201238672 六、發明說明： 【發明所屬之技術領域】 本發明，是關於鋼管的冷拉方法，詳細來說’是關於 改善鋼管的不平均厚度，使拉拔處理後的外徑精度 ' 胃® 精度提升的鋼管的冷拉方法。 只要沒有其他記載，本說明書的用語的定義如下° 「不平均厚度量」：在鋼管的任意剖面的厚度的最大 値與最小値的差値（參考後述第1圖）。 「不平均厚度改善量」：冷拉前的粗加工管的不平均 厚度量與拉拔後的管的不平均厚度量的差値。 【先前技術】 作爲用作機械構造用零件的鋼管，通常適用的鋼管是 藉由實施冷拉而對其內外面實施加工。例如在驅動軸等的 汽車用零件等，以輕量化爲目的，在取代棒鋼而使用鋼管 的情況，也通常用進行過冷拉的鋼管。 在實施冷拉的情況，如果拉拔後的不平均厚度量較大 ，則爲了確保最低厚度來確保強度及安全率，則必須增加 加工量而將全體的厚度增厚，如果是以輕量化爲目的的情 況，則會減少輕量化的效果。 以往提出過各種方法，爲了抑制金屬管的不平均厚度 ，而使尺寸精度提升。例如在專利文獻1提出了進行冷拉 的無縫鋼管的製造方法，在加熱爐以預定時間均熱爲預定 溫度的低合金鋼的鋼胚進行穿孔軋製及延伸軋製，而成爲 -5- 201238672 粗加工管，於再加熱爐將該粗加工管以預定時間均熱爲預 定溫度後，進行定徑軋製，最好進一步進行冷拉讓厚度加 工度爲6%以上30%以下。 藉由專利文獻1的方法，能有效地抑制厚度4mm以 下的薄壁管的不平均厚度的產生。可是，在該提出的方法 ，針對在後述的本發明的冷拉所採用的外徑加工並沒有任 何記載，疏忽了使外徑加工度增加所導致的效果。 在專利文獻2，揭示有內外徑精度良好的焊接管的製 造方法，在將高頻焊接管切削成讓其內面邊緣成爲預定高 度以下後，進行拉芯加工成讓管外徑減少率爲1 0%以上且 厚度減少率爲5%以上。可是在該揭示方法，並沒有針對 使厚度精度提升（也就是防止不平均厚度）的記載，在不 平均厚度量較大的情況，無法保證能藉由上述拉芯加工來 改善不平均厚度。 而在專利文獻3揭示一種鋼管的製造方法，在最終步 驟’使厚度加工度/外徑加工度成爲1.5以下而進行冷加工 。在專利文獻3的方法’是用來得到：讓以冷加工所得到 的油井管的L方向的壓縮強度爲拉伸強度的8〇%以上的鋼 管之方法’雖然其規定含有在後述的本發明的冷拉採用爲 參數的厚度加工度/外徑加工度，可是完全沒有提到關於 抑制不平均厚度。 〔先前技術文獻〕 〔專利文獻〕 -6- 201238672 〔專利文獻1〕日本特開2006 — 150452號公報 〔專利文獻2〕日本特開平9— 239433號公報 〔專利文獻3〕日本特開平1〇-80715號公報 【發明內容】 〔發明欲解決的課題〕 本發明鑑於這種情形，其目的要提供一種鋼管的冷拉 方法，防止冷拉後的不平均厚度，並且使外徑及厚度的精 度提升。 〔用以解決課題的手段〕 本發明的內容如下： (1)鋼管的冷拉方法，實施冷拉時的外徑的加工度 爲Rd ( % )，厚壁側厚度的加工度爲RT ( % )時，則以符 合下述（1)式子及（2)式子的條件來實施冷拉。 20(%) ^ R〇^ 35 ( % ) ... ( 1 ) 5 ( % ) ^ RT ^ 25 ( % ) ... ( 2 ) 這裡 RD= 1〇〇 ( D〇— D ) /D〇 Rt = 100 ( T〇 — T) /Τ〇 D〇 :冷拉前的外徑（mm ) D :冷拉後的外徑（mm ) T〇 ··冷拉前的厚壁側厚度（mm ) Τ :冷拉後的厚壁側厚度（mm ) (2 )在上述（1 )記載的鋼管的冷拉方法中，以符合 201238672 上述（1)式子及（2)式子且符合下述（3)式子的條件 來實施冷拉》201238672 VI. Description of the Invention: [Technical Field of the Invention] The present invention relates to a cold drawing method for a steel pipe, and in detail "is concerned with improving the uneven thickness of the steel pipe and making the outer diameter accuracy after the drawing process" stomach® precision Cold drawing method for elevated steel pipes. Unless otherwise stated, the terms used in this specification are defined as follows: "Uneven thickness": The difference between the maximum 値 and the minimum 厚度 of the thickness of any cross section of the steel pipe (refer to Fig. 1 which will be described later). "Uneven thickness improvement amount": The difference between the amount of uneven thickness of the rough-processed pipe before cold drawing and the amount of uneven thickness of the pipe after drawing. [Prior Art] As a steel pipe used as a component for mechanical construction, a steel pipe which is generally applied is processed by being subjected to cold drawing to perform inner and outer surfaces thereof. For example, in the case of a steel component such as a drive shaft, for the purpose of weight reduction, when a steel pipe is used instead of a steel bar, a steel pipe that has been subjected to cold drawing is usually used. In the case of performing cold drawing, if the amount of uneven thickness after drawing is large, in order to secure the minimum thickness and ensure the strength and safety rate, it is necessary to increase the amount of processing and increase the thickness of the entire thickness. In the case of the purpose, the effect of weight reduction is reduced. Various methods have been proposed in the past to increase the dimensional accuracy in order to suppress the uneven thickness of the metal pipe. For example, Patent Document 1 proposes a method for producing a seamless steel pipe which is subjected to cold drawing, in which a steel blank of a low-alloy steel which is heated to a predetermined temperature for a predetermined time is subjected to piercing and rolling and extension rolling, and becomes -5- 201238672 Roughing pipe, after the rough