TWI665310B - Carburizing steel sheet and manufacturing method of carburizing steel sheet - Google Patents

Carburizing steel sheet and manufacturing method of carburizing steel sheet Download PDF

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TWI665310B
TWI665310B TW107134864A TW107134864A TWI665310B TW I665310 B TWI665310 B TW I665310B TW 107134864 A TW107134864 A TW 107134864A TW 107134864 A TW107134864 A TW 107134864A TW I665310 B TWI665310 B TW I665310B
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steel sheet
carburizing
carbides
average
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TW202014526A (en
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匹田和夫
戶田由梨
橋本元仙
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日商新日鐵住金股份有限公司
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Abstract

課題：提供一種成形性及滲碳後之韌性較為優異的滲碳用鋼板及其製造方法。
解決手段：一種鋼板，以質量%計含有C：0.02%以上且小於0.30%、Si：0.005%以上且0.5%以下、Mn：0.01%以上且3.0%以下、P：0.1%以下、S：0.1%以下、sol.Al：0.0002%以上且3.0%以下、N：0.0001以上且0.035%以下，剩餘部分由Fe及不純物所構成；肥粒鐵的平均結晶粒徑小於10μm；碳化物的平均等效圓直徑為5.0μm以下；相對於總碳化物，縱橫比2.0以下之碳化物個數比率為80%以上；相對於總碳化物，存在於肥粒鐵結晶粒內之碳化物個數比率為60%以上；從鋼板最外表面起算在深度方向上至50μm為止的區域中，平均氮濃度為0.040質量%以上且0.200質量%以下。 Problem: To provide a steel sheet for carburizing, which has excellent formability and toughness after carburizing, and a method for manufacturing the same.
Solution: A steel sheet containing C: 0.02% or more and less than 0.30% by mass%, Si: 0.005% or more and 0.5% or less, Mn: 0.01% or more and 3.0% or less, P: 0.1% or less, and S: 0.1 % Or less, sol.Al: 0.0002% or more and 3.0% or less, N: 0.0001% or more and 0.035% or less, the remainder is composed of Fe and impurities; the average grain size of the ferrous iron is less than 10 μm; the average equivalent of carbides The diameter of the circle is 5.0 μm or less; the ratio of the number of carbides with an aspect ratio of 2.0 or less is 80% or more with respect to the total carbides; the ratio of the number of carbides present in the iron crystal grains of the fertilizer particles is 60 with respect to the total carbide % Or more; in the region from the outermost surface of the steel plate in the depth direction to 50 μm, the average nitrogen concentration is 0.040% by mass or more and 0.200% by mass or less.

Description

滲碳用鋼板及滲碳用鋼板的製造方法Carburizing steel sheet and manufacturing method of carburizing steel sheet

發明領域
本發明是有關於一種滲碳用鋼板及滲碳用鋼板的製造方法。 FIELD OF THE INVENTION The present invention relates to a steel sheet for carburizing and a method for manufacturing a steel sheet for carburizing.

發明背景
近年來，對於汽車齒輪、離合器板、阻尼器等的機械構造零件，除了耐久性高之外，還要求要能廉價製造。一般而言，作為此等零件的製造方法，是施行了使用熱鍛造材的切削及滲碳處理。然而，受到降低成本要求的日漸高漲，下述技術的開發正逐漸進展：以熱輥軋鋼板、冷輥軋鋼板作為素材，並在進行冷加工而成形為構件形狀後，進行滲碳處理。在冷加工中，會對素材進行衝孔，接著經過彎曲加工、引伸加工、擴孔加工等來成形出構件。此時，就加工所使用的滲碳用鋼板而言，會要求最基本的變形模式即彎曲性要為良好。更甚者，就扭矩轉換器(torque converter)之阻尼器等這類汽車零件而言，則會要求韌性等的優異耐衝撃特性。由這種觀點出發，近年來已提出了各種技術。 BACKGROUND OF THE INVENTION In recent years, in addition to high durability, mechanical structural parts such as automobile gears, clutch plates, and dampers have been required to be inexpensively manufactured. Generally, as a method of manufacturing these parts, cutting and carburizing using a hot-forged material are performed. However, due to the increasing demand for cost reduction, the development of the following technologies is gradually progressing: hot-rolled steel sheets and cold-rolled steel sheets are used as materials, and after cold working to form the shape of the components, carburizing treatment is performed. In the cold working, the material is punched, and then the member is formed by bending, drawing, expanding, etc. At this time, the carburizing steel sheet used for processing is required to have good bendability, which is the most basic deformation mode. Furthermore, automotive parts such as torque converters and torque converters require excellent impact resistance such as toughness. From this viewpoint, various technologies have been proposed in recent years.

例如，在以下的專利文獻1中提出了一種技術，是由肥粒鐵與波來鐵來構成熱輥軋鋼板的組織，之後，施行球狀化退火來將碳化物球狀化。For example, the following patent document 1 proposes a technique in which the structure of a hot rolled steel sheet is composed of ferritic iron and boron iron, and then spheroidizing annealing is performed to spheroidize carbides.

另外，在以下的專利文獻2中則提出了下述技術：除了控制碳化物粒徑之外，還控制肥粒鐵晶界碳化物個數相對於肥粒鐵晶粒內碳化物個數的比率，並進一步控制母相即肥粒鐵其結晶粒徑，藉此提升構件滲碳後的衝撃特性。In addition, in the following Patent Document 2, the following technology is proposed: in addition to controlling the carbide particle size, the ratio of the number of carbides in the grain boundary of the ferrous grains to the number of carbides in the grains of the ferrous grains is also controlled. , And further control the crystal grain size of the parent phase, ie, ferrous iron, thereby improving the flushing characteristics of the component after carburizing.

又，在以下的專利文獻3中提出下述技術：控制碳化物之粒徑及縱橫比和控制母相即肥粒鐵其結晶粒徑之外，還進一步控制肥粒鐵的縱橫比，藉此提升冷加工性。In addition, in the following Patent Document 3, the following techniques are proposed: in addition to controlling the particle size and aspect ratio of carbides and controlling the crystal grain size of ferrous iron, which is the parent phase, the aspect ratio of ferrous iron is further controlled, thereby improving Cold workability.

先前技術文獻
專利文獻
專利文獻1：日本特許第3094856號公報
專利文獻2：國際公開第2016/190370號
專利文獻3：國際公開第2016/148037號 Prior Art Literature Patent Literature Patent Literature 1: Japanese Patent No. 3094856 Patent Literature 2: International Publication No. 2016/190370 Patent Literature 3: International Publication No. 2016/148037

發明概要 Summary of invention         

就上述那種機械構造零件來說，為了提高強度而對淬火性有所要求。也就是說，就機械構造零件所使用的素材而言，會要求在維持淬火性的同時也要確保成形性。更甚者，對於滲碳後的機械構造零件而言，會要求耐衝撃特性(特別是滲碳後之韌性)。 In the above-mentioned mechanical structural parts, hardenability is required in order to improve the strength. That is to say, the materials used for mechanical structural parts are required to maintain formability while maintaining hardenability. What's more, for mechanical structural parts after carburizing, impact resistance characteristics (especially toughness after carburizing) are required.       

然而，在上述專利文獻1以控制碳化物的顯微組織為主體的製造方法中，雖能改善耐衝撃特性，且該耐衝撃特性是以冷加工所導入之龜裂為起點，但是，卻無法期待關於提升滲碳後之韌性的效果。再者，上述專利文獻2提出以控制碳化物及肥粒鐵之顯微組織為主體的製造方法中，成形性雖獲改善，但在適用到汽車扭矩轉換器之阻尼器等這類耐衝撃性被要求到高等級的特定汽車零件時，要獲得更為優異的韌性，仍有改善餘地。更甚者，使用上述專利文獻3所提出的技術雖能改善成形性，但在適用到汽車扭矩轉換器之阻尼器等這類耐衝撃性被要求到高等級的特定汽車零件時，要獲得更為優異的韌性，仍有改善餘地。如此一來，在習知所提案的技術中，要想擔保滲碳用鋼板的成形性、淬火性同時還獲得滲碳後充分的韌性，仍是有改善餘地，因此，尤其對於扭矩轉換器之阻尼器零件等這類耐衝撃性被要求到高等級的特定汽車零件而言，期許一種能更適合使用的滲碳用鋼板。However, in the above-mentioned Patent Document 1, a manufacturing method mainly focusing on controlling the microstructure of carbides, although the impact resistance characteristics can be improved, and the impact resistance characteristics are based on cracks introduced by cold working, it cannot be expected. The effect of improving toughness after carburizing. Furthermore, the above-mentioned Patent Document 2 proposes a manufacturing method that mainly controls the microstructure of carbides and ferrous iron. Although the formability is improved, the impact resistance such as a damper applied to an automotive torque converter is improved. When it comes to high-grade specific automotive parts, there is still room for improvement in order to obtain more excellent toughness. Furthermore, although the technology proposed in the aforementioned Patent Document 3 can improve the formability, when the impact resistance such as a damper applied to an automotive torque converter is required to a high level of specific automotive parts, it is necessary to obtain more For excellent toughness, there is still room for improvement. As such, there is still room for improvement in the conventionally proposed technology to ensure the formability and hardenability of the carburizing steel sheet and to obtain sufficient toughness after carburizing. Therefore, especially for torque converters, Carburizing steel sheets are expected to be more suitable for specific automotive parts where shock resistance such as damper parts is required to a high level.

於是，本發明即是鑑於上述問題所完成者，本發明之目的在於，提供一種成形性及滲碳後之韌性較為優異的滲碳用鋼板及其製造方法。Then, this invention is made in view of the said subject, and an object of this invention is to provide the steel plate for carburizing which is excellent in formability and toughness after carburizing, and its manufacturing method.

本案發明人等就解決上述課題之方法，進行了精心探討。結果獲得下述構想：如以下所詳述，透過適切控制碳化物朝肥粒鐵結晶粒中生成的位置、與適切控制鋼板表層部中的氮濃度，藉此可在維持淬火性的同時，提升冷加工時之成形性與滲碳後之韌性，終至完成本發明。The inventors of the present case have carefully studied the methods to solve the above problems. As a result, as described in detail below, by appropriately controlling the position where carbides are generated in the iron crystal grains of the fertilizer grains and by appropriately controlling the nitrogen concentration in the surface layer portion of the steel sheet, the hardenability can be improved while maintaining the hardenability. The formability during cold working and the toughness after carburizing have finally completed the present invention.

基於這種構想所完成的本發明，其要旨如下所述。 The gist of the present invention completed based on this concept is as follows.

[1]一種滲碳用鋼板，以質量%計含有C：0.02%以上且小於0.30%、Si：0.005%以上且0.5%以下、Mn：0.01%以上且3.0%以下、P：0.1%以下、S：0.1%以下、sol.Al：0.0002%以上且3.0%以下、N：0.0001%以上且0.035%以下，剩餘部分由Fe及不純物所構成；肥粒鐵的平均結晶粒徑小於10μm；碳化物的平均等效圓直徑為5.0μm以下；相對於總碳化物，縱橫比2.0以下之碳化物個數比率為80%以上；相對於總碳化物，存在於肥粒鐵結晶粒內之碳化物個數比率為60%以上；從鋼板最外表面起算在深度方向上至50μm為止的區域中，平均氮濃度為0.040質量%以上且0.200質量%以下。 [1] A steel sheet for carburizing, containing C: 0.02% or more and less than 0.30%, Si: 0.005% or more and 0.5% or less, Mn: 0.01% or more and 3.0% or less, P: 0.1% or less, S: 0.1% or less, sol.Al: 0.0002% or more and 3.0% or less, N: 0.0001% or more and 0.035% or less, the remainder is composed of Fe and impurities; the average grain size of ferrous iron is less than 10 μm; carbides The average equivalent circle diameter is less than 5.0 μm; the ratio of the number of carbides with an aspect ratio of 2.0 or less is 80% or more with respect to the total carbides; the number of carbides present in the iron crystal grains of the fertilizer with respect to the total carbides The number ratio is 60% or more; in the region from the outermost surface of the steel plate in the depth direction to 50 μm, the average nitrogen concentration is 0.040% by mass or more and 0.200% by mass or less.

[2]如[1]所記載之滲碳用鋼板，其以質量%計更含有下述之1種或2種以上來取代剩餘部分之Fe的一部分：Cr：0.005%以上且3.0%以下、Mo：0.005%以上且1.0%以下、Ni：0.010%以上且3.0%以下、Cu：0.001%以上且2.0%以下、Co：0.001%以上且2.0%以下、Nb：0.010%以上且0.150%以下、Ti：0.010%以上且0.150%以下、V：0.0005%以上且1.0%以下、B：0.0005%以上且0.01%以下。 [2] The steel sheet for carburizing according to [1], further comprising, in mass%, one or two or more of the following to replace part of the remaining Fe: Cr: 0.005% or more and 3.0% or less, Mo: 0.005% or more and 1.0% or less, Ni: 0.010% or more and 3.0% or less, Cu: 0.001% or more and 2.0% or less, Co: 0.001% or more and 2.0% or less, Nb: 0.010% or more and 0.150% or less, Ti: 0.010% or more and 0.150% or less, V: 0.0005% or more and 1.0% or less, and B: 0.0005% or more and 0.01% or less.

[3]如[1]或[2]所記載之滲碳用鋼板，其以質量%計更含有下述之至少一者來取代剩餘部分之Fe的一部分：W：1.0%以下、Ca：0.01%以下。 [3] The steel sheet for carburizing as described in [1] or [2], further comprising at least one of the following in place of a part of Fe in mass%: W: 1.0% or less, Ca: 0.01 %the following.

