JP2004292922A - Method for manufacturing high tensile strength steel pipe of excellent combined secondary workability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a high tensile strength steel pipe of excellent combined secondary workability. <P>SOLUTION: A steel pipe formed by performing electroseaming of a steel strip of the composition consisting of 0.01-0.60% C, 0.01-2.0% Si, 0.01-3.0% Mn, 0.005-0.10% Al, ≤ 0.0050% S, and ≤ 0.1% P, or further containing one kind or two or more kinds of Cu, Ni, Cr, Mo, Nb, V, Ti, B, REM and Ca is subjected to diameter-reduction rolling under the condition of the diameter-reduction ratio of ≥ 25%. The pipe is firstly subjected to the diameter-reduction rolling under the condition that the contraction ratio is ≥ 10%, the rolling start temperature is A<SB>3</SB>point or over, the rolling completion temperature is not lower than (A<SB>1</SB>point - 50°C) and not higher than A<SB>3</SB>point, and then, re-heated to not lower than A<SB>3</SB>point, and subjected to the diameter-reduction rolling under the condition that the diameter-reduction ratio is ≥ 5%, the rolling completion temperature is not lower than A<SB>3</SB>point. Here, A<SB>3</SB>point and A<SB>1</SB>point are defined by the relational expression to the composition. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００２７】
【表２】

【００２８】
【発明の効果】
本発明によれば、ＴＳ：５５０ＭＰａ以上の高引張強度を有し、かつ複合二次加工を受けたときに割れを生じない、複合二次加工性に優れた高張力鋼管を安定製造できるようになるという優れた効果を奏する。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing a high-strength steel pipe having excellent composite secondary workability, and more particularly, to a composite secondary processing comprising a combination of bending, diameter reduction, and pipe end flattening, wherein a surface and / or end portion is formed. The present invention relates to a method for producing a high-strength steel pipe having a tensile strength of 550 MPa or more, which is excellent in composite secondary workability, which is a property capable of completing processing without generating cracks in the steel pipe.
[0002]
[Prior art]
As a conventional technique for producing a steel pipe by hot reduction rolling using a welded steel pipe as a mother pipe, when an ERW welded steel pipe is used and hot drawn and rolled to produce a coiled steel pipe, an inner surface of the coiled steel pipe is used. In order to prevent cracks at the time of cold working due to inner surface wrinkles generated in the bead portion, the inner surface bead portion of the ERW welded steel pipe is cut or / and ground so that the bead height is -200 to +20 μm. Hot-rolling, hot-rolling, and coiling to produce a coiled steel pipe (Patent Document 1), and secondary processing can be performed without annealing. In order to manufacture a long coiled steel pipe without scale, the steel pipe is hot drawn and rolled by a stretch reducer, the rolling is completed at a temperature of _one or more points A, and then the coil is wound and allowed to cool. And carp In a method for manufacturing a tubular steel pipe (Patent Document 2) and a method for manufacturing a hot ERW pipe provided with a pipe making step, a heating step, and a rolling finishing step, hot joints free of local corrosion (grooved corrosion) at joints are used. For the purpose of stable production of an electric resistance welded pipe, after the pipe production, prior to the rolling finishing of the obtained mother pipe, the mother pipe is heated by a combustion heating furnace and an induction heating device to secure a predetermined rolling finishing temperature. In order to obtain a method of manufacturing a hot ERW pipe (Patent Document 3) and obtain a steel pipe excellent in hydroform formability, a steel pipe manufactured in an ERW pipe manufacturing process is mainly made of an ERW weld. as both sides, the minimum thickness at five times the range of the thickness in the circumferential direction performed bead cutting so as not smaller than the average thickness of the entire circumferential direction, the base tube Ac ₃ point -30 ° C. or more Heat to austenite, or austenite A method for producing a steel pipe excellent in formability, characterized in that diameter-reducing rolling is performed at a temperature at which a + ferrite two-phase structure is formed and in a temperature range of 700 ° C. or higher, followed by cooling (Patent Document 4). ing.