processing pipe is uniformly heated to a predetermined temperature for a predetermined time in a reheating furnace, sizing rolling is performed, and it is preferable to further perform cold drawing to make the thickness processing degree 6% or more and 30% or less. According to the method of Patent Document 1, the occurrence of the uneven thickness of the thin-walled tube having a thickness of 4 mm or less can be effectively suppressed. However, in the proposed method, the outer diameter machining used in the cold drawing of the present invention to be described later is not described, and the effect of increasing the outer diameter machining degree is neglected. Patent Document 2 discloses a method of manufacturing a welded pipe having excellent inner and outer diameters. After the high-frequency welded pipe is cut so that the inner edge thereof becomes a predetermined height or less, the core is processed to reduce the outer diameter of the pipe by one. 0% or more and the thickness reduction rate is 5% or more. However, in the disclosure method, there is no description for improving the thickness accuracy (i.e., preventing the uneven thickness), and in the case where the thickness is not large, it is not guaranteed that the uneven thickness can be improved by the above-described core processing. Further, Patent Document 3 discloses a method for producing a steel pipe, which is subjected to cold working in a final step of making the thickness working degree/outer diameter working degree 1.5 or less. The method of the patent document 3 is a method for obtaining a steel pipe having a compression strength in the L direction of the oil well pipe obtained by cold working of 8 % or more of the tensile strength, although it is intended to be included in the invention of the present invention to be described later. The cold drawing adopts the thickness processing degree/outer diameter processing degree as a parameter, but there is no mention at all about suppressing the uneven thickness. [Prior Art Document] [Patent Document] -6-201238672 [Patent Document 1] JP-A-2006-150452 (Patent Document 2) Japanese Laid-Open Patent Publication No. Hei 9-239433 (Patent Document 3) SUMMARY OF THE INVENTION [Problem to be Solved by the Invention] The present invention has been made in view of such circumstances, and an object thereof is to provide a cold drawing method of a steel pipe, which prevents uneven thickness after cold drawing and improves accuracy of outer diameter and thickness . [Means for Solving the Problem] The contents of the present invention are as follows: (1) The cold drawing method of the steel pipe, the degree of processing of the outer diameter when performing cold drawing is Rd (%), and the degree of processing of the thickness of the thick wall side is RT (%) In the case of the above, the cold drawing is carried out under the conditions of the following formula (1) and (2). 20(%) ^ R〇^ 35 ( % ) ... ( 1 ) 5 ( % ) ^ RT ^ 25 ( % ) ... ( 2 ) where RD = 1〇〇( D〇— D ) /D〇 Rt = 100 ( T〇 - T) /Τ〇D〇: outer diameter before cold drawing (mm) D : outer diameter after cold drawing (mm) T〇·· thickness of thick side before cold drawing (mm) Τ : Thickness of the thick wall side after cold drawing (mm) (2) In the cold drawing method of the steel pipe described in the above (1), it conforms to the above formula (1) and (2) of 201238672 and conforms to the following ( 3) The condition of the formula to implement cold drawing
