JP2008150669A - Method for manufacturing high-strength steel tube - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a high-strength steel tube with which the high strength steel tube having ≥590 MPa TS, suitable to a round tube, a square tube and a deformed tube, etc., used for constructive members for automobile. <P>SOLUTION: The steel tube composed by mass% of 0.005-0.50% C, ≤0.6% Si, 0.3-3.0% Mn, ≤0.02% P, ≤0.006% S, 0.001-0.08% Al, 0.001-0.006% N, ≤0.006% O and further, at least one or more among 0.5-1.2% Cu, 0.005-0.10% Nb and 0.01-0.10% V and the balance Fe with inevitable impurities, is made to be a raw tube stock, and after performing tube extending work at cold-rolling to this raw tube stock so as to become 10-60% reduction area ratio, a heat-treatment in the temperature range of 500-650°C for 30 sec, is applied and successively, air-cooling is performed as the manufacturing method. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for manufacturing a high-strength steel pipe suitable for manufacturing round tubes, square tubes, deformed tubes and the like having a tensile strength (TS) of 590 MPa or more, which are used for structural members such as automobiles.

In recent years, from the viewpoint of preventing global warming by suppressing carbon dioxide emitted from automobiles, weight reduction of vehicle bodies for the purpose of improving fuel efficiency has been studied. Furthermore, since the standards for collision safety are becoming stricter, studies for improving the safety have been actively conducted. As a method for achieving both weight reduction and safety of automobiles, studies for applying high-strength steel to structural members of automobiles have been activated.
Moreover, the shape of the structural member for automobiles as described above is complicated, and not only a round tube but also a square tube and a deformed tube are required.

  As a method for increasing the strength of a steel pipe, for example, a method for increasing the strength of a hot-rolled steel sheet as a material as described in Patent Document 1, or a method for increasing the strength of a steel pipe as described in Non-Patent Document 1. There is a method of quenching and tempering after making a sewn steel pipe.

However, in the method for increasing the strength of the hot-rolled steel sheet described in Patent Document 1, the amount of alloying element added to the steel sheet must be increased, and the spring shape during pipe making is large and the formed shape is not stabilized. Therefore, there is a problem that the quality of the ERW welded portion is deteriorated.
On the other hand, in the method of quenching and tempering after pipe making described in Non-Patent Document 1, it is necessary to increase the amount of alloying elements to ensure hardenability, and the quality of the ERW welds deteriorates. In addition, since the steel pipe is bent at the time of quenching, it is necessary to correct the steel pipe, which increases the manufacturing cost.

  With respect to these problems, Patent Document 2 discloses a method for producing a high-strength ERW steel pipe, in which a pipe is formed using a hot rolled steel sheet containing Cu or Mo, and the steel pipe is heat-treated at 500 ° C. to 650 ° C. ing. The manufacturing method described in Patent Document 2 is a method of forming a pipe from a hot-rolled steel sheet through forming, welding, and forming, followed by heat treatment to increase the strength, and then performing a cold drawing process and annealing. A process is given.

However, in the manufacturing method described in Patent Document 2, since the strength of the steel pipe is increased by heat treatment, a large reduction in area cannot be obtained in the cold drawing process, and productivity is increased when manufacturing to a predetermined steel pipe size. There exists a problem that it falls and manufacturing cost becomes high. In addition, in the method described in Patent Document 2, an example is disclosed in which the amount of Cu added is increased to about 1.5% in order to increase the strength of the steel pipe. There is a problem that cracking of the pieces is likely to occur. Furthermore, the method described in Patent Document 2 has a problem that the production cost increases because a large amount of Ni is required to prevent cracking.
JP 52-114519 A JP-A-4-103718 “Nisshin Steel Making Technique”, Nisshin Steel Corporation, No. 48, p. 88

  The present invention has been made in view of the above-mentioned problems. In particular, a high-strength steel pipe having a TS of 590 MPa or more, which is suitable for round tubes, rectangular tubes and deformed tubes used for structural members for automobiles, is inexpensively manufactured. An object of the present invention is to provide a method for producing a high-strength steel pipe.

  The gist of the present invention is as follows.

(1) By mass%, C: 0.005% to 0.50%, Si: 0.6% or less, Mn: 0.3 to 3.0%, P: 0.02% or less, S: 0.00. 006% or less, Al: 0.001 to 0.08%, N: 0.001 to 0.006%, O: 0.006% or less, and Cu: 0.5 to 1.2% , Nb: 0.005 to 0.10%, V: 0.01 to 0.10% of at least one or two or more types of steel pipes with the balance of iron and unavoidable impurities The tube is subjected to cold drawing so that the area reduction rate is 10% or more and 60% or less, and then heat treatment is performed at a temperature range of 500 ° C. to 650 ° C. for 30 seconds or more, and then air cooling is performed. A method for producing a high-strength steel pipe.

