JPH06179912A - Resistance welded tube of cr type ultrahigh tensile strength steel and its production - Google Patents

Abstract

PURPOSE:To produce a resistance welded tube of ultrahigh tensile strength steel without causing problems in productivity and economical efficiency by finishing finish rolling at specific temp. at the time of hot rolling of steel plate for tube having specific composition, coiling the resulting steel plate, and then performing forming and resistance welding. CONSTITUTION:The steel has a composition consisting of, by weight, 0.15-0.40% C, 0.05-1.5% Si, 2.0-3.0% Mn, <=0.02% P, <=0.006% S, 0.01-0.08% Al, 0.0003-0.003% B, <=0.005% N, 0.5-3.0% Cr, and the balance Fe with inevitable elements. At the time of hot rolling of a steel plate for tube consisting of the above steel, finish rolling is finished at a temp. between 950 deg.C and the Ar1 transformation point. Successively, the resulting steel plate is coiled at <=400 deg.C, followed by forming and resistance welding. Either or both of 0.01-0.20% each of Ti and Nb are further incorporated into the steel. By this method, the resistance welded tube of ultrahigh tensile strength steel, well balanced between ductility and toughness, can be obtained.

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 Japanese Patent Laid-Open No. 3-122219, 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 during electric resistance welded pipe due to lack of 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.

[0008]

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 1.5%, Mn: 2.0
To 3.0%, P: 0.02% or less, S: 0.006% or less, Al: 0.01 to 0.08%, B: 0.0003 to
0.003%, N: 0.005% or less, Cr: 0.5 to
3.0%, if necessary, Ti: 0.01 to 0.20
%, Nb: 0.01 to 0.20% of one or two, and the balance of Fe and unavoidable elements is used for pipe steel rolling at a hot plate thickness of 950 ° C. or less and finish rolling at an Ar ₁ transformation point or more. A method for manufacturing an ultra-high-strength electric resistance welded steel pipe, which comprises terminating and subsequently winding at 400 ° C. or lower, followed by forming and electric resistance welding.

(2) An ultra-high-strength electric resistance welded steel pipe manufactured by the method for manufacturing an electric resistance welded steel pipe according to the item (1), which has 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 invention of claim 1.

In the conventional process, in order to manufacture an ultra high strength steel pipe as described above, it is necessary to quench or quench and temper 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%. On the other hand, 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 1.5%, it becomes difficult to ignore the deterioration of surface properties due to scale formation. The upper limit was 5%.

Regarding 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 in during steelmaking. 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 oxide-based giant 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 also in the composition of the steel of the present invention, if it is less than 0.0003%, the strength is insufficient, and 0.003%. Boro beyond
n Constituent is generated and the ductility is remarkably reduced, so the lower limit is 0.0003% and the upper limit is 0.00
It was 3%.

N is an element that is unavoidably mixed 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.
%.

Cr 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 3.0% and the lower limit is 0.5%.

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 is 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 because the balance between strength and ductility is optimized 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 at the time of hot plate rolling.
℃ by winding under conditions of continued present invention completed the finish rolling below Ar ₁ transformation point or more, it can be an excellent material for the strength and extension toughness balance.

The coiling temperature is 400 ° C. or lower, which is intended to obtain the strength increase while maintaining the 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, strength is obtained while maintaining ductility by low temperature finishing and low temperature winding.

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

[0031]

[Embodiment] As a comparative example, an electric resistance welded steel pipe of size φ34.0 × t2.1 was produced by a conventional method as a comparative example.
Shown in the table.

As shown in Table 1, according to the present invention, a raw material steel sheet having an excellent balance of strength and ductility is manufactured by appropriately controlling the chemical composition, the finish rolling temperature and the winding temperature in hot strip rolling. Thus, it is possible to obtain an ultra-high-strength electric resistance welded steel pipe having an excellent balance of ductility and toughness.

[0033]

[Table 1]

[0034]

EFFECT 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 pipe, which requires a dedicated heat treatment facility. Not only is it necessary to pay special attention to securing dimensions and materials, 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 according to claim 1.

FIG. 2 is a conventional manufacturing process diagram.

FIG. 3 is a conventional manufacturing process diagram.

Claims (3)

[Claims] 1. A weight ratio of C: 0.15 to 0.40%, Si: 0.05 to 1.5%, Mn: 2.0 to 3.0%, P: 0.02% or less. , S: 0.006% or less, Al: 0.01 to 0.08%, B: 0.0003 to 0.003%, N: 0.005% or less, Cr: 0.5 to 3.0% During the hot plate thickness rolling of the steel for pipes containing the balance Fe and unavoidable elements, finish rolling is completed at 950 ° C or lower and Ar ₁ transformation point or higher, followed by winding at 400 ° C or lower, followed by forming and electric resistance welding. A method for manufacturing an ultra-high-strength electric resistance welded steel pipe, which is characterized by carrying out. 2. The component composition by weight is C: 0.15 to 0.40%, Si: 0.05 to 1.5%, Mn: 2.0 to 3.0%, P: 0.02% or less. , S: 0.006% or less, Al: 0.01 to 0.08%, B: 0.0003 to 0.003%, N: 0.005% or less, Cr: 0.5 to 3.0% Ti: 0.01 to 0.20%, Nb: 0.01 to 0.20% of 1 type or 2 types, and the balance Fe and unavoidable elements at the time of hot plate thickness rolling of a pipe steel material at 950 ° C. Below Ar ₁
A method for producing an ultra-high-strength electric resistance welded steel pipe, characterized in that finish rolling is completed at a transformation point or higher, followed by winding at 400 ° C. or lower, followed by forming and electric resistance welding. 3. By weight, the component composition is C: 0.15 to 0.40%, Si: 0.05 to 1.5%, Mn: 2.0 to 3.0%, P: 0.02% or less. , S: 0.006% or less, Al: 0.01 to 0.08%, B: 0.0003 to 0.003%, N: 0.005% or less, Cr: 0.5 to 3.0%, balance Consists of Fe and unavoidable elements, and has a tensile strength of 120.
Ultra-high-strength electric resistance welded steel pipe characterized by being ~ 180 kgf / mm ² .