Rt/Rd ^ 0.5 ... ( 3 ) (3)在上述（2)記載的鋼管的冷拉方法中，以符合 上述（1)式子〜（3)式子，且當實施冷拉時的管部的剖 面積減少率爲S P ( % )時，符合下述（4 )式子的條件， 來實施冷拉。 SP 客 50 ( %) ... (4) 這裡 Sp = 1 00 ( S。— s ) /SoRt/Rd ^ 0.5 (3) In the cold drawing method of the steel pipe according to the above (2), the formula (1) to the formula (1) are satisfied, and when cold drawing is performed, When the sectional area reduction rate of the tube portion is SP (%), the cold drawing is performed in accordance with the condition of the following formula (4). SP guest 50 (%) ... (4) where Sp = 1 00 (S. — s ) /So
So :冷拉前的管部的剖面積（mm) S :冷拉後的管部的剖面積（mm ) 〔發明效果〕 藉由本發明的鋼管的冷拉方法，以機械構造用零件或 汽車用零件等所用的鋼管作爲對象，改善粗加工管的不平 均厚度’防止冷拉後的不平均厚度，並且使外徑及厚度的 精度提升。 【實施方式】 本發明者認爲，爲了抑制上述課題也就是抑制冷拉後 的不平均厚度，不只是厚壁側厚度的加工度RT (以下僅 稱爲「厚度加工度RT」），也必須將外徑加工度RD適當 化，尤其著眼於外徑加工度、厚度加工度/外徑加工度的 比（Rt/Rd )對於不平均厚度的防止的影響，來進行調査 201238672 。具體來說，使用藉由機械加工導致偏心、不平均厚度的 粗加工管，調査：當涵蓋廣闊範圍使加工度（外徑加工度 及厚度加工度）變化時的不平均厚度改善量的變化。 第1圖爲在加工度所導致不平均厚度改善量的調查所 使用的偏心不平均厚度粗加工管的剖面形狀的示意性顯示 圖。該粗加工管的尺寸’外徑爲38.1mm’厚度爲4.0 mm ，如該圖所示’不平均厚度量爲(不平均厚度率： 15%)。 詳細調查結果在後述實施例顯示，而爲了將第1圖所 示的偏心不平均厚度粗加工管進行冷拉而改善不平均厚度 的最適當的外徑加工度Rd及厚度加工度Rt，判斷要符合 下述（1)式子及（2)式子。 2 0 ( % ) ^ RD ^ 35 ( % ) ... ( 1 ) 5 ( % ) ^ RT^ 25 ( % ) -(2) 並且判斷要符合下述（3)式子，並且管部的剖面積 減少率SP符合下述（4)式子。So: sectional area (mm) of the pipe portion before cold drawing S: sectional area (mm) of the pipe portion after cold drawing [Effect of the invention] The cold drawing method of the steel pipe according to the present invention, for mechanical structural parts or for automobiles The steel pipe used for parts and the like is used as an object to improve the uneven thickness of the rough-worked pipe, to prevent uneven thickness after cold drawing, and to improve the accuracy of the outer diameter and the thickness. [Embodiment] The present inventors have considered that it is necessary to suppress the above-mentioned problem, that is, to suppress the uneven thickness after cold drawing, and it is necessary to perform the processing degree RT (hereinafter simply referred to as "thickness processing degree RT") of the thickness of the thick side. The outer diameter machining degree RD was appropriately adjusted, and in particular, the effect of the outer diameter machining degree, the thickness machining degree/the outer diameter machining degree ratio (Rt/Rd) on the prevention of the uneven thickness was investigated, and 201238672 was investigated. Specifically, a roughing pipe having an eccentricity and an uneven thickness by machining is used, and a change in the amount of uneven thickness improvement when the machining degree (outer diameter machining degree and thickness machining degree) is changed over a wide range is investigated. Fig. 1 is a schematic view showing the cross-sectional shape of the eccentric uneven thickness roughing pipe used for the investigation of the uneven thickness improvement amount due to the degree of work. The roughened tube had a size 'outer diameter of 38.1 mm' and a thickness of 4.0 mm. As shown in the figure, the amount of uneven thickness was (uneven thickness ratio: 15%). The results of the detailed investigation are shown in the examples to be described later, and in order to improve the optimum outer diameter processing degree Rd and the thickness processing degree Rt of the uneven thickness by cold drawing the eccentric uneven thickness roughing pipe shown in Fig. 1, it is judged that It conforms to the following formula (1) and (2). 2 0 ( % ) ^ RD ^ 35 ( % ) ... ( 1 ) 5 ( % ) ^ RT^ 25 ( % ) - (2) and judged to conform to the following formula (3), and the section of the tube The area reduction rate SP conforms to the following formula (4).
Rt/Rd^ 0.5 ... ( 3 )Rt/Rd^ 0.5 ... ( 3 )
