JPH06184693A - Mo-based ultra-high-strength electric resistance welded steel pipe and method for manufacturing the same - Google Patents

Abstract

(57) [Abstract] [Purpose] Ultra-high-strength electric resistance welded steel pipes used for structural members of automobiles and the manufacturing method thereof. [Structure] An electric resistance welded steel pipe composed of a composition containing Mo. During hot plate thickness rolling of pipe steel, finish rolling is completed at 950 ° C or lower and Ar ₁ transformation point or higher, and is continuously wound at 400 ° C or lower. A method for producing a characteristic ultra-high-strength electric resistance welded steel pipe. An ultra-high-strength electric resistance welded steel pipe manufactured by the method for manufacturing an electric resistance welded steel pipe according to the above item, and having a tensile strength of 120 to 180 kgf / mm ² . [Effect] Since it becomes possible to manufacture an ultra-high-strength electric resistance welded steel pipe without problems in industrial productivity and economical efficiency, the industrial contribution 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 ultra-high-strength electric resistance welded steel pipe used for structural members such as automobiles and a method for manufacturing the same.

[0002]

2. Description of the Related Art Regarding structural members such as automobiles, thorough weight reduction is being considered to improve fuel efficiency and environmental measures, and some members have 120 kgf / mm as one measure to achieve both safety. More than ² ultra high strength steel pipes are being adopted.

Generally, as a method of increasing the strength of an electric resistance welded steel pipe, a method of tempering after electric resistance welded pipe, that is, quenching or quenching and tempering as described in JP-A-3-122219, etc., There is a method of increasing the strength of the hot-rolled sheet which is a raw material by the method described in JP-A-4-289122.

[0004]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
There are two types as described in the prior art section.
First, as described in JP-A-3-122219 and the like, there is a method of tempering after electric-sewn pipe, that is, quenching or quenching and tempering. The manufacturing process in this case is shown in FIG. This method requires a special heat treatment facility, requires special attention to secure the dimensions and shape, and the material, and is inevitably costly in terms of capital investment and productivity. This is a major obstacle to the spread of ultra-high-strength ERW pipes. Furthermore, it is completely unsuitable as a manufacturing method for square steel pipes and deformed steel pipes, which are attracting attention as structural members having higher rigidity.

Next, there is a method for increasing the strength of the hot-rolled sheet which is a raw material by the method described in JP-A-52-114519. The manufacturing process in this case is shown in FIG. In this method, with the components of the present invention, (1) the strength of the hot-rolled sheet is not sufficiently increased to obtain an ultra-high-strength electric resistance welded steel pipe, and (2) even when the strength of the hot-rolled sheet is sufficient. ,
There is a problem such as cracking at the time of electric resistance welded pipe due to insufficient ductility, and the manufacturing process of the present invention, that is, hot rolling, cold rolling, quenching by continuous annealing, and tempering It is necessary, requires a long process, requires quenching and tempering equipment, is costly, and is not industrially viable as a method for producing an ultra-high-strength electric resistance welded steel pipe.

An object of the present invention is to solve the problems in the method of manufacturing such an ultrahigh tensile electric resistance welded steel pipe.

[0007]

The subject matter of the present invention is as follows.

(1) The component composition by weight is C: 0.15 to 0.15.
0.40%, Si: 0.05 to 0.50%, Mn: 2.
0-3.0%, P: 0.02% or less, S: 0.006%
Hereinafter, Al: 0.01 to 0.08%, B: 0.001 to
0.003%, N: 0.005% or less, Mo: 0.3-
Ti: 0.01 to 0.20, if necessary, to 1.5%
%, Nb: the balance Fe containing 0.01 to 0.20%
And an electric resistance welded steel pipe consisting of unavoidable elements, characterized in that finish rolling is finished at 950 ° C or lower and Ar ₁ transformation point or higher during hot plate thickness rolling of steel for pipes, and is continuously wound at 400 ° C or lower. Ultra high strength ERW steel pipe manufacturing method.

