JP2000312907A - Method of manufacturing steel pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel pipe with isotropic and superior ductility and collision-impact resistance characteristics without requiring a large-scale process modification by combining stretch-rolling and skew-rolling at the rolling temperature from Ac3 transformation point to specific temperature after the material containing C, Si, Mn and Al of specific compositions and having a composition composed of Fe and unavoidable impurities of the remaining portion is made into a material pipe. SOLUTION: The material containing 0.005 to 0.30% C, 0.01 to 3.0% Si, 0.01 to 4.0% Mn, and 0.001 to 0.10% Al in weight is rolled by combining stretch- rolling and skew-rolling at the rolling temperature from Ac3 transformation point to 400 deg.C. It is preferable to add one type or more selected from 1% or less Cu, 2% or less Ni, 2% or less Cr, and 1% or less Mo, and/or one type or more selected from 0.1% or less Nb, 0.3$ or less V, 0.2% or less Ti, and 0.004% or less B, and/or one or two types selected from 0.02% or less REM and 0.01% or less Ca.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a steel pipe, and more particularly to a method for manufacturing a steel pipe having excellent mechanical properties.

[0002]

2. Description of the Related Art In order to increase the strength of steel, Mn,
Addition of alloy elements such as Si, heat treatment such as controlled rolling, controlled cooling, quenching and tempering, and addition of precipitation hardening elements such as Nb and V are utilized. However, steel materials need to have high ductility and toughness as well as strength.
For some time, there has been a demand for steel materials in which strength, ductility, and toughness are improved in a well-balanced manner.

[0003] Refinement of crystal grains is important as a few means capable of improving both strength, ductility and toughness. As a method for refining crystal grains, coarse austenite grains are prevented, and fine austenite is converted to austenite-.
There are a method of making ferrite grains fine by utilizing ferrite transformation, a method of making austenite grains fine by working to make ferrite grains fine, and a method of using martensite and lower bainite by quenching and tempering.

[0004] Above all, controlled rolling in which ferrite grains are refined by strong working in the austenite region and subsequent austenite-ferrite transformation is widely used in the production of steel products. Further, a small amount of Nb is added to suppress recrystallization of austenite grains to further refine ferrite grains. By processing in the austenite non-recrystallization temperature range, the austenite grains elongate and deformed bands are formed in the grains, and ferrite grains are generated from the deformed bands, and the ferrite grains are further refined. In order to further refine the ferrite grains, a step of cooling during or after the processing, that is, a controlled cooling is also used.

[0005]

However, according to the above-mentioned method, in order to manufacture a steel pipe having a high impact-resistant property as a component member suitable for a highly-safety automobile which has been strongly demanded recently, it is necessary to modify equipment. And significant process remodeling was required, and there was a limit in cost. In addition, in the conventional tube rolling method, the product tube has a large difference between the crystal grain size in the axial direction and the crystal grain size in the circumferential direction, so that the mechanical properties exhibit remarkable anisotropy, resulting in complicated processing. There was a worry that he was not suitable.

The present invention advantageously solves the above-mentioned problem, and provides a method for manufacturing a steel pipe capable of imparting isotropic and excellent ductility and impact resistance to the steel pipe without requiring a major process modification. The purpose is to:

[0007]

The present inventors have conducted intensive studies on a steel pipe manufacturing process capable of producing a high-strength steel pipe excellent in ductility at a high pipe-forming speed. By performing a process combining the reduction rolling and the tilt rolling, the microstructure becomes a fine and uniform ferrite and cementite structure with a ferrite grain size of 2 μm or less, and a steel pipe having high strength and excellent ductility and toughness can be obtained. Found and made the present invention.

[0008] That is, the present invention is a method for manufacturing a steel pipe described in each of the following items. (1) By weight%, C: 0.005 to 0.30%, Si: 0.01 to 3.0
%, Mn: 0.01 to 4.0%, Al: 0.001 to 0.10%,
After the material having a chemical composition the balance being Fe and unavoidable impurities was processed into raw tube, characterized by processing the product tube by rolling in combination with a reducing rolling inclined-rolling and the rolling temperature of Ac ₃ transformation point to 400 ° C. It is a method of manufacturing a steel pipe.