[4]一種滲碳用鋼板的製造方法，是製造如[1]~[3]中任1項所記載之滲碳用鋼板的方法，包含：熱輥軋步驟，是將具有如[1]~[3]中任1項所記載之化學組成的鋼材進行加熱，在800℃以上且小於920℃之溫度區域結束熱精輥軋，並在700℃以下之溫度進行捲取；退火步驟，是將經前述熱輥軋步驟而得的鋼板、或將前述熱輥軋步驟後施行過冷輥軋的鋼板，透過已將氮濃度控制在體積分率計25%以上之氣體環境，以5℃/h以上且100℃/h以下之平均加熱速度，加熱至下述式(1)所定義的Ac 1點以下之溫度區域為止，並施行在該Ac 1點以下之溫度區域保持10h以上且100h以下的退火處理後，施行下述冷卻，該冷卻是自退火結束時之溫度起至550℃為止之溫度區域的平均冷卻速度設為5℃/h以上且100℃/h以下；在前述熱輥軋步驟中，是自前述熱精輥軋結束時起算1秒以內，開始進行平均冷卻速度大於50℃/s的冷卻；將前述退火處理後的肥粒鐵平均粒徑控制在小於10μm。
[5]如[4]所記載之滲碳用鋼板的製造方法，其在用以獲得供給至前述熱輥軋步驟之前述鋼材的連續鑄造步驟中，施行鋼材健全化處理；該鋼材健全化處理是生成預定夾雜物、或降低預定元素中心偏析的處理之至少一者。 [4] A method for manufacturing a steel sheet for carburizing, which is a method for manufacturing a steel sheet for carburizing as described in any one of [1] to [3], including a hot rolling step, and The steel with the chemical composition described in any one of ~ [3] is heated, hot finish rolling is finished in a temperature range of 800 ° C to less than 920 ° C, and coiling is performed at a temperature of 700 ° C or less; the annealing step is The steel plate obtained through the hot rolling step or the super cold rolling steel plate after the hot rolling step is passed through a gas environment in which the nitrogen concentration has been controlled to 25% or more by volume fraction, at 5 ° C / The average heating rate above h and 100 ° C / h is heated to a temperature range below Ac 1 point defined by the following formula (1), and it is maintained in the temperature range below Ac 1 point for 10 hours to 100 hours. After the annealing treatment, the following cooling is performed. The average cooling rate in the temperature range from the temperature at the end of the annealing to 550 ° C is set to 5 ° C / h to 100 ° C / h; in the aforementioned hot rolling In the step, the average cooling rate is started within 1 second from the end of the hot finish rolling. Greater than 50 ℃ / s cooling; and annealing the ferrite average particle size after treatment controlled to less than 10μm.
[5] The method for manufacturing a steel sheet for carburizing according to [4], wherein in the continuous casting step for obtaining the steel material supplied to the hot-rolling step, the steel material is subjected to a tempering treatment; It is at least one of a process for generating a predetermined inclusion or reducing a center segregation of a predetermined element.

[數學式1]
[Mathematical formula 1]

此處，在上述式(1)中，[X]之標記是表示元素X的含量(單位：質量%)，不含該元素時則代入零。Here, in the above formula (1), the mark of [X] indicates the content (unit: mass%) of the element X, and when the element is not included, zero is substituted.

如同以上說明，依照本發明，就能提供一種成形性及滲碳後之韌性較為優異的滲碳用鋼板。 As described above, according to the present invention, it is possible to provide a steel sheet for carburizing which is excellent in formability and toughness after carburizing.       

以實施發明之形態
以下，詳細說明本發明適宜的實施形態。 Mode for Carrying Out the Invention Hereinafter, preferred embodiments of the present invention will be described in detail.

(關於本案發明人等進行檢討的內容及所獲得的構想)
在說明本發明之滲碳用鋼板及其製造方法之前，先就本案發明人等為了解決上述課題進行檢討之內容，詳細說明如下。
在進行上述檢討時，本案發明人等首先針對用以提升滲碳前的成形性(尤其是彎曲性)的方法進行檢討。 (About the content of the review by the inventors of this case and the ideas obtained)
Before describing the steel sheet for carburizing of the present invention and the manufacturing method thereof, the content of the review by the inventors of the present invention to solve the above problems will be described in detail as follows.
When conducting the above-mentioned review, the inventors of the present case first reviewed a method for improving formability (particularly bendability) before carburizing.

為了提升滲碳前的成形性(尤其是彎曲性)，下述事項很重要：抑制彎曲變形時產生龜裂，此外，於產生龜裂時，抑制所產生龜裂的伸展。為了抑制龜裂產生，控制鋼板中所生成之碳化物的縱橫比(長軸/短軸)一事很有效，且藉由球狀化退火來降低碳化物的縱橫比一事很重要。又，為了龜裂的伸展，抑制粗大碳化物生成的同時控制碳化物析出位置一事很有效。也就是說，當肥粒鐵的晶界生成碳化物時，以晶界作為傳遞路徑的龜裂其伸展便會獲得助長。因此，使碳化物在肥粒鐵結晶粒內生成一事很重要。吾人認為，透過使碳化物在肥粒鐵結晶粒內生成，便可抑制龜裂在晶界傳遞。In order to improve the formability (especially bendability) before carburizing, it is important to suppress the occurrence of cracks during bending deformation, and to suppress the extension of the cracks generated when cracks occur. In order to suppress the occurrence of cracks, it is effective to control the aspect ratio (long axis / short axis) of carbides generated in the steel sheet, and it is important to reduce the aspect ratio of carbides by spheroidizing annealing. In addition, in order to extend the crack, it is effective to control the carbide precipitation position while suppressing the formation of coarse carbides. In other words, when carbides are formed at the grain boundaries of ferrous iron, cracks whose grain boundaries are used as a transmission path will be promoted to expand. Therefore, it is important to generate carbides in the iron crystal grains of the fertilizer. I believe that by making carbides in the iron crystal grains of the fertilizer grains, the propagation of cracks at the grain boundaries can be suppressed.

本案發明人等實施了如上所述的組織控制之外，更著眼於一種能提升滲碳後耐衝撃特性的方法，透過滲碳用鋼板表層的氮變濃來提升韌性，並針對鋼板表層中氮變濃所帶來的作用效果進行了詳細調査及研究。結果發現，透過控制鋼板表層之氮濃度，則滲碳後之韌性(尤其是在室溫下的衝撃值)便會有飛躍性的提升。具體而言，將熱輥軋鋼板或冷輥軋鋼板進行退火時，將氣體環境中氮濃度控制到預定臨界值以上，藉此能使氮在滲碳用鋼板之鋼板表層變濃，結果發現，就從滲碳用鋼板所成形的滲碳構件而言，其在室溫下的衝撃值便會有飛躍性的提升。In addition to implementing the above-mentioned structure control, the inventors of the present case have focused on a method that can improve the impact resistance after carburizing. The toughness can be improved by increasing the nitrogen in the surface layer of the steel sheet for carburizing. The effects of thickening were investigated and studied in detail. It was found that by controlling the nitrogen concentration on the surface of the steel sheet, the toughness after carburizing (especially the impact value at room temperature) will be greatly improved. Specifically, when the hot-rolled steel sheet or the cold-rolled steel sheet is annealed, the nitrogen concentration in the gaseous environment is controlled to be higher than a predetermined threshold value, so that nitrogen can be enriched in the surface layer of the steel sheet for carburizing. As a result, As for the carburized member formed from the carburizing steel sheet, the impact value at room temperature will be greatly improved.

滲碳後韌性會提升之理由，其機制(mechanism)認為如下。將鋼板在含氮率高的氣體環境中進行退火，氣體環境中所含氮會入侵鋼板，而在鋼板表層會形成氮化物。所生成的氮化物因為是微細AlN為主體，故在滲碳熱處理中會展現出抑制舊沃斯田鐵的粒成長之效果。舊沃斯田鐵粒徑與變態後麻田散鐵的粒徑之間，會成立一比例關係。因此，若能透過微細AlN來抑制舊沃斯田鐵的粒成長，則滲碳構件之組織中麻田散鐵的粒徑也會微細化，結果認為，衝撃值便會有飛躍性的增加。本案發明人等精心探討的結果下述變得明朗：在滲碳用鋼板之鋼板表層形成有微細AlN，而滲碳構件之衝撃值會提升。The reason for the increase in toughness after carburizing is as follows. The steel sheet is annealed in a gas environment with a high nitrogen content. The nitrogen contained in the gas environment will invade the steel sheet, and nitrides will be formed on the surface of the steel sheet. Since the generated nitride is mainly composed of fine AlN, the effect of suppressing grain growth of old Vosted iron is exhibited during the carburizing heat treatment. There is a proportional relationship between the particle size of the old Vostian iron and the particle size of the loose Asada iron after metamorphosis. Therefore, if the grain growth of the old Vostian iron can be suppressed by the fine AlN, the particle size of the Asada loose iron in the structure of the carburized member will also be refined, and as a result, it is considered that the impact value will increase dramatically. As a result of careful study by the inventors of the present case, it became clear that fine AlN was formed on the surface layer of the steel sheet for carburizing steel, and the impact value of the carburizing member was increased.

另外，一旦鋼板的強度提高，則上述彎曲性及滲碳後之韌性就會處於劣勢。另一方面，就確保滲碳用鋼板所要求的淬火性之觀點來看，宜使鋼板高強度化。為了兼具此等相反之特性，透過上述概略說明之組織控制來維持淬火性，同時提升彎曲性及滲碳後之韌性一事很重要。因此，透過上述概略說明之組織控制，就能獲得一種兼具淬火性、彎曲性及滲碳後之韌性的滲碳用鋼板。In addition, if the strength of the steel sheet is increased, the above-mentioned bendability and toughness after carburization are inferior. On the other hand, from the viewpoint of ensuring the hardenability required for the steel sheet for carburizing, it is desirable to increase the strength of the steel sheet. In order to have these opposite characteristics, it is important to maintain the hardenability through the structure control described above and to improve the bendability and toughness after carburizing. Therefore, through the microstructure control described above, a carburized steel sheet having both hardenability, bendability, and toughness after carburizing can be obtained.

本案發明人等透過以上的鋼板組織控制，而成功維持淬火性同時提升冷加工中的彎曲性與滲碳後之韌性。藉此，就能獲得一種兼具淬火性、成形性及滲碳後之韌性的滲碳用鋼板。The inventors of the present case succeeded in maintaining the hardenability while improving the bendability during cold working and the toughness after carburizing by controlling the structure of the steel sheet. Thereby, a carburizing steel sheet having both hardenability, formability, and toughness after carburization can be obtained.

以下詳述之本發明實施形態的滲碳用鋼板及其製造方法，便是基於如上所述見解所完成者。基於這種見解所完成之本實施形態的滲碳用鋼板及其製造方法，詳細說明如下。The steel sheet for carburizing and the manufacturing method thereof according to the embodiments of the present invention described in detail below were completed based on the findings described above. The steel sheet for carburizing and the manufacturing method of this embodiment completed based on this knowledge are explained in detail below.

(關於滲碳用鋼板)
首先，詳細說明本發明實施形態的滲碳用鋼板。
本實施形態的滲碳用鋼板，具有如以下所詳述之預定化學成分。除此之外，本實施形態的滲碳用鋼板具有下述特定顯微組織：碳化物的平均等效圓直徑為5.0μm以下；相對於總碳化物，縱橫比2.0以下之碳化物個數比率為80%以上；相對於總碳化物，存在於肥粒鐵結晶粒內之碳化物個數比率為60%以上；並且，從鋼板最外表面起算在深度方向上至50μm為止的區域中的氮濃度為0.040質量%以上且0.200質量%以下。藉此，本實施形態的滲碳用鋼板就會在維持淬火性的同時，顯示出更為優異的成形性及滲碳後之韌性。 (About carburizing steel sheet)
First, a carburizing steel sheet according to an embodiment of the present invention will be described in detail.
The steel sheet for carburizing in this embodiment has a predetermined chemical composition as described in detail below. In addition, the steel sheet for carburizing in this embodiment has the following specific microstructure: the average equivalent circle diameter of carbides is 5.0 μm or less; and the ratio of the number of carbides to the total carbides is 2.0 or less in aspect ratio. 80% or more; 60% or more of the number of carbides present in the iron crystal grains of the ferrite grains relative to the total carbides; and nitrogen in the region from the outermost surface of the steel sheet in the depth direction to 50 μm The concentration is 0.040 mass% or more and 0.200 mass% or less. Thereby, the steel sheet for carburizing in this embodiment exhibits more excellent formability and toughness after carburizing while maintaining the hardenability.

＜關於滲碳用鋼板的化學成分＞
首先，就本實施形態的滲碳用鋼板之板厚中央部中的化學成分，進行詳細說明。又，在以下的說明中，有關化學成分的「%」，只要無特別聲明便是意指「質量%」。 ＜ About chemical composition of carburizing steel sheet ＞
First, the chemical components in the center portion of the thickness of the steel sheet for carburizing according to this embodiment will be described in detail. In the following description, "%" of the chemical composition means "mass%" unless otherwise specified.

[C：0.02%以上且小於0.30%]
C(碳)是一種為了確保最終獲得之滲碳構件的板厚中央部強度所必須的元素。又，在滲碳用鋼板中，C是一種會固溶於肥粒鐵晶界而提高晶界強度，並有助於提升彎曲性的元素。 [C: 0.02% or more and less than 0.30%]
C (carbon) is an element necessary to ensure the strength of the central portion of the plate thickness of the carburized member to be finally obtained. Further, in the steel sheet for carburizing, C is an element which solidly dissolves in the grain boundaries of the ferrous grains to increase the strength of the grain boundaries and contributes to the improvement of the bendability.

