JPH11279697A - ERW steel pipe with excellent workability and its manufacturing method - Google Patents

Abstract

(57) [Problem] To provide an electric resistance welded steel pipe excellent in workability and a method for manufacturing the same. SOLUTION: In weight%, C: 0.02 to 0.15%,
Si: 0.1 to 1.5%, Mn: 1.0 to 2.0%,
S: 0.02% or less, P: 0.03% or less, Al: 0.
01 to 0.10%, Cr: 1.5% or less, Ti: 0.0
1% or less, N: 0.0050% or less, if necessary, C
an electric resistance having excellent workability, characterized by containing 0.0002 to 0.0020% of a, and having a composite structure consisting of ferrite having the workability and the balance consisting of iron and unavoidable impurities and the balance consisting of the remaining martensite and bainite. Steel pipe and its manufacturing method. The hot-rolled ERW steel pipe manufactured by the method of the present invention is an ERW steel pipe excellent in workability characterized by having a composite structure composed of ferrite having workability and the remaining martensite and bainite. is there. In the future, there will be an increasing demand for ERW pipes having high ductility that can withstand complicated processing such as hydroforming. Therefore, the effect of the electric resistance welded steel pipe manufactured according to the present invention is extremely large.

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 for an automobile or a machine structure requiring excellent workability. The invention relates to a steel comprising a specific component, specific hot rolling conditions and high frequency electric resistance welding conditions. The present invention relates to an electric resistance welded steel pipe having excellent workability and a method for producing the same.

[0002]

2. Description of the Related Art In Europe and the United States, hydrophobes have been developed for the purpose of cost reduction by reducing the weight of automobiles and the number of parts.
System technology is being put to practical use. Active efforts are being made in Japan as one of the cores of next-generation vehicle production technology. Against this background, the application of underbody parts and body parts made of ERW steel tubes and formed by hydroforming is increasing. Hydropho
It is important that the ERW steel pipe for the rubber has good ductility that can withstand complicated processing.

There have been many researches and developments aimed at improving ductility while maintaining the same strength, that is, improving strength-ductility balance. In the past,
What has drastically improved the strength-ductility balance is a so-called duplex stainless steel having a mixed structure of ferrite and martensite. For example, JP-A-51-12317 discloses a technique of duplex stainless steel obtained by hot-rolling followed by rapid cooling, and JP-B-57-45454 discloses a technique obtained by continuous annealing. Currently, duplex stainless steels take advantage of their excellent properties,
It is attracting attention as an application for reducing the weight of materials for automobiles and the like.

[0004]

It is important that an ERW steel pipe for a hydroform has good ductility to withstand complicated machining. The present invention provides an electric resistance welded steel pipe excellent in workability and a method for manufacturing the same.

[0005]

SUMMARY OF THE INVENTION The present invention provides, in weight percent,
C: 0.02 to 0.15%, Si: 0.1 to 1.5%,
Mn: 1.0 to 2.0%, S: 0.02% or less, P:
0.03% or less, Al: 0.01 to 0.10%, Cr:
1.5% or less, Ti: 0.02% or less, N: 0.005
0% or less, if necessary, 0.0002 to 0.0
An electric resistance welded steel pipe made of a steel excellent in workability characterized by containing 020%, the balance being iron and unavoidable impurities, and having a composite structure composed of ferrite having workability and the remainder consisting of martensite and bainite. As a method of manufacturing an electric resistance welded steel pipe, an attempt is made to solve the problem by taking specific hot rolling conditions. Further, the present invention is also a method for producing an electric resistance welded steel pipe excellent in workability by welding the above steel to an inert gas atmosphere having an oxygen concentration of 100 ppm or less when performing electric resistance welding at high frequency to form a pipe.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, an electric resistance welded steel pipe excellent in workability and a method for manufacturing the same according to the present invention will be described in detail. First, the reasons for limiting the chemical components will be described. C forms the basis of the present invention to form a martensitic phase containing C, and requires 0.02% or more. However, if C exceeds 0.15%, the maximum hardness of the electric resistance welded portion becomes too high as compared with the base metal, which is not preferable for severe processing such as hydroforming. Therefore, the upper limit of C is 0.15
%.

When Si is used as a deoxidizer, 0.1
% Or less, but since the ferrite is strengthened without significantly impairing the ductility of the ferrite, the lower limit of Si is set to 0.1%. However, when the content of Si exceeds 1.5%, ductility deteriorates greatly. Therefore, the upper limit of Si is set to 1.5%.

