TWI326714B

TWI326714B - Low-carbon resulfurized free-machining steel excellent in machinability

Info

Publication number: TWI326714B
Application number: TW095145464A
Authority: TW
Inventors: Koichi Sakamoto; Atsuhiko Yoshida
Original assignee: Kobe Steel Ltd
Priority date: 2005-12-16
Filing date: 2006-12-06
Publication date: 2010-07-01
Also published as: EP1964939A4; US20090169414A1; EP1964939B1; JP4203068B2; US20120121454A1; JP2007162119A; KR101033073B1; EP1964939A1; TW200738892A; WO2007069386A1; KR20080068750A; CN101331242A

Description

1326714 Ο) 九、發明說明【發明所屬之技術領域】本發明係關於低碳再硫化易切削鋼’其不含有害的鈴 (Pb)且展現良好之完工面粗糙度。【先前技術】低碳再硫化易切削鋼經廣泛用爲汽車齒輪箱單元之液壓零件以及小零件諸如螺絲和印表機軸的鋼料，這些用途不需要很高的強度。當需要較好的完工面粗糙度及切屑可處置性（chip disposablity)時，係使用包含與鉛（Pb)結合的低碳再硫化易切削鋼之鉛-硫易切削鋼。包含在易切削鋼內的鉛（Pb)係一種改良切削性非常有效的元素，但對人體有害。此外，含鉛易切削鋼在製錠及切屑可處置性上典型地具有鉛煙氣問題。因此，需要有不添加鉛（Pb)(無鉛）且展現良好切削性之易切削鋼。有多種技術業經提出以改良沒有鉛的無鉛低碳再硫化易切削鋼之切削性。例如，專利文件1揭示一種經由控制硫化物夾雜物之尺寸而改良切削性（完工面粗糙度及切屑可處置性）的技術。專利文件2教示導鋼料中的氧含量必須予以適當控制以控制硫化物夾雜物的尺寸。一種經由指定鋼料中的氧化物夾雜物而改良切削性之技術業已例如在專利文件3中提出。專利文件4提出一種經由指定錳（Μη)對硫 (S)之比例及經由在澆鑄前一刻控制游離氧含量來改良切削性之技術。 -4- (2) (2)1326714 例如，專利文件5至7中各提出一種經由適當地指定鋼料之化學組成而改良切削性之技術。此等習用技術從改良易切削鋼的切削性之觀點來看都很有用，但它們都不能實現從形成時的完工面粗糙度之觀點來看與含鉛鋼料一樣好的切削性。重要的是，此等無鉛鋼料除了有令人滿意的切削性之外，還應具有良好的生產性。自此觀點來看，它們必須經由連續鑄造方法生產，典型地不含表面缺陷且能易於輥軋。該連續鑄造方法咸信不利於改良鋼料之切削性。因此，能以良好生產力經由連續鑄造方法製造切削性極佳之易切削鋼也很重要。該連續鑄造方法可實現良好的表面品質、內部品質、及良好的產率。專利文件8揭示一種技術，其經由連續鑄造方法提供切削性極佳（完工面粗糙度）之易切削鋼。此技術指出經由在一鋼料中倂入相當大量的100至300 ppm之氧及倂入比常見的相等物更大量之氮（N)，可以良好產率由連續鑄造法獲得切削性極佳之易切削鋼。經由滿足此點，可遏制刃口積屑緣（built-up edge)，此刃口積屑緣係在切削時出現於工具表面中。不過，若一鋼料的氧含量及氮含量兩者都高時，常會形成經由一氧化碳氣體（CO氣體）及氮氣（N2氣體）所引起的氣孔（bloxv holes)，其會惡化鋼料之完工面粗糙度。專利文件1:日本專利公開第200 3 -25 3 3 90號專利文件2 :日本專利公開第H09-3 1 522號 (3) 1326714 專利文件3 :日本專利公開第Η 1 Ο · 1 5 8 7 1號專利文件4:日本專利公開第2005-23342號專利文件5:日本專利公開第2001 - 1 5228 1號專利文件6:日本專利公開第200 1 - 1 52282號專利文件7:日本專利公開第200 1 - 1 5 22 8 3號專利文件8 :日本專利公開第Η05 -3 45 9 5 1號IX. INSTRUCTION DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a low carbon re-vulcanized free-cutting steel which does not contain harmful bells (Pb) and exhibits a good finished surface roughness. [Prior Art] Low carbon re-vulcanized free-cutting steels are widely used as hydraulic parts for automobile gearbox units and steels for small parts such as screws and printer shafts, which do not require high strength. When a good finish surface roughness and chip disposablity are required, lead-sulfur free-cutting steel containing low carbon re-vulcanized free-cutting steel combined with lead (Pb) is used. Lead (Pb) contained in free-cutting steel is a very effective element for improving machinability, but it is harmful to humans. In addition, lead-containing free-cutting steels typically have lead fumes in the handling of ingots and chips. Therefore, there is a need for a free-cutting steel that does not contain lead (Pb) (lead-free) and exhibits good machinability. A variety of techniques have been proposed to improve the machinability of lead-free low carbon re-vulcanized free-cutting steels without lead. For example, Patent Document 1 discloses a technique for improving machinability (finished surface roughness and chip disposability) by controlling the size of sulfide inclusions. Patent Document 2 teaches that the oxygen content of the steel guide must be properly controlled to control the size of the sulfide inclusions. A technique for improving machinability by specifying oxide inclusions in steel materials has been proposed, for example, in Patent Document 3. Patent Document 4 proposes a technique for improving the machinability by specifying the ratio of manganese (?n) to sulfur (S) and controlling the free oxygen content at the moment before casting. -4- (2) (2) 1326714 For example, each of Patent Documents 5 to 7 proposes a technique for improving machinability by appropriately specifying the chemical composition of the steel. These conventional techniques are useful from the viewpoint of improving the machinability of the free-cutting steel, but they are not capable of achieving the same machinability as the lead-containing steel material from the viewpoint of the finish surface roughness at the time of formation. Importantly, these lead-free steels should have good productivity in addition to satisfactory machinability. From this point of view, they must be produced by a continuous casting process, typically free of surface defects and can be easily rolled. The continuous casting method is not conducive to improving the machinability of the steel. Therefore, it is also important to be able to produce a highly machinable easy-cut steel by a continuous casting method with good productivity. This continuous casting method achieves good surface quality, internal quality, and good yield. Patent Document 8 discloses a technique for providing a free-cutting steel having excellent machinability (finished surface roughness) via a continuous casting method. This technique states that by injecting a relatively large amount of 100 to 300 ppm of oxygen into a steel material and injecting a larger amount of nitrogen (N) than a common equivalent, it is possible to obtain excellent machinability by continuous casting in good yield. Free cutting steel. By satisfying this, the built-up edge of the cutting edge can be suppressed, which occurs in the surface of the tool during cutting. However, if both the oxygen content and the nitrogen content of a steel material are high, bloxv holes caused by carbon monoxide gas (CO gas) and nitrogen gas (N2 gas) are often formed, which deteriorates the finished surface of the steel material. Roughness. Patent Document 1: Japanese Patent Publication No. 2003-35-3, Patent Document 2: Japanese Patent Publication No. H09-3 1 522 (3) 1326714 Patent Document 3: Japanese Patent Publication No. 1 Ο · 1 5 8 7 Patent Document No. 1: Japanese Patent Publication No. 2005-23342 Patent Document 5: Japanese Patent Publication No. 2001 - 1 5228 No. 1 Patent Document No.: Japanese Patent Publication No. 2001- 1 52282 Patent Document No. 7: Japanese Patent Publication No. 200 1 - 1 5 22 8 Patent Document No. 8: Japanese Patent Publication No. 05 -3 45 9 5 1