(2) A Mo-based ultra-high-strength electric resistance welded steel pipe manufactured by the method for manufacturing an electric resistance welded steel pipe according to the above item (1) and having a tensile strength of 120 to 180 kgf / mm ² .

The present invention will be described in detail below. FIG. 1 shows a manufacturing process according to the present invention.

In the conventional process, in order to manufacture an ultra-high-strength steel pipe as described above, it is necessary to perform quenching or quenching and tempering after rolling or electric resistance welding. This method requires a special heat treatment facility, requires special attention to secure the dimensions and shape, and the material, and is inevitably costly in terms of capital investment and productivity.

Therefore, in the present invention, an ultrahigh-strength electric resistance welded steel pipe is manufactured by defining the finish rolling temperature and the winding temperature in hot rolling without performing quenching and tempering treatment after rolling or electric resistance welding. It is what

The reasons for limiting the components of the steel sheet used in the present invention will be described.

If the C content is small, the ductility is good and the workability is excellent, but the required strength cannot be obtained, so the lower limit was made 0.15%. Further, if it exceeds 0.40%, cold workability such as formability at the time of pipe making and toughness tend to be deteriorated, and the heat-affected zone is hardened at the time of pipe making welding of the electric resistance welded steel pipe, resulting in workability Therefore, the upper limit was made 0.40%.

In the case of killed steel, it is difficult to control Si to less than 0.05% in terms of steelmaking technology, and if it exceeds 0.50%, deterioration of ductility becomes difficult to ignore, so 0.50.
% Was set as the upper limit.

With respect to Mn, if it is less than 2.0%, the strength becomes insufficient, and if it exceeds 3.0%, cracks may occur due to insufficient ductility during processing such as forming during pipe forming. The lower limit was 2.0% and the upper limit was 3.0%.

P is an element that is unavoidably mixed during steelmaking, but if it exceeds 0.02%, cracks in the welded portion tend to occur particularly during electric resistance welding of ultra-high strength steel pipes, so the upper limit is 0.02%.
And

Similar to P, S is an element that is inevitably mixed during steelmaking, and if it exceeds 0.006%, cracks in the welded portion tend to occur during electric resistance welding, so the upper limit was made 0.006%. To further suppress the cracks caused by S during electric resistance welding, use M
Ca, which is an element for controlling the morphology of nS, may be added.

In the case of killed steel, it is difficult to control Al to less than 0.01% in terms of steelmaking technology, and when it exceeds 0.08%, slab cracking, internal defects due to formation of large oxide inclusions, etc. Therefore, 0.08% was made the upper limit.

B is an essential element for delaying the ferrite transformation in the cooling process to obtain a high-strength transformation structure, but even in the composition of the steel of the present invention, if it is less than 0.001%, the strength becomes insufficient and 0.003%. Boron beyond
Constitutant forms and ductility decreases significantly, so the lower limit is 0.001% and the upper limit is 0.003%.
And

N is an element that is unavoidably mixed in during steelmaking, but if it exceeds 0.005%, the strength increasing effect of Ti and B is hindered and the strength becomes insufficient, so the upper limit is 0.005.
%.

Mo is an element that increases the strength of the steel material, and addition in an amount that does not excessively impair the ductility is effective for the production of ultra-high-strength electric resistance welded steel pipe. However, if it is too low, the strength will be insufficient. Therefore, the upper limit is set to 0.3% and the lower limit is set to 1.5%.

The following Ti and Nb are added as needed.

Ti is an important element for controlling the strength, but if it is less than 0.01%, the strength becomes insufficient, and 0.20
%, The effect is not improved, so the lower limit was made 0.01% and the upper limit was made 0.20%.

Nb, like Cr, is an element that increases the strength of the steel material, and addition within a range that does not excessively impair the ductility is effective for the production of ultra-high-strength electric resistance welded steel pipe. However, if it is too low, the strength will be insufficient. Therefore, the upper limit is 0.20% and the lower limit is 0.01%.