(2) The steel pipe used as the raw material is further in mass%, Ni: 0.1-2.0%, Cr: 0.1-2.0%, Mo: 0.1-2.5% , Ti: containing at least one or more of 0.005 to 0.030%, The method for producing a high-strength steel pipe according to the above (1).

(3) The method described in (1) or (2) above, further comprising a step of heating and cooling to a temperature equal to or higher than the Ac3 transformation point before the tube is cold-drawn. Manufacturing method of high strength steel pipe.

  According to the method for producing a high-strength steel pipe of the present invention, a high-strength steel pipe having a TS of 590 MPa or more, which is suitable for a round pipe, a square pipe, a deformed pipe and the like, can be produced at a low cost. By adopting such a high-strength steel pipe obtained by the method for producing a high-strength steel pipe of the present invention for a structural member such as an automobile, it becomes possible to reduce the weight of the automobile or the like and to significantly improve the collision safety. In addition, since the fuel efficiency of automobiles and the like is improved, resources can be effectively used.

Hereinafter, an embodiment of a method for producing a high-strength steel pipe according to the present invention will be described in detail.
In addition, since this embodiment is described in detail for better understanding of the gist of the invention, the present invention is not limited unless otherwise specified.

  The manufacturing method of the high strength steel pipe of this invention is the mass%, C: 0.005-0.50%, Si: 0.6% or less, Mn: 0.3-3.0%, P: 0.00. 02% or less, S: 0.006% or less, Al: 0.001 to 0.08%, N: 0.001 to 0.006%, O: 0.006% or less, and Cu: 0.5 to 1.2%, Nb: 0.005 to 0.10% or less, V: 0.01 to 0.10% or less of at least one or two or more types, with the balance being iron and A steel pipe made of inevitable impurities is used as a raw pipe, and the raw pipe is cold-drawn so that the area reduction rate is 10% or more and 60% or less, and then at a temperature range of 500 ° C to 650 ° C for 30 seconds or more. In this method, heat treatment is performed, followed by air cooling.

  The method for producing a high-strength steel pipe according to the present invention includes a steel pipe containing at least one or more of Cu, Nb, and V as a base pipe, and the steel pipe is cold-drawn and then heat-treated. Thus, it is possible to achieve the high strength of the steel pipe by maximizing the precipitation strengthening of Cu, Nb and V.

"Composition of ingredients"
Below, the reason for limitation of the component composition of the raw material (steel pipe base material) prescribed | regulated by this invention is explained in full detail.

(C: 0.005% to 0.50%)
C (carbon) needs to be added in an amount of 0.005% or more in order to ensure the strength of the base material. However, if the amount of addition of C exceeds 0.50%, the workability and toughness of the green will deteriorate, so 0.50% was made the upper limit.

(Si: 0.6% or less)
Si (silicon) is an element added for deoxidation and strength improvement, but if the addition amount is too large, the surface properties deteriorate due to scale formation, so the upper limit was made 0.6%.

(Mn: 0.3% to 3.0%)
Mn (manganese) is an indispensable element for securing strength, and the lower limit of the amount of addition is 0.3%. However, if the amount of Mn added is too large, the center segregation of the continuously cast steel piece is promoted and cracks are generated at the time of molding, so the upper limit was made 3.0%.

(P: 0.02% or less)
In the present invention, the content of P (phosphorus), which is an inevitable impurity, is set to 0.02% or less. The main reason is to prevent the occurrence of cracks in the weld. Moreover, the reduction of the P content has the effect of reducing the center segregation of the continuously cast slab, preventing the grain boundary fracture and improving the low temperature toughness.

(S: 0.006% or less)
S (sulfur) is an element inevitably mixed in the same way as P. If the content of S (sulfur) exceeds 0.006%, cracks occur in the weld zone, so the upper limit value was set to 0.006%.

(Al: 0.001 to 0.08%)
In the case of Al (aluminum), in the case of killed steel, it is difficult to suppress the content to less than 0.001% in terms of steelmaking technology, and when it exceeds 0.08%, cracks in slabs and large oxide inclusions Since an internal defect due to formation is caused, the upper limit value is set to 0.08%.

(N: 0.001 to 0.006%)
N (nitrogen) forms TiN together with Ti and has the effect of improving the low temperature toughness by suppressing the coarsening of γ grains during slab reheating, and the minimum addition amount required for this is 0.001% is there. However, if the amount of N added is too large, it causes slab surface flaws, so the upper limit value must be limited to 0.006%.

