JPH0711029B2 - Manufacturing method of high strength ERW steel pipe excellent in low temperature toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention has improved the low temperature toughness of the electric resistance welded portion, which is the greatest weak point of the electric resistance welded steel pipe, to the same level as or higher than that of the base metal portion, and has a high strength excellent in low temperature toughness. The present invention relates to a method for manufacturing an electric resistance welded portion.

(Prior Art) Since the oil crisis, oil extraction has been actively carried out in frigid regions such as the North Sea, Canada, Alaska, etc., and the wells have become deeper. Therefore, the demand for high-strength steel pipes excellent in low-temperature toughness is increasing, and more recently, the demand for high-strength steel pipes excellent in low-temperature toughness, which are inexpensive electric resistance welded steel pipes, is increasing.

However, with high strength ERW pipes with a yield strength of 80 ksi or more, it was very difficult to guarantee the low temperature toughness of the ERW welds, and it was necessary to adopt a method of quenching and tempering the ERW pipe.
However, electric resistance welded steel pipe is characterized by its low price, and if the method of quenching and tempering is adopted, it is the current situation that the demands of customers cannot be satisfied in terms of price.

(Problems to be Solved by the Invention) The present invention is capable of improving the low temperature toughness of the electric resistance welded portion to the same level as or higher than that of the base metal portion without depending on quenching and tempering, and high strength electric resistance welding having excellent low temperature toughness. It is intended to provide a method for manufacturing a steel pipe at low cost.

(Means for Solving Problems) The present invention is configured as follows in order to solve the problems.

First, regarding the components of the raw material, C is set to 0.05% or more in order to secure the required strength. However, if C is too high, the low temperature toughness is adversely affected, so the content was made 0.15% or less. If Si is too high, it adversely affects the low temperature toughness, so the content was made 0.05% or less. Mn to ensure the required strength
It was set to 1.2% or more. However, if Mn is too high, the low temperature toughness is adversely affected, so the content was made 2.0% or less. Other P and S should be as little as possible from segregation and inclusions. Nb, V, T
i is necessary to secure the strength, and Nb is 0.050% or less, V is 0.050% or less, and Ti is 0.050% or less.
The material is deoxidized with Al and contains the usual amount of Al remaining at that time.

Next, the heating and cooling conditions after electric resistance welding will be described.

The present invention destroys the cast structure generated by electric resistance welding by heating the electric resistance welded portion at 850 ° C to 1050 ° C after the electric resistance welding. That is, the temperature at which the entire electric resistance welded portion is transformed into an austenitic structure must be 850 ° C or higher. However, if the temperature is too high, the crystal grains become coarse, so it is necessary to heat at 1050 ° C or lower. Destruction of the cast structure generated by electric resistance welding by the above heating is almost completely performed, but since this heating is local heating only for the electric resistance welding, its cooling rate is air cooling. Nevertheless, it will be quite large. That is, in high-strength ERW steel pipes with relatively high C and Mn, the occurrence of bainite structure, etc. may be observed even when air-cooled, and this fact is also due to the fact that this heating is localized heating only at the electric resistance welded portion. ,
This indicates that the cooling rate is considerably high. That is, even if the bainite structure or the like is not generated in the high strength ERW steel pipe having a relatively high C and Mn, the low temperature toughness is deteriorated because the ferrite structure contains a large amount of excessive C due to a considerably large cooling rate. . The present invention pays attention to this point and devises so as not to destroy the cast structure generated by electric resistance welding which is harmful to the low temperature toughness, and at the same time, to prevent the structure from becoming harmful even in the subsequent cooling. The point is that α, which is considered to change into a harmful tissue upon cooling,
It is considered that this is due to the increased cooling in the γ two-phase region. According to the present invention, as shown in FIG. 3, a new method of maintaining the α and γ two-phase regions during cooling for a certain period of time is used to destroy the cast structure generated by electric resistance welding that is harmful to low temperature toughness. It is intended to provide a manufacturing method which does not change into a harmful structure even in the subsequent cooling.

Next, the above will be described metallurgically.

Now, assuming that the content of C in steel is 0.09%, it is expected that ferrite will contain C in excess of the equilibrium state due to the large cooling of the heat treatment after electric resistance welding. Assuming that 0.07% of C is present in the ferrite, the solid content of C in the equilibrium state of the ferrite is 0.02%, so 0.05% contains an excessive amount of C. This is not good for the low temperature toughness, and it is better to keep the ferrite content in the carbon content as low as possible for the low temperature toughness. Therefore, if the ferrite containing excess C is held at the α, γ two-phase region temperature, the first
As shown in the figure, excess C existing in α, that is, ferrite (indicated by a thin line in the figure) is diffused toward γ, that is, austenite (indicated by a thick line in the figure) in an attempt to return to the equilibrium state. This is because γ, that is, austenite, has the ability to contain a larger amount of C than α, that is, ferrite. This austenite can contain C up to 2.0%.

According to the method of maintaining the α, γ two-phase region for a certain period of time during cooling according to the present invention, a structure having a very clean ferrite containing only 0.02% C can be formed. That is, in the α, γ two-phase region, 700 ° C to 800 ° C
By heating within 30 minutes, a structure with clean ferrite can be formed. The experimental result is the second
Shown in the figure. If it is less than 700 ° C, the low temperature toughness is not improved because it does not reach the α, γ two-phase region, but at 700 ° C to 800 ° C, the low temperature toughness is remarkably improved, and the low temperature toughness is equal to or higher than that of the base metal part which is the object of the present invention. It is an electric resistance welded steel pipe with an electric resistance welded part with improved toughness. However, above 800 ° C, most of it changes to γ, that is, austenite, so clean ferrite cannot be obtained, and the low temperature toughness is hardly improved.

This is in the α, γ two-phase region during cooling as in the present invention,
A simple method of holding for a certain period of time has an excellent effect. It is desirable that the holding time at the α and γ two-phase temperature is as long as possible, but for economic reasons it should be within 30 minutes. However, when the heat treatment of the present invention is performed online, it is economically more advantageous if it is within 1 minute.

Next, Table 1 shows examples of the present invention.

(Effects of the Invention) As described above, according to the present invention, the low temperature toughness of the electric resistance welded portion of the electric resistance welded steel pipe is maintained in the α, γ two-phase region for a certain period of time to improve the low temperature toughness without quenching and tempering. An excellent high strength electric resistance welded steel pipe can be manufactured at low cost.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing the movement of C by diffusion in the α and γ two-phase region temperature, FIG. 2 is a diagram showing the relationship between the reheatable temperature of the electric resistance welded portion and the Charpy low temperature toughness, and FIG. 3 is It is explanatory drawing which shows the heat processing pattern of this invention.

Claims (1)

[Claims] 1. C: 0.05 to 0.15%, Si: 0.5% or less, Mn: 1.2 to
2.0% as a basic component, Nb: 0.050% or less, V: 0.050% or less, Ti: 0.050% or less, and 1 or 2 or more, balance Fe
And the electric resistance welded portion of the electric resistance welded steel pipe made of unavoidable impurities.
After heating at 0 ℃ to 1050 ℃, cool it by air cooling to the temperature range of 700 ℃ to 800 ℃, and within the temperature range of 700 ℃ to 800 ℃.
A method for producing a high-strength electric resistance welded steel pipe excellent in low temperature toughness, which is characterized by holding for 30 minutes or less.