Mn enhances the stability of austenite and the hardenability of austenite, but the required amount varies depending on the cooling rate. In the present invention, Mn is required to be 1.0% or more to increase the stability of austenite at a cooling rate and stably obtain martensite. However, Mn
Exceeds 2.0%, the hardenability in the electric resistance welded portion increases, and the hardness becomes too high as compared with the base metal.
It is not preferable for severe processing such as rubber. Therefore,
The upper limit of Mn was set to 2.0%.

S is an element that forms MnS and reduces toughness and press workability. In particular, when the strength is high, the influence of deterioration in toughness and press workability is likely to be strong.
Therefore, it must be as low as possible. So S
Is set to 0.02% or less. P is effective for strengthening steel, but if added excessively exceeding 0.03%, the resistance to electric resistance welding is impaired.

When Al is used as a deoxidizing agent, Al
At 1% or less, there is no effect, so the lower limit of Al is set to 0.1%.
01%. However, when the content of Al exceeds 0.1%, the amount of alumina-based inclusions increases and the workability deteriorates. Therefore, A
The upper limit of 1 was set to 0.1%. Cr is an element that enhances the hardenability and facilitates the formation of martensite, and its upper limit is set to 1.5% from the viewpoint of economy.

Since Ti is an effective element for fixing solid solution N in steel and preventing deterioration of elongation due to aging, its upper limit is made 0.02%. Ca is effective in adjusting the form of inclusions to improve cold workability, but too much Ca increases inclusions in the steel and conversely deteriorates cold workability. -0.0020%.

Next, the reasons for limiting the hot rolling conditions of the steel sheet will be described. First, the reasons for limiting the hot rolling conditions of the cold rolled steel sheet and the continuous annealing conditions after cold rolling will be described. Under the hot rolling conditions of the hot rolled material, the finishing temperature was set to the Ar ₃ transformation point temperature −50.
The reason for setting the temperature range from ℃ to the Ar ₃ transformation point temperature + 50 ℃ is that the precipitation nuclei of ferrite are increased by such low-temperature rolling, and 50% or more of polygonal ferrite and C and This is for generating austenite in which other hardening elements are concentrated. If the finishing temperature exceeds the Ar ₃ transformation point temperature + 50 ° C., the generation of ferrite precipitation nuclei decreases, and good polygonal ferrite cannot be obtained. Conversely, when rolling is performed in a low-temperature region of the α + γ two-phase region, work distortion of ferrite is not sufficiently removed, and ductility is deteriorated. The limit is the Ar ₃ transformation point temperature −50 ° C. In order to obtain stable austenite enriched in C and the like during the pre-stage cooling, it is preferable to add rolling during the progress of ferrite precipitation. In this sense, the finishing temperature is preferably lower than the Ar ₃ transformation point temperature. preferable.

Next, the average cooling rate in the former stage of the temperature range from the finishing temperature to 650 ° C. means 1 to 50 ° C./s. The terminating temperature of 650 ° C. is the ferrite precipitation terminating temperature and the cooling of hot rolling. It is a temperature at which no pearlite is formed during the process. The pre-cooling has the function of promoting the precipitation of polygonal ferrite in combination with low-temperature rolling, and measuring and stabilizing the concentration of C and Mn in the remaining austenite. For that purpose, the cooling rate should be slower and the average cooling rate should be at least 50 ° C./s or less. However, if it is too slow, productivity becomes a problem, and it is practically impossible to realize it with existing equipment. Therefore, the average cooling rate was set to 1 ° C./s or more.

Next, the average cooling rate in the latter stage of the temperature range from 650 ° C. to the winding temperature means 10 to 100 ° C./s. It has a function of causing martensitic transformation in combination with a low winding temperature of not more than ° C.
Therefore, the average cooling rate in the subsequent stage needs to be 10 ° C./s or more. However, if the cooling rate is too fast, solid solution C remains in the ferrite and the ductility of the ferrite deteriorates. Therefore, the upper limit of the average cooling rate must be 100 ° C./s.

If the winding temperature is not lower than 250 ° C., pearlite transformation or the like occurs, and the predetermined characteristics cannot be obtained. Further, the average cooling rate in the first stage needs to be lower than the average cooling rate in the second stage. It is presumed that the reason for this is that in the continuous cooling transformation diagram, the precipitation noise of ferrite and pearlite varies in a complicated manner depending on the rolling temperature and the pre-stage and post-stage cooling rates, and pearlite is generated when the conditions are not satisfied.