當C含量小於0.02%時，便無法獲得如上所述的提升彎曲性效果。因此，本實施形態的滲碳用鋼板中，C含量是設為0.02%以上。C含量宜為0.05%以上。另一方面，當C含量達0.30%以上時，碳化物的平均等效圓直徑會大於5.0μm，而彎曲性會劣化。因此，本實施形態的滲碳用鋼板中，C含量是設為小於0.30%。C含量宜為0.20%以下。又，考量到彎曲性及淬火性的均衡，C含量更宜為0.10%以下。When the C content is less than 0.02%, the effect of improving the bendability as described above cannot be obtained. Therefore, in the steel sheet for carburizing in this embodiment, the C content is set to 0.02% or more. The C content should be above 0.05%. On the other hand, when the C content is 0.30% or more, the average equivalent circle diameter of the carbide is larger than 5.0 μm, and the bendability is deteriorated. Therefore, in the steel sheet for carburizing in this embodiment, the C content is set to less than 0.30%. The C content should be 0.20% or less. In consideration of the balance between bendability and hardenability, the C content is more preferably 0.10% or less.

[Si：0.005%以上且0.5%以下]
Si(矽)是一種會發揮出對熔融鋼脫氧而使鋼健全化之作用的元素。當Si含量小於0.005%時，並無法對熔融鋼進行充分脫氧。因此，本實施形態的滲碳用鋼板中，Si含量是設為0.005%以上。Si含量宜為0.01%以上。另一方面，當Si含量大於0.5%時，固溶於碳化物的Si會讓碳化物安定化，且碳化物的平均等效圓直徑會大於5.0μm而損害彎曲性。因此，本實施形態的滲碳用鋼板中，Si含量是設為0.5%以下。Si含量宜為0.3%以下。 [Si: 0.005% or more and 0.5% or less]
Si (silicon) is an element that exerts the effect of deoxidizing molten steel to make the steel sound. When the Si content is less than 0.005%, sufficient deoxidation of the molten steel cannot be performed. Therefore, in the steel sheet for carburizing in this embodiment, the Si content is set to 0.005% or more. The Si content is preferably 0.01% or more. On the other hand, when the Si content is greater than 0.5%, the solid solution of Si in the carbide will stabilize the carbide, and the average equivalent circle diameter of the carbide will be greater than 5.0 μm, thereby impairing the bendability. Therefore, in the steel sheet for carburizing in this embodiment, the Si content is set to 0.5% or less. The Si content is preferably 0.3% or less.

[Mn：0.01%以上且3.0%以下]
Mn(錳)是一種會發揮出對熔融鋼脫氧而使鋼健全化之作用的元素。當Mn含量小於0.01%時，並無法對熔融鋼進行充分脫氧。因此，本實施形態的滲碳用鋼板中，Mn含量是設為0.01%以上。Mn含量宜為0.1%以上。另一方面，當Mn含量大於3.0%時，固溶於碳化物的Mn會讓碳化物安定化，且碳化物的平均等效圓直徑會大於5.0μm而導致彎曲性劣化。因此，Mn含量是設為3.0以下。Mn含量宜為2.0%以下，較宜為1.0%以下。 [Mn: 0.01% or more and 3.0% or less]
Mn (manganese) is an element that exerts the effect of deoxidizing molten steel and strengthening the steel. When the Mn content is less than 0.01%, sufficient deoxidation of the molten steel cannot be performed. Therefore, in the steel sheet for carburizing in this embodiment, the Mn content is set to 0.01% or more. The Mn content is preferably above 0.1%. On the other hand, when the Mn content is more than 3.0%, Mn solid-dissolved in the carbide will stabilize the carbide, and the average equivalent circle diameter of the carbide will be greater than 5.0 μm, resulting in deterioration of bendability. Therefore, the Mn content is set to 3.0 or less. The Mn content is preferably 2.0% or less, and more preferably 1.0% or less.

[P：0.1%以下]
P(磷)是一種會在肥粒鐵的晶界偏析而使彎曲性劣化的元素。當P含量大於0.1%時，晶界強度會明顯下降，而彎曲性會劣化。因此，本實施形態的滲碳用鋼板中，P含量是設為0.1%以下。P含量宜為0.050%以下，較宜為0.020%以下。另外，P含量的下限並不特別限定。但是，一旦將P含量減低至小於0.0001%，則脫P成本會大幅提高，在經濟上並不利。因此，實際使用的鋼板上，P含量則是0.0001%為實質下限。 [P: 0.1% or less]
P (phosphorus) is an element that segregates at the grain boundaries of fertilized iron and deteriorates bendability. When the P content is more than 0.1%, the grain boundary strength is significantly reduced, and the bendability is deteriorated. Therefore, in the steel sheet for carburizing in this embodiment, the P content is set to 0.1% or less. The P content should be 0.050% or less, and more preferably 0.020% or less. The lower limit of the P content is not particularly limited. However, once the P content is reduced to less than 0.0001%, the cost of P removal will increase significantly, which is not economically beneficial. Therefore, on the steel sheet actually used, the P content is 0.0001% as a practical lower limit.

[S：0.1%以下]
S(硫)是一種會形成夾雜物而使彎曲性劣化的元素。當S含量大於0.1%時，粗大夾雜物會生成而彎曲性會下降。因此，本實施形態的滲碳用鋼板中，S含量是設為0.1%以下。S含量宜為0.010%以下，較宜為0.008%以下。另外，S含量的下限並不特別限定。但是，一旦將S含量減低至小於0.0005%，則脫S成本會大幅提高，在經濟上並不利。因此，實際使用的鋼板上，S含量則是0.0005%為實質下限。 [S: 0.1% or less]
S (sulfur) is an element that forms inclusions and deteriorates flexibility. When the S content is more than 0.1%, coarse inclusions are formed and the bendability is reduced. Therefore, in the steel sheet for carburizing in this embodiment, the S content is set to 0.1% or less. The S content is preferably 0.010% or less, and more preferably 0.008% or less. The lower limit of the S content is not particularly limited. However, once the S content is reduced to less than 0.0005%, the cost of de-S will increase significantly, which is not economically beneficial. Therefore, in the actual steel sheet, the S content is 0.0005% as the lower limit.

[sol.Al：0.0002%以上且3.0%以下]
Al(鋁)是一種會發揮出對熔融鋼脫氧而使鋼健全化之作用的元素。當Al含量小於0.0002%時，並無法對熔融鋼進行充分脫氧。因此，本實施形態的滲碳用鋼板中，Al含量(較詳細來說是sol.Al含量)設為0.0002%以上。Al含量宜為0.0010%以上，較宜為0.0050%以上，更宜為0.010%以上。另一方面，當Al含量大於3.0%時，會生成粗大氧化物而損害彎曲性。因此，Al含量是設為3.0%以下。Al含量宜為2.5%以下，較宜為1.0%以下，更宜為0.2%以下，更加適宜為0.05%以下。 [sol.Al: 0.0002% or more and 3.0% or less]
Al (aluminum) is an element that exerts the effect of deoxidizing molten steel and strengthening the steel. When the Al content is less than 0.0002%, sufficient deoxidation of the molten steel cannot be performed. Therefore, in the steel sheet for carburizing in this embodiment, the Al content (more specifically, the sol.Al content) is set to 0.0002% or more. The Al content is preferably 0.0010% or more, more preferably 0.0050% or more, and more preferably 0.010% or more. On the other hand, when the Al content is more than 3.0%, coarse oxides are formed and the bendability is impaired. Therefore, the Al content is set to 3.0% or less. The Al content is preferably 2.5% or less, more preferably 1.0% or less, more preferably 0.2% or less, and still more preferably 0.05% or less.

[N：0.0001%以上且0.035%以下]
本實施形態的滲碳用鋼板中，N(氮)含量必須為0.035%以下。又，在此所定義的N含量，是橫跨鋼板板厚方向整體所存在之N的平均值(N含量之板厚方向平均值)。當N含量大於0.035%時，氮化物會橫跨滲碳用鋼板板厚方向整體大量析出，而難以獲得所欲的彎曲性。因此，本實施形態的滲碳用鋼板中，N含量是設為0.035%以下。N含量宜為0.030%以下，較宜為0.020%以下，更宜為0.010%以下。N含量下限，並不特別限定。但是，一旦將N含量降低至小於0.0001%，則脫N成本會大幅提高，在經濟上並不利。因此，實際使用的鋼板上，N含量是0.0001%為實質下限。又，若考量到要使鋼板表層充分含有氮，則N含量亦可設為0.0020%以上。 [N: 0.0001% or more and 0.035% or less]
In the steel sheet for carburizing in this embodiment, the N (nitrogen) content must be 0.035% or less. The N content defined here is an average value of N existing across the entire thickness direction of the steel sheet (the average value of the N content in the thickness direction). When the N content is more than 0.035%, nitrides are largely precipitated across the entire thickness direction of the steel sheet for carburizing, and it is difficult to obtain desired bendability. Therefore, in the steel sheet for carburizing in this embodiment, the N content is set to 0.035% or less. The N content is preferably 0.030% or less, more preferably 0.020% or less, and more preferably 0.010% or less. The lower limit of the N content is not particularly limited. However, once the N content is reduced to less than 0.0001%, the cost of removing N will be greatly increased, which is not economically beneficial. Therefore, in the steel sheet actually used, the N content is 0.0001% as a substantial lower limit. In addition, if it is considered that the surface layer of the steel sheet sufficiently contains nitrogen, the N content may be set to 0.0020% or more.

[Cr：0.005%以上且3.0%以下]
Cr(鉻)是一種在最終獲得之滲碳構件中具有提高淬火性效果之元素，同時也是一種在滲碳用鋼板中會使肥粒鐵結晶粒微細化而有助於更為提升滲碳後之韌性的元素。因此，本實施形態的滲碳用鋼板中，亦可視需求而含有Cr。在含有Cr之情況下，為了獲得更為提升滲碳後之韌性的效果，宜將Cr含量設為0.005%以上。Cr含量較宜為0.010%以上。又，考量到生成碳化物、氮化物之影響，並為了獲得更加提升滲碳後之韌性的效果，Cr含量宜設為3.0%以下。Cr含量較宜為2.0%以下，更宜為1.6%以下。 [Cr: 0.005% or more and 3.0% or less]
Cr (chromium) is an element that has the effect of improving the hardenability in the finally obtained carburizing member. At the same time, it is a kind of fine-grained iron crystal grains in the carburizing steel sheet, which helps to further improve carburization. Of toughness. Therefore, the steel sheet for carburizing in this embodiment may contain Cr as required. In the case of containing Cr, in order to obtain the effect of further improving the toughness after carburizing, the Cr content should be set to 0.005% or more. The Cr content is more preferably 0.010% or more. In addition, considering the influence of the formation of carbides and nitrides, and in order to obtain the effect of further improving the toughness after carburizing, the Cr content should be 3.0% or less. The Cr content is more preferably 2.0% or less, and more preferably 1.6% or less.

[Mo：0.005%以上且1.0%以下]
Mo(鉬)是一種在最終獲得之滲碳構件中具有提高淬火性效果之元素，同時也是一種在滲碳用鋼板中會使肥粒鐵結晶粒微細化而有助於更為提升滲碳後之韌性的元素。因此，本實施形態的滲碳用鋼板中，亦可視需求而含有Mo。在含有Mo之情況下，為了獲得更加提升滲碳後之韌性的效果，宜將Mo含量設為0.005%以上。Mo含量較宜為0.010%以上。又，考量到生成碳化物、氮化物之影響，並為了獲得更加提升滲碳後之韌性的效果，Mo含量宜設為1.0%以下。Mo含量較宜為0.8%以下。 [Mo: 0.005% or more and 1.0% or less]
Mo (molybdenum) is an element that has the effect of improving the hardenability in the finally obtained carburizing member. At the same time, it is a kind of fine-grained iron crystal grains in the carburizing steel sheet, which helps to further improve carburization Of toughness. Therefore, the steel sheet for carburizing in this embodiment may contain Mo as required. In the case of containing Mo, in order to obtain the effect of further improving the toughness after carburizing, the Mo content should preferably be 0.005% or more. The Mo content is more preferably 0.010% or more. In addition, considering the influence of the formation of carbides and nitrides, and in order to obtain the effect of further improving the toughness after carburizing, the Mo content should be 1.0% or less. The Mo content is more preferably 0.8% or less.

[Ni：0.010%以上且3.0%以下]
Ni(鎳)是一種在最終獲得之滲碳構件中具有提高淬火性效果之元素，同時也是一種在滲碳用鋼板中會使肥粒鐵結晶粒微細化而有助於更為提升滲碳後之韌性的元素。因此，本實施形態的滲碳用鋼板中，亦可視需求而含有Ni。在含有Ni之情況下，為了獲得更加提升滲碳後之韌性的效果，宜將Ni含量設為0.010%以上。Ni含量較宜為0.050%以上。又，考量到Ni在肥粒鐵晶界偏析的影響，並為了獲得更加提升滲碳後之韌性的效果，Ni含量宜設為3.0%以下。Ni含量較宜為2.0%以下，更宜為1.0%以下，更加適宜為0.5%以下。 [Ni: 0.010% or more and 3.0% or less]
Ni (nickel) is an element that has the effect of improving the hardenability in the finally obtained carburizing member, and it is also a kind of finer iron crystal grains in the carburizing steel sheet, which helps to further improve carburization. Of toughness. Therefore, the steel sheet for carburizing in this embodiment may contain Ni as required. In the case of containing Ni, in order to obtain the effect of further improving the toughness after carburizing, the Ni content should be set to 0.010% or more. The Ni content is more preferably 0.050% or more. In addition, considering the influence of Ni segregation at the grain boundary of the ferrous grains, and in order to obtain the effect of further improving the toughness after carburizing, the Ni content should be set to 3.0% or less. The Ni content is more preferably 2.0% or less, more preferably 1.0% or less, and still more preferably 0.5% or less.