本發明要解決的問題本發明係在此等情況下完成，且本發明之一目的係提供一種低碳再硫化易切削鋼，其展現以完工面粗糙度爲特點的良好切削性，即使不含鉛亦然，且可經由連續鑄造方法以良好生產力製造，同時可揭制氣孔。解決問題之手段 # 本發明業經完成以達到上述目的且提供一種切削性極佳之低碳再硫化易切削鋼，其含有： 0.02%至0.15%(以質量百分比爲基準計；後文同此）之碳（C); 0.004%或更低（不包括0%)之矽（Si); 0.6%至 3 %之錳（Μη); 0.0 2 % 至 0 · 2 % 之磷（Ρ ); 0.3 5 % 至 1 % 之硫（S ); 0.005%或更低（不包括0%)之鋁（Α1); 0.008% 至 0.03% 之氧（O);及 (4) (4)1326714 0.007%至0.03%之氮（N)，其餘爲鐵及無法避免的雜質，其中錳含量[Μη]對硫含量[S]之比例[Mn]/[S]係在3至4之範圍內，且其中該碳含量[C]、錳含量[Μη]及氮含量[N]滿足下面的式（1): l〇[C]x[Mn]'094+1226 [N]2^1.2 (1) 其中[C]、[Μη]及[N]分別代表以質量百分比爲基準之碳、錳及氮之含量。本發明之低碳再硫化易切削鋼，每一者較佳地都具有一化學組成，其中（1)可溶性氮的含量是0.002%至0.02%及 /或（2)選自由Ti、cr、Nb、V、Zr、及Β組成之群組中之至少一種元素的總含量是0.02%或更低（包括0%)。經由滿足這些條件，本發明之低碳再硫化易切削鋼可具有進一步的改良性質。該鋼料較佳爲經由施以電磁攪拌而製得者，其中讓電磁攪拌係在鑄造期間施加100至5 00高斯（gauss)之磁場。所得鋼料具有更好的表面品質。優點本發明控制鋼中的碳 '錳、及氮之含量以滿足特定的關係式。經由滿足此點，即使根據連續鑄造方法也可以用良好的生產力製造具有良好完工面粗糙度的低碳再硫化之易切削鋼，同時可遏制氣孔。 (5) (5)1326714 實施本發明之最佳模式易切削鋼之完工面粗糙度會因刃口積屑緣之產生、尺寸、形狀及一致性而明顯改變。刃口積屑緣之產生係一種現象，爲工件之部分貼附於一工具之表面且實際上表現如該工具之部分（切邊）。其可能不利地影響工作材料之完工面粗糙度。刃口積屑緣僅在特定的條件下產生，但易切削鋼在技藝中通常於此等條件下被切割，因而引發刃口積屑緣。由於彼等尺寸之變異咸信刃口積屑緣會導致嚴重的缺陷。於另一方面，該刃口積屑緣扮演一保護工具之邊緣的角色，從而延長工具之壽命。在考量所有因素之下，完全移除掉此等刃口積屑緣係不利者，且該刃口積屑緣必須穩定地形成爲具有一致的尺寸及形狀。要穩定地形成具有一致尺寸及形狀之刃口積屑緣時，必須在要待切斷區域的初級及次級切變區內形成很多的細微裂痕（fine crack)。因此必須引進很多的裂痕-形成部位以形成許多細微裂痕。MnS夾雜物係已知可用作形成細微裂痕之部位。不是所有MnS夾雜物而是大尺寸（寬的）球形 MnS夾雜物可作爲細微裂痕-形成部位。此等MnS夾雜物會在初級及次級切變區內伸長，但若它們變成太薄而如基質一樣薄，則無法作爲細微裂痕-形成部位。因此，工件（待切割的鋼料）在切斷前必須包含大尺寸球狀MnS夾雜物〇鋼料中之氧（總氧）會影響MnS夾雜物的尺寸及球面性 -8 - (6) (6)1326714 (例如，請參見專利文件2 )，且咸信硫化物之尺寸（直徑）會隨鋼料氧含量之增加而增加。結果，爲了使MnS夾雜物更大及更具球形，鋼料之氧含量必須增加至某種程度。此外，錳含量及硫含量必須高於在常見的易切削鋼中之値，諸如日本工業標準（Japanese Industrial Standards)(JIS) SUM 23鋼及SUM 24L鋼，以增加有細微裂痕-形成部位作用的MnS夾雜物。本案發明人發現鋼料中之可溶性氮也會顯著地影響細微裂痕之形成且經由適當地調整可溶性氮的含量可實現有良好切削性的易切削鋼。初級及次級切變區內之溫度會自一位置至另一位置地明顯變化。當可溶性氮以某一量存在時，變形抗性會視個別位置的溫度而變化。變形抗性之差異（變化）會產生細微裂痕-形成部位。因此，經由將Ti、 Cr、Nb、V、Zr、及B之總量控制於特定的含量或更低，便可有效地確保某一或更大的可溶性氮含量。此係因爲此等組分作用於固定可溶性氮，亦即，它們作用而形成氮化物。具體而言，本案發明人發現例如經由兩種現象，亦即 :(1)使MnS夾雜物變成更大及具球形，和（2)增加可溶性氮，便可穩定地形成具有一致尺寸及形狀之刃口積屑緣》其所產生的鋼料在形成程序中具有經大幅改良之完工面粗糙度且從而展現與鉛易切削鋼一樣好的性質。本發明的該易切削鋼必須具有經恰當規範的化學組成。規範基本組分C、Si、Μη、P、S、A1、0、及N之含量 -9- (7) 的理由如下。碳（C) : 0 · 0 2 % 至 0 · 1 5 % 碳（C)係一種確保鋼料強度的必要元素，而且若添加特定的量或更多時，可用於改良完工面粗糙度。碳含量必須爲0.02 %或更多以展現此等活性。不過，其過高的含量在切割上可能縮短工具之壽命從而惡化切削性，且可能由於在鑄造時的一氧化碳（CO)氣體而引發缺陷。自此等觀點來看，該碳含量較佳爲0.15 %或更低。該碳含量之下限及上限較佳地分別爲0.0 5 %及0.1 2 %。矽（Si) : 0.004%或更低（不包括0%) 矽（Si)係一種因固體-溶液強化作用之結果而有效確保鋼料強度的元素，不過其基本上係作爲去氧劑以形成二氧化矽Si02。該二氧化矽（Si〇2)係用於形成Mn0-Si02_ MnS夾雜物。若矽含量超過〇_〇〇4%，則夾雜物中的 Si02 含量變成太高而不能確保MnS夾雜物中所需之氧含量。因此，完工面粗糙度會惡化。自此等觀點來看，該矽含量必須爲0.0 0 4 %或更低且較佳爲0.0 0 3 %或更低。錳（Μ η) : 〇 · 6 % 至 3 % 錳（Μη)係用於改良硬化性，以提高度韌鐵（bainite)之形成，及改良切削性。其係一種有效確保鋼料強度之元素。再者，其與硫結合形成MnS及與氧結合形成MnO，從 -10- (8) (8)1326714 而形成Μη O-MnS複合夾雜物。因此，其可用於改良切削性。爲展現此等作用，錳含量必須爲0.6%或更高，若其超過3%，強度會過度的增加而惡化切削性。錳含量之下限及上限較佳地分別爲1 %及2 %。磷（P) : 0.0 2 % 至 0 · 2 % 磷（P)係用於改良完工面粗糙度。其亦經由促進切屑中的裂痕增長而用於大幅改良切屑可處置性。爲展現此等優點，該磷含量必須爲0.02 %或更高。不過，過高的磷含量會惡化熱加工性，故該磷含量必須爲0.2%或更低。磷含量之較佳下限及上限分別爲（K 0 5 %及0.1 5 %。硫（S ) : 0，3 5 % 至 1 % 硫（S)係一種元素，其在鋼料中與錳結合以形成硫化錳（MnS)，從而作爲在切斷時之應力集中點。如此，切屑被分割從而改良切削性。爲展現此等作用，該硫含量必須爲0.3 5 %或更高，若硫含量過高而超過1%，則可能惡化熱加工性。因此，硫含量之較佳上限是〇. 8 %。總鋁（A 1 ) : 0 · 〇〇 5 %或更低（不包括〇 %) 鋁（A1)係一種可用於因固體—溶液強化之結果而確保鋼料強度且用於去氧之元素。其亦充當強去氧劑以形成氧化物（Al2〇3)。該氧化物（Al2〇3)構成 MnO-Al 203-MnS 夾雜物。若鋁含量超過0.005 %，夾雜物之Al2〇3含量變成太 -11 - 1326714 Ο) 高而不能確保MnS夾雜物中的所需氧含量，從而不利地影響完工面粗糙度。該鋁含量較佳爲0.0 03 %或更低且更佳爲0.001%或更低。氧（0) : 0 · 0 0 8 % 至 0.0 3 % 氧（〇)會與錳（Μη)組合形成氧化錳（MnO)。該MnO含有大量硫，從而構成 MnO-MnS複合夾雜物。MnO-MnS 複合夾雜物在輥軋之際可抗拒延長，其係以相當球形夾雜物的形式存在且從而在切割時作爲應力集中點。因此，氧添加於鋼料中具有正面作用。若氧含量低於0.008%，此等作用會不足，但若其超過0.03%，則可能在鋼錠中出現因一氧化碳氣體引起的內部缺陷。因此，氧含量（總氧含量）必須在〇 . 〇〇 8 %至0.0 3 %之範圍內。氧（總氧）在熔融鋼料中會形成氧化錳（MnO)，且該 MnO含有大量之硫從而形成MnO-MnS複合夾雜物。此等 MnO-MnS複合夾雜物充當核，以在凝固期間沉澱MnS夾雜物。如此，所生成之小鋼坯（billet)(由連續鑄造所製備的鑄錠）含有主要包含MnS的MnO-MnS複合夾雜物。該小鋼坯接著歷經加熱、坯軋、及線條（wire rod)輥軋或棒磨輥軋。隨著遞增的氧含量，主要包括MnS的MnO-MnS 複合夾雜物可在坯軋與線條輥軋或棒磨輥軋中更抗拒伸長，且它們會在最終產品諸如鋼線及鋼條中構成大尺寸球形 MnS夾雜物。氧（總氧）之下限從此等機制著眼來設定，其中該氧含 -12- 1326714 do) 量較佳地爲高的。不過，氧含量之上限亦於實際中要設定。其理由將解釋如下。氧（總氧）包括氧化物形式之氧、及溶解於熔融鋼料中的可溶性氧（游離氧）。該氧化物形式之氧，亦即MnO中之氧係非常有用的。相較之下，游離氧 (〇)會與熔融鋼料中之碳（C)反應形成CO氣體[C + 0 = C0(氣體）]，且該CO氣體，若未充分釋出，會導致氣孔。此外 ’根據本發明係增加鋼之氮含量，在固化程序期間在且熔融鋼料中之可溶性氮會形成N2(氣體）[N + N = N2(氣體）]，因爲熔融鋼料中的氮溶解度會隨著下降的溫度而減少。該 N2氣體亦會造成氣孔。具體言之，氣孔主要包含CO(氣體 )及N2(氣體）。本發明之一項特點（槪念）係游離氧（0)及氮（N)含量係經設定在CO(氣體）及N2(氣體）不會形成氣孔的此等範圍內之最高値。除了調定鋼料之化學組成之外，鋼料中氣孔之形成亦可經由進行電磁攪拌予以改良。此係因爲氣孔，若形成後，可經由在連續鑄造的模具中進行電磁攪拌而從鋼料排除之故。在此等槪念下，本案發明人進行硏究以確定何者會影響游離氧（0)含量’而發現主要爲錳含量[Μη]及硫[S]含量影響游離氧（〇)含量。據此，C0(氣體）之量可經由[C]、 [Μη]、和[S]予以控制’且C0(氣體）+ Ν2(氣體）之量可根據式（1)定出，其中將氮含量[Ν]加到此等參數中。如此，可控制氣孔。其細節將隨後說明。熔融鋼料中之游離氧（0)含量，由防止C0氣體所引 -13- (11) (11)1326714 起內部缺陷之觀點來看，較佳爲經控制在約〇. 〇〇 5 Ο %或更低’而其係會依照碳及氮含量[C]及[Ν]或電磁攪拌條件而變化。鋼料氧含量（總氧含量）之較佳下限與上限分別爲 Ο . Ο 1 % 及〇 · 〇 3 %。氮（Ν) : 0.007% 至 0.03% 氮（Ν)係一種會影響刃口積屑緣的量之元素，且其含量會影響完工面粗糙度。若氮含量低於0.007%，會發生過量的刃口積屑緣，從而不利地影響完工面粗糙度。氮易於在基質內的差排（dislocation)中偏析（segregate)。其在切割期間於差排中偏析，從而使該基質易碎及促進裂痕增長。因此，氮係用來改良切屑易碎性（Breakability)(切屑可處置性）。不過，超過0.03 %的過高氮含量在鑄造時會引起氣泡（氣孔），其往往變成所得鑄錠的內部及表面缺陷。所以，該氮含量必須控制至0.03 %或更低。氮含量之較佳下限及上限分別爲0 · 00 5 %及0 · 02 5 %。僅按上述規範本發明低-碳再硫化易切削鋼之化學組成並不足以達成本發明目的。除此之外，錳含量[Μη]對硫含量之比例[Mn]/[S]也必須控制在特定的適當範圍內且此等參數必須滿足式（1)所示的條件。設定此等要求的理由如下。 [Mn]/[S]比：3至 4 [Mn]/[S]比係一項影響例如在熱加工期間破裂之重要 .14- (12) (12)1326714 因素。若錳含量相對於硫含量爲不足時，亦即[Μη]/[S]低於3，則常會形成FeS，此會造成熱裂。當[Mn]/[S]比在3 至4之範圍內時，該錳含量相對於硫含量爲足夠者，這可防止FeS之形成’從而防止熱裂。若[Mn]/[S]比超過4, 此作用會飽和且游離氧（〇)含量會減低，從而不利地影響完工面粗糙度。該游離氧含量係依[Μη]及[S]而變異。 10[C] X [Μη]'0 94 + 1 226 [N]2 ^ 1.2 上述條件必須滿足以防止氣孔及確保令人滿意的切削性。若該左手邊値（lOtCnxtMnrGM + iaSGtN]2)超過1.2，則可能形成氣孔。該左手邊値較佳爲1 . 1或更低且更佳爲0.9 或更低。式（1)所示條件業經各種實驗後定出，且其理由將說明如下。溶解在熔融鋼料中的碳（C)、氧、及氮（N)會歷經由固體-液體分離所致微偏析且在液體中增濃。可溶性氧近乎等於游離氧（〇)，而游離氧意指氧活性。液體中的碳、氧、和氮之溶解度會隨著遞降的溫度而減低。具體而言，由於微偏析所增濃的碳、氧、和氮會在遞降溫度所致遞減溶解度之下發生如C + 0 = C0(氣體）及Ν=1/2Ν2(氣體）之反應。所生成的氣體，若克服局部的壓力，會在熔融鋼料之液體部份內形成氣泡（氣孔）。該局部壓力主要包括大氣壓力、熔融鋼料靜壓力、及（液體與氣體之間的介面能量 )/(氣泡之直徑）之總和。該氣泡常在熔融鋼料靜態壓力低的彎月面附近形成。氣體（氣泡）包含C0(氣體）及n2(氣體） -15- (13) 1326714 。若氣體（氣泡）由於密度之不同而浮起且自熔融鋼料逸散至大氣，則其不會以氣孔形式保留在小鋼坯中。不過，若其例如被凝固的結晶體所吞噬，則其會留存爲氣孔且成爲小鋼坯中的缺陷。在上述機制之假設下，氣孔之形成可能會依碳含量 [C]、游離氧含量[〇]及氮含量[N]而變異。因此，該現象可由下列式（2)至（7)以熱力學表示： C0(氣體）= [c] + [o] (2) Kco~(aca〇)/Pco=fc[C]f〇[0]/Pco (3) l〇g(Kc〇)=-l160/T-2.003 (4) CcL = Cc〇/{l-(l-kc)f} (5) C°L = C°〇/{ 1 -(1 -k〇)f} (6)PROBLEM TO BE SOLVED BY THE INVENTION The present invention has been accomplished in such circumstances, and it is an object of the present invention to provide a low carbon re-vulcanized free-cutting steel which exhibits good machinability characterized by finished surface roughness, even if it does not contain Lead is also available and can be manufactured with good productivity via a continuous casting process while at the same time exposing the pores. Means for Solving the Problem # The present invention has been completed to achieve the above object and to provide a low carbon re-vulcanized free-cutting steel having excellent machinability, which contains: 0.02% to 0.15% (based on mass percentage; later) Carbon (C); 0.004% or less (excluding 0%) of bismuth (Si); 0.6% to 3% of manganese (Μη); 0.0 2% to 0 · 2 % of phosphorus (Ρ); 0.3 5 % to 1% sulfur (S); 0.005% or lower (excluding 0%) of aluminum (Α1); 0.008% to 0.03% of oxygen (O); and (4) (4) 1326714 0.007% to 0.03 % of nitrogen (N), the balance being iron and unavoidable impurities, wherein the ratio of manganese content [Μη] to sulfur content [S] [Mn]/[S] is in the range of 3 to 4, and the carbon The content [C], the manganese content [Μη], and the nitrogen content [N] satisfy the following formula (1): l〇[C]x[Mn]'094+1226 [N]2^1.2 (1) where [C] , [Μη] and [N] represent the content of carbon, manganese and nitrogen based on the mass percentage, respectively. The low carbon re-vulcanized free-cutting steel of the present invention preferably each has a chemical composition wherein (1) the soluble nitrogen content is 0.002% to 0.02% and/or (2) is selected from the group consisting of Ti, Cr, Nb. The total content of at least one of the groups of V, Zr, and yttrium is 0.02% or less (including 0%). By satisfying these conditions, the low carbon re-vulcanized free-cutting steel of the present invention can have further improved properties. The steel material is preferably produced by applying electromagnetic stirring, wherein the electromagnetic stirring system applies a magnetic field of 100 to 500 gauss during casting. The resulting steel has a better surface quality. Advantages The present invention controls the content of carbon 'manganese, and nitrogen in the steel to satisfy a specific relationship. By satisfying this, even a low-carbon re-vulcanized free-cutting steel having a good finished surface roughness can be manufactured with good productivity even according to the continuous casting method, while suppressing the pores. (5) (5) 1326714 The best mode for carrying out the invention The finished surface roughness of the free-cutting steel is significantly changed by the generation, size, shape and consistency of the edge of the cutting edge. The edge of the blade is a phenomenon in which a part of the workpiece is attached to the surface of a tool and actually appears as a part of the tool (cut edge). It may adversely affect the finished surface roughness of the working material. The cutting edge of the cutting edge is produced only under certain conditions, but the free-cutting steel is usually cut under such conditions in the art, thereby causing the chip edge to be chipped. Due to the variation of their size, the edges of the edges can cause serious defects. On the other hand, the blade edge acts as a protective tool to the edge of the tool, thereby extending the life of the tool. Under all factors, the edges of these edges are completely removed, and the edge of the edge must be stably formed to have a uniform size and shape. To stably form the edge of the edge having a uniform size and shape, a large number of fine cracks must be formed in the primary and secondary shear regions of the region to be cut. Therefore, many crack-forming sites must be introduced to form many fine cracks. The MnS inclusions are known to be useful as sites for the formation of fine cracks. Not all MnS inclusions but large (wide) spherical MnS inclusions can serve as a fine crack-forming site. These MnS inclusions will elongate in the primary and secondary shear zones, but if they become too thin and as thin as the matrix, they cannot be used as fine crack-forming sites. Therefore, the workpiece (the steel to be cut) must contain large-size spherical MnS inclusions before cutting. The oxygen (total oxygen) in the steel will affect the size and sphericality of the MnS inclusions - 8 - (6) ( 6) 1326714 (see, for example, Patent Document 2), and the size (diameter) of the sulfide is increased as the oxygen content of the steel increases. As a result, in order to make the MnS inclusion larger and more spherical, the oxygen content of the steel must be increased to some extent. In addition, the manganese content and sulfur content must be higher than those in common free-cutting steels, such as Japanese Industrial Standards (JIS) SUM 23 steel and SUM 24L steel to increase the role of fine crack-forming sites. MnS inclusions. The inventors of the present invention found that the soluble nitrogen in the steel material also significantly affects the formation of fine cracks and can realize a free-cutting steel with good machinability by appropriately adjusting the content of soluble nitrogen. The temperature in the primary and secondary shear zones varies significantly from one location to another. When soluble nitrogen is present in a certain amount, the deformation resistance varies depending on the temperature of the individual locations. The difference (change) in deformation resistance produces a fine crack-forming site. Therefore, by controlling the total amount of Ti, Cr, Nb, V, Zr, and B to a specific content or lower, it is possible to effectively ensure a certain or larger soluble nitrogen content. This is because these components act on the fixed soluble nitrogen, i.e., they act to form a nitride. Specifically, the inventors of the present invention have found that, for example, through two phenomena, namely, (1) making MnS inclusions larger and spherical, and (2) increasing soluble nitrogen, it is possible to stably form uniform sizes and shapes. The resulting steel material has a greatly improved finished surface roughness in the forming process and thus exhibits as good properties as lead free-cutting steel. The free cutting steel of the present invention must have a properly defined chemical composition. The reason for standardizing the contents of the basic components C, Si, Μη, P, S, A1, 0, and N -9- (7) is as follows. Carbon (C): 0 · 0 2 % to 0 · 1 5 % Carbon (C) is an essential element for ensuring the strength of steel, and can be used to improve the finish surface roughness when a specific amount or more is added. The carbon content must be 0.02% or more to exhibit such activity. However, its excessive content may shorten the life of the tool in the cutting to deteriorate the machinability, and may cause defects due to carbon monoxide (CO) gas during casting. From this point of view, the carbon content is preferably 0.15% or less. The lower limit and the upper limit of the carbon content are preferably 0.05% and 0.12%, respectively.