(O: 0.006% or less)
O (oxygen) is an unavoidable impurity that deteriorates the toughness of the steel pipe, so the smaller the content, the better. In particular, if it exceeds 0.006%, a large amount of coarse oxides are produced in the steel and the toughness deteriorates, so the upper limit value was made 0.006%.

(Cu: 0.5-1.2%)
Cu (copper) is an important element that brings about precipitation hardening at the time of heat treatment after cold working, and 0.5% or more of addition is necessary in order to sufficiently exhibit such an effect. However, if Cu is added excessively, Cu-cracks are generated during hot rolling, so the upper limit value was set to 1.2%.

(Nb: 0.005-0.10%)
Nb (niobium), like Cu, is an important element that brings about precipitation hardening during heat treatment after cold working, and at least 0.005% Nb is necessary to obtain such an effect. However, if the amount of Nb added is too large, the ductility deteriorates, so the upper limit value was limited to 0.10%.

(V: 0.01-0.10%)
V (vanadium) has substantially the same effect as Nb, and 0.01% or more of addition is necessary to exert the effect. Further, the upper limit is allowable up to 0.10% from the viewpoint of ductility.

  Next, the reason for adding Ni, Cr, Mo, Ti will be described. In the present invention, the main purpose of adding these elements to the above basic components is to improve the strength without impairing the characteristics of the steel of the present invention. Therefore, the amount of addition is naturally limited.

(Ni: 0.1-2.0%)
Ni (nickel) has the effect of improving the strength of the base material without adversely affecting the weldability, but the effect cannot be sufficiently obtained at 0.1% or less, and addition exceeding 2.0% Is not preferable for weldability, so the upper limit value was made 2.0%.

(Cr: 0.1-2.0%)
Cr (chromium) has the effect of increasing the strength of the base material, and it is necessary to add 0.1% or more. However, if the amount of Cr added is too large, the weldability is remarkably deteriorated, so the upper limit value was set to 2.0%.

(Mo: 0.1-2.5%)
Mo (molybdenum) is an element that increases the strength of the base metal, but if added over 2.5%, the weldability is deteriorated in the same manner as Cr. Cannot be obtained sufficiently.

(Ti: 0.005 to 0.030%)
Ti (titanium) is an element that forms fine TiN together with N, suppresses the coarsening of γ grains during slab reheating, refines the microstructure, and contributes to the improvement of the strength and toughness of the base material. In order to exhibit such an effect, addition of 0.005% or more is necessary. Moreover, since the coarsening of TiN and the precipitation hardening by TiC will arise if there is too much addition amount of Ti and low temperature toughness will be deteriorated, the upper limit was limited to 0.030%.

"Production method"
Below, each manufacturing condition prescribed | regulated with the manufacturing method of the high intensity | strength exchange of this invention is demonstrated.

In the production method of the present invention, a steel pipe having the above component range is used as a raw pipe, and the raw pipe is cold-drawn so that the area reduction rate is 10% or more and 60% or less. This is a method of performing heat treatment for 30 seconds or more in the range and then air-cooling.
In the present invention, strain is introduced into the steel by cold-drawing the raw tube, and then heat treatment in the above temperature range is performed to finely precipitate carbonitrides such as Cu, Nb, and V on the dislocations. By doing so, a steel pipe with increased strength can be obtained.

If the area reduction during cold drawing is less than 10%, sufficient strain cannot be introduced into the steel, so that the effect of increasing the strength by precipitation strengthening cannot be obtained sufficiently. Further, when the area reduction rate exceeds 60%, the steel is broken during cold working, so the area reduction rate is limited to 60% or less.
If the temperature of the heat treatment is less than 500 ° C., Cu, Nb and V are not sufficiently precipitated, and if it exceeds 650 ° C., overaging occurs and sufficient strength cannot be obtained. Further, if the heat treatment time is less than 30 seconds, the precipitation of Cu, Nb, and V becomes insufficient.

  In addition to the above manufacturing conditions, the manufacturing method of the present invention further includes a step of heating and cooling to a temperature equal to or higher than the Ac3 transformation point before the tube is cold-drawn. However, it is preferable at the point from which the high strength steel pipe which has the outstanding strength characteristic and ductility (elongation) is obtained.

  As described above, according to the method for manufacturing a high-strength steel pipe of the present invention, a high-strength steel pipe having a TS of 590 MPa or more suitable for round tubes, square tubes, deformed tubes, and the like is manufactured at low cost by the above configuration. be able to. By adopting such a high-strength steel pipe obtained by the method for producing a high-strength steel pipe of the present invention for a structural member such as an automobile, it becomes possible to reduce the weight of the automobile or the like and to significantly improve the collision safety. In addition, since the fuel efficiency of automobiles and the like is improved, resources can be effectively used.