[Cu：0.001%以上且2.0%以下]
Cu(銅)是一種在最終獲得之滲碳構件中具有提高淬火性效果之元素，同時也是一種在滲碳用鋼板中會使肥粒鐵結晶粒微細化而有助於更為提升滲碳後之韌性的元素。因此，本實施形態的滲碳用鋼板中，亦可視需求而含有Cu。在含有Cu之情況下，為了獲得更加提升滲碳後之韌性的效果，宜將Cu含量設為0.001%以上。Cu含量較宜為0.010%以上。又，考量到Cu在肥粒鐵晶界偏析的影響，並為了獲得更加提升滲碳後之韌性的效果，Cu含量宜設為2.0%以下。Cu含量較宜為0.80%以下。 [Cu: 0.001% or more and 2.0% or less]
Cu (copper) is an element that has the effect of improving the hardenability in the finally obtained carburizing member. At the same time, it is also a kind of fine-grained iron crystal grains in the carburizing steel sheet, which helps to further improve carburization. Of toughness. Therefore, the steel sheet for carburizing in this embodiment may contain Cu as needed. In the case of containing Cu, in order to obtain the effect of further improving the toughness after carburizing, the Cu content should be set to 0.001% or more. The Cu content is more preferably 0.010% or more. In addition, considering the influence of Cu segregation at the grain boundaries of the fertile grains, and in order to obtain the effect of further improving the toughness after carburizing, the Cu content should be set to 2.0% or less. The Cu content is more preferably 0.80% or less.

[Co：0.001%以上且2.0%以下]
Co(鈷)是一種在最終獲得之滲碳構件中具有提高淬火性效果之元素，同時也是一種在滲碳用鋼板中會使結晶粒微細化而有助於更為提升滲碳後之韌性的元素。因此，本實施形態的滲碳用鋼板中，亦可視需求而含有Co。在含有Co之情況下，為了獲得更加提升滲碳後之韌性的效果，宜將Co含量設為0.001%以上。Co含量較宜為0.010%以上。又，考量到Co在肥粒鐵晶界偏析的影響，並為了獲得更加提升滲碳後之韌性的效果，Co含量宜設為2.0%以下。Co含量較宜為0.80%以下。 [Co: 0.001% or more and 2.0% or less]
Co (cobalt) is an element that has the effect of improving the hardenability in the finally obtained carburized member, and it is also a kind of finer crystal grains in the carburizing steel sheet, which helps to further improve the toughness after carburizing element. Therefore, Co may be contained in the steel sheet for carburizing in this embodiment as needed. In the case where Co is contained, in order to obtain the effect of further improving the toughness after carburizing, the Co content should preferably be 0.001% or more. The Co content is more preferably 0.010% or more. In addition, considering the influence of Co segregation at the grain boundaries of the ferrous grains, and in order to obtain the effect of further improving the toughness after carburizing, the Co content should be set to 2.0% or less. The Co content is more preferably 0.80% or less.

[Nb：0.010%以上且0.150%以下]
Nb(鈮)是一種會使肥粒鐵結晶粒微細化而有助於更為提升彎曲性的元素。因此，本實施形態的滲碳用鋼板中，亦可視需求而含有Nb。在含有Nb之情況下，為了獲得更為提升彎曲性之效果，宜將Nb含量設為0.010%以上。Nb含量較宜為0.035%以上。又，考量到生成碳化物、氮化物之影響，並為了獲得更為提升彎曲性之效果，Nb含量宜設為0.150%以下。Nb含量較宜為0.120%以下，更宜為0.100%以下，更加適宜為0.050%以下。 [Nb: 0.010% or more and 0.150% or less]
Nb (niobium) is an element which makes the iron crystal grains of the fertilizer grains finer and helps to improve the bendability. Therefore, Nb may be contained in the steel sheet for carburizing in this embodiment as needed. In the case where Nb is contained, in order to obtain the effect of improving the flexibility, the Nb content should preferably be 0.010% or more. The Nb content is more preferably 0.035% or more. In addition, considering the influence of the formation of carbides and nitrides, and in order to obtain the effect of improving the flexibility, the Nb content should be set to 0.150% or less. The Nb content is more preferably 0.120% or less, more preferably 0.100% or less, and still more preferably 0.050% or less.

[Ti：0.010%以上且0.150%以下]
Ti(鈦)是一種會使肥粒鐵結晶粒微細化而有助於更為提升彎曲性的元素。因此，本實施形態的滲碳用鋼板中，亦可視需求而含有Ti。在含有Ti之情況下，為了獲得更為提升彎曲性之效果，宜將Ti含量設為0.010%以上。Ti含量較宜為0.035%以上。又，考量到生成碳化物、氮化物之影響，並為了獲得更為提升彎曲性之效果，Ti含量宜設為0.150%以下。Ti含量較宜為0.120%以下，更宜為0.050%以下，更加適宜為0.020%以下。 [Ti: 0.010% or more and 0.150% or less]
Ti (titanium) is an element that makes the iron crystal grains of the fertilizer grains finer and helps to improve the bendability. Therefore, the steel sheet for carburizing in this embodiment may also contain Ti as required. In the case where Ti is contained, in order to obtain the effect of improving the flexibility, the Ti content should be set to 0.010% or more. The Ti content is more preferably 0.035% or more. In consideration of the influence of the formation of carbides and nitrides, and in order to obtain the effect of improving the flexibility, the Ti content should be 0.150% or less. The Ti content is more preferably 0.120% or less, more preferably 0.050% or less, and still more preferably 0.020% or less.

[V：0.0005%以上且1.0%以下]
V(釩)是一種會使肥粒鐵結晶粒微細化而有助於更為提升彎曲性的元素。因此，本實施形態的滲碳用鋼板中，亦可視需求而含有V。在含有V之情況下，為了獲得更為提升彎曲性之效果，宜將V含量設為0.0005%以上。V含量較宜為0.0010%以上。又，考量到生成碳化物、氮化物之影響，並為了獲得更為提升彎曲性之效果，V含量宜設為1.0%以下。V含量較宜為0.80%以下。 [V: 0.0005% or more and 1.0% or less]
V (Vanadium) is an element that makes the iron crystal grains of the fertilizer grains finer and helps to improve the bendability. Therefore, V may be contained in the steel sheet for carburizing in this embodiment as required. In the case where V is contained, in order to obtain the effect of improving the flexibility, the V content should be set to 0.0005% or more. The V content is more preferably 0.0010% or more. In consideration of the influence of the formation of carbides and nitrides, and in order to obtain the effect of improving the flexibility, the V content should be 1.0% or less. The V content is more preferably 0.80% or less.

[B：0.0005%以上且0.01%以下]
B(硼)是一種會在肥粒鐵的晶界偏析來提升晶界強度，並更為提升彎曲性的元素。因此，本實施形態的滲碳用鋼板中，亦可視需求而含有B。在含有B之情況下，為了獲得更為提升彎曲性之效果，宜將B含量設為0.0005%以上。B含量較宜為0.0010%以上。又，就算將B添加至大於0.01%，如上所述的更為提升彎曲性效果仍達飽和，故B含量宜設為0.01%以下。B含量較宜為0.0075%以下，更宜為0.0050%以下，更加適宜為0.0020%以下。 [B: 0.0005% or more and 0.01% or less]
B (boron) is an element that will segregate at the grain boundaries of ferrous iron to increase the strength of the grain boundaries and further improve the bendability. Therefore, B may be contained in the steel sheet for carburizing in this embodiment as needed. In the case where B is contained, in order to obtain the effect of improving the flexibility, the B content should be set to 0.0005% or more. The B content is more preferably 0.0010% or more. In addition, even if B is added to more than 0.01%, the effect of improving the bendability as described above is still saturated, so the B content should be set to 0.01% or less. The B content is more preferably 0.0075% or less, more preferably 0.0050% or less, and still more preferably 0.0020% or less.

[W：1.0%以下]
W(鎢)是一種會發揮出對熔融鋼脫氧而使鋼更為健全化之作用的元素。因此，本實施形態的滲碳用鋼板中，亦可視需求而以1.0%為上限來含有W。W含量較宜為0.5%以下。 [W: 1.0% or less]
W (tungsten) is an element that exerts the effect of deoxidizing molten steel to make the steel more robust. Therefore, in the steel sheet for carburizing in this embodiment, W may be contained with 1.0% as an upper limit as needed. The W content is preferably 0.5% or less.

[Ca：0.01%以下]
Ca(鈣)是一種會發揮出對熔融鋼脫氧而使鋼更為健全化之作用的元素。因此，本實施形態的滲碳用鋼板中，亦可視需求而以0.01%為上限來含有Ca。Ca含量較宜為0.005%以下。 [Ca: 0.01% or less]
Ca (calcium) is an element that exerts the effect of deoxidizing molten steel to make the steel more sound. Therefore, in the steel sheet for carburizing in this embodiment, Ca may be contained with an upper limit of 0.01% as needed. The Ca content is more preferably 0.005% or less.

[剩餘部分：Fe及不純物]
板厚中央部之成分組成的剩餘部分為Fe及不純物。不純物可舉例如：從鋼原料、廢料，及/或，製鋼過程中混入，且在不阻礙本實施形態的滲碳用鋼板特性之範圍下所允許的元素。 [Remaining part: Fe and impurities]
The remainder of the component composition at the center of the plate thickness is Fe and impurities. Impurities include, for example, elements that are mixed from steel raw materials, scraps, and / or during steel making, and that are allowed within a range that does not impair the characteristics of the steel sheet for carburizing in this embodiment.

以上，詳細說明完本實施形態的滲碳用鋼板所具有的化學成分。The chemical components of the steel sheet for carburizing in this embodiment have been described in detail above.

＜關於滲碳用鋼板的顯微組織＞
接著，針對構成本實施形態滲碳用鋼板的顯微組織，進行詳細說明。
本實施形態的滲碳用鋼板之顯微組織，實質上是以肥粒鐵與碳化物所構成。更詳細來說，本實施形態的滲碳用鋼板之顯微組織是構成如下：肥粒鐵的平均結晶粒徑小於10μm，肥粒鐵面積率為例如80~95%之範圍內，碳化物面積率為例如5~20%之範圍內，並且，肥粒鐵與碳化物合計面積率不大於100%。 ＜ About the microstructure of carburizing steel sheet ＞
Next, the microstructure constituting the steel sheet for carburizing according to this embodiment will be described in detail.
The microstructure of the steel sheet for carburizing in this embodiment is substantially composed of ferrous iron and carbides. More specifically, the microstructure of the steel sheet for carburizing in this embodiment is composed as follows: the average grain size of the ferrous iron is less than 10 μm, the area ratio of the ferrous iron is, for example, 80 to 95%, and the carbide area is The ratio is, for example, in a range of 5 to 20%, and the total area ratio of the ferrous iron and the carbide is not more than 100%.

如上所述的肥粒鐵及碳化物的面積率，是以垂直於滲碳用鋼板寬度方向之剖面作為觀察面所採取的樣本來進行測定。樣本的長度雖是視測定裝置而定，不過10mm~25mm左右即可。樣本在研磨觀察面後，以硝太蝕劑進行蝕刻。以熱場發射型掃描式顯微鏡(例如，JEOL製JSM-7001F)，觀察經硝太蝕劑蝕刻後觀察面的下述位置：板厚1/4位置(意指：從滲碳用鋼板表面起算在鋼板厚度方向上鋼板厚度1/4的位置)、板厚3/8位置及板厚1/2位置的範圍。The area ratio of the ferrous iron and carbides as described above was measured using a sample taken as a viewing surface with a cross section perpendicular to the width direction of the steel sheet for carburizing. Although the length of the sample depends on the measurement device, it may be about 10 mm to 25 mm. After grinding the observation surface, the sample was etched with nitrate. Using a thermal field emission scanning microscope (for example, JSM-7001F manufactured by JEOL), observe the following positions on the observation surface after being etched by nitric acid etchant: 1/4 position of plate thickness (meaning: from the surface of the steel sheet for carburizing) In the thickness direction of the steel sheet, the range of the thickness of the steel sheet is 1/4), the thickness of the steel sheet is 3/8, and the thickness of the steel sheet is 1/2.

對於各個樣本的觀察對象範圍以2500μm 2之範圍觀察10個視野，於各個視野，測定視野面積中肥粒鐵及碳化物所占面積的比率。然後，將總視野中肥粒鐵所占面積比率的平均值、及總視野中碳化物所占面積比率的平均值，分別作為肥粒鐵的面積率及碳化物的面積率。 Ten fields of view were observed in a range of 2500 μm 2 for each sample, and the ratio of the area occupied by ferrous iron and carbides in the field of view was measured in each field of view. Then, the average value of the area ratio of the ferrous iron in the total field of view and the average value of the area ratio of the carbide in the total field of view were used as the area ratio of the ferrous iron and the area ratio of the carbide, respectively.

在本案中，本實施形態之顯微組織中的碳化物主要為：鐵與碳的化合物即雪明碳鐵(Fe 3C)、及ε系碳化物(Fe 2~3C)等的鐵系碳化物。又，除了上述鐵系碳化物之外，顯微組織中的碳化物有時也含有：雪明碳鐵中的Fe原子經Mn、Cr等置換後的化合物、或合金碳化物(M 23C 6、M 6C、MC等；M是Fe及其他金屬元素)。本實施形態之顯微組織中的碳化物，其大致上是由鐵系碳化物所構成。因此，就如上所述的碳化物而言，若著眼於如以下所詳述之個數時，其個數可為如上所述之各種碳化物的合計個數，亦可僅為鐵系碳化物的個數。也就是說，如以下所詳述之關於碳化物的各種個數比率，可為含有鐵系碳化物的各種碳化物作為母集團者，也可為僅以鐵系碳化物作為母集團者。鐵系碳化物能以例如對樣品使用繞射分析或EDS(Energy dispersive X-ray spectrometry)來進行測定。 In this case, the carbides in the microstructure of the present embodiment are mainly iron-based compounds such as cuming carbon iron (Fe 3 C) and ε-based carbides (Fe 2 ~ 3 C), which are compounds of iron and carbon. carbide. In addition to the above-mentioned iron-based carbides, the carbides in the microstructure may also include: compounds in which Fe atoms in the clear carbon iron are replaced by Mn, Cr, or the like, or alloy carbides (M 23 C 6 , M 6 C, MC, etc .; M is Fe and other metal elements). The carbides in the microstructure of this embodiment are roughly composed of iron-based carbides. Therefore, when focusing on the number of carbides as described above, the number may be the total number of various carbides as described above, or it may be only iron-based carbides. Number of. That is, as described in detail below, the various number ratios of carbides may be those in which various carbides containing iron-based carbides are used as the parent group, or those in which only iron-based carbides are used as the parent group. The iron-based carbide can be measured by using diffraction analysis or EDS (Energy dispersive X-ray spectrometry) on the sample, for example.