矽(Si) : 0.004% or less (excluding 0%) 矽(Si) is an element which effectively ensures the strength of the steel as a result of solid-solution strengthening, but it is basically used as an oxygen scavenger to form Ceria SiO 2 . The cerium oxide (Si〇2) is used to form Mn0-SiO 2 MnS inclusions. If the cerium content exceeds 〇_〇〇4%, the SiO2 content in the inclusions becomes too high to ensure the desired oxygen content in the MnS inclusions. Therefore, the roughness of the finished surface deteriorates. From this point of view, the niobium content must be 0.04% or less and preferably 0.03% or less. Manganese (Μ η) : 〇 · 6 % to 3% Manganese (Μη) is used to improve hardenability to improve the formation of bainite and to improve machinability. It is an element that effectively ensures the strength of the steel. Further, it combines with sulfur to form MnS and combines with oxygen to form MnO, and forms Μη O-MnS composite inclusions from -10-(8)(8)1326714. Therefore, it can be used to improve machinability. In order to exhibit such effects, the manganese content must be 0.6% or more, and if it exceeds 3%, the strength excessively increases to deteriorate the machinability. The lower limit and the upper limit of the manganese content are preferably 1% and 2%, respectively. Phosphorus (P): 0.0 2 % to 0 · 2 % Phosphorus (P) is used to improve the finish surface roughness. It is also used to greatly improve chip disposability by promoting the growth of cracks in the chips. To exhibit these advantages, the phosphorus content must be 0.02% or higher. However, an excessively high phosphorus content deteriorates hot workability, so the phosphorus content must be 0.2% or less. The preferred lower and upper limits of the phosphorus content are (K 0 5 % and 0.1 5 %, respectively. Sulfur (S ) : 0,3 5 % to 1 %. Sulfur (S) is an element which is combined with manganese in the steel. Manganese sulfide (MnS) is formed as a stress concentration point at the time of cutting. Thus, the chips are divided to improve machinability. To exhibit such effects, the sulfur content must be 0.35% or higher, if the sulfur content is excessive High and more than 1% may deteriorate the hot workability. Therefore, the upper limit of the sulfur content is 〇. 8 %. Total aluminum (A 1 ) : 0 · 〇〇 5% or less (excluding 〇%) Aluminum (A1) is an element which can be used to ensure the strength of steel due to the solid-solution strengthening and is used for deoxidation. It also acts as a strong deoxidizer to form an oxide (Al2〇3). This oxide (Al2〇) 3) MnO-Al 203-MnS inclusions are formed. If the aluminum content exceeds 0.005%, the Al2〇3 content of the inclusions becomes too -11 - 1326714 Ο) high and the required oxygen content in the MnS inclusions cannot be ensured, thereby being disadvantageous The ground affects the roughness of the finished surface. The aluminum content is preferably 0.03% by weight or less and more preferably 0.001% or less. Oxygen (0) : 0 · 0 0 8 % to 0.0 3 % Oxygen (〇) combines with manganese (Μη) to form manganese oxide (MnO). The MnO contains a large amount of sulfur to constitute a MnO-MnS composite inclusion. The MnO-MnS composite inclusions are resistant to elongation during rolling, which is present in the form of relatively spherical inclusions and thus acts as a stress concentration point during cutting. Therefore, the addition of oxygen to the steel material has a positive effect. If the oxygen content is less than 0.008%, these effects may be insufficient, but if it exceeds 0.03%, internal defects due to carbon monoxide gas may occur in the ingot. Therefore, the oxygen content (total oxygen content) must be in the range of 〇〇〇 8 % to 0.0 3 %. Oxygen (total oxygen) forms manganese oxide (MnO) in the molten steel, and the MnO contains a large amount of sulfur to form a MnO-MnS composite inclusion. These MnO-MnS composite inclusions act as nuclei to precipitate MnS inclusions during solidification. Thus, the generated billet (ingot prepared by continuous casting) contains MnO-MnS composite inclusion mainly containing MnS. The billet is then subjected to heating, billet rolling, and wire rod rolling or bar rolling. With increasing oxygen content, MnO-MnS composite inclusions, which mainly include MnS, can resist elongation in billet rolling and line rolling or bar rolling, and they can form large in final products such as steel wire and steel bars. Size spherical MnS inclusions. The lower limit of oxygen (total oxygen) is set by the mechanism in which the amount of oxygen contains -12-1326714 do) is preferably high. However, the upper limit of the oxygen content is also set in practice. The reason will be explained as follows. Oxygen (total oxygen) includes oxygen in the form of oxides and soluble oxygen (free oxygen) dissolved in the molten steel. Oxygen in the form of an oxide, i.e., oxygen in MnO, is very useful. In contrast, free oxygen (〇) reacts with carbon (C) in the molten steel to form CO gas [C + 0 = C0 (gas)], and if the CO gas is not fully released, it will cause pores. . Furthermore, according to the invention, the nitrogen content of the steel is increased, and the soluble nitrogen in the molten steel during the solidification process forms N2 (gas) [N + N = N2 (gas)] because of the nitrogen solubility in the molten steel. Will decrease with decreasing temperature. The N2 gas also causes pores. Specifically, the pores mainly contain CO (gas) and N 2 (gas). A feature (memory) of the present invention is that the free oxygen (0) and nitrogen (N) contents are set to the highest enthalpy in the range in which CO (gas) and N2 (gas) do not form pores. In addition to setting the chemical composition of the steel, the formation of pores in the steel can also be improved by performing electromagnetic stirring. This is because the pores, if formed, can be removed from the steel by electromagnetic stirring in a continuous casting mold. Under this circumstance, the inventors of the present case conducted a study to determine which would affect the free oxygen (0) content and found that the main manganese content [Μη] and sulfur [S] content affected the free oxygen (〇) content. Accordingly, the amount of C0 (gas) can be controlled via [C], [Μη], and [S]' and the amount of C0 (gas) + Ν2 (gas) can be determined according to formula (1), wherein nitrogen is The content [Ν] is added to these parameters. In this way, the air holes can be controlled. The details will be explained later. The content of free oxygen (0) in the molten steel is preferably controlled at about 〇. 〇〇5 Ο % from the viewpoint of preventing internal defects caused by C0 gas-13-(11) (11) 1326714. Or lower' and its system will vary according to carbon and nitrogen content [C] and [Ν] or electromagnetic stirring conditions. The preferred lower and upper limits of the oxygen content (total oxygen content) of the steel are Ο 1 % and 〇 · 〇 3 %, respectively. Nitrogen (Ν): 0.007% to 0.03% Nitrogen (Ν) is an element that affects the amount of edge of the cutting edge and its content affects the roughness of the finished surface. If the nitrogen content is less than 0.007%, excessive edge formation will occur, which adversely affects the finish surface roughness. Nitrogen tends to segregate in the dislocation within the matrix. It segregates in the row during the cutting process, making the matrix brittle and promoting crack growth. Therefore, nitrogen is used to improve chip breakability (chip disposability). However, an excessively high nitrogen content of more than 0.03% causes bubbles (pores) during casting, which tend to become internal and surface defects of the resulting ingot. Therefore, the nitrogen content must be controlled to 0.03% or less. The lower and upper limits of the nitrogen content are 0 · 00 5 % and 0 · 02 5 %, respectively. The chemical composition of the low-carbon re-vulcanized free-cutting steel of the present invention according to the above specification alone is not sufficient for the purpose of the present invention. In addition, the ratio of the manganese content [Μη] to the sulfur content [Mn]/[S] must also be controlled within a specific appropriate range and these parameters must satisfy the conditions shown in the formula (1). The reasons for setting these requirements are as follows. [Mn]/[S] ratio: 3 to 4 [Mn]/[S] ratio is an important influence such as cracking during hot working. 14- (12) (12) 1326714 Factor. If the manganese content is insufficient relative to the sulfur content, that is, [Μη]/[S] is less than 3, FeS is often formed, which causes thermal cracking. When the [Mn] / [S] ratio is in the range of 3 to 4, the manganese content is sufficient with respect to the sulfur content, which prevents the formation of FeS' thereby preventing thermal cracking. If the [Mn]/[S] ratio exceeds 4, the effect is saturated and the free oxygen (〇) content is reduced, thereby adversely affecting the finish surface roughness. The free oxygen content varies depending on [Μη] and [S]. 10[C] X [Μη]'0 94 + 1 226 [N]2 ^ 1.2 The above conditions must be met to prevent porosity and to ensure satisfactory machinability. If the left-hand side 値 (lOtCnxtMnrGM + iaSGtN) 2) exceeds 1.2, pores may be formed. The left-hand side 値 is preferably 1.1 or less and more preferably 0.9 or less. The conditions shown in the formula (1) are determined after various experiments, and the reasons are as follows. The carbon (C), oxygen, and nitrogen (N) dissolved in the molten steel are subjected to microsegregation by solid-liquid separation and enriched in the liquid. Soluble oxygen is nearly equal to free oxygen (〇), while free oxygen means oxygen activity. The solubility of carbon, oxygen, and nitrogen in the liquid decreases with decreasing temperature. Specifically, carbon, oxygen, and nitrogen which are enriched by microsegregation may undergo a reaction such as C + 0 = C0 (gas) and Ν = 1/2 Ν 2 (gas) under the decreasing solubility due to the descending temperature. The generated gas, if it overcomes the local pressure, forms bubbles (pores) in the liquid portion of the molten steel. The partial pressure mainly includes the sum of atmospheric pressure, molten steel static pressure, and (intermediate energy between liquid and gas) / (diameter of bubbles). This bubble is often formed near the meniscus where the static pressure of the molten steel is low. The gas (bubble) contains C0 (gas) and n2 (gas) -15- (13) 1326714. If the gas (bubble) floats due to the difference in density and escapes from the molten steel to the atmosphere, it does not remain in the form of pores in the billet. However, if it is swallowed, for example, by solidified crystals, it will remain as pores and become a defect in the billet. Under the assumption of the above mechanism, the formation of pores may vary depending on the carbon content [C], the free oxygen content [〇], and the nitrogen content [N]. Therefore, this phenomenon can be expressed thermodynamically by the following equations (2) to (7): C0 (gas) = [c] + [o] (2) Kco~(aca〇)/Pco=fc[C]f〇[0 ]/Pco (3) l〇g(Kc〇)=-l160/T-2.003 (4) CcL = Cc〇/{l-(l-kc)f} (5) C°L = C°〇/{ 1 -(1 -k〇)f} (6)