[0003]
[Patent Document 1]
JP-A-6-238488 [Patent Document 2]
JP-B-63-53248 [Patent Document 3]
Japanese Patent No. 3031233 [Patent Document 4]
JP-A-2002-115029
[Problems to be solved by the invention]
On the other hand, for example, in application fields such as structural steel for automobiles (for example, undercarriage parts), composite secondary processing (: bending, diameter reduction, It is becoming more and more common to use two or more types of pipe end flattening to form a complicated shape and to use it. There is an increasing demand for high-strength steel pipes that do not crack even when they are cracked (ie, have excellent composite secondary workability). However, in the above-described conventional technology, the composite secondary workability is not considered, and a high-strength steel pipe having a sufficient composite secondary workability cannot always be obtained. Therefore, the present invention provides a method for producing a high-strength steel pipe excellent in composite secondary workability, which can stably produce a steel pipe having TS: 550 MPa or more and which does not crack even when subjected to composite secondary processing. The purpose is to:
[0005]
[Means for Solving the Problems]
The inventors have conducted intensive studies to achieve the above object, and as a result, shallow-roll a steel pipe formed by ERW welding a steel strip having a specific composition (= chemical composition) at a specific reduction ratio, In addition, this diameter reduction rolling is divided into a former stage and a latter stage, and reheating is performed in the middle thereof. In the first stage rolling, rolling is performed in a γ region or γ + α two-phase region. The present inventors have found that by rolling in the γ region, it is possible to stably produce a high-tensile steel pipe having excellent composite secondary workability, and have made the present invention.
[0006]
That is, in the present invention, in mass%, C: 0.01 to 0.60%, Si: 0.01 to 2.0%, Mn: 0.01 to 3.0%, Al: 0.005 to 0%. .10%, S: 0.0050% or less, P: 0.1% or less, or further, Cu: 1% or less, Ni: 1% or less, Cr: 2% or less, Mo: 1% or less, Nb : 0.1% or less, V: 0.5% or less, Ti: 0.2% or less, B: 0.005% or less, REM: 0.02% or less, Ca: 0.01% or less Alternatively, when a steel pipe formed by electric resistance welding of a steel strip having two or more kinds of compositions is subjected to diameter reduction at a diameter reduction rate of 25% or more, first, a diameter reduction rate of 10% or more, and a rolling start temperature: [equation 1] being defined a ₃ points or more, the rolling end temperature :( below a ₁ point -50 ° C. defined by [formula 2]) or the a ₃ point following conditions Cast径圧, then reheated to above the A ₃ point, then radial contraction rate: 5% or more, the rolling end temperature: conjugated secondary workability, characterized by diameter reduction rolling at the A ₃ point or more conditions This is a method for manufacturing a high-tensile steel pipe excellent in quality.
[0007]
(Equation 1)
A ₃ points (° C.) = 910 −203 × Δ (% C) −15.2 × (% Ni) + 44.7 × (% Si) + 104 × (% V) + 31.5 × (% Mo) − [30 × (% Mn) + 11 × (% Cr) + 20 × (% Cu) −700 × (% P) −400 × (% Al) −400 × (% Ti)]
[Equation 2]
A ₁ point (° C.) = 723 −10.7 × (% Mn) −16.9 × (% Ni) + 29.1 × (% Si) + 16.9 × (% Cr)
[0008]
BEST MODE FOR CARRYING OUT THE INVENTION
First, the reason why the composition (= chemical composition) of the steel strip is limited to the above range in the present invention will be described.
C: 0.01 to 0.60%
C is an element that dissolves in the matrix or precipitates as a carbide to increase the strength of steel, and also precipitates as a hard second phase (second phase particles or second phase structure) such as cementite, pearlite, Bainite and martensite contribute to higher strength and ductility. In order to secure the desired strength and obtain the effect of improving ductility due to the cementite or the like precipitated as the second phase, the content of C is required to be 0.01% or more, preferably 0.04% or more. If the content exceeds 0.60%, the ductility deteriorates. For this reason, C is limited to the range of 0.01 to 0.60%.
[0009]
Si: 0.01 to 2.0%
Si acts as a deoxidizing agent and also forms a solid solution in the matrix to increase the strength of the steel. This effect is observed at a content of 0.01% or more, preferably 0.1% or more, but a content of more than 2.0% deteriorates the ductility. For this reason, Si was limited to the range of 0.01 to 2.0%. Preferably, it is in the range of 0.10 to 1.5% from the viewpoint of strength-ductility balance.
