel is usually used as the material of the mold.

At the time of casting, a refractory coating agent 3 using sodium silicate or the like as a binder is applied to the inner surface of the mold 1 and the upper surface of the mold 2. The mold 1 may be provided with a water-cooling jacket in order to increase the cooling capacity. The mold temperature in this case is about 100 ° C. Expecting a decrease in mold filling capacity due to quenching, making the molten steel cast into the mold as laminar as possible, finishing the casting quietly and quickly, and promoting gas emission are the same as those in the conventional mold casting. Is the same. The cast material 5 obtained by opening the mold is externally cut into a predetermined tool shape to form a steel base.

If the tool is a piercer plug, its shape is a pointed bullet. Therefore, in the same casting as in the past, the cooling of the tip is too fast to obtain the feeder effect,
Solidification defects such as cavities due to solidification shrinkage are likely to occur. Therefore, it is necessary to devise ways to enhance the feeder effect, such as casting with the tip end down, increasing the thickness of the coating agent at the tip end to lower the cooling ability. Also, in the application of the mold wash, if the thickness of a part of the film is too thick, a temperature gradient will occur, and only a part of the feeder will make the cavity elongated and crack easily, so careful film thickness management is necessary.

When a tool base is manufactured by die casting, a solidified structure with a small number of segregation is obtained by rapid solidification, and the tool life is extended as compared with the case of sand casting. Further, since columnar crystals grow long from the surface to the inside and fine crystal grains continue to the inside, the same structure is formed from the surface layer portion to the inner layer portion.

In order to clarify the quenching effect in die casting, Table 1 shows the value of the thermal diffusivity α in Fourier's equation representing thermal diffusion in comparison with the case of sand casting. The Fourier equation is as shown in Equation 1. In addition, Table 2 shows the mechanical properties of the 3Cr-1Ni steel plugs produced by die casting in comparison with those produced by sand casting.

As can be seen from Table 2, the plug made by die casting has better surface layer mechanical properties than the plug made by sand casting, and the mechanical properties are maintained up to the inner layer. Even at a distance of 100 mm from the surface, it has properties much better than the surface layer of the sand mold material.

[0034]

[Table 1]

[0035]

(Equation 1)

[0036]

[Table 2]

In the heat treatment for forming the scale layer on the surface of the steel base, the heat treatment atmosphere affects the structure of the scale to be generated. In the method of the present invention, it is desirable that the oxygen concentration in the atmosphere be adjusted to the atmospheric oxygen concentration or less. If the oxygen concentration in the atmosphere is higher than the atmospheric oxygen concentration, the iron oxide grows rapidly during the heat treatment, so that it becomes a thick but poorly dense porous scale and easily peels off. In general, the oxygen concentration is desirably 1.0% or less, and CO gas and H ₂ O (water vapor) are mixed as necessary.

The temperature of the heat treatment affects the structure of the formed scale and the growth rate. In the method of the present invention, the heat treatment temperature is 800
It is desirably set to １1100 ° C. The reason is 800
When the temperature is lower than 0 ° C., the oxide forming ability is inferior, so that a very long heat treatment time is required, which is not suitable for actual operation.
If the temperature exceeds 00 ° C., the scale growth becomes remarkably fast, the scale becomes unstable and porous, and peeling easily occurs.

The heat treatment time affects the thickness and structure of the formed scale. In the method of the present invention, the heat treatment time is 4 to 10 hours.
r is desirable. The reason is that if it is less than 4 hours, the component system will not have a sufficient scale thickness, while
If it exceeds hr, the scale growth becomes excessive and the scale thickness becomes too thick, and the scale becomes porous and peeling is liable to occur.

The cooling rate after soaking affects the compactness of the scale. In the method of the present invention, after soaking, it is preferable to gradually cool to 800 to 500 ° C. at a cooling rate of 50 ° C./hr or less. If the cooling rate exceeds 50 ° C./hr, cracking of the scale and peeling-off occur. Particularly desirable is 20 to 30 ° C.
It is preferable to gradually cool at a cooling rate of / hr or less. If the end temperature of the slow cooling is higher than 800 ° C., peeling and falling off of the scale will occur in the subsequent rapid cooling.
If the temperature is lower than 00 ° C., the cooling time becomes too long, which causes a problem in operation.

Through such heat treatment, and further through shot blasting before heat treatment, the thickness of the surface oxide scale layer becomes 100 μm or more. If it is less than 100 μm, the lubricity of the scale and the heat resistance to the base material cannot be obtained, and the desired tool life for drilling cannot be obtained. In addition, when the oxide scale layer is 500
If it exceeds μm, it becomes too thick and the scale layer is porous and easily peeled off.