Pco = (fcfoCc〇C0〇)/[{l-(l-kc)f}{ l-(l-k〇)f}Kc〇] (7) φ 首先，假設反應自右進行至左之下來探討式（2)。式 (2)中的平衡常數KC0係由碳的活性係數（fc)、碳含量[c] 、氧之活性因數（f〇)、氧含量[0]、及C0分壓（pco)給出。該平衡常數係根據式（4)定出，其中T表絕對溫度。碳含量[C]及氧含量[〇]係指微偏析後之含量且係根據Sheil Equation如式（5)及（6)而測定。在式（5)及（6)中，Cc〇及 CQ〇分別表示熔融鋼料鑄造前之起始碳含量[c]及氧含量 [〇]，且ccL及c0L分別表示在固化期間（其中固相及液相同時共存）液相中之碳含量[C]及氧含量[0]。CcL及c〇L表 -16- (14) (14)1326714 示由於微偏析增濃後的含量。經由將此等參數代入式（3) 中’ C0分壓（Pco)可由式（7)表出。在這些式中，"f"表示固相之分數；且kc及k〇分別表示碳及氧之平衡分配係數〇與氮有關的現象可由下列式（8)至（12)表示。 1/2N2(氣體）= [N] (8) K.N2 = (aN)/-y/_ ^N2 = fN [N]/ (9) log(KN2)=*518/T-l.063 (10) CNL = CN〇/{l-(l-kN)f} (11) V- PN2 = (fNCN〇)/{l-(l-kN)f}KN2 (12) 具體而言，式（8)中的平衡常數KN2可由式（9)表示’ 且該平衡常數可由式（10)表出。微偏析後熔融鋼料之氮含量[N]可由式（1 1)表出，且經由將此代入式（9)中’ &分JS (Pn2)可由式（1 2)表出。當經如此估測的式（7)及（12)所表示之分β總、$ (Pco + PN2)超過外部壓力（大氣壓力）、熔融鋼料靜壓力、及 (液體與氣體之間的介面能量）/(氣泡直徑）的總和時（$口 T 面式（13)所表示者），會形成氣孔：Pco = (fcfoCc〇C0〇)/[{l-(l-kc)f}{ l-(lk〇)f}Kc〇] (7) φ First, assume that the reaction proceeds from the right to the bottom left to explore the formula ( 2). The equilibrium constant KC0 in the formula (2) is given by the carbon activity coefficient (fc), the carbon content [c], the oxygen activity factor (f〇), the oxygen content [0], and the C0 partial pressure (pco). The equilibrium constant is determined according to formula (4), where T is the absolute temperature. The carbon content [C] and the oxygen content [〇] refer to the content after the microsegregation and are determined according to the Sheil Equation according to the formulas (5) and (6). In the formulas (5) and (6), Cc〇 and CQ〇 respectively represent the initial carbon content [c] and oxygen content [〇] of the molten steel before casting, and ccL and c0L respectively indicate during solidification (wherein The phase and the liquid phase coexist simultaneously) the carbon content [C] and the oxygen content [0] in the liquid phase. CcL and c〇L Table -16- (14) (14) 1326714 shows the content after enrichment by microsegregation. Substituting these parameters into equation (3) 'C0 partial pressure (Pco) can be expressed by equation (7). In these formulas, "f" represents the fraction of the solid phase; and kc and k〇 represent the equilibrium partition coefficient of carbon and oxygen, respectively. 〇 The phenomenon related to nitrogen can be expressed by the following formulas (8) to (12). 1/2N2 (gas) = [N] (8) K.N2 = (aN)/-y/_ ^N2 = fN [N]/ (9) log(KN2)=*518/Tl.063 (10) CNL = CN〇/{l-(l-kN)f} (11) V- PN2 = (fNCN〇)/{l-(l-kN)f}KN2 (12) Specifically, in equation (8) The equilibrium constant KN2 can be represented by the formula (9) and the equilibrium constant can be expressed by the formula (10). The nitrogen content [N] of the molten steel after microsegregation can be expressed by the formula (1 1), and by substituting this into the formula (9), the & JS (Pn2) can be expressed by the formula (1 2). When the thus estimated equations (7) and (12) represent the total β, $ (Pco + PN2) exceeds the external pressure (atmospheric pressure), the molten steel static pressure, and (the interface between liquid and gas) When the sum of energy)/(bubble diameter) (expressed by $port T (13)), pores are formed:

