JPH08295989A - High carbon resistance welded tube excellent in wear resistance - Google Patents

Abstract

PURPOSE: To provide a resistance welded tube, after water hardening, with excellent wear resistance by specifying the composition of an Mn-type low alloy steel strip forming the steel tube. CONSTITUTION: The steel strip has a composition consisting of, by weight ratio, 0.40-0.50% C, 0.15-0.30% Si, 0.75-1.20% Mn, <=0.005% S, <=0.015% P, further at least one kind among <=0.10% of at least one kind among Cu, Ni, Cr, and Mo, and the balance Fe with inevitable impurities. A resistance welded tube of >=8mm wall thickness is formed by using this steel strip. At the time of coiling the hot rolled steel strip, water cooling for preventing cracking is not performed but slow cooling is performed to accelerate the precipitation of AlN, etc., by which the growth of austenite crystalline grains at subsequent heat treatment can be inhibited. Moreover, after resistance welding, the resulting weld zone of the steel tube is annealed, and then, the steel pipe is heated to the austenitizing temp. and subjected to water cooling and hardening from the inside of the steel tube, by which the wear resistance required of a tube for pipe transport can be provided and this tube can be used without tempering treatment.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric resistance welded steel pipe having excellent wear resistance, which is used for transportation of slurries such as coal and pneumatic transportation of dust.

[0002]

2. Description of the Related Art In recent years, slurry of earth and sand, ore, coal,
BACKGROUND ART Pipelines using steel pipes are becoming popular for pneumatic transportation of dust, pipe transportation of a mixture of solid and liquid, and the like. The inner surface of a steel pipe used in such a pipeline is constantly in contact with a solid substance transported in the pipe, and as a result, is constantly ground by the solid substance. Therefore, the wear of the inner surface of the steel pipe is extremely large, and therefore the life of the steel pipe is short, and frequent replacement is required. As a result, the cost is not so high and the operating rate of the plant is lowered.

Various methods have been developed or proposed in order to enhance the wear resistance of the inner surface of the steel pipe.
Coatings on the inner surface of the pipe, coating with a hard alloy such as stellite, and coating pipes with a resin are known. However, these methods have problems such as high cost and measures around the welded portion.

The wear resistance of steel pipes is generally improved by increasing their surface hardness. For example, it is known that the wear resistance of a mechanical structure component or the like is improved by forming a hardened layer on its surface by carburizing or the like. However, when the product is large, there is a problem that it is difficult to apply such a method and the cost becomes high.

Attempts have been made to improve the wear resistance even for steel products having a large unit weight.
Japanese Unexamined Patent Publication No. Hei 6-17188 discloses a technique in which a steel pipe having a specified C equivalent is heated, cooled from the inner surface, and tempered at a low temperature.
A wear resistant steel containing C, Si and Mn is disclosed. In this technique, steel has a mixed structure of a ferrite phase and a martensite phase, and a hard martensite phase is dispersed in a soft ferrite phase. Further, Japanese Patent Application Laid-Open No. 5-98351 discloses a technique of rolling a slab obtained by wrapping a soft steel having a low C content with a hard steel having a high C content to form a steel strip, and manufacturing a welded steel pipe from the steel strip. Has been done.

[0006]

However, the conventional techniques including the above-mentioned technique are not necessarily suitable for obtaining an electric resistance welded steel pipe having excellent wear resistance. That is, the electric resistance welded steel pipe excellent in abrasion resistance is required to be sound as a steel pipe and to be able to be manufactured efficiently and inexpensively. Also has the following drawbacks.

The technique disclosed in Japanese Patent Laid-Open No. 62-270725 is premised on tempering at a low temperature, but it is known that low temperature tempering is often a heat treatment for simultaneously lowering hardness and toughness. However, it is problematic to perform such heat treatment at a cost.

In the technique disclosed in Japanese Patent Laid-Open No. 6-17188, since a steel having a high Si content is used, it is not always easy to obtain a sound electric resistance welded portion, resulting in an increase in manufacturing cost. Further, in the technique disclosed in Japanese Patent Laid-Open No. 5-98351, the slab assembly is difficult,
Not an economical method.

The most common way to improve the wear resistance of steel is to increase the hardness of the steel. The hardness of the martensitic steel increases as the C content increases. Therefore, the higher the C content, the better the abrasion resistance. on the other hand,
Weldability, workability, etc. decrease as the C content increases. Further, quenching is effective for making the steel into a martensite phase, but if sufficient care is not taken, quench cracking is likely to occur. Incidentally, the above-mentioned JP-A-62-270.
The technique disclosed in Japanese Patent No. 725 is also said to be a technique for preventing this quench cracking, but it is not always clear in which state the crack is prevented from occurring.