(2) In addition to the composition of the material (meaning chemical composition; the same applies hereinafter), Cu: 1% or less, Ni: 2% by weight.
Hereafter, one or more selected from Cr: 2% or less, Mo: 1% or less, and / or Nb: 0.1% or less,
V: at least 0.3%, Ti: at most 0.2%, B: at least one selected from at most 0.004%, and / or
The method according to (1), wherein one or two selected from among REM: 0.02% or less and Ca: 0.01% or less are added.

(3) The method according to (1) or (2), wherein the raw tube before rolling or the tube to be rolled during rolling is heated to a temperature not higher than the Ac ₁ transformation point + 50 ° C. (4) Make the total reduction of rolling area 20% or more (1)-
The method according to any one of (3). (5) Make the total reduction of rolling area 60% or more (1)-
The method according to any one of (3).

(6) At least one pass having a reduction ratio of 6% or more per pass is included in all rolling passes.
The method according to any one of (5). (7) Make the total reduction of area by drawing rolling 60% or more (1)-
The method according to any one of (6). (8) Make the total area reduction rate of inclined rolling 50% or more (1)-
The method according to any one of (6).

(9) The method according to any one of (1) to (6), wherein the total area reduction rate of the reduction rolling and the total area reduction rate of the inclined rolling are each 30% or more. (10) The method according to any one of (1) to (6), wherein the total area reduction rate of the reduction rolling and the total area reduction rate of the inclined rolling are each 60% or more. Here, reduction of area r _i per pass, the total area reduction rate R is defined by the following formula.

Note that r _i = (1−X _i ) × 100 (%) R = {1 − {(X _i )} × 100 (%) X _i = S _i / S _i−1 where S: rolled Pipe cross-sectional area, subscripts i-1, i: entrance side and exit side of the i-th path, Π (X _i ): X _a × X _b × ‥‥ × when i = (a, b, ‥‥, m) X _m

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, steel having a specific composition is used as a raw material, but there is no particular limitation on means (tube forming method) for processing the raw material into a raw pipe (steel pipe). Electric resistance welding method using high-frequency current (base pipe name: ERW steel pipe), solid phase pressure welding method in which both ends of open pipe are heated to the solid-state pressure welding temperature range and pressure-welded (base pipe name: solid-phase pressure welded steel pipe) , Forged welding method (base pipe name: forged steel pipe) and Mannesmann piercing and rolling method (base pipe name: seamless steel pipe) can be suitably used.

Next, the reasons for limiting the composition of the material will be described. C: 0.005 to 0.30% C is an element which increases the strength of steel by being precipitated as a solid solution or carbide in the matrix, and fine cementite precipitated as a hard second phase has ductility (uniform elongation). Contribute to improvement. 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 must be 0.005% or more, more preferably 0.04% or more. If the content exceeds this, the strength becomes too high and the ductility deteriorates. Because of this, C
Is limited to the range of 0.005 to 0.30%. In addition, a more preferable range is 0.04 to 0.30%.

Si: 0.01-3.0% Si acts as a deoxidizing element and forms a solid solution in the matrix to increase the strength of steel. This effect is observed when the content is 0.01% or more, preferably 0.1% or more, but the content exceeding 3.0% deteriorates the ductility. From this, Si is 0.
Limited to the range of 01-3.0%. In order to improve the fatigue resistance, the content of Si is preferably set to 1.5% or less. 1.5
%, Inclusions are generated, and the fatigue resistance deteriorates. Therefore, the preferred range is 0.1 to 1.5%.

Mn: 0.01 to 4.0% Mn is an element that increases the strength of steel, and promotes fine precipitation of cementite as the second phase in the present invention. 0.
If it is less than 01%, desired strength cannot be ensured and fine precipitation of cementite is hindered. On the other hand, if it exceeds 4.0%, the strength is excessively increased and ductility is deteriorated. For this reason, Mn
Was limited to the range of 0.01 to 4.0%. From the viewpoint of strength-elongation balance, Mn is preferably in the range of 0.2 to 1.3%, more preferably in the range of 0.6 to 1.3%.

Al: 0.001 to 0.10% Al has an effect of reducing the crystal grain size. To refine the crystal grains, the content must be at least 0.001% or more. However, if the content exceeds 0.10%, the amount of oxide-based inclusions increases and the cleanliness deteriorates. For this reason, Al was limited to the range of 0.001 to 0.10%. Incidentally, the content is preferably 0.015 to 0.06%.