在彎曲變形中，變形應力會集中於軟質組織與硬質組織之界面。因此，必須極力縮小軟質組織與硬質組織之間的硬度差，或者，為了緩和應力集中而控制硬質組織的形狀。其中，透過球狀化退火來降低碳化物的縱橫比，藉此能抑制龜裂產生。一旦彎曲變形更為進行，則所產生的龜裂便會伸展。由於龜裂會在容易產生破壞之區域傳遞下去，故肥粒鐵之晶界、及肥粒鐵與碳化物之界面就會成為傳遞路徑。此時，一旦在肥粒鐵的晶界生成碳化物，則以晶界作為傳遞路徑的龜裂其伸展便會獲得助長，因此使碳化物在肥粒鐵結晶粒內生成一事很重要。吾人認為，透過使碳化物在肥粒鐵結晶粒內生成，便可抑制龜裂在晶界傳遞。In bending deformation, deformation stress is concentrated at the interface between soft tissue and hard tissue. Therefore, it is necessary to minimize the difference in hardness between the soft tissue and the hard tissue, or to control the shape of the hard tissue in order to reduce the stress concentration. Among them, the aspect ratio of carbides is reduced by spheroidizing annealing, thereby suppressing the occurrence of cracks. Once the bending deformation progresses, the cracks generated will stretch. Since cracks will be transmitted in the area prone to damage, the grain boundaries of the ferrous iron and the interface between the ferrous iron and the carbide will become the transmission path. At this time, once carbides are formed at the grain boundaries of the ferrous iron, cracks that use the grain boundaries as a transmission path will be promoted. Therefore, it is important to generate carbides within the ferrous iron crystal grains. I believe that by making carbides in the iron crystal grains of the fertilizer grains, the propagation of cracks at the grain boundaries can be suppressed.

滲碳構件是透過滲碳而在表層導入碳，故構件表層的強度會變高，另一方面，成為滲碳構件素材的鋼材，若其強度提高就會變脆。因此，成為素材的滲碳用鋼板而言，表層之韌性很重要。關於此點，將鋼板表層之結晶粒微細化，藉此韌性會提升。如以下所詳述，以含氮率高的氣體環境對鋼板進行退火，藉此氣體環境中所含氮會入侵鋼板，而在鋼板表層形成氮化物。所生成的氮化物由於是微細AlN為主體，故在滲碳熱處理中會展現出抑制舊沃斯田鐵之粒成長的效果。舊沃斯田鐵粒徑與變態後麻田散鐵的粒徑之間，會成立一比例關係，因此，若能透過微細AlN來抑制舊沃斯田鐵的粒成長，則滲碳構件之組織中麻田散鐵的粒徑也會微細化，此事遂為明朗。
以下，針對構成本實施形態滲碳用鋼板之顯微組織其限定理由，進行詳細說明。 Carburizing members introduce carbon into the surface layer by carburizing, so the strength of the surface layer of the member becomes higher. On the other hand, the steel that becomes the material of the carburizing member becomes brittle if its strength is increased. Therefore, the toughness of the surface layer is important for the carburizing steel sheet to be used as a material. In this regard, the crystal grains on the surface layer of the steel sheet are made finer, thereby improving the toughness. As described in detail below, the steel sheet is annealed in a gas environment with a high nitrogen content, whereby the nitrogen contained in the gas environment will invade the steel sheet and form nitrides on the surface layer of the steel sheet. Since the generated nitride is mainly composed of fine AlN, the effect of suppressing grain growth of old Vosted iron is exhibited during the carburizing heat treatment. There is a proportional relationship between the particle size of the old Vostian iron and the particle size of the loose Asada iron after metamorphosis. Therefore, if the grain growth of the old Vostian iron can be suppressed through the fine AlN, the carburized structure will It is clear that the particle size of Asada loose iron will also become finer.
Hereinafter, the reasons for limiting the microstructure of the steel sheet for carburizing in this embodiment will be described in detail.

[肥粒鐵的平均結晶粒徑：小於10μm]
本實施形態的滲碳用鋼板的顯微組織中，肥粒鐵的平均結晶粒徑如上所述為小於10μm。藉由將肥粒鐵的平均結晶粒徑設為小於10μm，便能展現出如上所述結晶粒微細化所帶來的效果，且能提升滲碳後的衝撃值。一旦肥粒鐵的平均結晶粒徑為10μm以上，便無法展現出如上所述的結晶粒微細化所帶來的效果，且無法提升滲碳後的衝撃值。肥粒鐵的平均結晶粒徑宜小於8μm。肥粒鐵的平均結晶粒徑之下限值，並未特別規定。但是，實際機械作業上，要將肥粒鐵的平均結晶粒徑控制在小於0.1μm是有困難的，故0.1μm為實質下限。 [Average crystal grain size of ferrous iron: less than 10 μm]
In the microstructure of the steel sheet for carburizing in this embodiment, the average crystal grain size of the ferrous iron is less than 10 μm as described above. By setting the average crystal grain size of the ferrous iron to less than 10 μm, the effect brought by the miniaturization of the crystal grains as described above can be exhibited, and the impact value after carburization can be increased. Once the average grain size of the ferrous iron is 10 μm or more, the effect of miniaturizing the crystal grains as described above cannot be exhibited, and the impact value after carburization cannot be increased. The average grain size of the ferrous iron should be less than 8 μm. The lower limit of the average crystal grain size of the ferrous iron is not particularly specified. However, in actual mechanical operations, it is difficult to control the average crystal grain size of ferrous iron to less than 0.1 μm, so 0.1 μm is the substantial lower limit.

[總碳化物中縱橫比2.0以下之碳化物個數比率：80%以上]
如前所提及，本實施形態中的碳化物是主要由雪明碳鐵(Fe 3C)及ε系碳化物(Fe 2~3C)等的鐵系碳化物所構成。本案發明人等檢討的結果下述變得明朗：若總碳化物中縱橫比2.0以下之碳化物個數比率為80%以上，就能獲得良好的彎曲性。當總碳化物中縱橫比2.0以下之碳化物個數比率小於80%時，便會助長龜裂於彎曲變形時產生而無法獲得良好的彎曲性。因此，在本實施形態的滲碳用鋼板中，是將總碳化物中縱橫比2.0以下之碳化物個數比率下限設為80%。以更加提升彎曲性為目的，總碳化物中縱橫比2.0以下之碳化物個數比率宜為85%以上。另外，總碳化物中縱橫比2.0以下之碳化物個數比率上限並未特別規定。但是，實際機械作業中要設為98%以上是有困難的，故98%為實質上限。 [Number of carbides with a total aspect ratio of 2.0 or less: 80% or more]
As mentioned above, the carbides in this embodiment are mainly composed of iron-based carbides such as cis-carbon iron (Fe 3 C) and ε-based carbides (Fe 2 to 3 C). As a result of the review by the inventors of the present case, it became clear that if the ratio of the number of carbides having an aspect ratio of 2.0 or less in the total carbides is 80% or more, good bendability can be obtained. When the ratio of the number of carbides with an aspect ratio of 2.0 or less in the total carbides is less than 80%, cracks are promoted during bending deformation, and good bendability cannot be obtained. Therefore, in the steel sheet for carburizing in this embodiment, the lower limit of the number ratio of carbides in the total carbides with an aspect ratio of 2.0 or less is set to 80%. In order to further improve the bendability, the ratio of the number of carbides with an aspect ratio of 2.0 or less in the total carbides should be 85% or more. In addition, the upper limit of the number-of-carbides ratio of the total carbides with an aspect ratio of 2.0 or less is not particularly specified. However, it is difficult to set it to 98% or more in actual mechanical operations, so 98% is substantially limited.

[總碳化物中存在於肥粒鐵結晶粒內之碳化物個數比率：60%以上]
本案發明人等檢討的結果下述變得明朗：總碳化物中存在於肥粒鐵結晶粒內的碳化物之個數比率若為60%以上，就能獲得良好的彎曲性。當總碳化物中存在於肥粒鐵結晶粒內之碳化物的個數比率小於60%時，會助長龜裂於彎曲變形時伸展而無法獲得良好的彎曲性。因此，在本實施形態的滲碳用鋼板中，總碳化物中存在於肥粒鐵結晶粒內之碳化物的個數比率下限設為60%。以更加提升彎曲性為目的，總碳化物中存在於肥粒鐵結晶粒內之碳化物的個數比率宜為65%以上。另外，總碳化物中存在於肥粒鐵結晶粒內之碳化物的個數比率上限並未特別規定。但是，實際機械作業中要設為98%以上是有困難的，故98%為實質上限。 [The ratio of the number of carbides in the total carbides present in the iron crystal grains of the fertilizer grains: 60% or more]
As a result of the review by the inventors of the present case, it became clear that if the ratio of the number of carbides present in the iron crystal grains of the ferrite grains in the total carbides is 60% or more, good bendability can be obtained. When the ratio of the number of carbides in the total carbides that are present in the ferrous iron crystal grains is less than 60%, it will promote cracks to stretch during bending deformation, and good bendability cannot be obtained. Therefore, in the steel sheet for carburizing in this embodiment, the lower limit of the ratio of the number of carbides present in the iron grains of the ferrite grains among the total carbides is set to 60%. In order to further improve the bendability, the ratio of the number of carbides in the total iron carbides present in the iron crystal grains of the fertile grains should be 65% or more. In addition, the upper limit of the ratio of the number of carbides in the total carbides present in the iron crystal grains of the fertilizer grains is not particularly limited. However, it is difficult to set it to 98% or more in actual mechanical operations, so 98% is substantially limited.

[碳化物的平均等效圓直徑：5.0μm以下]
在本實施形態的滲碳用鋼板之顯微組織中，碳化物的平均等效圓直徑必須為5.0μm以下。當碳化物的平均等效圓直徑大於5.0μm時，於彎曲變形時會產生裂紋而無法獲得良好的彎曲性。碳化物的平均等效圓直徑越小，則彎曲性就越良好；碳化物的平均等效圓直徑宜為1.0μm以下，較宜為0.8μm以下，更宜為0.6μm以下。碳化物的平均等效圓直徑下限，並未特別規定。但是，實際機械作業中，要將碳化物的平均等效圓直徑設為0.01μm以下是有困難的，故0.01μm為實質下限。 [Average equivalent circle diameter of carbides: 5.0 μm or less]
In the microstructure of the steel sheet for carburizing in this embodiment, the average equivalent circle diameter of the carbide must be 5.0 μm or less. When the average equivalent circle diameter of the carbide is more than 5.0 μm, cracks may occur during bending deformation, and good bendability cannot be obtained. The smaller the average equivalent circle diameter of the carbide is, the better the bendability is; the average equivalent circle diameter of the carbide is preferably 1.0 μm or less, more preferably 0.8 μm or less, and more preferably 0.6 μm or less. The lower limit of the average equivalent circle diameter of the carbide is not particularly specified. However, in actual mechanical operations, it is difficult to set the average equivalent circle diameter of carbides to 0.01 μm or less, so 0.01 μm is the substantial lower limit.

接著，就顯微組織中的肥粒鐵平均粒徑、以及碳化物的各種個數比率及碳化物的平均等效圓直徑之測定方法，進行詳細說明。另外，在以下測定中，雖有規定樣本的觀察位置，不過，在樣本中，所測定之肥粒鐵及碳化物狀態、與本實施形態之鋼板表層部分(氮呈現變濃之部分)中的肥粒鐵及碳化物狀態，兩者之間並不存在有明顯差異。Next, a method for measuring the average particle diameter of ferrous iron in the microstructure, the various number ratios of carbides, and the average equivalent circle diameter of carbides will be described in detail. In addition, in the following measurement, the observation position of the sample is specified. However, in the sample, the state of the ferrous iron and carbides measured in the sample is similar to that in the surface layer portion (the portion where the nitrogen appears to be thickened) of the steel sheet of this embodiment. There is no obvious difference between the iron and carbide state of the fertilizer.

首先，從滲碳用鋼板，以垂直於其表面的剖面(板厚剖面)可供觀察之方式，切出樣本。樣本的長度雖是視測定裝置而定，但10mm左右即可。將剖面進行研磨及腐蝕，並供給至下述測定：碳化物的析出位置、縱橫比與平均等效圓直徑。就研磨而言，例如，使用粒度600到粒度1500的碳化矽紙(Silicon carbide paper)對測定面進行研磨後，使用下述液體修飾成鏡面即可；該液體是已將粒徑為1μm至6μm的鑽石粉末分散至醇等稀釋液或純水中。就腐蝕而言，只要是能觀察到碳化物的形狀與析出位置的方法，便無特別限制，例如，將碳化物與基鐵之晶界進行腐蝕的手段，可使用飽和苦味酸-醇溶液進行蝕刻，亦可採用下述方法：藉由非水溶劑系電解液進行定電位電解蝕刻法(黑澤文夫等，日本金屬學會誌，43，1068，(1979))等，將基鐵除去數微米左右並僅使碳化物殘留下來。First, a sample was cut out from a steel sheet for carburizing so that a cross section (plate thickness cross section) perpendicular to the surface thereof could be observed. Although the length of the sample depends on the measurement device, it may be about 10 mm. The cross section was ground and etched, and was supplied to the following measurements: the precipitation position of the carbide, the aspect ratio, and the average equivalent circle diameter. In terms of grinding, for example, after polishing the measurement surface with a silicon carbide paper having a particle size of 600 to 1500, the following liquid can be used to modify the mirror surface; the liquid has a particle diameter of 1 μm to 6 μm The diamond powder is dispersed in diluent such as alcohol or pure water. As far as corrosion is concerned, there is no particular limitation as long as the shape and precipitation location of the carbide can be observed. For example, the means for corroding the grain boundary between carbide and base iron can be carried out using a saturated picric acid-alcohol solution. For etching, the following method can also be used: a constant-potential electrolytic etching method using a non-aqueous solvent-based electrolyte solution (Kurozawa Fumio et al., Japan Society of Metals, 43, 1068, (1979)), etc. Left and right and only the carbides remain.