The scale layer is generally divided into two layers. The outer layer is a relatively porous scale mainly composed of FeO, and the inner layer is a dense scale layer of a spinel / composite oxide type. The interface between the inner layer scale and the base material is of a grain boundary oxidation type or an intragranular oxidation type.

[0043]

Embodiments of the present invention will be described below to clarify the effects.

Using a 500 kg high frequency induction electric furnace, steels having the chemical composition shown in Table 3 were melted, and each ingot obtained by die casting and sand die casting was machined to form a piercer plug having a maximum outer diameter of 147 mm. Finished on a steel substrate.

Next, the manufactured steel substrate was placed at 720 ° C. × 2
After a softening treatment of Hr was performed and the surface layer scale was once removed by shot blasting, heat treatment for scaling was performed. In this heat treatment, CO =
The test was performed at a predetermined temperature, time, cooling rate, and slow cooling end temperature at 1%.

Then, for the obtained piercer plug,
The thickness of the surface scale layer was actually measured from a cross-sectional microphotograph of the center of the plug, and the scale structure was specified. The mechanical properties were also investigated. Further, the piercer plug thus produced was subjected to a piercing test on an actual machine to evaluate its life.

[0047]

[Table 3]

In the drilling test, an S45C billet having an outer diameter of 187 mm was used for a steel piercer plug as a material to be rolled, and an outer diameter of 18 mm was used for a steel piercer plug.
7 mm SUS420 and SUS304 billets were used. The rolling conditions were as follows: In the case of S45C, the billet length was 3380 mm and the shell length was 10500 mm.
Billet length 2100m for 20 and SUS304
m, and the shell length was 6500 mm. Table 4 shows the heat treatment conditions and the oxide scale thickness for the steel to piercer plugs, and Table 5 shows the mechanical properties and the number of drilling passes.

[0049]

[Table 4]

[0050]

[Table 5]

The steel is a 3Cr-1Ni steel. When the steel substrate is a die casting material, the inner layer scale becomes denser, and the mechanical properties of the substrate are improved both at room temperature and high temperature.
The number of perforations increased significantly from 122 in sand casting to 205.

The steel is Mo-W added steel in which the total amount of Mo and W is increased to 1.5% or more, and 3Cr-1Ni is used for drilling alloy steel.
High temperature strength is improved over steel. The number of piercings in the case of a sand casting material was four times in SUS420 piercing and rolling,
The piercing and rolling in US 304 was twice. However, the use of the mold casting material further improved the mechanical properties at high temperatures and further increased the thickness of the inner layer scale, so that the number of perforations increased to six and three times, respectively.

[0053] The steel is also a Mo-W added steel in which the total amount of Mo and W is increased to 1.5% or more, but the mechanical properties at high temperature are further enhanced and the scale thickness is increased. Therefore, even in the case of a sand casting material, SUS420, SUS30
The number of perforations was increased to 10 and 6 in each perforation of No. 4, however, in the case of the die casting material, the mechanical properties at high temperatures were further improved, and the inner layer scale was further thickened. The number of perforations increased to eight.

As described above, by manufacturing the tool base by die casting, the tool life is extended 1.5 times or more as compared with the case of manufacturing by the conventional sand die casting. When the tool base is made by sand casting, the number of re-uses of the tool is 1 to
It was 2 times, but when it was manufactured by die casting,
It could be reused more than once.

[0055]

As described above, the method for manufacturing a hot pipe making tool according to the present invention improves the mechanical properties of the steel base by producing the steel base by die casting, and forms the steel base on the surface. By improving the oxidation scale to be used, the tool life can be greatly extended, and the tool can be used repeatedly, which is very effective in improving the tool unit consumption and reducing the cost of pipe production.

[Brief description of the drawings]

FIG. 1 is a schematic view illustrating the state of die casting.

[Explanation of symbols]

 1, 2 Mold 3 Coating agent 4 Material steel (Molten metal) 5 Casting material (Steel substrate before finish cutting) 6 Runner refractory 7 Refractory ceramic 8 Gas vent hole

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazuo Kawaguchi 5-13-1, Shinjuku-cho, Kawagoe-shi, Saitama In-house Shinpokoku Steel Corporation (56) References JP-A-4-74848 (JP, A) JP-A Heisei 4-270003 (JP, A) JP-A-63-290695 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) B21B 25/00 B22C 9/22 B22D 25/02

Claims (1)

(57) [Claims] 1. A method for manufacturing an oxide scale type hot pipe forming tool using an oxide scale formed on a surface of a steel base, wherein the steel base is manufactured by die casting. A method of manufacturing a pipe making tool.