In addition to the above-mentioned basic composition of the material steel pipe, the following alloying element group may be added alone or in combination. Cu: 1% or less, Ni: 2% or less, Cr: 2% or less, Mo: 1%
One or more selected from the following Cu, Ni, Cr, and Mo are all elements that increase the strength, and one or more of them can be added as necessary. These elements have the effect of lowering the transformation point and miniaturizing the ferrite grains or the second phase. However, the hot workability deteriorates when a large amount of Cu is added, so the upper limit is 1%. Ni improves toughness with increasing strength, but adding more than 2% saturates the effect and increases cost, so 2%
Was set as the upper limit. If large amounts of Cr and Mo are added, the weldability and ductility will deteriorate and the cost will increase.
Was set as the upper limit. Preferably, Cu: 0.1-0.6%, N
i: 0.1 to 1.0%, Cr: 0.1 to 1.5%, Mo: 0.05 to 0.5
%.

Nb: 0.1% or less, V: 0.3% or less, Ti: 0.
2% or less, B: one or two or more selected from 0.004% or less Nb, V, Ti, and B precipitate as carbides, nitrides, or carbonitrides, resulting in finer grains and higher strength. Element, which contributes to the formation of
There is also an effect of miniaturizing grains in a heating process at the time of joining and a precipitation nucleus of ferrite in a cooling process to prevent hardening of a joined portion, and one or more kinds can be added as necessary.
However, when added in large amounts, both weldability and toughness deteriorate, so that Nb is 0.1%, V is 0.3%, Ti is 0.2%, and B is 0.004%.
% Was the upper limit. Preferably, Nb: 0.005
~ 0.05%, V: 0.05 ~ 0.1%, Ti: 0.005 ~ 0.10%,
B: 0.0005 to 0.002%.

One or two kinds of REM and Ca selected from REM: 0.02% or less and Ca: 0.01% or less, both have an effect of adjusting the shape of inclusions and improving workability. Furthermore, it also has the effect of precipitating out as a sulfide, oxide or sulfate and preventing hardening of a joint in a steel pipe having a joint, and one or more kinds can be added as necessary. REM exceeds 0.02% or Ca is 0.
If it exceeds 01%, the amount of inclusions becomes too large and the cleanliness decreases,
Ductility deteriorates. REM is less than 0.004% and Ca is less than 0.004%.
Below 1%, the effect of this effect is small, so REM:
The content is preferably 0.004% or more and Ca: 0.001% or more.

The composition portion (remaining portion) other than the above component elements in the material composition is composed of Fe and unavoidable impurities. As inevitable impurities, N: 0.010% or less, O: 0.006%
% Or less, P: 0.025% or less, S: 0.020% or less. N: 0.010% or less N is an amount necessary for bonding with Al and refining crystal grains.
Up to 010% is acceptable, but more than 10% will reduce ductility, so it is preferred to reduce it to 0.010% or less.
More preferably, N is 0.002 to 0.006%.

O: 0.006% or less O deteriorates cleanliness as an oxide, so it is preferable to reduce it as much as possible, but up to 0.006% is acceptable. P: 0.025% or less P segregates at grain boundaries and degrades toughness, so it is preferable to reduce P as much as possible, but up to 0.025% is acceptable.

S: not more than 0.020% S is preferably reduced as much as possible because it increases sulfides and degrades cleanliness. However, S is allowable up to 0.020%. Next, the rolling step of the present invention will be described. In the present invention, after processing the raw material having the above composition into a raw tube, Ac
₃ (meaning the Ac ₃ transformation point; the same applies hereinafter) Rolling using a combination of rolling using a rolling mill and tilt rolling using a tilt rolling mill is performed at ~ 400 ° C. to form a product tube.

The reducing mill is preferably a three-roll or four-roll reducing mill. The reducer is preferably one that can be continuously rolled with a plurality of stands arranged in tandem. The number of stands is appropriately determined according to the target dimensions at the reducer entrance side and exit side of the rolled tube. The inclined rolling mill may be any of a rotary elongator, a reeler, a disher mill, and an assel mill.