Pg ^ Pa + pLgh + 2a/r (13) 其中Pg表示熔融鋼料中各氣體之分壓總和； -17- (15) (15)1326714Pg ^ Pa + pLgh + 2a/r (13) where Pg represents the partial pressure of each gas in the molten steel; -17- (15) (15) 1326714

Pa表示外部壓力： pLgh表示液體靜壓力； σ表示液體與氣體之間的介面能量； "r"表示氣泡直徑。本案發明人探討氣孔發生頻率如何依照上述有此等物理學意義之方法計算出的總分壓（PC0 + PN2)而變異。結果，他們發現當分壓總和（PC0 + PN2)超過1.2大氣壓（atm)時，會出現氣孔。本案發明人嘗試將分壓總和（PC0 + PN2)轉換成爲一指數。該碳及氮含量[C]及[N]可經由線上分析而容易地測定，但游離氧含量必須使用游離氧分析儀測定。其在某些測定程序中可能伴隨大誤差。本案發明人探討何者會影響游離氧含量[〇]且發現錳含量[Μη]及硫含量[S]會影響游離氧含量[〇]。此也從下述事實明顯可知：在熔融鋼料中氧會形成MnO-MnS氧化物-硫化物夾雜物。這顯示氣孔之形成可由[C]、[Mn]、[S]、及[N]之間的關係式表示。此外，锰及硫含量[Μη]及[S]具有關聯，其中[Mn]/[S]比爲3至4。從此關聯來看’氣孔之形成可由在[C]、[Μη]、及[N]之間的關聯式表出。在此等構想下，式（7)之右手邊値及含量數據諸如 [Μη]經實驗顯示Pco等於l〇[C]x[MnrQ94，因爲式（7)顯示 Pco正比於[C]和[0]，且[〇]係依[Μη]而變異。式（12)之右手邊値之平方及含量數據諸如[Ν]顯示氮分壓ΡΝ2等於 1 226[Ν]2，因爲式（12)顯示ΡΝ：^正比於[ν]2。 -18- (16) (16)1326714 在一氧化碳及氮氣之分壓總和 Pco + PN2(l〇[C]x[Mn]-Q 94+ 1 22 6 [N]2)增加之下’氣孔出現且從而引起表面缺陷。氣孔的形成自然地影響完工面粗糙度。一氧化碳及氮氣之分壓總和Pco + Pn2與完工面粗糙度之間具有如後文提及的圖1中所顯示之關係。圖1顯示從表面缺陷之形成與完工面粗糙度來看，分壓總合之限値爲約 1.2° 本發明低碳再硫化之易切削鋼包括上述諸組分，其餘組分基本上爲鐵。不過，除了此等組分之外，它們可進一步包含微量組分，且進一步包含此等微量組分者也包括在本發明範圍之內。本發明低碳再硫化之易切削鋼包含無法避免的雜質諸如Cu、Sn、和Ni，且此等無法避免的雜質在不會不利地影響本發明優點之範圍內係可接受者。本發明低碳再硫化之易切削鋼，可根據需要較佳地具有（1)0.002 %至0.02 %之可溶性氮之含量及/或（2)0.02 %或更低（〇%除外）之選自由Ti、Cr、Nb、V、Zr、及B所組成之群組中的至少一者之總含量。設定此等特定範圍之理由如下。可溶性氮之含量：0.002%至0.02% 如上所述者，鋼料中之可溶性氮會影響細微裂痕之形成，且經由適當的控制可溶性氮之含量可實現有良好切削性的易切削鋼。爲展現此等優點，鋼料中之可溶性氮含量較佳爲經控制到〇 · 〇〇 2 %或更大。不過，若其超過〇〇 2 %， -19- (17) (17)1326714 表面缺陷可能增加。選自由Ti、Cr、Nb、V、Zr、及B所組成之群組中的至少一種元素之總含量：0 · 0 2 %或更低（0 %除外）此等元素會與氮結合形成氮化物，且若其含量過高，則可溶性氮之含量會變成太小而低於所需的可溶性氮之含量。自此觀點來看，此等元素之總含量較佳爲控制至 0.0 2 %或更低。本發明低碳再硫化之易切削鋼基本上係經由連續鑄造法製成。它們可特別地例如根據下列程序來製造。起初，於一轉爐內將碳排放到0.04%或更低之碳含量以使熔融鋼料具有高游離氧含量（可溶性氧含量）。在此，游離氧含量較佳爲500 ppm或更多。其次，在出鐵時添加合金諸如 Fe-Mn合金及Fe-S合金。此等合金含有矽及鋁等雜質。經由在從轉換器出鐵時，添加此等合金至高氧熔融鋼料中，可將矽及鋁氧化成3丨02及Al2〇3。它們會漂浮並在隨後的熔融鋼料之澆桶精煉程序時分離入爐渣中。如此，鋼料中留存之矽及鋁會減低到目標含量。在此製程中重要的是，70 %或更多的添加組分諸如Fe-Mn合金及Fe-S合金等經加入以調整化學組成者係在從轉爐出鐵時添加以減少鋁及矽，且其之餘量（30 %或更低）係在熔融鋼料之澆桶中精煉程序中添加。經由進行此等程序，該鋼料可具有0.004% 或更低之矽含量。在鋼料之製造中，較佳爲進行電磁攪拌，其中係在鑄造時，於鋼料上施加一預定的磁場。該電磁攪拌係從減少 -20- (18) 1326714 氣孔之觀點上進行，從而預防缺陷及提供良好的表面。結合電磁攪拌之鋼料製造非常有用於製造大尺寸且的MnS夾雜物且可用於防止氣孔之形成。要在電磁中施用的磁場較佳爲具有約1〇〇至約500高斯之強度。場強度低於1〇〇高斯，則可能無法展現出電磁攪拌之。相較之下，若其超過500高斯，則在連續鑄造模內融鋼料可能以過高的速率流動，且該鋼料可能包含模促使鑄造困難。下面要參照數個實施例進一步詳細闡明本發明。意的是，無論如何，下面的實施例絕無意用來限制本之範圍，除非脫離本發明之精神及範圍，否則可有各改及變異，且這些修改及變異都包括在本發明之技術內。【實施方式】 [實施例] 使用一個3-噸感應電爐，一個100-噸轉爐，及一括澆桶的熔融鋼料精煉設施，製造具有各種含量的， ’ Si、Mn、S、A1、及N之一系列熔融鋼料。在此程，經由分別改變要添加的Fe-Mn合金及Fe-S合金內及鋁含量，來調整矽及鋁含量。於緊接在鑄造入預定具內之前，使用游離氧探針（probe)( Heraeus Electro 在"HYOP 10A-C150"商品名稱下之產品）測定在所得鋼料內之游離氧含量，且將它們定義爲游離氧含量。品質球形攪拌若磁效用之熔粉而應注發明種修範圍個包例如序中之矽的模 -Nite 熔融 -21 - (19) 1326714 使用一具有3〇〇毫米寬及430毫米長之斷面尺寸的鑄模 (坯軋型）之對熔融鋼料施以連續鑄造。或者，在3-噸感應電爐內予以鑄造，其中使用一具有3 〇〇毫米寬及430毫米長之斷面尺寸的鑄鐵模，該模係經設計成達到如在坯軋小鋼胚中之冷卻速率。於需要的情況中，在鑄造期間施加一磁場於該鑄模中以進行電磁攪拌。自所得小鋼胚或鑄錠之表面層中的淬火部份採取樣品且予化學分析以測定它們的化學組成。結果顯示於下表1 中〇 1326714Pa represents the external pressure: pLgh represents the hydrostatic pressure; σ represents the interface energy between the liquid and the gas; "r" represents the bubble diameter. The inventors of the present invention investigated how the frequency of occurrence of stomata varies according to the total partial pressure (PC0 + PN2) calculated by the above-described methods having such physical significance. As a result, they found that when the partial pressure sum (PC0 + PN2) exceeded 1.2 atm (atm), pores appeared. The inventor of the present invention tried to convert the partial pressure sum (PC0 + PN2) into an index. The carbon and nitrogen contents [C] and [N] can be easily determined by on-line analysis, but the free oxygen content must be measured using a free oxygen analyzer. It may be accompanied by large errors in some measurement procedures. The inventors of the present invention explored which influences the free oxygen content [〇] and found that the manganese content [Μη] and the sulfur content [S] affect the free oxygen content [〇]. This is also apparent from the fact that oxygen forms MnO-MnS oxide-sulfide inclusions in the molten steel. This shows that the formation of pores can be expressed by the relationship between [C], [Mn], [S], and [N]. Further, manganese and sulfur contents [Μη] and [S] are related, wherein the [Mn]/[S] ratio is 3 to 4. From this correlation, the formation of pores can be expressed by the correlation between [C], [Μη], and [N]. Under these assumptions, the right-hand side enthalpy of equation (7) and the content data such as [Μη] have been experimentally shown to have Pco equal to l〇[C]x[MnrQ94, since equation (7) shows that Pco is proportional to [C] and [0 ], and [〇] is mutated according to [Μη]. The square of the right hand side of equation (12) and the content data such as [Ν] shows that the partial pressure of nitrogen ΡΝ 2 is equal to 1 226 [Ν] 2 because equation (12) shows that ΡΝ: ^ is proportional to [ν] 2 . -18- (16) (16) 1326714 Under the increase of the partial pressure of carbon monoxide and nitrogen Pco + PN2 (l〇[C]x[Mn]-Q 94+ 1 22 6 [N]2), the pores appear and This causes surface defects. The formation of pores naturally affects the roughness of the finished surface. The partial pressure sum of PCO + Pn2 of carbon monoxide and nitrogen and the roughness of the finished surface have a relationship as shown in Fig. 1 mentioned later. Figure 1 shows that from the formation of surface defects and the roughness of the finished surface, the partial pressure of the partial pressure is about 1.2. The low carbon re-vulcanized free cutting steel of the present invention comprises the above components, and the remaining components are substantially iron. . However, in addition to these components, they may further contain minor components, and further inclusion of such minor components is also included in the scope of the present invention. The low carbon re-vulcanized free-cutting steel of the present invention contains unavoidable impurities such as Cu, Sn, and Ni, and such unavoidable impurities are acceptable to those who do not adversely affect the advantages of the present invention. The low carbon re-vulcanized free-cutting steel of the present invention may preferably have a content of soluble nitrogen of (1) 0.002% to 0.02% and/or (2) 0.02% or less (excluding 〇%), as needed. The total content of at least one of the group consisting of Ti, Cr, Nb, V, Zr, and B. The reasons for setting these specific ranges are as follows. Soluble Nitrogen Content: 0.002% to 0.02% As mentioned above, soluble nitrogen in the steel affects the formation of fine cracks, and a free-cutting steel with good machinability can be achieved by appropriately controlling the content of soluble nitrogen. In order to exhibit these advantages, the soluble nitrogen content in the steel is preferably controlled to 〇 · 〇〇 2 % or more. However, if it exceeds 〇〇 2 %, -19- (17) (17) 1326714 surface defects may increase. The total content of at least one element selected from the group consisting of Ti, Cr, Nb, V, Zr, and B: 0 · 0 2 % or lower (except 0%). These elements combine with nitrogen to form nitrogen. The compound, and if its content is too high, the soluble nitrogen content will become too small and lower than the desired soluble nitrogen content. From this point of view, the total content of these elements is preferably controlled to 0.0 2 % or less. The low carbon re-vulcanized free cutting steel of the present invention is basically produced by a continuous casting method. They can be manufactured in particular, for example, according to the following procedure. Initially, carbon is discharged to a carbon content of 0.04% or less in a converter to impart a high free oxygen content (soluble oxygen content) to the molten steel. Here, the free oxygen content is preferably 500 ppm or more. Next, alloys such as Fe-Mn alloy and Fe-S alloy are added at the time of tapping. These alloys contain impurities such as barium and aluminum. Niobium and aluminum can be oxidized to 3丨02 and Al2〇3 by adding these alloys to the high-oxygen molten steel when iron is discharged from the converter. They float and separate into the slag during the subsequent refining process of the molten steel. In this way, the ruthenium and aluminum retained in the steel will be reduced to the target content. It is important in this process that 70% or more of the added components such as Fe-Mn alloy and Fe-S alloy are added to adjust the chemical composition to be added to reduce aluminum and bismuth when tapping from the converter, and The remainder (30% or less) is added to the refining process in the ladle of molten steel. By performing such procedures, the steel material may have a niobium content of 0.004% or less. In the manufacture of steel materials, electromagnetic stirring is preferably carried out in which a predetermined magnetic field is applied to the steel material during casting. The electromagnetic stirring is performed from the viewpoint of reducing the pores of -20-(18) 1326714, thereby preventing defects and providing a good surface. The steel material combined with electromagnetic stirring is very useful for manufacturing large-sized and MnS inclusions and can be used to prevent the formation of pores. The magnetic field to be applied in the electromagnetic is preferably of a strength of from about 1 Torr to about 500 gauss. If the field strength is less than 1 〇〇 Gauss, it may not exhibit electromagnetic stirring. In contrast, if it exceeds 500 gauss, the molten steel may flow at an excessive rate in the continuous casting mold, and the steel may contain a mold to facilitate casting. The invention will now be explained in further detail with reference to a number of examples. It is intended that the following examples are not intended to limit the scope of the invention, and that modifications and variations may be included in the teachings of the present invention without departing from the spirit and scope of the invention. . [Embodiment] [Examples] A 3-ton induction furnace, a 100-ton converter, and a molten steel refining facility including a ladle were fabricated to produce various contents, 'Si, Mn, S, A1, and N. A series of molten steel. In this process, the niobium and aluminum contents were adjusted by changing the contents of the Fe-Mn alloy and the Fe-S alloy to be added and the aluminum content, respectively. The free oxygen content in the resulting steel is measured using a free oxygen probe (here by Heraeus Electro under the trade name "HYOP 10A-C150" under the trade name) and placed before the casting into the predetermined tool. Defined as the free oxygen content. Quality spherical agitation, if the powder is used for magnetic effect, should be invented. The package of the invention is in the form of a mold, such as the mold of the order - Nite Melt-21 - (19) 1326714 A section with a width of 3 mm and a length of 430 mm is used. The size of the mold (blank rolling type) is applied to the molten steel by continuous casting. Alternatively, casting in a 3-ton induction furnace using a cast iron mold having a section width of 3 mm and a length of 430 mm, which is designed to achieve cooling as in a billet mill rate. Where necessary, a magnetic field is applied to the mold during casting for electromagnetic stirring. Samples were taken from the quenched portion of the surface layer of the resulting small steel or ingot and chemically analyzed to determine their chemical composition. The results are shown in Table 1 below 〇 1326714