SpS 50 ( %) ... ( 4) 在本發明的鋼管的冷拉方法，之所以讓外徑加工度 RD符合上述（1 )式子，如從後述實施例可了解，藉由讓 外徑加工度RD爲20 %以上，則可得到大致50%以上的很 大的不平均厚度改善效果。另一方面，之所以讓外徑加工 度Rd爲35%以下，是因爲當超過35%時，則拉拔時管部 斷裂的可能性很大，而無法穩定製造。 -9- 201238672 之所以讓厚度加工度RT符合上述（2)式子，是因爲 在厚度加工度RT小於5 %時，外徑、厚度的尺寸精度降低 ，而超過25 %時，則拉拔時管部斷裂的可能性很大。 在本發明的冷拉方法，並且如果以符合上述（3)式 子的條件來實施冷拉，則可得到更大的不平均厚度改善效 果。 並且在本發明的冷拉方法，如果以符合上述（3)式 子，且符合上述（4)式子的條件來實施冷拉的話，除了 可得到更大的不平均厚度改善效果之外，且拉拔時管部則 沒有斷裂的可能性。 如上述，在本發明的鋼管的冷拉方法，藉由將外徑加 工度Rd增大到20%以上則可得到很大的不平均厚度改善 效果。爲了了解該作用原理，針對：厚度加工度爲一定（ 1 0.0 % )，外徑加工度爲符合本發明的規定的2 1 . 1 %的情 況、與不符本發明的規定的10.0%的情況，進行途中停止 件的厚壁側及薄壁側的厚度測定。 所謂「途中停止件」’是當拉拔時，從材料（粗加工 管）接觸於拉模接近部的位置直到拉模直線部的結束位置 之間的途中，將拉拔處理停止的材料。 在將具有偏心不平均厚度的粗加工管進行拉拔加工的 情況，通過拉模中的粗加工管，如後述第2圖、第3圖所 示，藉由粗加工管與工具的接觸狀態分成三個區間（區域 )，調査的結果，如以下所述，判斷外徑縮徑區域與厚壁 側厚度加工區域的延長是有效於改善不平均厚度。 -10- 201238672 這裡所謂的「外徑縮徑區域與厚壁側厚度加工區域的 延長」’稱爲讓該兩區域變長的狀態。 第2圖及第3圖，分別爲外徑加工度爲21.1 %及10.0% 的情況的途中停止件的厚度測定結果之顯示圖。在兩圖中 ，（a)爲加工中的厚度變化的顯示圖，（b)爲粗加工管 與拉模及插塞的位置關係的顯示圖。 如第2圖及第3圖（b)所示，區間（i)是藉由拉模 2將粗加工管1的外徑縮徑的外徑縮徑區域，爲從粗加工 管1的厚壁側的外面與拉模2接觸的位置，直到厚壁側的 內面與插塞3接觸的位置的區間。區間（ii )爲藉由拉模 2與插塞3讓粗加工管1的厚壁側受到加工的厚度加工區 域，爲從上述厚壁側的內面與插塞3接觸的位置，直到薄 壁側的內面與插塞3接觸的位置的區間。區間（iii )爲從 薄壁側的內面與插塞3接觸的位置直到拉模2的直線部的 結束位置的區間。 在第2圖及第3圖（a )，橫軸爲以拉模直線部的開 始位置作爲基準（0mm )表示的軸方向位置。 比較第2圖（a )與第3圖（a )，外徑加工度爲2 1 · 1 % 的情況（第2圖（a ))，相較於外徑加工度爲1 0·0%的情 況（第3圖（a ))，特徵爲區間（i )的外徑縮徑區域與 區間（ii )的厚壁側厚度加工區域較長。因此，容易產生 從厚壁側朝薄壁側的金屬流移，在粗加工管通過（i )與 (ii )期間其不平均厚度量會減少。 也就是說，在第2圖（a )，在區間（i )，由於加工 -11 - 201238672 度較大的內面側沒有被限制，所以粗加工管1的厚壁側也 稍微增厚，而由於區間（i)較長，所以藉由朝薄壁側的 金屬流移，讓薄壁側大幅增厚而讓不平均厚度量減少。在 下個區間（ii )，內面側是被插塞3所限制，所以藉由厚 壁側的厚度加工，讓厚壁側的厚度大幅減少，在薄壁側持 續金屬流移而增厚。結果讓不平均厚度量大幅減少（在第 2圖（a )中以橢圓圍繞的部分）。因此，認爲拉模中心與 插塞中心的偏移變少，會抑制拉拔後的不平均厚度。 相對地’在第3圖（a )，加工度較小，區間（i )與 區間（Π )較短，所以不易產生朝薄壁側的金屬流移，幾 乎沒有產生不平均厚度量的變化。因此，拉模中心與插塞 中心的偏移較大，並未改善粗加工管的不平均厚度。 表1 ’以區間（i )的外徑縮徑以及區間（i i )的厚壁 側厚度加工來分別整理第2圖（a )所示的厚度變化》 〔表1〕 厚度變化 不平均厚度量的變化 考察 厚壁up 薄壁側 (0外徑縮徑 厚度增加 (少） 厚度增加 (多） 薄壁側其厚度增加量 較多，所以不平均厚 度量減少 藉由縮徑而產生從厚 壁側朝薄壁側的圓周 方向的金屬流移 U)厚壁側 厚度加工 厚度減少 厚度增加 只有厚壁側藉由厚度 加工讓厚度減少，不 平均厚度量減少 產生從經過厚度加工 的厚壁側朝薄壁側的 圓周方向的金屬流移 如以±所述’認爲將偏心不平均厚度粗加工管進行拉 拔力U X B寺的不平均厚度改善效果，藉由外徑縮徑或厚壁側 -12- 201238672 厚度加工而產生的從厚壁側朝薄壁側的圓周方向的金屬流 移，而讓厚度均勻化，減少不平均厚度量。 在本發明的冷拉方法，作爲拉拔的對象鋼種，例如在 JIS G 344 1所規定的機械構造用合金鋼鋼管，在其他機械 構造用零件所用的低合金鋼管。 〔實施例〕 使用：具有上述第1圖所示的形狀、尺寸的偏心不平 均厚度材料（外徑：38.1mm，厚度：4.0mm，不平均厚度 量：0.6mm (不平均厚度率·· 15% ))，涵蓋廣闊範圍將 外徑加工度及厚度加工變更，來調查對不平均厚度改善量 造成的影響。所用的粗加工管，是以lCr— 0·8Μ〇材，·進 行一次冷拉（抽伸）後，將管部的外側實施機械加工而作 成偏心不平均厚度粗加工管。而所謂的「不平均厚度改善 量」，如上述，是拉拔的前後的不平均厚度量的差。 在表2是顯示冷拉程序以及調査結果（不平均厚度改 善量）。拉拔都是使用錐形（25° )拉模以及圓筒插塞來 進行。 -13- 201238672 U谳〕 摘要 外徑、厚度的尺寸精度降低 1 1 1 1 管部斷裂的可能性大 不平均厚度量改善效果較/』、 1 管部斷裂的可能性大 不平均厚度改善量 (mm) 0.08 0.10 0.205 0.24 0.29 1 0.03 0.15 0.38 1 剖面積減少率 Sp (%) 26.0 26.9 30.3 33.1 37.0 45.0 18.8 23.9 34.0 44.3 Rt/Rd (-) 0.18 0.24 0.47 0.68 0.95 1.48 1.02 0.68 0.40 0.28 厚度加工度 RT (%) 〇〇 10.0 14.4 20.0 31.3 10.0 外徑加工度 Rd (%) 21.1 00 σ； 14.8 24.9 35.2 尺寸 厚度 3.85 1_ 3.80 3.60 3.425 3.20 2.75 3.60 外徑 30.05 1 34.35 32.45 28.60 24.70 試驗號碼 JJ3 J-X A3 Λ3 ΛΛ ΔΛ AJ CN 寸 yn 卜 00 Os 〇 莩鎰丑：qq :屮匡5:餾-14:柃(拋銮) -14- 201238672 在表2，試驗號碼1〜6，是用來調查外徑加工度Rd 爲一定（21.1%)而厚度加工度RT對不平均厚度改善量造 成的影響的試驗，試驗號碼7〜10，是用來調査厚度加工 度爲一定（10.0%)而外徑加工度對不平均厚度改善量造 成的影響的試驗。 第4圖及第5圖是用來圖示在表2顯示的結果。第4 圖爲外徑加工度一定時的厚度加工度與不平均厚度改善量 的關係的顯示圖。第5圖爲厚度加工度一定時的外徑加工 度與不平均厚度改善量的關係的顯示圖。 如表2及第4圖所示，當外徑加工度一定而使厚度加 工度變化時，會連同厚度加工度的增大，不平均厚度改善 量也一起增大。尤其是，厚度加工度在10%以上，不平均 厚度改善量較大，粗加工管的不平均厚度量爲0.6mm，而 厚度加工度爲1 〇%，可得到超過3 0%的改善效果。 厚度加工度較小，而小於5 %的話，外徑、厚度的尺 寸精度會降低。另一方面，厚度加工度超過25%的話，在 拉拔時管部斷裂的可能性變高。於是，厚度加工度需要在 5〜25 %的範圍內。 如表2及第5圖所示，在厚度加工度一定而使外徑加 工度變化時，連同外徑加工度的增大，不平均厚度改善量 也會急遽增大。該情況，判斷爲外徑加工度某程度增大， 在20%左右以上，不平均厚度的改善效果會變大。另一方 面，外徑加工度超過3 5 %的話，拉拔時管部斷裂的可能性 變高，而無法穩定製造。於是，外徑加工度需要在20〜 -15- 201238672 3 5 %的範圍內* 第6圖是圖示在表2顯示的不平均厚度改善量的增大 比率較大的試驗號碼7〜9及試驗號碼3的結果’且厚度 加工度/外徑加工度（RWRd )與不平均厚度改善量的關係 的顯示圖。從第6圖可得知當RT/RD爲0 · 5以下時可得到 較大的不平均厚度改善效果。 剖面積減少率SP超過50%的話，拉拔時管部可能會 斷裂。 〔產業上的可利用性〕 本發明的鋼管的冷拉方法，爲適合作爲在機械構造用 零件所使用的鋼管的冷拉方法，在相關產業領域能有效利 用。 【圖式簡單說明】 第1圖爲在加工度所導致不平均厚度改善量的調査所 使用的偏心不平均厚度粗加工管的剖面形狀的示意性顯示 圖。 第2圖（a )爲根據外徑加工度爲2 1 . 