  Hereinafter, examples of the method for producing a high-strength steel pipe according to the present invention will be given and the present invention will be described more specifically. However, the present invention is not originally limited to the following examples, but the purpose described above and below The present invention can be carried out with appropriate modifications within a range that can be adapted to the above, and all of them are included in the technical scope of the present invention.

[Sample preparation]
Using steel from 1 to 23 having the chemical composition shown in Table 1 below, a steel pipe (base steel pipe) having a steel pipe size of 63.5 mm (φ) × 2.2 mm (t) is prepared. Stretching and heat treatment were performed under the conditions shown in Table 2 to obtain a high-strength steel pipe made of the steel of the present invention (steels 1 to 12) and a steel pipe made of comparative steel (steels 13 to 23).

[Evaluation test]
Each steel pipe sample manufactured by the above method was examined for mechanical properties by conducting a tensile test.
The tensile test was performed in accordance with JIS Z 2241 and sampled No. 5 test piece from the longitudinal direction of the steel pipe and in accordance with JIS Z 2241. In Table 2, YS (MPa) is the yield strength, TS (MPa) is the tensile strength, and EL (%) is the total elongation.

  Table 1 shows a list of chemical composition of each steel of this example, and Table 2 shows a list of manufacturing conditions and mechanical properties of the steel pipe.

[Evaluation results]
As is clear from the results shown in Tables 1 and 2, steel pipes (steel 1 to 12) having a chemical composition within the range specified in the present invention are used, and a steel pipe is manufactured according to the manufacturing conditions within the range specified in the present invention. In this case, it is understood that a high-strength steel pipe having high strength characteristics and good ductility (elongation) can be obtained.

In contrast, a steel pipe made of comparative steel (steel 13 to 23) whose chemical component composition or each manufacturing condition is outside the specified range of the present invention results in inferior mechanical properties. ing.
Steel 13 has a low tensile strength TS of 500 (MPa) because the amount of Cu added is as low as 0.35% by mass. Moreover, since the addition amount of Cu was too much with 1.3%, the result of which the Cu-crack generate | occur | produced was produced for the steel 14.
Steel 15 has a low tensile strength TS of 480 (MPa) because the amount of Nb added is as low as 0.002%. Moreover, since the amount of Nb added is too large at 0.110%, the total elongation EL is inferior at 4%.
Steel 17 has a low tensile strength TS of 470 (MPa) because the amount of V added is small. Moreover, since the addition amount of V is too much at 0.11%, the total elongation EL is inferior at 4%.

Since steel 19 has a small area reduction rate of 5% during the drawing process, the tensile strength TS is as low as 560 (MPa). Moreover, since the area reduction rate in the drawing process of steel 20 was too large at 65%, it resulted in a fracture during the drawing process.
Since the heat treatment temperature of steel 21 is as low as 450 ° C., the tensile strength TS is as low as 565 (MPa). Moreover, since the heat processing temperature of steel 22 is too high with 700 degreeC, it has a low tensile strength TS of 560 (MPa).
Steel 23 has a low tensile strength TS of 555 (MPa) because the heat treatment time is as short as 20 seconds.

  The high-strength steel pipe obtained by the production method of the present invention is excellent in strength characteristics and ductility, and can be applied to structural members in various fields in addition to the automobile field.

Claims (3)

% By mass
C: 0.005% to 0.50%,
Si: 0.6% or less,
Mn: 0.3-3.0%
P: 0.02% or less,
S: 0.006% or less,
Al: 0.001 to 0.08%,
N: 0.001 to 0.006%,
O: each containing 0.006% or less,
Cu: 0.5 to 1.2%,
Nb: 0.005 to 0.10%,
V: 0.01-0.10%
A steel pipe containing at least one or more of the above, with the balance being iron and inevitable impurities,
The base tube is subjected to cold drawing so that the area reduction rate is 10% or more and 60% or less, then heat-treated at a temperature range of 500 ° C. to 650 ° C. for 30 seconds or more, and then air-cooled. To manufacture high strength steel pipe. The steel pipe used as the raw material is further in mass%,
Ni: 0.1 to 2.0%,
Cr: 0.1 to 2.0%,
Mo: 0.1 to 2.5%,
Ti: 0.005-0.030%
2. The method for producing a high-strength steel pipe according to claim 1, comprising at least one or more of the above. The high-strength steel pipe according to claim 1 or 2, further comprising a step of heating and cooling to a temperature equal to or higher than the Ac3 transformation point before the pipe is cold-drawn. Method.