Whichever of the drawing rolling and the tilt rolling is performed first,
It may be performed any number of times in any combination. By this rolling, so-called multiaxial processing in which the processing in the pipe axis direction and the processing in the pipe circumferential direction are combined is realized, crystal grains are easily divided, and the grain structure is more equiaxially refined to produce a product pipe. The mechanical properties are improved while maintaining good isotropy.

The rolling temperature of the drawing rolling and the gradient rolling is Ac _{3 to} 400 ° C. in the ferrite recovery / recrystallization temperature range, preferably (Ac ₁ (meaning the Ac ₁ transformation point; the same applies hereinafter) + 50 ° C.) to 400.
° C, more preferably in the range of 750 to 400 ° C. If the rolling temperature exceeds Ac ₃ , growth of ferrite grains after recrystallization becomes remarkable, and ductility does not improve at the expense of strength. For this reason, the rolling temperature is Ac ₃ or less, preferably (Ac ₁ + 50 ° C.)
The temperature is more preferably set to 750 ° C. or lower. On the other hand, if the rolling temperature is less than 400 ° C., the material becomes brittle due to blue embrittlement and the material may be broken during rolling. In addition, if the rolling temperature is lower than 400 ° C., the deformation resistance of the material increases, making rolling difficult.
Insufficient recrystallization causes processing strain to remain easily. For this reason, the rolling temperature is from Ac _{3 to} 400 ° C., preferably (Ac ₁ +
(50 ° C) to 400 ° C, more preferably 750 to 400 ° C. The temperature is most preferably 600 to 700 ° C.

When it is necessary to heat the raw tube before rolling or the tube to be rolled during rolling in order to secure the above-mentioned rolling temperature, the heating temperature is Ac _{3 to} 400 ° C., preferably (Ac ₁ + 50 ° C) to 400 ° C, more preferably 750 to
The temperature should be 400 ° C. If the heating temperature exceeds Ac ₃ , the surface properties deteriorate and the crystal grains become coarse. Therefore, the heating temperature of the raw tube is set to Ac ₃ or less, preferably (Ac ₁ + 50 ° C.) or less, and more preferably 750 ° C. or less. If the heating temperature is lower than 400 ° C., a suitable rolling temperature cannot be secured, so that the heating temperature is preferably 400 ° C. or higher.

Further, the total area reduction rate in rolling and one pass per
As for the reduction in area, the following
It is preferable to adopt the case. According to these conditions,
In particular, the crystal grain size is reduced to 2 μm or less, and further to 1 μm or less.
Along with the axial grain size d._LAnd around
Grain size d_CDeviation (long and short side deviation: average value ((d _L
+ D_C) / 2) to less than 10%
And reduce the anisotropy of mechanical properties,
It can be used sufficiently for rough processing.

[Total area reduction rate] The total area reduction rate of the entire rolling is 20
%, The crystal grains are not sufficiently refined by recovery and recrystallization, and it is difficult to obtain a product pipe having high ductility. In addition, the pipe forming speed is low and the production efficiency is low. Therefore, the total area reduction rate of the entire rolling is preferably set to 20% or more. Further, when the total area reduction rate of the entire rolling is 60% or more, the microstructure becomes remarkable in addition to the increase in strength due to work hardening, and a product pipe excellent in both strength and ductility can be obtained even from a low alloy steel pipe. Therefore, the total area reduction rate of the entire rolling is more preferably set to 60% or more.

Further, instead of making the total area reduction rate of the entire rolling 60% or more, the total area reduction rate of the reduction rolling is 60% or more, or
The total area reduction rate of the inclined rolling may be 50% or more. These three
The same effect can be obtained by any of the two rolling conditions. It should be noted that a greater shear strain is applied in inclined rolling than in drawn rolling, so that the condition that the total area reduction rate of inclined rolling is 50% or more is equivalent to the condition that the total area reduction rate of drawn rolling is 60% or more.

Further, since the processing in the tube axis direction is performed in the reduction rolling and the processing in the circumferential direction of the pipe is performed in the inclination rolling, both the total area reduction rate of the reduction rolling and the total area reduction rate of the inclination rolling are both 30%. With the above, the microstructure is extremely refined, and an effect equal to or greater than that obtained when the total area reduction rate of the entire rolling is 60% or more is obtained, which is more preferable. Further, when the total area reduction rate is 50% or more in each of the reduction rolling and the inclined rolling, the structure is extremely refined and the strength is improved, so that it is even more preferable.