From the above, the development of a wear-resistant ERW steel pipe that does not differ greatly from the ordinary ERW steel pipe production method, requires no change in production equipment, etc., and can be produced inexpensively and efficiently. Is waiting.

Therefore, an object of the present invention is to solve the above-mentioned problems and to be suitable as a steel pipe for transporting slurries of earth and sand, ores, coal, etc., solid pipes such as garbage, and pipe transport of a mixture of solid and liquid, An object of the present invention is to provide an electric resistance welded steel pipe having excellent wear resistance and capable of being manufactured economically and efficiently by a method similar to a usual manufacturing method.

[0012]

From the above viewpoints, the inventors of the present invention have developed a long-term electro-resistance welded steel pipe having an excellent wear resistance on the inner surface, which can be efficiently produced and is economical. The present invention has been completed as a result of intensive research over a period of time.

The invention of the present application is C: 0.40 to 0.
50 wt.%, Si: 0.15 to 0.30 wt.%, Mn: 0.
75 to 1.20 wt.%, S: 0.005 wt.% Or less, P
: 0.015 wt.% Or less, at least one component selected from the group consisting of: Cu: 0.10 wt.% Or less (including 0), Ni: 0.10 wt.% Or less (including 0), Cr:
0.10 wt.% Or less (including 0) and Mo: 0.1
It is characterized by being a high carbon ERW steel pipe having an excellent wear resistance, which is 0 wt.% Or less (including 0), the rest: Fe and unavoidable impurities and has a wall thickness of 8 mm or more.

[0014]

The electric resistance welded steel pipe of the present invention is produced by melting the steel having the above-mentioned chemical composition and forming a slab by continuous casting. The slab is heated to a predetermined temperature and hot-rolled to form a hot-rolled steel strip. The obtained hot-rolled steel strip is formed into an open pipe, then formed into an electric resistance welded steel pipe by high frequency induction welding or electric resistance welding, heated to an austenitizing temperature, and then the inner surface of the steel pipe is formed. It is manufactured by cooling with more water.

The metallic structure of the electric resistance welded steel pipe of the present invention contains the as-quenched martensite phase. Here, the term "as-quenched" has a positive meaning that normal tempering after quenching is not performed. If the ERW steel pipe is left to undergo the martensitic transformation, cracking often occurs. This crack may occur simply during storage of the steel pipe, but frequently occurs when welding or the like is performed. Such cracks hardly occur when the steel pipe is appropriately tempered, but on the other hand, the hardness of the steel pipe decreases and the wear resistance thereof deteriorates.

The inventors of the present invention have conducted research based on the idea that tempering treatment after quenching is not the only countermeasure for preventing the occurrence of cracks, and completed the present invention. The electric resistance welded steel pipe of the present invention is intended to be used for transportation of slurries and the like for pipelines, and therefore, it is firstly required to have excellent abrasion resistance, and further, the minimum required. Made to have weldability. Of course, the connection by the flange is also a consideration.

The most common method for imparting wear resistance to a steel pipe is to increase its hardness, as described above. And, the most common method for obtaining a steel pipe with high hardness is to utilize the martensitic transformation of steel. By the way, a steel pipe that has undergone martensitic transformation is generally brittle, and not only cannot withstand various processing, but also cracks occur during storage.

The above phenomenon is considered to be caused by the difference in time during which martensitic transformation occurs when quenching is carried out under the condition that quenching occurs in the entire steel pipe.
In order to prevent the occurrence of cracks, it has been found that it is effective to limit the component composition of the steel pipe to a specific range and control heating conditions and cooling conditions.

That is, the amount of martensite transformation is large in the vicinity of the inner surface of the steel pipe and has sufficient abrasion resistance, and it is effective to reduce the amount of martensite transformation as it enters the outer surface side in the wall thickness direction of the steel pipe. By limiting the composition of the steel to a specific range and controlling the structure of the steel before quenching and the quenching conditions, we succeeded in obtaining a steel pipe with excellent wear resistance.

Next, the reason why the composition of the steel pipe in the present invention is limited to the above range will be described. (1) C: C is an important element that imparts wear resistance to steel. However, if the C content is less than 0.4 wt.%, The desired effect cannot be obtained, while if the C content exceeds 0.5 wt.%, Quench cracking tends to occur. Therefore, the C content should be limited to the range of 0.4 to 0.5 wt.%.

(2) Mn: Mn is an essential element for ensuring the hardenability of steel. However, if the Mn content is less than 0.75 wt.%, The hardness near the inner surface of the steel will be Hv40.
It cannot be 0 or more. On the other hand, the Mn content is 1.
If it exceeds 20 wt.%, The inside of the steel is excessively quenched and cracks occur. Therefore, the Mn content is 0.75
It should be limited to the range of 1.20 wt.%. The upper limit value is almost equal to the upper limit value for ensuring manufacturability and weldability.