[Mn]/[S] 〇 rn rn rS - iTi VO rn in cn cn cn ITi cn VO rn rn 卜 cn rn cn rn CO m m 化學組成(質量％) 其他組分 Ti:0.06, Cr:0.005, Zr:0.005, V:0.005 1 1 1 O o g o 1 g o o U s" o o Ti:0.002, Cr:0.006 S 〇〇 N cn o o 〇 cn o o 1 s o o u <N o o 1 1 〇〇 U O o Ti:0.003, Cr:0.005, Nb: 0.001 1 S O o U o o Ti:0.003,Cr:0.005,Nb: 0.001 Ti:0.003, Cr:0.004 1 Ti:0.002, Cr:0.004 £ 咖 1 1 1 1 0.300 1 1 1 1 1 1 1 1 1 1 1 I 1 1 1 :0.0124 1_ _______ ! 0.0160 0.0189 1 I 0.0210 I | 0.0241 | 0.0070 0.0186 | I 0.0173 | 0.0234 | 0,0150 I 0.0125 0.0180 0.0143 0.0156 0.0135 0.0155 0.0135 0.0167 0.0165 0.0205 0.0155 游離氧 0.0066 1 1 0.0054 1 1 0.0046 1 I 0.0040 | | 0.0029 | 0.0049 | 0.0036 | | 0.00321 | 0.0027 | | 0.0046 | | 0.0040 | 0.0036 | 0.0032 | 0.0029 ! 0.0027 0.0046 i 0.0040 0.0036 0.0032 0.0029 0.0027 i 總氧 0.0284 i 0.0223 1 0.0185 1 0.0160 |〇.0114| 0.0208 | 0.0168 1 |〇.0155 | 丨 0.0111 | 0.0198 1 0.0181 0.0146 0.0125 0.0117 0.0104 0.0222 0.0192 0.0160 0.0145 0.0135 0.0111 < 0.003 0.001 1 0.002 1 I 0.002 I | 0.003 | 0.001 | 0.001 | I 0.003 I I 0.009 I 1 o.ooi I I 0.002 | 0.004 | 0.002 | 0.005 | Γα〇〇3 0.001 ! 0.001 0.002 0.003 0.002 0.001 0.30 1 0.32 I 0.36 1 0.42 0.59 | 0.31 i 0.49 | 1 0.53 | I 0.65 I | 0.37 | 0.45 | 0.50 0.55 | 0.59 i 0.65 0.35 0.43 0.48 0.55 0.61 0.70 0.080 0.087 0.081 0.086 ! 0.079 1 0.082 ! 0.080 | | 0.081 | | 0,082 | | 0.079 | | 0.081 | 0.084 0.077 0.079 i 0.075 0.083 0.077 0.083 0.079 0.078 0.077 C S 〇\ ο rn CN Os m (N T— m 卜 On (N fN Os ri tN 0.002 0.002 0.001 0.003 0.007 0.003 0.003 1 0.004 | ,0.007 | 0.003 | m o o | 0.004 | | 0.003 I 0.003 I 0.004 0.002 0.001 1 0.003 I 1 o.ooi ! | 0.004 | 0.003 〇 o.io 1 〇 o o 0.11 0.08 on 1 0.12 | o 1 o.io | 1 o.io | | 0.08 | 0.08 1 0.08 I 0.08 0.10 i 0.08 0.09 ; 0.10 0.10 1 ο-is 樣品編號 — <N m 寸卜〇〇 On o (N 寸 VO 卜〇〇 Os (N -23- (21) (21)1326714 將所得小鋼胚和鑄錠在1 25 0 °C下加熱1小時，施以坯軋到155毫米寬及155毫米長之斷面尺寸，輥軋到25毫米之直徑，施以酸洗以產生具有22毫米直徑之冷完工鋼條，及施以切削試驗。其中之輥軋係在1 000 °C下進行且以約1.5 eC 每秒之冷卻速率將軋鋼由800°C強迫地冷卻到5 00°C。使用幅射高溫計測定鋼料之溫度。測量該鋼料的可溶性氮含量且在下列條件下施以切削試驗。根據下列準則評定切削試驗後該鋼料之完工表面及表面缺陷。 [可溶性氮含量之測定] 可溶性氮之含量係經測定爲在總氮與在化合物內的氮之間的差値。根據一種使用惰性氣體熔化熱之傳導係數的方法測定總氮，且經由含有10%乙醯丙酮及1%氯化四甲銨之甲醇溶液溶解及萃取一樣品，透過1-微米過濾器收集氮且使用靛酚-吸光計測定氮而測定化合物內的氮。 [切削試驗條件] 工具：局速工具鋼SKH4A 切削速率：100米/分鐘進料比例：0.01毫米每轉切削深度：〇. 5毫米切削油：含氯-水不溶性切削油切削長度：5 00米 -24- (22) (22)1326714 [準則] 完工表面評定：根據JIS B 060 1 (2001 )的最大不平整高度Rz評定表面粗糙度。表面缺陷評定：具有155毫米寬及155毫米長之斷面尺寸的坯軋樣品鋼坯上之表面缺陷係使用一自動缺陷偵測器偵測。將自動缺陷偵測器沒有偵測到缺陷的樣品評定爲" 好"：將具有可由加工移除的若干缺陷之樣品評定爲"普通且將具有不可移除的缺陷之樣品評定爲"不及格"。切削試驗結果與其他資料，諸如式（1)左手邊値及磁場之強度，都顯不在下表2中。[Mn]/[S] 〇rn rn rS - iTi VO rn in cn cn cn ITi cn VO rn rn 卜 rn cn rn CO mm Chemical composition (% by mass) Other components Ti: 0.06, Cr: 0.005, Zr: 0.005, V:0.005 1 1 1 O ogo 1 goo U s" oo Ti:0.002, Cr:0.006 S 〇〇N cn oo 〇cn oo 1 soou <N oo 1 1 〇〇UO o Ti:0.003, Cr: 0.005, Nb: 0.001 1 SO o U oo Ti: 0.003, Cr: 0.005, Nb: 0.001 Ti: 0.003, Cr: 0.004 1 Ti: 0.002, Cr: 0.004 £ Coffee 1 1 1 1 0.300 1 1 1 1 1 1 1 1 1 1 1 I 1 1 1 :0.0124 1_ _______ ! 0.0160 0.0189 1 I 0.0210 I | 0.0241 | 0.0070 0.0186 | I 0.0173 | 0.0234 | 0,0150 I 0.0125 0.0180 0.0143 0.0156 0.0135 0.0155 0.0135 0.0167 0.0165 0.0205 0.0155 Free oxygen 0.0066 1 1 0.0054 1 1 0.0046 1 0.000 | | 0.0029 | 0.0049 | 0.0036 | | 0.00321 | 0.0027 | | 0.0046 | | 0.0040 | 0.0036 | 0.0032 | 0.0029 ! 0.0027 0.0046 i 0.0040 0.0036 0.0032 0.0029 0.0027 i Total Oxygen 0.0284 i 0.0223 1 0.0185 1 0.0160 |〇.0114| 0.0208 | 0.0168 1 |〇.0155 | 丨0.0111 | 0.0198 1 0.0181 0.0146 0.0125 0.0117 0.0104 0.0222 0.0192 0.0160 0. 0145 0.0135 0.0111 < 0.003 0.001 1 0.002 1 I 0.002 I | 0.003 | 0.001 | 0.001 | I 0.003 II 0.009 I 1 o.ooi II 0.002 | 0.004 | 0.002 | 0.005 | Γα〇〇3 0.001 ! 0.001 0.002 0.003 0.002 0.001 0.30 1 0.32 I 0.36 1 0.42 0.59 | 0.31 i 0.49 | 1 0.53 | I 0.65 I | 0.37 | 0.45 | 0.50 0.55 | 0.59 i 0.65 0.35 0.43 0.48 0.55 0.61 0.70 0.080 0.087 0.081 0.086 ! 0.079 1 0.082 ! 0.080 | | 0.081 | 0,082 | | 0.079 | | 0.081 | 0.084 0.077 0.079 i 0.075 0.083 0.077 0.083 0.079 0.078 0.077 CS 〇\ ο rn CN Os m (NT- m 卜 On (N fN Os ri tN 0.002 0.002 0.001 0.003 0.007 0.003 0.003 1 0.004 | , 0.007 | 0.003 | moo | 0.004 | | 0.003 I 0.003 I 0.004 0.002 0.001 1 0.003 I 1 o.ooi ! | 0.004 | 0.003 〇o.io 1 〇oo 0.11 0.08 on 1 0.12 | o 1 o.io | 1 o. Io | | 0.08 | 0.08 1 0.08 I 0.08 0.10 i 0.08 0.09 ; 0.10 0.10 1 ο-is Sample Number — <N m Inch On O (N Inch VO Divination Os (N -23- (21) (21) 1326714 The obtained small steel embryo and ingot were heated at 1500 ° C for 1 hour. Rolling the billet to cross-sectional dimensions 155 mm wide and 155 mm long, the roll diameter of 25 mm, subjected to pickling to produce steel strip having cold completed diameter of 22 mm, and subjected to a cutting test. The rolling was carried out at 1 000 ° C and the rolling of the steel was forcedly cooled from 800 ° C to 500 ° C at a cooling rate of about 1.5 eC per second. The temperature of the steel was measured using a radiant pyrometer. The soluble nitrogen content of the steel was measured and subjected to a cutting test under the following conditions. The finished surface and surface defects of the steel after the cutting test were evaluated according to the following criteria. [Measurement of Soluble Nitrogen Content] The content of soluble nitrogen was determined as the difference between total nitrogen and nitrogen in the compound. The total nitrogen is determined according to a method of melting the heat transfer coefficient by using an inert gas, and a sample is dissolved and extracted through a methanol solution containing 10% acetamidine acetone and 1% tetramethylammonium chloride, and nitrogen is collected through a 1-micron filter. The nitrogen in the compound was determined by measuring nitrogen using a phenol-absorber. [Cutting test conditions] Tool: Local speed tool steel SKH4A Cutting rate: 100 m / min Feed ratio: 0.01 mm Depth of cut per revolution: 〇. 5 mm cutting oil: Chlorine-water insoluble cutting oil Cutting length: 5000 m -24- (22) (22) 1326714 [Guidelines] Completion surface evaluation: The surface roughness was evaluated according to the maximum unevenness height Rz of JIS B 060 1 (2001). Surface Defect Assessment: Surface defects on billet samples with 155 mm wide and 155 mm long cross-section dimensions were detected using an automatic defect detector. A sample that does not detect a defect in the automatic defect detector is rated as "good": a sample with several defects that can be removed by processing is rated as "normal" and a sample with non-removable defects is rated as " Failed ". The results of the cutting test and other data, such as the left-hand side of the equation (1) and the strength of the magnetic field, are not shown in Table 2 below.