1 %的情況的途中 停止件的厚度測定結果之加工中的厚度變化的顯示圖。 第2圖（b )爲根據外徑加工度爲2 1 · 1 %的情況的途 中停止件的厚度測定結果之拉模及插塞與材料的位置關係 的顯示圖。 第3圖爲外徑加工度爲1 〇 . 〇 %的情況的途中停止件的 -16- .201238672 厚度測定結果的顯示圖，（a)爲加工中的厚度變化的顯 示圖’ （b)爲拉模及插塞與材料的位置關係的顯示圖。 第4圖爲外徑加工度一定時的厚度加工度與不平均厚 度改善量的關係的顯示圖。 第5圖爲厚度加工度一定時的外徑加工度與不平均厚 度改善量的關係的顯示圖。 第6圖爲厚度加工度/外徑加工度與不平均厚度改善 量的關係的顯示圖。 【主要元件符號說明】 1 :粗加工管 2 :拉模 3 :插塞 -17-SpS 50 (%) (4) In the cold drawing method of the steel pipe according to the present invention, the outer diameter machining degree RD is made to conform to the above formula (1), as can be understood from the later-described embodiment, by letting the outer diameter When the degree of work RD is 20% or more, a large uneven thickness improvement effect of substantially 50% or more can be obtained. On the other hand, the reason why the outer diameter processing degree Rd is 35% or less is that when it exceeds 35%, the tube portion is likely to be broken at the time of drawing, and it is not stable. -9- 201238672 The reason why the thickness processing degree RT conforms to the above formula (2) is that the dimensional accuracy of the outer diameter and the thickness is lowered when the thickness processing degree RT is less than 5%, and when the thickness is more than 25%, the drawing is performed. The possibility of pipe breakage is great. In the cold drawing method of the present invention, and if cold drawing is carried out under the conditions conforming to the above formula (3), a larger uneven thickness improving effect can be obtained. Further, in the cold drawing method of the present invention, if cold drawing is carried out in accordance with the above formula (3) and the conditions of the above formula (4) are satisfied, in addition to obtaining a larger uneven thickness improving effect, and There is no possibility of breakage when drawing the tube. As described above, in the cold drawing method of the steel pipe according to the present invention, a large uneven thickness improving effect can be obtained by increasing the outer diameter processing degree Rd to 20% or more. In order to understand the principle of operation, the thickness processing degree is constant (10.0%), the outer diameter processing degree is 21.1% in accordance with the provisions of the present invention, and the case is not in accordance with the provisions of the present invention, 10.0%. The thickness of the thick-wall side and the thin-wall side of the stop in the middle was measured. The "stop in the middle" is a material that stops the drawing process from the position where the material (roughing pipe) contacts the portion of the die close to the end position of the straight portion of the die when drawing. In the case of drawing a rough-processed pipe having an eccentric uneven thickness, the rough-processed pipe in the die is divided by the contact state of the rough-worked pipe and the tool as shown in FIG. 2 and FIG. 3 which will be described later. The three sections (areas), as a result of the investigation, as described below, it is judged that the extension of the outer diameter reduction area and the thick wall side thickness processing area is effective for improving the uneven thickness. -10- 201238672 Here, the "extension of the outer diameter reduction area and the thick side thickness processing area" is referred to as a state in which the two regions are lengthened. Fig. 2 and Fig. 3 are graphs showing the results of measuring the thickness of the in-transit stop in the case where the outer diameter machining degree is 21.1% and 10.0%, respectively. In the two figures, (a) is a display diagram of the thickness variation during machining, and (b) is a display diagram of