[0010]
Mn: 0.01 to 3.0%
Mn is an element that increases the strength of steel and promotes fine precipitation of cementite as a second phase or precipitation of martensite and bainite. Such an effect is observed at a content of 0.01% or more, but a content of more than 3.0% deteriorates ductility. For this reason, Mn was limited to the range of 0.01 to 3.0%. From the viewpoint of strength-ductility balance, Mn is preferably in the range of 0.2 to 1.3%, and more preferably in the range of 0.6 to 1.3%.
[0011]
Al: 0.005 to 0.10%
Al has a function of making crystal grains fine. Thereby, the dispersion of the second phase structure in the material steel pipe stage is made fine dispersion, and the effect of the present invention is further increased. For this purpose, the content must be at least 0.005% or more, but if it exceeds 0.10%, the amount of oxide-based inclusions increases and the cleanliness deteriorates. For this reason, Al was limited to the range of 0.005 to 0.10%. Incidentally, the content is preferably 0.015 to 0.06%.
[0012]
S: 0.0050% or less S increases the sulfide and deteriorates the cleanliness, so it is desirable to reduce it as much as possible. However, if the content exceeds 0.0050%, the composite secondary workability is significantly impaired. Therefore, S is set to 0.0050% or less.
P: 0.1% or less P increases the strength of the steel when it is contained in a trace amount, but when it is excessively contained, it segregates at the grain boundaries and significantly deteriorates the composite secondary workability. .
[0013]
In addition to the above-described basic composition, the following alloy element group (first group to third group) may be added alone or in combination. That is, a first group consisting of one or more of Cu: 1% or less, Ni: 1% or less, Cr: 2% or less, Mo: 1% or less, Nb: 0.1% or less, V: 0. A second group consisting of one or more of 5% or less, Ti: 0.2% or less, B: 0.005% or less, REM: 0.02% or less, Ca: 0.01% or less Or a third group consisting of two types.
[0014]
The first group of elements Cu, Ni, Cr and Mo are all elements that increase the strength, and one or more of them can be added as needed. These elements have the effect of lowering the transformation point and refining the ferrite grains or the second phase structure. However, when Cu is added in a large amount, hot workability deteriorates, so the upper limit is 1%. Ni improves the toughness as the strength increases. However, even if added over 1%, the effect saturates as compared with the high cost, so the upper limit is 1%. If Cr and Mo are added in large amounts, the weldability and ductility are deteriorated and the cost is increased. Therefore, the upper limits are 2% and 1%, respectively. Preferably, Cu: 0.1 to 0.6%, Ni: 0.1 to 0.7%, Cr: 0.1 to 1.5%, Mo: 0.05 to 0.5%.
[0015]
The elements of the second group, Nb, V, Ti, and B, all precipitate as carbides, nitrides, or carbonitrides, and contribute to the refinement of crystal grains and, consequently, the fine dispersion of the second phase structure, as well as to increase the strength. Especially in steel pipes that have joints that are heated to high temperatures, in addition to the effect of grain refinement during the heating process during joining, acts as ferrite precipitation nuclei during the cooling process, It also has the effect of preventing curing, and one or more of them can be added as necessary. However, if a large amount is added, both weldability and toughness are deteriorated. Therefore, the upper limits of Nb are 0.1%, V is 0.5%, Ti is 0.2%, and B is 0.005%. Preferably, Nb is 0.005 to 0.05%, V is 0.05 to 0.1%, Ti is 0.005 to 0.10%, and B is 0.0005 to 0.002%.
[0016]
The third group of elements REM and Ca both have the effect of adjusting the shape of inclusions and improving workability, and further precipitate as sulfides, oxides, or oxysulfides, and It also has the effect of preventing the hardening of the joints in steel pipes having the above, and one or two kinds can be added as necessary. However, if REM exceeds 0.02% or Ca exceeds 0.01%, the number of inclusions becomes too large, the cleanliness decreases, and the ductility deteriorates. Therefore, REM is 0.02% and Ca is 0%. 0.01% was set as the upper limit. When the REM is less than 0.004% and the Ca is less than 0.001%, the effect of the above effect is small. Therefore, it is preferable that the REM is 0.004% or more and the Ca is 0.001% or more.