(3) Si: Si is a deoxidizing element, and 0.1
It is contained in the range of 5 to 0.30 wt.%. If the Si content is less than 0.15 wt.%, The deoxidizing effect is not sufficient, while
When the Si content exceeds 0.30 wt.%, Not only the deoxidizing effect is saturated, but also adverse effects such as deterioration of toughness occur.

(4) Cu, Ni, Cr, Mo: Cu, N
i, Cr, and Mo are also elements that enhance the hardenability of steel. If necessary, at least one of the above-mentioned elements should be 0.
It is contained within the range of 1 wt.% Or less. If the content of at least one of the above elements exceeds 0.1 wt.%, Quenching tends to occur, causing quenching even inside the steel and causing cracking. The total content of the above elements is 0.3 wt.% As a guide. If the total content exceeds 0.3 wt.%, Cracking may occur for the same reason.

(5) S: The content of S as an impurity is 0.
It should be limited to 005 wt.% Or less. S is known to be harmful to workability, but like the steel pipe of the present invention, S
It is particularly harmful when the content is high. S content is 0.
If it exceeds 005 wt.%, Cracks are likely to occur during hot rolling, and the edge of the steel strip is largely cracked.

(6) P: The content of P as an impurity is 0.
It should be limited to 015 wt.% Or less. When the P content exceeds the above value, the mechanical properties also deteriorate. Furthermore, P
Has an effect of enhancing the hardenability, so if the P content exceeds 0.015 wt.%, The problem of excessively deep quenching occurs.

(7) Al: Al is a deoxidizing agent and at the same time N
At the same time, it has a function of forming AlN and refining austenite crystal grains of steel to improve mechanical properties. The refining of the austenite crystal grains reduces the hardenability of the steel and reduces the quenching depth. Therefore, it is possible to obtain a structure in which a large amount of baking is formed near the surface and a small amount of baking is formed inside. However, the Al content is 0.005w
If it is less than t.%, the desired effect cannot be obtained. On the other hand, if the Al content exceeds 0.05 wt.%, the embrittlement action due to the increase in the amount of inclusions becomes large. Therefore, the Al content is 0.
It is preferably in the range of 005 to 0.05 wt.%.

(8) N: N forms AlN together with Al, has the effect of making the austenite crystal grains of the steel finer, improving the mechanical properties, and reducing the hardenability. However, if the N content exceeds 0.01 wt.%, The mechanical properties deteriorate. There is no need to set the lower limit,
Usually, 0.001 wt.% Or more is mixed when the steel is melted. Therefore, the N content is preferably in the range of 0.001 to 0.01 wt.%.

(9) O: O deteriorates the ductility and toughness of steel, and its adverse effect becomes remarkable when the O content exceeds 0.007 wt.%. Therefore, the O content is preferably 0.007 wt.% Or less.

(10) H: When H is contained in the steel, it causes cracking, and when the amount exceeds 0.0001 wt.%, Cracking is particularly likely to occur. Therefore, the H content is preferably limited to 0.0001 wt.% Or less.

Even if Ti and V are contained in an amount of about 0.1 wt.%, They do not affect the essence of the present invention. 0.01
The same is true for wt.% or less Ca and Mg. However, since B is an element that greatly affects the hardenability, it is necessary to limit it to 0.0001 wt.% Or less even when it enters as an impurity.

In the production of the electric resistance welded steel pipe of the present invention, water cooling is not carried out when the hot rolled steel strip to be processed into the steel pipe is wound up.
The reason for this is to prevent the occurrence of cracks, but to further promote the precipitation of AlN and the like by performing slow cooling and suppress the growth of austenite crystal grains during the subsequent heat treatment.

In the electric resistance welded steel pipe of the present invention, a slab having the above-mentioned composition is prepared, and then the slab is hot-rolled to prepare a hot-rolled steel strip, and the hot-rolled steel strip is continuously formed by a multi-stage forming roll. Manufactured by forming an open pipe into an open pipe, and then forming the ERW steel pipe by electric resistance welding, annealing the welded part of the obtained ERW steel pipe, heating the annealed steel pipe, and then water-cooling the inner surface. .

[0033]

EXAMPLES Next, the present invention will be further described with reference to Examples and comparison with Comparative Examples. Steel having a chemical composition within the scope of the present invention is melted by a converter and formed into a slab by continuous casting, and the slab is heated to a predetermined temperature and hot-rolled into a hot-rolled steel strip, which is water-cooled. It was wound up without doing.