肥粒鐵的平均結晶粒徑，是使用熱場發射型掃描式顯微鏡(例如，JEOL製JSM-7001F)，對樣本的板厚1/4位置以2500μm 2之範圍進行攝影，對於所得到的圖像應用線段法來算出。 The average crystal grain size of the ferrous iron was measured using a thermal field emission scanning microscope (for example, JSM-7001F manufactured by JEOL) at a position of 1/4 of the plate thickness of the sample in the range of 2500 μm 2 . It is calculated using the line segment method.

碳化物之縱橫比的算出，則是使用熱場發射型掃描式顯微鏡(例如，JEOL製JSM-7001F)，對樣本的板厚1/4位置觀察10000μm 2之範圍來進行。就觀察到的視野所含全部碳化物，測定長軸與短軸再算出縱橫比(長軸/短軸)，並求出其平均值。在5個視野實施上述觀察，並將5個視野的平均值作為樣本的碳化物縱橫比。參考所得到的碳化物之縱橫比，從縱橫比2.0以下的碳化物總個數與存在於上述5個視野中的碳化物合計數，算出總碳化物中縱橫比2.0以下之碳化物的個數比率。 The calculation of the aspect ratio of the carbides was performed using a thermal field emission scanning microscope (for example, JSM-7001F manufactured by JEOL) and observing a range of 10,000 μm 2 of the plate thickness 1/4 position of the sample. For all carbides contained in the observed field of view, the major axis and the minor axis were measured, and the aspect ratio (major axis / minor axis) was calculated, and the average value was calculated. The observation was performed in five fields of view, and the average value of the five fields of view was used as the carbide aspect ratio of the sample. With reference to the obtained aspect ratio of the carbides, the total number of carbides having an aspect ratio of 2.0 or less and the total number of carbides present in the above five fields of view are used to calculate the number of carbides having an aspect ratio of 2.0 or less in the total carbides. ratio.

碳化物析出位置的確認，是使用熱場發射型掃描式顯微鏡(例如，JEOL製JSM-7001F)，對樣本的板厚1/4位置觀察10000μm 2之範圍來進行。就觀察到的視野所含全部碳化物，觀察析出位置，並算出總碳化物中析出於肥粒鐵晶粒內之碳化物比率。在5個視野實施上述觀察，並將5個視野的平均值作為碳化物中形成在肥粒鐵結晶粒內的碳化物比率(即，總碳化物中存在於肥粒鐵結晶粒內之碳化物的個數比率)。 Confirmation of the carbide precipitation position was performed using a thermal field emission scanning microscope (for example, JSM-7001F manufactured by JEOL) and observing a range of 10,000 μm 2 of the plate thickness 1/4 position of the sample. With respect to all carbides contained in the observed field of view, the precipitation position was observed, and the ratio of carbides precipitated in the iron grains of the ferrite grains in the total carbides was calculated. The above observations were performed in five fields of view, and the average value of the five fields of view was taken as the ratio of carbides formed in the iron crystal grains of the ferrite grains (that is, the carbides present in the iron crystal grains of the grains of the total carbides) Number ratio).

碳化物的平均等效圓直徑，則是使用熱場發射型掃描式顯微鏡(例如，JEOL製JSM-7001F)，對樣本的板厚1/4位置就600μm 2之範圍攝影4個視野來進行。對各個視野，使用圖像解析軟體(例如，Media Cybernetics製IMage-Pro Plus)，分別測定所拍到碳化物之長軸與短軸。就視野中各個碳化物，以所得到的長軸與短軸之平均值作為該碳化物的直徑；就視野中所拍到的全部碳化物，算出所得到的直徑之平均值。如此所獲得在4個視野中碳化物直徑平均值再以視野數量作平均，來作為碳化物的平均等效圓直徑。 The average equivalent circle diameter of the carbide is measured using a thermal field emission scanning microscope (for example, JSM-7001F manufactured by JEOL), and photographing four fields of view in a range of 600 μm 2 from the plate thickness 1/4 position of the sample. For each field of view, image analysis software (for example, IMage-Pro Plus manufactured by Media Cybernetics) was used to measure the major axis and minor axis of the captured carbide, respectively. For each carbide in the field of view, the average value of the obtained long axis and short axis is taken as the diameter of the carbide; for all carbides captured in the field of view, the average value of the obtained diameter is calculated. The average value of the diameter of the carbides obtained in the 4 fields of view is then averaged by the number of fields of view to be used as the average equivalent circle diameter of the carbides.

以上，已詳細說明完本實施形態滲碳用鋼板所具有的顯微組織。The microstructure of the steel sheet for carburizing according to this embodiment has been described in detail.

[鋼板表層的平均氮濃度：0.040質量%以上且0.20質量%以下]
接著，針對滲碳用鋼板表層的平均氮濃度進行說明。經本案發明人等檢討的結果，下述變得明朗：若滲碳用鋼板表層的平均氮濃度為0.040質量%以上，則從滲碳用鋼板所製造出的滲碳構件中，就能獲得良好的韌性。以下，詳細說明此一見解。 [Average nitrogen concentration on the surface of the steel sheet: 0.040% by mass or more and 0.20% by mass or less]
Next, the average nitrogen concentration in the surface layer of the steel sheet for carburizing will be described. As a result of the review by the inventors of the present case, it became clear that if the average nitrogen concentration in the surface layer of the carburizing steel sheet is 0.040% by mass or more, good results can be obtained from the carburizing member produced from the carburizing steel sheet. Of toughness. This insight is explained in detail below.

本案發明人等從已獲得良好韌性之滲碳構件其表層附近，使用聚焦離子束加工觀察裝置，採取長40μm×深25μm的薄膜樣本，並使用穿透式電子顯微鏡來調查顯微組織。結果確認到，在薄膜樣本中生成有平均直徑50nm以下之微細AlN。The inventors of the present case used a focused ion beam machining observation device from the vicinity of the surface layer of the carburized member that had obtained good toughness, and took a thin film sample of 40 μm × 25 μm in depth, and investigated the microstructure using a transmission electron microscope. As a result, it was confirmed that fine AlN having an average diameter of 50 nm or less was generated in the thin film sample.

本案發明人等更為了調查AlN生成位置與母相組織之對應，實施了以下的分析。也就是說，使用聚焦離子束加工觀察裝置採取長100μm×深100μm的薄膜樣本，將該薄膜樣本固定於銅製網座(mesh holder)上之外，還供給到搭載有熱場發射型掃描式顯微鏡(JEOL製JSM-6500F)之穿透式電子背向散射繞射裝置來實施分析。從電子背向散射繞射法所得到的測定結果，再次構築出舊沃斯田鐵之結晶方位分布圖，並與穿透式電子顯微鏡圖像進行比較。於是，結果下述變得明朗：微細AlN是存在於舊沃斯田鐵晶界附近，還有，有析出微細AlN的舊沃斯田鐵晶界，其是存在於從鋼板最外表面起算至深度50μm左右為止之位置。也就是說，吾人認為，生成在鋼板表層(從鋼板最外表面起算至50μm為止的區域)的微細AlN，抑制了舊沃斯田鐵在滲碳熱處理時的粒成長後，結果在滲碳構件之組織中麻田散鐵的粒徑會微細化，而衝撃值會有飛躍性的增加。另外，在此所謂鋼板最外表面，乃意指鋼板母材之表面，而不包含鏽垢層等這類可能存在於鋼板母材表面的各種層體。The inventors of this case investigated the correspondence between the AlN generation position and the parent structure, and performed the following analysis. That is, a film sample with a length of 100 μm × depth of 100 μm is taken using a focused ion beam processing observation device, and the film sample is fixed to a copper mesh holder and supplied to a scanning microscope equipped with a thermal field emission. (JSM-6500F manufactured by JEOL) was analyzed with a penetrating electron backscatter diffraction device. From the measurement results obtained by the electron backscatter diffraction method, the crystal orientation distribution map of the old Vostian iron was constructed again, and compared with the image of a transmission electron microscope. As a result, it became clear that fine AlN exists near the old Vosstian iron grain boundary, and there is an old Vostian iron grain boundary where fine AlN is deposited, which exists from the outermost surface of the steel sheet to a depth of about 50 μm. Its location. In other words, I think that the fine AlN formed on the surface of the steel plate (the area from the outermost surface of the steel plate to 50 μm) inhibits the grain growth of the old Vosted iron during carburizing heat treatment, and as a result, the carburized member The particle size of Asada loose iron in the microstructure will be refined, and the impulse value will increase dramatically. In addition, the outermost surface of the steel plate herein means the surface of the steel plate base material, and does not include various layers such as a rust layer that may exist on the surface of the steel plate base material.

本案發明人等還使用已獲得良好韌性之滲碳構件，並使用搭載有波長分散型X射線光譜儀與場發射型電子槍的電子探針微量分析器，測定了自鋼板表面起至鋼板中心部為止的氮濃度分布剖面(profile)。結果確認到，鋼板表層(即，從鋼板最外表面起算至50μm為止的區域)之平均氮濃度達0.040質量%以上。The inventors of the present case also used a carburizing member that has obtained good toughness, and measured the distance from the surface of the steel plate to the center of the steel plate using an electron probe microanalyzer equipped with a wavelength-dispersive X-ray spectrometer and a field emission electron gun. Nitrogen concentration profile. As a result, it was confirmed that the average nitrogen concentration of the steel sheet surface layer (that is, the area from the outermost surface of the steel sheet to 50 μm) reached 0.040% by mass or more.

本案發明人等反覆精心探討之結果確認到，若將板厚中央部之平均氮濃度(更詳細來說，是從板厚中央部起算朝向表面側至100μm之位置為止的平均氮濃度)控制在0.2質量%以下後，以此鋼板作為素材，並在已將氮濃度控制在體積分率計25%以上之氣體環境中，以5℃/h以上且100℃/h以下之平均加熱速度，將作為素材的鋼板加熱至Ac 1點以下之溫度區域為止，並在該Ac 1點以下之溫度區域保持10h以上且100h以下之後，以5℃/h以上且100℃/h以下之平均冷卻速度進行冷卻，若為如此，則鋼板表層的平均氮濃度就會在0.040質量%以上且0.200質量%以下。也就是說，在氮濃度已控制於體積分率計25%以上之氣體環境下，以5℃/h以上且100℃/h以下之平均加熱速度，將鋼板加熱至Ac 1點以下之溫度區域為止，並在此Ac 1點以下之溫度區域保持10h以上且100h以下之後，以5℃/h以上且100℃/h以下之平均冷卻速度進行冷卻，藉此，在鋼板表層中會生成50nm以下之微細AlN。結果認為，鋼板表層的平均氮濃度就會在0.040質量%以上且0.200質量%以下。又，透過如上所述的退火所生成之微細AlN組織，其大致不因冷加工而產生變化，而有助於抑制舊沃斯田鐵在滲碳熱處理時的粒成長。 The inventors of the present case have repeatedly and carefully examined the results. It has been confirmed that if the average nitrogen concentration in the central portion of the plate thickness (more specifically, the average nitrogen concentration from the central portion of the plate thickness toward the surface side to a position of 100 μm) is controlled to After 0.2% by mass or less, use this steel plate as a material, and in a gas environment in which the nitrogen concentration has been controlled to 25% by volume fraction, at an average heating rate of 5 ° C / h or more and 100 ° C / h or less, The steel plate used as the material is heated to a temperature range below Ac 1 point, and maintained at a temperature range below Ac 1 point for 10 hours to 100 hours, and then performed at an average cooling rate of 5 ° C / h or more and 100 ° C / h or less. When cooling, the average nitrogen concentration in the surface layer of the steel sheet is 0.040 mass% or more and 0.200 mass% or less. That is, in a gas environment in which the nitrogen concentration has been controlled at 25% or more by volume fraction, the steel plate is heated to a temperature range below Ac 1 point at an average heating rate of 5 ° C / h to 100 ° C / h. After keeping the temperature range below Ac 1 point for more than 10 hours and less than 100 hours, cooling is performed at an average cooling rate of 5 ° C / h or more and 100 ° C / h or less, thereby generating 50 nm or less in the surface layer of the steel sheet. Fine AlN. As a result, it is considered that the average nitrogen concentration in the surface layer of the steel sheet is 0.040 mass% or more and 0.200 mass% or less. In addition, the fine AlN structure generated by the annealing described above does not change substantially due to cold working, and contributes to suppressing grain growth of old Vosted iron during carburizing heat treatment.