the positional relationship between the roughing pipe and the die and the plug. As shown in Fig. 2 and Fig. 3(b), the section (i) is an outer diameter reduction area in which the outer diameter of the roughing pipe 1 is reduced by the die 2, and is a thick wall from the roughing pipe 1. The outer side of the side is in contact with the die 2 until the interval of the inner surface of the thick wall side in contact with the plug 3. The section (ii) is a thickness-processed region where the thick-walled side of the roughing pipe 1 is processed by the die 2 and the plug 3, and is a position in contact with the plug 3 from the inner surface of the thick-walled side until thin wall The section of the position where the inner surface of the side is in contact with the plug 3. The section (iii) is a section from the position where the inner surface of the thin-walled side comes into contact with the plug 3 to the end position of the straight portion of the die 2. In Fig. 2 and Fig. 3(a), the horizontal axis represents the axial direction position indicated by the start position of the straight portion of the die (0 mm). Comparing Fig. 2 (a) and Fig. 3 (a), when the outer diameter machining degree is 2 1 · 1 % (Fig. 2 (a )), the outer diameter machining degree is 1 0·0%. In the case (Fig. 3(a)), it is characterized in that the outer diameter reduced diameter region of the section (i) and the thick wall side thickness processing region of the section (ii) are longer. Therefore, it is easy to cause metal flow from the thick wall side to the thin wall side, and the amount of uneven thickness is reduced during the passage of the roughing pipe through (i) and (ii). In other words, in the second figure (a), in the section (i), since the inner side of the processing -11 - 201238672 is not restricted, the thick side of the roughing pipe 1 is slightly thickened, and Since the section (i) is long, the thin wall side is greatly thickened by the metal flow toward the thin wall side, and the amount of uneven thickness is reduced. In the next section (ii), since the inner surface side is restricted by the plug 3, the thickness of the thick wall side is greatly reduced by the thickness processing on the thick wall side, and the metal is continuously moved and thickened on the thin wall side. As a result, the amount of uneven thickness is greatly reduced (the portion surrounded by an ellipse in Fig. 2(a)). Therefore, it is considered that the offset between the center of the die and the center of the plug is small, and the uneven thickness after the drawing is suppressed. In the third figure (a), the degree of processing is small, and the section (i) and the section (Π) are short. Therefore, metal migration toward the thin side is less likely to occur, and there is almost no change in the amount of uneven thickness. Therefore, the offset between the center of the die and the center of the plug is large, and the uneven thickness of the roughed pipe is not improved. Table 1 'The thickness variation shown in Fig. 2(a) is separately processed by the outer diameter reduction of the section (i) and the thick side thickness of the section (ii). [Table 1] The thickness variation is not the average thickness. Change the thickness of the thick-walled up thin wall side (0 outer diameter reduction diameter increase (less) thickness increase (more) thin wall side thickness increase, so the uneven thickness reduction is reduced by the diameter of the thick wall side The metal flow in the circumferential direction toward the thin-walled side U) Thick-walled side thickness processing Thickness reduction Thickness increase Only the thick-walled