Next, the obtained hot-rolled steel strip is continuously formed into an open pipe by a multi-stage forming roll, and then both edge portions thereof are electrically heated and electric resistance welded to form an electric resistance welded steel pipe. After the electric resistance welding, heat treatment was performed to prevent cracking of the welded portion, and the specimens of the steel pipe of the present invention (hereinafter referred to as the present invention specimen) Nos. 1 to 7 shown in Table 1 were prepared. Since steel with a high C content is used and cracks are likely to occur during welding, a device was devised to prevent cooling water from flowing to the welded portion (so-called V throat).

[0035]

[Table 1]

Although either high-frequency induction welding or electric resistance welding can be used for welding, the electric resistance welding method was used in this embodiment. In order to prevent the occurrence of cracks in the welded part after welding, the vicinity of the welded part was annealed after welding at 550 to 650 ° C. The pipe may be water-cooled after the welded part is annealed.

After cooling, it was inspected by ultrasonic waves and the like, and after confirming that there was no defect, the temperature was raised to a predetermined temperature by high frequency heating and water cooling was performed from the inner surface. For heating, while moving the coil, a heating zone is formed, and a nozzle that ejects high-pressure cooling water evenly in the circumferential direction on the inner surface of the pipe is kept at a constant speed and a constant distance from the heating coil. It was done by moving. Steel pipe sizes are No. 1 to No.
No. 5 has an outer diameter of 609.6φ and a wall thickness of 11.1 mm, No. 6 has an outer diameter of 609.6φ and a wall thickness of 8.7 mm, and No. 7 has an outer diameter of 609.6φ and a wall thickness of 12. It is 7 mm.

Regarding the above-mentioned specimens No. 1 to 7 of the present invention, the austenite grain size, the amount of martensite and the hardness (H
v), abrasion resistance, and the state of quench cracking were examined and shown in Table 1. As is clear from Table 1, each of the test pieces had a wear resistance three times or more that of the carbon steel pipe, and no crack was observed during storage or welding.

Table 2 shows the comparative steel pipe specimens (hereinafter referred to as comparative specimens) Nos. 11 to 19 in which at least one of the chemical composition and the wall thickness is outside the scope of the present invention.
The results of examining the austenite grain size, the amount of martensite, the hardness (Hv), the wear resistance, and the state of quench cracking were shown for each of them. The size of the steel pipe is the comparative specimen No. 11
Have an outer diameter of 609.6φ and a wall thickness of 7.9 mm, and all have an outer diameter of 609.6φ and a wall thickness of 11.1 mm.

[0040]

[Table 2]

As is clear from Table 2, the comparative sample N
In No. 11, the wall thickness of the steel pipe was out of the range of the present invention, and quench cracking occurred. Comparative sample No. 13 had a large amount of C outside the range of the present invention, and similarly, cracking occurred. Comparative sample No. 14 had a large amount of Mn outside the range of the present invention,
Similarly, fire cracking occurred. Comparative specimen No. 16 is P
The amount was out of the range of the present invention, and similarly, quench cracking occurred. Comparative specimens No. 17 and 18 are Cu, N
The amounts of i, Cr, and Mo were out of the range of the present invention, and similarly, quench cracking occurred.

The comparative sample No. 12 had a small amount of C outside the range of the present invention, and the abrasion resistance was not sufficient. Comparative sample No. 15 also had a small amount of Mn outside the range of the present invention, and the abrasion resistance was not sufficient. Specimen for comparison No. 19
The amount of S was out of the range of the present invention, and the occurrence of cracks during hot rolling was remarkable, and the production of steel pipes was abandoned.

[0043]

As described above, according to the present invention,
Excellent abrasion resistance while maintaining the mechanical properties as a steel pipe, suitable as a steel pipe for transporting slurries of earth and sand, ores, coal, solids such as dust, and pipes transporting mixtures of solids and liquids. The electric resistance welded steel pipe having the above can be obtained economically and efficiently by a method similar to the ordinary manufacturing method, which brings industrially useful effects.

Claims (1)

[Claims] 1. C: 0.40 to 0.50 wt.%, Si: 0.15 to 0.30 wt.%, Mn: 0.75 to 1.20 wt.%, S: 0.005 wt.% Or less, P: 0.015 wt.% Or less, at least one component selected from the group consisting of: Cu: 0.10 wt.% Or less (including 0), Ni: 0.10 wt.% Or less (including 0), Cr : 0.10 wt.% Or less (including 0) and Mo: 0.10 wt.% Or less (including 0), the rest: Fe and inevitable impurities, and a wall thickness of 8 mm
A high carbon ERW steel pipe with excellent wear resistance, characterized by the above.