如以上所述，本案發明人等精心探討之結果，下述變得明朗：若滲碳用鋼板之鋼板表層(從鋼板最外表面起算至50μm為止的區域)中的平均氮濃度為0.040質量%以上，則在鋼板表層中會有微細AlN生成，而滲碳構件的衝撃值會提升。鋼板表層中的平均氮濃度宜為0.045質量%以上。另一方面，當鋼板表層中的平均氮濃度大於0.200質量%時，會生成粗大氮化物而韌性會劣化。因此，鋼板表層中的平均氮濃度是以0.200質量%為上限。鋼板表層中的平均氮濃度宜為0.150質量%以下。As described above, as a result of careful study by the inventors of the present case, it became clear that the average nitrogen concentration in the surface layer of the steel sheet (area from the outermost surface of the steel sheet to 50 μm) of carburizing steel was 0.040% by mass. As described above, fine AlN is generated in the surface layer of the steel sheet, and the impact value of the carburized member is increased. The average nitrogen concentration in the surface layer of the steel sheet is preferably 0.045% by mass or more. On the other hand, when the average nitrogen concentration in the surface layer of the steel sheet is more than 0.200% by mass, coarse nitrides are generated and toughness is deteriorated. Therefore, the average nitrogen concentration in the surface layer of the steel sheet is 0.200% by mass as an upper limit. The average nitrogen concentration in the surface layer of the steel sheet is preferably 0.150% by mass or less.

接著，說明鋼板表面中的平均氮濃度之測定方法。
如先前所提及，透過退火所生成的微細AlN組織，其大致不因冷加工而產生變化，而在滲碳熱處理時會有助於抑制舊沃斯田鐵的粒成長。因此，將熱輥軋鋼板或冷輥軋鋼板供給至退火後的滲碳用鋼板用來調查氮分布剖面即可。 Next, a method for measuring the average nitrogen concentration in the surface of the steel sheet will be described.
As mentioned earlier, the fine AlN structure produced by annealing does not change substantially due to cold working, and it will help suppress the growth of old Vosted iron grains during carburizing heat treatment. Therefore, the hot-rolled steel sheet or cold-rolled steel sheet may be supplied to the annealed steel sheet for carburizing to investigate the nitrogen distribution profile.

具體而言，首先，從滲碳用鋼板，以垂直其表面的剖面(板厚剖面)可供觀察之方式，切出樣本。樣本的長度雖是視測定裝置而定，但10mm~25mm左右即可。使用日本電子製的截面拋光儀(Cross section polisher)與日本電子製的樣品旋轉台，透過氬離子束，將測定面調整至使測定面不會產生條紋狀凹凸。之後，使用搭載有波長分散型X射線光譜儀及場發射型電子槍的電子探針微量分析器，以50nm之間隔而測定出從鋼板最外表面起至板厚中央部(板厚1/2位置)為止的氮濃度分布剖面。之後，算出從鋼板最表層起至深度50μm之位置為止的氮濃度(單位：質量%)的平均值，並作為如先所提及之鋼板表層中的平均氮濃度。還有，從板厚中央部起朝向表面側至100μm為止的氮濃度(單位：質量%)平均值，則作為板厚中央部的平均氮濃度。另外，由於氮在退火步驟中的入侵量在捲材的正反面並無明顯差異，故上述測定只要在鋼板正反面的一者或另一者進行實施即可。Specifically, first, a sample is cut out from a steel sheet for carburizing so that a cross section (plate thickness cross section) perpendicular to the surface thereof can be observed. Although the length of the sample depends on the measurement device, it may be about 10 mm to 25 mm. Using a cross section polisher made by Nippon Electronics and a sample turntable made by Nippon Electronics, the measurement surface was adjusted so that the measurement surface would not have streak-like irregularities through the argon ion beam. Then, using an electron probe microanalyzer equipped with a wavelength-dispersive X-ray spectrometer and a field emission electron gun, the distance from the outermost surface of the steel plate to the center of the plate thickness (the position of the plate thickness 1/2) was measured at 50 nm intervals. Nitrogen concentration profile. After that, the average value of the nitrogen concentration (unit: mass%) from the surface layer of the steel plate to a position having a depth of 50 μm was calculated and used as the average nitrogen concentration in the surface layer of the steel plate as mentioned earlier. The average nitrogen concentration (unit: mass%) from the center of the plate thickness toward the surface side to 100 μm is taken as the average nitrogen concentration of the center of the plate thickness. In addition, since the amount of nitrogen invasion during the annealing step is not significantly different between the front and back sides of the coil, the above measurement may be performed on one or the other side of the steel sheet.

＜關於滲碳用鋼板之板厚＞
以本實施形態滲碳用鋼板之板厚來說，並未特別限定，不過宜設為例如2mm以上。藉由將滲碳用鋼板之板厚設為2mm以上，就能使捲材寬度方向之板厚差變得較小。滲碳用鋼板之板厚較宜為2.3mm以上。又，滲碳用鋼板之板厚並未特別限定，不過宜設為6mm以下。藉由將滲碳用鋼板之板厚設為6mm以下，可降低壓製成形時的荷重，而能使零件的成形變得較容易進行。滲碳用鋼板之板厚較宜為5.8mm以下。 ＜ About the thickness of steel plate for carburizing ＞
The thickness of the steel sheet for carburizing in this embodiment is not particularly limited, but is preferably set to, for example, 2 mm or more. By setting the plate thickness of the steel sheet for carburizing to 2 mm or more, the plate thickness difference in the coil width direction can be made small. The thickness of the steel plate for carburizing is preferably 2.3 mm or more. The thickness of the steel sheet for carburizing is not particularly limited, but is preferably set to 6 mm or less. By setting the thickness of the steel sheet for carburizing to 6 mm or less, the load at the time of press forming can be reduced, and the forming of the part can be performed more easily. The thickness of the steel plate for carburizing is preferably 5.8 mm or less.

以上，已詳細說明完本實施形態的滲碳用鋼板。The steel sheet for carburizing of this embodiment has been described in detail above.

(關於滲碳用鋼板的製造方法)
接著，針對用以製造如以上說明之本實施形態滲碳用鋼板的方法，進行詳細說明。 (About the manufacturing method of the carburizing steel plate)
Next, a method for manufacturing the steel sheet for carburizing according to the embodiment described above will be described in detail.

用以製造如以上說明之本實施形態滲碳用鋼板的製造方法，包含：(A)熱輥軋步驟，是使用具有如先前說明之化學組成的鋼材，依據預定條件來製造熱輥軋鋼板；及(B)退火步驟，是對於所得到的熱輥軋鋼板、或對於在熱輥軋步驟後施行過冷輥軋的鋼板，依據預定熱處理條件來施行退火處理。
以下，針對上述熱輥軋步驟及退火步驟，進行詳細說明。 The manufacturing method for manufacturing the steel sheet for carburizing according to this embodiment as described above includes: (A) a hot rolling step for manufacturing a hot rolled steel sheet using a steel material having a chemical composition as described above according to predetermined conditions; (B) The annealing step is performed on the obtained hot-rolled steel sheet, or on the steel sheet subjected to super-cold rolling after the hot-rolling step, according to predetermined heat treatment conditions.
Hereinafter, the hot rolling step and the annealing step will be described in detail.

＜關於熱輥軋步驟＞
以下詳述的熱輥軋步驟，是使用具有預定化學組成的鋼材，依據預定條件來製造熱輥軋鋼板的步驟。 ＜ About hot rolling procedure ＞
The hot-rolling step detailed below is a step of manufacturing a hot-rolled steel sheet using a steel material having a predetermined chemical composition in accordance with predetermined conditions.

在此，供給至熱輥軋的鋼片(鋼材)為一般作法所製造之鋼片即可，例如可使用連續鑄造鋼胚、薄鋼胚鑄造機等一般方法所製造之鋼片。Here, the steel sheet (steel material) supplied to the hot rolling may be a steel sheet manufactured by a conventional method, and for example, a steel sheet manufactured by a general method such as a continuous casting steel billet or a thin steel billet casting machine can be used.

又，從提升韌性之觀點來看，就供給至熱輥軋的鋼材而言，MnS等的夾雜物、Mn的中心偏析宜越少越好。因此，例如，在用以獲得供給至熱輥軋之鋼片的連續鑄造步驟中，宜施行鋼材健全化處理，該鋼材健全化處理是如控制每單位時間的熔融鋼鑄造量來生成預定夾雜物，或者，在鑄片完全凝固前實施降低中心偏析的處理。From the viewpoint of improving toughness, it is preferable that the number of inclusions such as MnS and the center segregation of Mn should be as small as possible for the steel material supplied to the hot rolling. Therefore, for example, in a continuous casting step for obtaining a steel sheet to be supplied to hot rolling, it is desirable to perform a steel-hardening treatment, such as controlling a molten steel casting amount per unit time to generate a predetermined inclusion Or, to reduce the central segregation before the slab is completely solidified.

更詳細來說，是使用具有如前說明之化學組成的鋼材，將該鋼材加熱並供給至熱輥軋，在800℃以上且小於920℃之溫度區域結束熱精輥軋，並在700℃以下之溫度進行捲取來作成熱輥軋鋼板。此時，將熱精輥軋後的冷卻開始時間設為熱精輥軋結束時起算1秒以內，並且，將熱精輥軋後的平均冷卻速度設為大於50℃/s。In more detail, the steel material having the chemical composition as described above is used to heat and supply the steel material to hot rolling, and finish the hot finishing rolling in a temperature range of 800 ° C or higher and less than 920 ° C, and the temperature is 700 ° C or lower. It is coiled at this temperature to form a hot-rolled steel sheet. At this time, the cooling start time after the hot finishing rolling is set to be within 1 second from the end of the hot finishing rolling, and the average cooling rate after the hot finishing rolling is set to be greater than 50 ° C / s.

[熱精輥軋之輥軋溫度：800℃以上且小於920℃]
在本實施形態的熱輥軋步驟中，必須在800℃以上的輥軋溫度來施行熱精輥軋的輥軋。當熱精輥軋時之輥軋溫度(即，精輥軋溫度)小於800℃而低溫化時，由於肥粒鐵變態開始溫度也會降低，故會導致析出的碳化物產生粗大化。如此一來，在後段退火步驟中此等粗大碳化物的粒成長會受到助長，結果會導致彎曲性劣化。據此，在本實施形態的熱輥軋步驟中，是將精輥軋溫度設為800℃以上。精輥軋溫度宜為830℃以上。另一方面，當精輥軋溫度達920℃以上時，則沃斯田鐵粒就會顯著粗大化，而肥粒鐵的成核位置會減少，結果肥粒鐵的變態開始溫度會下降，而析出的碳化物會變得容易粗大化。在此情況下，在後段退火步驟中此等粗大碳化物的粒成長會受到助長，結果會導致彎曲性劣化。據此，在本實施形態的熱輥軋步驟中，是將精輥軋溫度設為小於920℃。精輥軋溫度宜小於900℃。 [Rolling temperature of hot finishing rolling: 800 ° C or more and less than 920 ° C]
In the hot rolling step of this embodiment, it is necessary to perform hot finishing rolling at a rolling temperature of 800 ° C or higher. When the rolling temperature during hot finishing rolling (ie, the finishing rolling temperature) is lower than 800 ° C. and the temperature is lowered, the ferrous grain iron metamorphic onset temperature will also decrease, which will cause coarsening of the precipitated carbides. In this way, the grain growth of these coarse carbides is promoted in the subsequent annealing step, and as a result, the bendability is deteriorated. Accordingly, in the hot rolling step of this embodiment, the finishing rolling temperature is set to 800 ° C or higher. The finishing rolling temperature should be above 830 ° C. On the other hand, when the finishing rolling temperature is above 920 ° C, the iron particles of Vostian will be significantly coarsened, and the nucleation position of the ferrous iron will be reduced. The precipitated carbides tend to coarsen. In this case, the grain growth of these coarse carbides is promoted in the subsequent annealing step, and as a result, the bendability is deteriorated. Accordingly, in the hot rolling step of this embodiment, the finishing rolling temperature is set to be lower than 920 ° C. The finishing rolling temperature should be less than 900 ° C.

[捲取溫度：700℃以下]
如先前所提及，滲碳用鋼板的顯微組織必須如下：總碳化物中縱橫比2.0以下之碳化物個數比率為80%以上，總碳化物中形成於肥粒鐵結晶粒內的碳化物個數比率為60%以上，碳化物的平均等效圓直徑為5.0μm以下，鋼板表層的平均氮濃度為0.040質量%以上且0.200質量%以下。為此，供給到後段退火步驟(更詳細來說，是球狀化退火)之前的鋼板組織(熱輥軋鋼板組織)宜為：主要含有面積率合計為100%以下的肥粒鐵與波來鐵，該肥粒鐵以面積率計為10%以上且80%以下，該波來鐵以面積率計為10%以上且60%以下；剩餘部分則由變韌鐵、麻田散鐵、回火麻田散鐵、及殘留沃斯田鐵之至少任一者所構成。 [Winding temperature: 700 ° C or less]
As mentioned earlier, the microstructure of the steel plate for carburizing must be as follows: the ratio of the number of carbides with an aspect ratio of 2.0 or less in the total carbides is 80% or more; The number of objects ratio is 60% or more, the average equivalent circle diameter of the carbide is 5.0 μm or less, and the average nitrogen concentration of the steel sheet surface layer is 0.040% by mass or more and 0.200% by mass or less. For this reason, the steel sheet structure (hot rolled steel sheet structure) supplied before the subsequent annealing step (more specifically, spheroidizing annealing) is preferably: mainly containing ferrous iron and wave iron with a total area ratio of 100% or less Iron, the fertile iron is 10% to 80% in terms of area ratio, and the wave iron is 10% to 60% in terms of area ratio; the remainder is made of toughened iron, Asada loose iron, tempering It is composed of at least one of the Asada loose iron and the remaining Vosda iron.