side is reduced in thickness by thickness processing, and the uneven thickness is reduced from thick-walled side to thin-thickness The metal flow in the circumferential direction of the wall side is considered to be an uneven thickness improvement effect of the drawing force UXB Temple which is considered to be eccentrically uneven thickness, by the outer diameter reduction or thick wall side -12 - 201238672 The metal flow in the circumferential direction from the thick wall side to the thin wall side produced by the thickness processing is made uniform, and the thickness is made uniform to reduce the uneven thickness. In the cold drawing method of the present invention, the alloy steel for drawing is, for example, an alloy steel pipe for mechanical construction specified in JIS G 344 1, and a low alloy steel pipe for other mechanical structural parts. [Examples] The eccentricity uneven thickness material having the shape and size shown in Fig. 1 was used (outer diameter: 38.1 mm, thickness: 4.0 mm, uneven thickness: 0.6 mm (uneven thickness ratio··15) %)), covering a wide range of changes in outer diameter machining and thickness processing to investigate the impact on uneven thickness improvement. The rough-worked pipe used is a lCr- 0·8 coffin, and after one cold drawing (extraction), the outer side of the pipe portion is machined to form an eccentrically uneven thickness roughing pipe. As described above, the "uneven thickness improvement amount" is the difference in the amount of uneven thickness before and after the drawing. Table 2 shows the cold drawing procedure and the survey results (uneven thickness improvement). Drawing is performed using a tapered (25°) die and a cylindrical plug. -13- 201238672 U谳] The dimensional accuracy of the outer diameter and thickness is reduced. 1 1 1 1 The possibility of tube breakage is large. The effect of uneven thickness is better than that. / 1 The possibility of tube breakage is large uneven thickness improvement (mm) 0.08 0.10 0.205 0.24 0.29 1 0.03 0.15 0.38 1 Area reduction rate Sp (%) 26.0 26.9 30.3 33.1 37.0 45.0 18.8 23.9 34.0 44.3 Rt/Rd (-) 0.18 0.24 0.47 0.68 0.95 1.48 1.02 0.68 0.40 0.28 Thickness processing RT (%) 〇〇10.0 14.4 20.0 31.3 10.0 Outer diameter processing degree Rd (%) 21.1 00 σ; 14.8 24.9 35.2 Dimensional thickness 3.85 1_ 3.80 3.60 3.425 3.20 2.75 3.60 Outer diameter 30.05 1 34.35 32.45 28.60 24.70 Test number JJ3 JX A3 Λ3 ΛΛ ΔΛ AJ CN inch yn 00 Os 〇莩镒 ugly: qq : 屮匡 5: Distillation-14: 柃 (throwing 銮) -14- 201238672 In Table 2, test numbers 1 to 6, are used to investigate the outer diameter processing The test of the effect of the thickness of the workability RT on the improvement of the uneven thickness, the test number 7 to 10, is used to investigate the thickness processing degree (10.0%) and the outer diameter processing degree. Uneven thickness improvement The impact of the test. Figures 4 and 5 are used to illustrate the results shown in Table 2. Fig. 4 is a graph showing the relationship between the thickness machining degree and the uneven thickness improvement amount when the outer diameter machining degree is constant. Fig. 5 is a graph showing the relationship between the outer diameter machining degree and the uneven thickness improvement amount when the thickness processing degree is constant. As shown in Tables 2 and 4, when the degree of processing of the outer diameter is constant and the thickness processing is changed, the uneven thickness improvement is also increased together with the increase in the thickness processing degree. In particular, the thickness processing degree is 10% or more, the uneven thickness