[0017]
The composition portion (remaining portion) other than the component elements described above is composed of Fe and unavoidable impurities. In the present invention, N: 0.010% or less and O: 0.006% or less are allowed as inevitable impurities. Describing these unavoidable impurity elements, N can be allowed up to 0.010%, which is effective for refining the crystal grains by combining with Al and thereby refining the second phase structure, but exceeds 0.010%. Is preferably reduced to 0.010% or less, and more preferably 0.002 to 0.006%, since the content of the steel deteriorates ductility. O is preferably reduced as much as possible because it deteriorates cleanliness as an oxide, but up to 0.006% is acceptable.
[0018]
Next, the steel pipe manufacturing method of the present invention will be described.
In the present invention, the steel strip having the above composition is welded by ERW to form a steel pipe (ERW pipe). The electric resistance welding method is not particularly limited, and may be performed either cold or hot. Then, the obtained ERW steel pipe is subjected to diameter reduction rolling (= reduction rolling) using a stretch reducer or the like to obtain a steel pipe having a desired outer diameter. The diameter reduction ratio (total diameter reduction; described as R) in the entire diameter reduction rolling is set to 25% or more from the viewpoint of securing the production efficiency.
[0019]
In the present invention, diameter reduction rolling is performed in two stages of pre-rolling and post-rolling, and intermediate heating is performed between pre-rolling and post-rolling. In the pre-rolling, rolling is started from the γ region and finished at the high temperature side of the α region to the α + γ region to sufficiently introduce rolling strain into a ferrite structure or a mixed structure of ferrite and austenite, thereby dispersing the second phase finely. Promotes the development of the rolling texture. Furthermore, by heating to the γ region by intermediate heating and rolling only in the γ region in the subsequent rolling, recrystallization is caused over the entire cross section of the tube, avoiding the formation of a mixed grain structure, and fine and uniform sized austenite structure And Thereby, the cooling transformation structure after rolling is changed to a texture in which the second phase is finely dispersed and suitable for ductility (for example, the {110} plane is orthogonal to the pipe diameter direction and the <111> direction is the pipe axis direction. (Having a crystal orientation along the same direction) can be formed into a fine grain structure mainly composed of ferrite, and a steel tube having high strength and excellent composite secondary workability can be obtained.
[0020]
The reasons for limiting the operating conditions relating to the above-mentioned first-stage rolling, intermediate heating and second-stage rolling employed in the present invention will be described below.
Front rolling radial contraction rate (referred to as r _1): less than 10% or more r ₁ is 10%, the material: rolled strain is not sufficiently introduced into the (rolled steel), a fine dispersion of second phase not terms of the well, the formation of rolling texture becomes insufficient, since the development of texture due to the subsequent steps will not be made, r ₁ is 10% or more. Incidentally, it is preferably at least 20%.
[0021]
Rolling start temperature of the pre-rolling (T ₀ and denoted): The A ₃ points or more T ₀ is A less than ₃ points, in a subsequent step the intermediate heat - so prone to mixed grain structure of extent have difficulty obliterate later rolled , _{T 0} is the _three or more points _a. Incidentally, preferably in the range of _{A 3} point to A _3-point +250 ° C. From the viewpoint of further grain refinement.
The front rolling (referred to as _{T 1)} rolling end temperature of :( _{A 1} point -50 ° C.) or higher _{A 3} points below _{T 1} is less than _{(A 1} point -50 ° C.), occurrence disturbances deterioration and shape of the surface texture It tends to, whereas, T ₁ is the previous stage rolling is performed only gamma zone at _three points than a, does not occur alpha → gamma transformation in the next intermediate heating, grain refining effect by nucleation of the transformation can not be expected since, _{T 1} is not more than ₃ points _a or _{(a 1} point -50 ° C.).
[0022]
The heating temperature of the intermediate heating (T referred to as _{the IH):} The A to less than ₃ points T _{the IH} is A ₃ points, the next subsequent rolling is performed in gamma + alpha region, because uniform fine recrystallized gamma grain structure can not be obtained , _{T the IH} is set to _three or more points _a. Incidentally, preferably in the range of _{A 3} point to A _3-point +250 ° C. From the viewpoint of further grain refinement.