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z« 備註比較實施例比較實施例比較實施例比較實施例比較實施例參考例實施例實施例實施例實施例實施例實施例實施例實施例實施例實施例實施例實施例實施例實施例實施例表面缺陷不及格不及格不及格不及格不及格 & 普通普通普通 !& Ι& & & 完工面粗糙度Rz (微米） tn 寸〇卜〇〇 ΓΊ a 〇〇卜卜 μ CN 卜 ON Π*ί (Ν (Ν 溶解氮 (質量％) 0.0052 0.0138 0.0162 0.0183 0.0198 0.0036 0.0140 0.0130 0.0190 0.0120 0.0090 0.0155 0.0113 0.0120 | 0.0080 0.0128 0.0101 0.0120 0.0125 0.0181 0.0113 磁場 (高斯） 1 1 1 1 1 〇 1 1 1 〇〇〇〇〇〇 100 〇〇 500 500 500 500 500 式(1)之左手邊値 1.293 1.320 1.298 1.292 1.260 0.734 1.092 1.023 1.057 0.875 0.833 0.688 0.697 0.589 1.076 1 0.770 0.888 j 0.881 1.013 0.980 樣品編號 — ΓΟ 寸卜〇〇〇\ 〇 C^l 寸 VO 卜 00 os (Ν -26- (24) (24)1326714 這些結果顯示滿足本發明指定的要求之樣品（樣品編號7至21)具有小完工面粗糙度（最大不平整高度Rz)且展現良好切削性；而且，其中有接受電磁攪拌的樣品（樣品編號10至2 1)在由氣孔引起的表面缺陷方面有減少。相較之下，沒有滿足本發明規範的諸要求之至少一項的樣品皆在至少一種性質上爲不良者。圖1顯示完工面粗糙度（最大不平整高度Rz)如何依照式（1)左手邊値及磁場之存在或不存在而變異。【圖式簡單說明】圖1係一圖表，顯示完工面粗糙度（最大不平整高€度 Rz)根據式（1)之左手邊値及磁場之存在或不存在而變異。Z« Remarks Comparative Embodiments Comparative Embodiments Comparative Embodiments Comparative Embodiments Comparative Embodiments Reference Examples Embodiments Examples Embodiments Examples Embodiments Examples Embodiments Examples Embodiments Examples Examples Examples Facial failure, failing, failing, failing, failing, &&&&&&&&&&&&&&&&&&&&&&&&< Π*ί (Ν (Ν Dissolved nitrogen (% by mass) 0.0052 0.0138 0.0162 0.0183 0.0198 0.0036 0.0140 0.0130 0.0190 0.0120 0.0090 0.0155 0.0113 0.0120 | 0.0080 0.0128 0.0101 0.0120 0.0125 0.0181 0.0113 Magnetic field (Gauss) 1 1 1 1 1 〇1 1 1 〇〇〇〇〇〇100 〇〇500 500 500 500 500 Type (1) Left-hand side 値 1.293 1.320 1.298 1.292 1.260 0.734 1.092 1.023 1.057 0.875 0.833 0.688 0.697 0.589 1.076 1 0.770 0.888 j 0.881 1.013 0.980 Sample No. — ΓΟ 〇〇〇\ 〇C^l inch VO 00 os (Ν -26- (24) (24)132 6714 These results show that samples satisfying the requirements specified in the present invention (sample Nos. 7 to 21) have small finish surface roughness (maximum unevenness height Rz) and exhibit good machinability; and, among them, samples subjected to electromagnetic stirring (sample number) 10 to 2 1) There is a reduction in surface defects caused by pores. In contrast, samples which do not satisfy at least one of the requirements of the specification of the present invention are inferior in at least one of the properties. Figure 1 shows the finished surface Roughness (maximum uneven height Rz) varies according to the presence or absence of the left-hand side 値 and the magnetic field of equation (1). [Simplified illustration] Figure 1 is a diagram showing the roughness of the finished surface (maximum unevenness) The degree Rz) varies according to the presence or absence of the left-hand side of the formula (1) and the magnetic field.