Next, the manufacturing process will be described. In accordance with the present invention, the steel of the above components is 950 ° C. or less during hot plate rolling.
Finish rolling is completed at r ₁ transformation point or higher. This is to optimize the balance between strength and ductility by performing appropriate low-temperature rolling. Above 950 ° C, there is no rolling in the unrecrystallized region, so the strength and ductility deteriorate and the Ar ₁ transformation point If it is less than 2, it does not lead to two-phase rolling, and an increase in strength cannot be expected. Therefore, the steel of the above composition is 950 ° C during hot plate rolling.
A material having an excellent balance of strength and ductility can be obtained by completing the finish rolling at the Ar ₁ transformation point or higher and continuously winding it under the conditions of the present invention.

The coiling temperature is 400 ° C. or lower, which is intended to increase strength while maintaining ductility by the formation of bainite and martensite.

As described above, the present invention is characterized in that the components have high hardenability, and in hot rolling, the strength is obtained while maintaining the ductility by low temperature finishing and low temperature winding.

The present invention has been described above.
Is manufactured by the method for manufacturing an electric resistance welded steel pipe according to claim 1 or 2,
It is an ultra-high-strength electric resistance welded steel pipe having a tensile strength of 120 to 180 kgf / mm ² .

[0030]

[Examples] Table 1 shows the results of pipe forming of an electric resistance welded steel pipe of size φ34.0 × t2.1 by the conventional method as a comparative example of the method of the present invention.
It was shown to.

As shown in Table 1, according to the present invention, a raw steel sheet having an excellent balance of strength and ductility is manufactured by appropriately controlling the chemical composition, the finish rolling temperature and the coiling temperature in hot strip rolling. As a result, an ultra-high-strength electric resistance welded steel pipe having an excellent ductility balance can be obtained.

[0032]

[Table 1]

[0033]

EFFECTS OF THE INVENTION In order to manufacture an ultra-high-strength steel pipe in the conventional process, it is necessary to quench or quench-temper after hot rolling or electric resistance welded steel pipe, and a dedicated heat treatment facility is required. Not only is it necessary to pay special attention to securing the shape and material, but it has also become extremely costly in terms of capital investment and productivity. According to the present invention, it is possible to manufacture an ultra-high-strength electric resistance welded steel pipe without the problems of industrial productivity and economic efficiency, so that the industrial contribution is extremely large.

[Brief description of drawings]

FIG. 1 is a manufacturing process diagram of the method of the present invention.

FIG. 2 is a conventional manufacturing process diagram.

FIG. 3 is a conventional manufacturing process diagram.

Claims (3)

[Claims] 1. Component composition by weight C: 0.15 to 0.40%, Si: 0.05 to 0.50%, Mn: 2.0 to 3.0%, P: 0.02% or less. , S: 0.006% or less, Al: 0.01 to 0.08%, B: 0.001 to 0.003%, N: 0.005% or less, Mo: 0.3 to 1.5% A steel material for pipes, which contains the balance Fe and unavoidable elements, is finished at the hot plate thickness of 950 ° C or lower at the Ar ₁ transformation point or higher, and the finish rolling is continued, and then rolled up at 400 ° C or lower, and then pipe-formed. A method for manufacturing a Mo-based ultra-high-strength electric resistance welded steel pipe. 2. The method for producing an ultra-high tensile electric resistance welded steel pipe according to claim 1, wherein the composition of the steel material for pipe is Ti:
A method for producing a Mo-based high-strength electric resistance welded steel pipe containing one or two of 0.01 to 0.20% and Nb: 0.01 to 0.20%. 3. By weight, the component composition is C: 0.15 to 0.40%, Si: 0.05 to 0.50%, Mn: 2.0 to 3.0%, P: 0.02% or less. , S: 0.006% or less, Al: 0.01 to 0.08%, B: 0.001 to 0.003%, N: 0.005% or less, Mo: 0.3 to 1.5% , Ti: 0.01 to 0.20%, Nb: 0.
01-0.20% of 1 type, or 2 types, or does not contain, the balance consists of Fe and unavoidable elements, and the tensile strength is 120-180 kgf / mm ² Mo-based ultrahigh Tension electric resistance welded steel pipe.