[Reduction of area per pass] If the reduction of area per pass is less than 6% over all rolling passes, the recovery is reduced.
In addition to insufficient refining of the crystal grains due to recrystallization, the balance between the strength and ductility of the product tube may be deteriorated, and the material temperature rise due to the heat generated during processing is poor, so that the rolling temperature tends to decrease. For this reason, it is preferable that at least one of all rolling passes has a surface reduction rate of 6% or more per pass. From the viewpoint of further refinement of crystal grains, a surface reduction rate of 8% or more per pass in at least one pass is more preferable.

In order to carry out the present invention, it is only necessary to add an inclined rolling step to the existing drawing rolling step, so that it is not necessary to remodel the step significantly.

[0035]

EXAMPLES Steel pipes (product pipes) were manufactured from materials having the compositions shown in Table 1 under various manufacturing conditions shown in Table 2. Table 2
In the “solid-phase” pipe-forming method, the material (hot-rolled steel strip) is preheated to 600 ° C in a preheating furnace, and then continuously formed into a tubular shape using a plurality of forming rolls. After preheating to ℃, it is heated to the unmelting temperature range of 1450 ℃, and upset with a squeeze roll to have an outer diameter of 148 mm and a wall thickness of 3.
A 5mm solid-state pressure welded steel pipe was made. In the "SML" pipe production method, the material (continuously cast billet) is heated and pierced and rolled by a Mannes mandrel mill to produce an outer diameter of 192 mm.
× A seamless steel pipe having a thickness of 15 mm was formed. In the "ERW" pipe forming method, a material (hot-rolled steel strip) is continuously formed into a tubular shape by a plurality of forming rolls, the joint is heated to a melting temperature range by induction heating, and then upset by a squeeze roll. An 114.3 mm × 6 mm thick ERW steel pipe was made.

These raw tubes were rolled by a rolling method (warm rolling step) shown in Table 2 to obtain product tubes. The details of the rolling method are as follows. Note: Warm rolling process :: Reheating → Drawing rolling → Inclined rolling (1 pass) ・: Reheating → Drawing rolling → Inclined rolling (2 passes) ・: Reheating → Drawing rolling → Inclined rolling (1 pass) → Intermediate heating ⇒Draw-rolling :: Reheating ⇒Draw-rolling ⇒Intermediate heating ⇒Tilting-rolling (1 pass) ・: Reheating ⇒Tilting-rolling (1 pass) ⇒Draw-rolling Rolling :: Reheating ⇒ Inclined rolling (2 passes) ⇒ Intermediate heating ⇒ Drawing rolling ：: Reheating ⇒ Drawing rolling *: Reheating ⇒ Inclined rolling (1 pass) Draw rolling is a 3-roll reducer (multi-stand tandem arrangement) ), And inclined rolling was performed using a rotary elongator. The rolling temperature was measured with a radiation thermometer arranged on the entry side of each rolling mill. Reheating was performed by a heating furnace, and intermediate heating was performed by a high-frequency induction coil. Table 2 shows the rolling temperature and the area reduction under each manufacturing condition.

The ferrite grain size (axial direction, circumferential direction), particle size ratio (axial / circumferential), tensile strength (TS), elongation (El), and impact energy absorption (E)
s) was investigated. JIS No. 11 test piece was used for tensile test,
The elongation value is calculated using the conversion formula El =
El ₀ (√ (a ₀ / a)) ^0.4 (where, El ₀ : measured elongation, a ₀ : constant
292 mm ² , a: Evaluated by a conversion value based on the test piece cross-sectional area (mm ² ). The impact impact characteristics were determined by performing a high-speed tensile test at a strain rate of 2000 s ⁻¹ , obtaining the absorbed energy up to a strain amount of 30% from the obtained stress-strain curve, and evaluating it as the impact impact absorbed energy. (The impact and impact characteristics are represented by the deformation energy of the material at a strain rate of 1000 to 2000 s ^-1 when an automobile actually collides. The larger the value, the better the characteristics.) The same investigation as in the example was conducted for those that did.