-27--27-

Claims

1326714 Π) X. Patent application scope 1. A low-carbon re-vulcanized free-cutting steel with excellent machinability, which comprises: 0.02 to 0.15 mass% of carbon (C); 0.004 mass% or less (excluding 0%)矽 (Si); 0.6 to 3% by mass of manganese (Μη); 0.02 to 0_2% by mass of phosphorus (Ρ): 0.35 to 1% by mass of sulfur (S); 0.0 05% by mass or less (excluding 0) %) aluminum (Α1); 0.008 to 0.03 mass% oxygen (0); and 0.007 to 0.03 mass% nitrogen (Ν), the balance being iron and unavoidable impurities; wherein the manganese content [Μη] versus sulfur content [ The ratio [Mn]/[S] of S] is in the range of 3 to 4, and the carbon content [C], the manganese content [Μη], and the nitrogen content [N] satisfy the following formula (1): 10 [ C]x[Mn]'094 + 1226 [N] 2^1.2 (1) wherein [C], [Μη] and [N] represent the contents of carbon, manganese and nitrogen, respectively, based on the mass percentage. 2. Low carbon re-vulcanized free-cutting steel according to item 1 of the patent application, wherein the content of soluble nitrogen is 0.002 to 0.02% by mass. 3 Low carbon re-vulcanized free-cutting steel according to item 2 of the patent application, which is selected from the group consisting of Ti, Cr, Nb, V, Zr, and Β to -28- (2) (2) 1326714

The total content of one less element is 0.02% by mass or less. 4. The low carbon re-vulcanized free-cutting steel according to any one of claims 1 to 3, which is a product subjected to electromagnetic stirring, wherein the electromagnetic stirring system applies a magnetic field of 1 〇〇 to 500 gauss during casting . -29-