Subsequent rolling of the radial contraction rate (referred to as r _2): When more than 5% r ₂ is less than 5%, it is difficult to uniformly recrystallized tube section throughout, so tends to mixed grain structure, r ₂ is 5% or more. Note that the radial contraction rate _{r 1} and _{r 2} and Zenchijimi diameter ratio R of the front rolling and subsequent rolling, the formula: R = is related with _{1- (1-r 1) ×} (1-r 2).
[0023]
(Referred to as T ₂₎ rolling end temperature of the subsequent rolling: When A ₃ point or more T ₂ is less than ₃ points A, occur gamma → alpha transformation during rolling, the ferrite generated transformation and work hardening are rolled, since ductility is deteriorated, _{T 2} is set to _three or more points _a.
After the second-stage rolling, cooling may be performed according to a conventional method. This cooling may be performed by either air cooling or water cooling.
[0024]
【Example】
A strip steel (thickness of a hot-rolled steel sheet) having a thickness of 2.5 mm having a composition shown in Table 1 was cold-sealed by a conventional method and subjected to electric resistance welding, followed by bead cutting to obtain an outer diameter of 146.0 mm. , And it was drawn and rolled under various conditions shown in Table 2 to obtain a product tube having a size shown in Table 2. In the rolling reduction, a 4-roll reducer having a tandem arrangement was used. Heating before pre-rolling was performed using a high-frequency induction coil. Intermediate heating was performed using a high frequency induction coil. The cooling after the second-stage rolling was air cooling.
[0025]
The obtained product pipe was examined for TS and composite workability. TS was measured by a tensile test of a JIS No. 12 A test piece (arc-shaped test piece, parallel portion width 19 mm, gauge length 50 mm) taken with the tube axis direction as the test direction. The composite secondary workability is as follows: For a 200 mm long steel tube test piece cut out from each product tube, after reducing the diameter by 30% with a punching die, bending 30 ° and flattening a 100 mm length from the pipe end And the crack occurrence ratio (denoted as the number of crack occurrences / 10) was investigated, and the result was used to evaluate. Table 2 shows the results of these investigations. Table 2 shows that the product pipe manufactured according to the present invention has high tensile strength and excellent composite secondary workability.
[0026]
[Table 1]

[0027]
[Table 2]

[0028]
【The invention's effect】
According to the present invention, it is possible to stably produce a high-strength steel pipe having a high tensile strength of TS: 550 MPa or more, and which does not crack when subjected to composite secondary processing and has excellent composite secondary workability. An excellent effect is achieved.

Claims (2)

In mass%, C: 0.01 to 0.60%, Si: 0.01 to 2.0%, Mn: 0.01 to 3.0%, Al: 0.005 to 0.10%, S: When a steel pipe formed by electric resistance welding of a steel strip having a composition containing 0.0050% or less and P: 0.1% or less is subjected to diameter reduction at a rate of 25% or more, first, the diameter reduction is performed. 10% or more, rolling start temperature: ₃ points or more of A defined by the following [Equation 1], rolling end temperature: ( ₁ point of -50 ° C defined by the following [2]) or more and the ₃ points of the above A And then re-heated to the above-mentioned A ₃ point or more, and then reduced-diameter rolling under the condition of a diameter reduction ratio: 5% or more and a rolling end temperature: the above A ₃ point or more. A method for manufacturing high-tensile steel pipes with excellent secondary workability.
(Equation 1)
A ₃ points (° C.) = 910 −203 × Δ (% C) −15.2 × (% Ni) + 44.7 × (% Si) + 104 × (% V) + 31.5 × (% Mo) − [30 × (% Mn) + 11 × (% Cr) + 20 × (% Cu) −700 × (% P) −400 × (% Al) −400 × (% Ti)]
[Equation 2]
A ₁ point (° C.) = 723 −10.7 × (% Mn) −16.9 × (% Ni) + 29.1 × (% Si) + 16.9 × (% Cr)
Here, (% element symbol) on the right side is the content of the element in steel (% by mass), and 0 is substituted for the element not contained. In addition to the above composition, in mass%, Cu: 1% or less, Ni: 1% or less, Cr: 2% or less, Mo: 1% or less, Nb: 0.1% or less, V: 0.5% or less, Ti : 0.2% or less, B: 0.005% or less, REM: 0.02% or less, Ca: 0.01% or less, wherein one or more of them are contained. Method for producing high-strength steel pipe excellent in composite secondary workability.