improvement amount is large, the rough processing tube has an uneven thickness of 0.6 mm, and the thickness processing degree is 1 〇%, and an improvement effect of more than 30% can be obtained. The thickness processing degree is small, and if it is less than 5%, the dimensional accuracy of the outer diameter and the thickness is lowered. On the other hand, if the thickness processing degree exceeds 25%, the possibility of breakage of the tube portion at the time of drawing becomes high. Therefore, the thickness processing degree needs to be in the range of 5 to 25%. As shown in Tables 2 and 5, when the thickness processing degree is constant and the outer diameter processing degree is changed, the uneven thickness improvement amount increases sharply as the outer diameter machining degree increases. In this case, it is determined that the degree of outer diameter processing is increased to some extent, and is about 20% or more, and the effect of improving the uneven thickness is increased. On the other hand, if the outer diameter processing degree exceeds 35 %, the possibility of breakage of the tube portion during drawing becomes high, and stable manufacturing cannot be achieved. Therefore, the outer diameter processing degree needs to be in the range of 20 to -15 - 201238672 3 5 % * Fig. 6 is a test number 7 to 9 which shows a large increase ratio of the uneven thickness improvement amount shown in Table 2 and A graph showing the relationship between the result of test number 3 and the relationship between the thickness working degree/outer diameter working degree (RWRd) and the uneven thickness improvement amount. It can be seen from Fig. 6 that a large uneven thickness improvement effect can be obtained when the RT/RD is 0.5 or less. If the sectional area reduction rate SP exceeds 50%, the tube may break during drawing. [Industrial Applicability] The cold drawing method of the steel pipe according to the present invention is a cold drawing method suitable as a steel pipe used for mechanical structural parts, and can be effectively utilized in the related industrial field. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic view showing a cross-sectional shape of an eccentrically uneven thickness roughing pipe used for investigation of an uneven thickness improvement amount due to a degree of workability. Fig. 2(a) is a view showing a change in thickness during processing of the thickness measurement result of the stopper in the case where the outer diameter machining degree is 21.1%. Fig. 2(b) is a view showing the relationship between the die of the thickness measurement of the stopper and the positional relationship between the plug and the material in the case where the outer diameter machining degree is 2 1 · 1 %. Fig. 3 is a diagram showing the thickness measurement results of the midway stopper in the case where the outer diameter machining degree is 1 〇. 〇%, and (a) is a display diagram of the thickness variation during machining. (b) A display of the relationship between the die and the plug and the material. Fig. 4 is a graph showing the relationship between the thickness processing degree and the uneven thickness improvement amount when the outer diameter machining degree is constant. Fig. 5 is a graph showing the relationship between the outer diameter machining degree and the uneven thickness improvement amount when the thickness processing degree is constant. Fig. 6 is a graph showing the relationship between the thickness working degree/outer diameter working degree and the uneven thickness improving amount. [Main component symbol description] 1 : Roughing pipe 2 : Die 3 : Plug -17-