Next, a description will be given of a process for forming an ERW steel pipe. The steel strip described above is subjected to electric resistance welding at a high frequency to form a raw tube. However, the steel plates described above are not compatible with Si and M
The addition amount of n is high, and the FeO-based oxide generated during the heating and melting of the electric resistance welded surface is reduced by Si, Mn and the like in the steel, and reaction products such as SiO ₂ and MnO are generated.
These reaction products remain as fine particulate oxides in the ERW weld. In order to prevent such welding defects, it is necessary to seal the welding atmosphere in the high-frequency electric resistance welding with an inert gas to reduce the oxygen concentration to 100 ppm or less. As described above, in the case where the steel described above is formed as an ERW steel pipe, it is possible to manufacture an ERW steel pipe having an excellent ERW weld by performing seal welding. ERW steel pipe with excellent workability that can withstand advanced and complicated forming by using a specific component system and a special ERW steel pipe manufacturing method such as hot rolling, continuous annealing conditions and seal welding as in the present invention. Is obtained.

[0017]

EXAMPLES Table 1 shows the chemical composition of the steel of the present invention and the comparative steel. Table 2 shows the hot rolling conditions of the hot rolled material. Table 3 shows the mechanical properties of the ERW steel pipe of the hot rolled material manufactured according to the present invention and the ERW steel pipe manufactured in comparison. As is apparent from Table 3, the ERW steel pipe manufactured according to the present invention has
L, uniform elongation is very high, and there is no welding defect in the ERW weld. On the other hand, the ERW steel pipe manufactured by the conventional method has lower EL and uniform elongation than the ERW steel pipe manufactured according to the present invention, and welding defects at the ERW weld are observed.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

[Table 3]

[0021]

As described above, the ERW steel pipe of the hot-rolled material manufactured by the method of the present invention has a workability characterized by having a composite structure of ferrite having workability, the remaining martensite and bainite. ERW steel pipe with excellent resistance. In the future, there will be an increasing demand for ERW pipes having high ductility that can withstand complicated processing such as hydroforming. Therefore, the effect of the electric resistance welded steel pipe manufactured according to the present invention is extremely large.

 ──────────────────────────────────────────────────続 き Continued on front page (72) Inventor Koji Sakuma 1 Kimitsu, Kimitsu City, Chiba Prefecture Nippon Steel Corporation Kimitsu Works

Claims (2)

[Claims] C .: 0.02 to 0.15%, Si: 0.1 to 1.5%, Mn: 1.0 to 2.0%, S: 0.02% or less by weight% P: 0.03% or less; Al: 0.01 to 0.10%; Cr: 1.5% or less; Ti: 0.01% or less; N: 0.0050% or less; An electric resistance welded steel pipe excellent in workability, containing 0.0002 to 0.0020%, the balance being iron and unavoidable impurities, and having a composite structure consisting of ferrite having workability and the remaining martensite and bainite. 2. In% by weight, C: 0.02 to 0.15%, Si: 0.1 to 1.5%, Mn: 1.0 to 2.0%, S: 0.02% or less, P: 0.03% or less; Al: 0.01 to 0.10%; Cr: 1.5% or less; Ti: 0.01% or less; N: 0.0050% or less; When hot rolling a steel containing 0.0002 to 0.0020%, the balance being iron and unavoidable impurities and having a workability of ferrite and the remainder consisting of martensite and bainite, the finishing temperature is changed to the Ar ₃ transformation point. Temperature -50 ° C to Ar ₃ transformation point temperature
Winding is performed at a temperature range of 50 ° C. or less and 250 ° C. or less. At this time, the average cooling rate in the former stage is divided into a former stage of a temperature range of 650 ° C. from the finishing temperature and a latter stage of a temperature range of 650 ° C. to 1-50. ° C / s, the average cooling rate in the latter stage is 10 to 100
When welding steel plates at ℃ / s and the average cooling rate of the former stage is smaller than the average cooling speed of the latter stage by high frequency welding, the welding atmosphere should be an inert gas atmosphere with an oxygen concentration of 100 ppm or less. A method for producing an electric resistance welded steel pipe excellent in workability characterized by the following.