The results are shown in Table 2. As shown in Table 2, according to the practice of the present invention in which a raw tube is made into a product tube by warm rolling in which drawing rolling and inclined rolling are combined, the strength is reduced without deteriorating ductility as compared with the conventional method of producing steel pipe. 30%
It is possible to manufacture a steel pipe in which the above-mentioned improvement and the impact energy absorption are improved by 50% or more, and the shape of the crystal grains can be suppressed to 10% or less in long and short sides, and complicated processing can be performed. Furthermore, the steel pipe whose strength was improved by 40% or more by the reduction of the area reduction rate by the methods (4) to (9) of the present invention doubled the strength by the restriction of the reduction of area by the method (10) of the present invention. Steel pipes can be manufactured.

[0039]

[Table 1]

[0040]

[Table 2]

[0041]

As described above, according to the present invention, there is provided an excellent effect that a steel pipe having a more equiaxial grain structure and excellent ductility and impact resistance can be produced without the need for significant process modification. .

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (reference) C22C 38/00 301 C22C 38/00 301Z 38/06 38/06 38/58 38/58 (72) Inventor Yoshikazu Kawabata Aichi 1-1, Kawasaki-cho, Handa-shi, Pref.Kawasaki Steel Corporation Chita Works (72) Inventor Masanori Nishimori 1-1-1, Kawasaki-cho, Handa-shi, Aichi Prefecture Kawasaki Steel Co., Ltd.Chita Works (72) Inventor Motoaki Itaya 1-1, Kawasaki-cho, Handa-shi, Aichi Prefecture, Chita Works, Kawasaki Steel Corporation (72) Inventor Masatoshi Aratani 1-1-1, Kawasaki-cho, Handa-shi, Aichi Prefecture, Chita Works, Kawasaki Steel Corporation (72) Inventor, Noh Okabe Knowledge 1-1-1, Kawasaki-cho, Handa-shi, Aichi Prefecture Kawasaki Steel Corporation Chita Works (72) Inventor Mikio Odaka 1-1-1 Kawasaki-cho, Handa City, Aichi Prefecture Kawasaki Steel Corporation Chita Works (72) Inventor Kanayama Taro 1-1, Kawasaki-cho, Handa-shi, Aichi Prefecture Kawasaki Steel Corporation Chita Manufacturing Co., Ltd. Office F term (reference) 4K032 AA01 AA02 AA04 AA05 AA08 AA11 AA12 AA14 AA16 AA17 AA19 AA21 AA22 AA23 AA24 AA26 AA27 AA29 AA31 AA32 AA35 AA36 AA40 BA03 CA01 CB02 CC01 CC02 CC03

Claims (10)

[Claims] (1) C: 0.005 to 0.30% by weight, Si: 0.
After processing a raw material containing 01-3.0%, Mn: 0.01-4.0%, and Al: 0.001-0.10% and having a chemical composition consisting of the balance of Fe and unavoidable impurities into a raw tube, the Ac ₃ transformation point is 400 ° C. A method for producing a steel pipe, comprising rolling at a rolling temperature of a combination of draw rolling and inclined rolling to process the product pipe. 2. The composition according to claim 1, further comprising:
And Cu: 1% or less, Ni: 2% or less, Cr: 2% or less, Mo:
One or more selected from 1% or less, and / or Nb: 0.1% or less, V: 0.3% or less, Ti: 0.2
% Or less, B: one or more selected from 0.004% or less, and / or REM: 0.02% or less, Ca: 0.
2. The method according to claim 1, wherein one or two selected from 01% or less are added. 3. The method according to claim 1, wherein the raw tube before rolling or the tube to be rolled during rolling is heated to a temperature not higher than the Ac ₁ transformation point + 50 ° C. 4. The method according to claim 1, wherein the total area reduction of the rolling is 20% or more. 5. The method according to claim 1, wherein the total area reduction of the rolling is 60% or more. 6. A reduction ratio of 6 per rolling during all rolling passes.
The method according to any one of claims 1 to 5, wherein at least one path includes at least one% path. 7. The method according to claim 1, wherein the total reduction of area in the reduction rolling is 60% or more. 8. The method according to claim 1, wherein a total area reduction rate of the inclined rolling is 50% or more. 9. The method according to claim 1, wherein the total area reduction rate of the reduction rolling and the total area reduction rate of the inclined rolling are both 30% or more. The method according to any one of claims 1 to 6, wherein the total area reduction rate of the reduction rolling and the total area reduction rate of the inclined rolling are both 60% or more.