在本實施形態的熱輥軋步驟中，當捲取溫度大於700℃時，會過度促進肥粒鐵變態，結果會導致波來鐵的生成受到抑制，而在退火後的滲碳用鋼板中，便會難以將總碳化物中縱橫比2.0以下之碳化物個數比率控制在80%以上。因此，在本實施形態的熱輥軋步驟中，是將捲取溫度上限設為700℃。就本實施形態的熱輥軋步驟之捲取溫度而言，下限並未特別限制。但是，由於實際機械作業上要在室溫以下進行捲取是有困難的，故室溫為實質下限。另外，為了使後段退火步驟後碳化物縱橫比變得更小，從此觀點出發，本實施形態的熱輥軋步驟之捲取溫度宜為400℃以上。In the hot rolling step of this embodiment, when the coiling temperature is higher than 700 ° C, the transformation of ferrous iron is excessively promoted, and as a result, the generation of sporadic iron is suppressed. In the carburized steel sheet after annealing, It will be difficult to control the ratio of the number of carbides with an aspect ratio of 2.0 or less in the total carbides to 80% or more. Therefore, in the hot rolling step of this embodiment, the upper limit of the coiling temperature is set to 700 ° C. The lower limit of the coiling temperature in the hot rolling step of this embodiment is not particularly limited. However, since it is difficult to take up coils at a temperature below room temperature in actual mechanical operations, room temperature is a substantial lower limit. In addition, in order to make the carbide aspect ratio smaller after the post-annealing step, from this point of view, the coiling temperature in the hot rolling step of this embodiment is preferably 400 ° C or higher.

[熱精輥軋後的冷卻開始時間：熱精輥軋結束時起算1秒以內]
[熱精輥軋後的平均冷卻速度：大於50℃/s]
在本實施形態的熱輥軋步驟中，是在熱精輥軋結束時起算1秒以內，開始進行平均冷卻速度大於50℃/s的冷卻。藉此，就能使熱精輥軋後的沃斯田鐵粒微細化。藉由令熱精輥軋後的沃斯田鐵粒微細化，就能將後段退火步驟(更詳細來說，是球狀化退火)後的肥粒鐵平均粒徑控制在小於10μm。 [Cooling start time after hot finishing rolling: within 1 second from the end of hot finishing rolling]
[Average cooling rate after hot finishing rolling: more than 50 ° C / s]
In the hot-rolling step of this embodiment, cooling with an average cooling rate of more than 50 ° C./s is started within 1 second from the end of the hot-rolling. This makes it possible to miniaturize the Vosted iron particles after the hot finishing rolling. By minimizing the Vosted iron particles after hot finishing rolling, the average particle size of the ferrous iron particles after the subsequent annealing step (more specifically, spheroidizing annealing) can be controlled to less than 10 μm.

當熱精輥軋後的冷卻開始時間是從結束時起算大於1秒時，則會導致沃斯田鐵粒粗大化，且球狀化退火後的肥粒鐵的平均結晶粒徑會大於10μm，而無法顯現出使結晶粒微細化所帶來的效果。熱精輥軋後的冷卻開始時間從結束時起算宜在0.8秒以內。冷卻開始時間之下限值並未特別限制。但是，由於實際機械作業上要將冷卻開始時間從結束時起算設為小於0.01秒是有困難的，故0.01秒為實質下限。When the cooling start time after hot finish rolling is greater than 1 second from the end, the Vostian iron particles will be coarsened, and the average crystal grain size of the ferrous iron after spheroidizing annealing will be greater than 10 μm. However, the effect of miniaturizing crystal grains cannot be exhibited. The cooling start time after hot finishing rolling should be within 0.8 seconds from the end. The lower limit of the cooling start time is not particularly limited. However, since it is difficult to set the cooling start time to less than 0.01 seconds from the end in actual mechanical work, 0.01 second is the substantial lower limit.

又，當熱精輥軋後的平均冷卻速度為50℃/s以下時，會導致沃斯田鐵粒粗大化，且後段球狀化退火後的肥粒鐵平均結晶粒徑會大於10μm。熱精輥軋後的平均冷卻速度宜為55℃/s以上。平均冷卻速度之上限值並未特別規定。但是，由於實際機械作業上要將平均冷卻速度設為300℃/s以上是有困難的，故300℃/s為實質上限。In addition, when the average cooling rate after hot finishing rolling is 50 ° C./s or less, the Vostedt iron particles are coarsened, and the average grain size of the ferrous iron particles after the subsequent spheroidizing annealing is greater than 10 μm. The average cooling rate after hot finishing rolling should be above 55 ° C / s. The upper limit of the average cooling rate is not particularly specified. However, since it is difficult to set the average cooling rate to 300 ° C / s or more in actual mechanical operation, 300 ° C / s is substantially limited.

另外，以如上所述熱輥軋步驟進行捲取後的鋼板(熱輥軋鋼板)，亦可回捲並酸洗，再施行冷輥軋。以酸洗來除去鋼板表面氧化物，藉此期待能更為提升擴孔性等。另外，酸洗可進行一次，也可分成數次來進行。冷輥軋可以是以一般軋縮率(例如，30~90%)來進行的冷輥軋。就熱輥軋鋼板及冷輥軋鋼板來說，除了經熱輥軋及冷輥軋後保持原樣者以外，也還包含以一般條件施行過調質輥軋的鋼板。In addition, the steel sheet (hot-rolled steel sheet) coiled in the hot-rolling step as described above may be rolled back and pickled, and then cold-rolled. By removing acid on the surface of the steel sheet by pickling, it is expected to further improve the hole expandability. In addition, the acid washing may be performed once or may be divided into several times. The cold rolling may be cold rolling performed at a general reduction rate (for example, 30 to 90%). The hot-rolled steel sheet and the cold-rolled steel sheet include steel sheets that have been subjected to quenching and tempering under normal conditions, in addition to those that have been left as they are after being hot-rolled and cold-rolled.

在本實施形態的熱輥軋步驟中，透過以上方式而可製造出熱輥軋鋼板。對於所製造出的熱輥軋鋼板、或者對於熱輥軋步驟後施行過冷輥軋的鋼板，進一步可藉由如以下所詳述之退火步驟來施行特定退火處理，藉此獲得本實施形態的滲碳用鋼板。In the hot-rolling step of this embodiment, a hot-rolled steel sheet can be manufactured by the above method. For the manufactured hot-rolled steel sheet, or for the steel sheet subjected to super-cold rolling after the hot-rolling step, a specific annealing treatment can be further performed by an annealing step as described in detail below, thereby obtaining the Carburizing steel sheet.

＜關於退火步驟＞
以下詳述的退火步驟，是對於上述熱輥軋步驟所得熱輥軋鋼板、或對於熱輥軋步驟後施行過冷輥軋的鋼板，依據預定熱處理條件施行退火處理(球狀化退火處理)的步驟。透過這種退火處理，使熱輥軋步驟中生成的波來鐵球狀，並將球狀化退火後肥粒鐵的平均結晶粒徑控制在小於10μm。 ＜ About annealing steps ＞
The annealing step detailed below is performed on the hot-rolled steel sheet obtained in the hot-rolling step described above, or on the steel sheet subjected to super-cold rolling after the hot-rolling step, according to a predetermined heat treatment condition (spheroidizing annealing treatment). step. Through this annealing treatment, the boron iron generated in the hot rolling step is spherical, and the average crystal grain size of the ferrous iron after the spheroidizing annealing is controlled to less than 10 μm.

更詳言之，將以上述方式所得熱輥軋鋼板、或將熱輥軋步驟後施行過冷輥軋的鋼板，透過已將氮濃度控制在體積分率計25%以上之氣體環境，以5℃/h以上且100℃/h以下之平均加熱速度，加熱至下述式(101)所定義的Ac 1點以下之溫度區域為止，並施行在Ac 1點以下之溫度區域保持10h以上且100h以下的退火處理後，施行下述冷卻，該冷卻是自退火結束時之溫度起至550℃為止之溫度區域的平均冷卻速度設為5℃/h以上且100℃/h以下。
此處，下述式(101)中，[X]之標記是表示元素X的含量(單位：質量%)，不含該元素時則代入零。 More specifically, the hot-rolled steel sheet obtained in the above manner, or the steel sheet subjected to super-cold rolling after the hot-rolling step, is passed through a gas environment in which the nitrogen concentration has been controlled to 25% or more by volume. The average heating rate above ℃ / h and below 100 ℃ / h is heated to a temperature range below Ac 1 point defined by the following formula (101), and it is maintained in the temperature range below Ac 1 point for 10 hours and 100 hours. After the following annealing treatment, cooling is performed in which the average cooling rate in a temperature range from the temperature at the end of the annealing to 550 ° C is set to 5 ° C / h or more and 100 ° C / h or less.
Here, in the following formula (101), the mark of [X] indicates the content (unit: mass%) of the element X, and when the element is not included, zero is substituted.

[數學式2]
[Mathematical formula 2]

[退火氣體環境：已將氮濃度控制在體積分率計25%以上之氣體環境]
在如上所述的退火步驟中，退火氣體環境是作成：已將氮濃度控制在體積分率計25%以上之氣體環境。當氮濃度以體積分率計小於25%時，便無法將鋼板表層的平均氮濃度控制在0.040質量%以上且0.200質量%以下。因此，在本實施形態的退火步驟中，是將退火氣體環境中的氮濃度設為以體積分率計25%以上。退火氣體環境中的氮濃度宜為以體積分率計75%以上，較宜為體積分率計80%以上。又，該氮濃度雖是越高越好，但由於要將氮濃度控制到以體積分率計99%以上，則在成本上並不利，故體積分率99%為實質上限。 [Annealing gas environment: the gas environment in which the nitrogen concentration has been controlled above 25% by volume fraction]
In the annealing step as described above, the annealing gas environment is prepared: the nitrogen concentration has been controlled to a gas environment with a volume fraction of 25% or more. When the nitrogen concentration is less than 25% in volume fraction, the average nitrogen concentration of the steel sheet surface layer cannot be controlled to be 0.040 mass% or more and 0.200 mass% or less. Therefore, in the annealing step of this embodiment, the nitrogen concentration in the annealing gas environment is set to 25% or more by volume fraction. The nitrogen concentration in the annealing gas environment should be more than 75% by volume fraction, and more preferably 80% by volume fraction. The higher the nitrogen concentration, the better, but it is not cost-effective to control the nitrogen concentration to 99% or more by volume fraction, so the volume fraction of 99% is substantially limited.

在本實施形態的退火步驟中，是將含氮原子之分子所構成的氣體作為氣體環境氣體來導入，並一邊控制退火氣體環境一邊進行熱處理。例如，使用設置於退火爐內的氣體濃度計，調整要導入退火步驟所用加熱爐中的氣體環境氣體的流量等，藉此來控制退火氣體環境即可。In the annealing step of this embodiment, a gas composed of molecules containing nitrogen atoms is introduced as a gas ambient gas, and the heat treatment is performed while controlling the atmosphere of the annealing gas. For example, the gas concentration meter installed in the annealing furnace may be used to control the annealing gas environment by adjusting the flow rate of the gas ambient gas to be introduced into the heating furnace used in the annealing step.

另外，氣體環境氣體的剩餘部分宜以氮之外的惰性氣體為主體，例如適宜使用氫、氬等的還元性氣體即可。更具體而言，就退火氣體環境來說，可將氮濃度以體積分率計設為75%以上，並以剩餘部分為氫。又，若量少，則就算氣體環境氣體中含有氧等氣體也不會有問題。In addition, the remainder of the gas environment gas should be mainly composed of an inert gas other than nitrogen. For example, a reducing gas such as hydrogen or argon can be suitably used. More specifically, in the case of an annealing gas environment, the nitrogen concentration can be set to 75% or more by volume fraction, and the remaining portion can be hydrogen. In addition, if the amount is small, there is no problem even if a gas such as oxygen is contained in the gas environment gas.

[加熱條件：以5℃/h以上且100℃/h以下之平均加熱速度，至Ac 1點以下之溫度區域為止]
本實施形態的退火步驟中，必須將如上所述的熱輥軋鋼板、或將熱輥軋步驟後施行過冷輥軋的鋼板，以5℃/h以上且100℃/h以下之平均加熱速度，加熱至以上述式(101)所界定的Ac 1點以下之溫度區域為止。當平均加熱速度小於5℃/h時，則碳化物的平均等效圓直徑會大於5.0μm，而彎曲性會劣化。另一方面，當平均加熱速度大於100℃/h時，便不會充分促進碳化物球狀化，而變得難以將總碳化物中縱橫比2.0以下之碳化物個數比率控制在80%以上。又，當加熱溫度大於上述式(101)所界定的Ac 1點時，會導致總碳化物中形成在肥粒鐵結晶粒內之碳化物的個數比率小於60%，而無法獲得良好的彎曲性。另外，加熱溫度之溫度區域的下限，並未特別規定，不過一旦加熱溫度之溫度區域小於600℃，則退火處理中的保持時間會變長，而製造成本會變得不利。因此，加熱溫度之溫度區域宜設為600℃以上。為了更適切控制碳化物的狀態，本實施形態退火步驟中的平均加熱速度宜設為20℃/h以上。又，為了更適切控制碳化物的狀態，本實施形態退火步驟中的平均加熱溫度宜設為50℃/h以下。為了更適切控制碳化物的狀態，本實施形態退火步驟中的加熱溫度之溫度區域較宜設為630℃以上。又，為了更適切控制碳化物的狀態，本實施形態退火步驟中的加熱溫度之溫度區域較宜設為670℃以下。 [Heating conditions: at an average heating rate of 5 ° C / h or more and 100 ° C / h or less, to a temperature range below Ac 1 point]
In the annealing step of this embodiment, the above-mentioned hot-rolled steel sheet or steel sheet subjected to super-cold rolling after the hot-rolling step must be heated at an average heating rate of 5 ° C / h or more and 100 ° C / h or less. , Heating to a temperature region below the Ac 1 point defined by the above formula (101). When the average heating rate is less than 5 ° C / h, the average equivalent circle diameter of the carbide will be greater than 5.0 μm, and the bendability will be deteriorated. On the other hand, when the average heating rate is more than 100 ° C / h, the spheroidization of carbides is not sufficiently promoted, and it becomes difficult to control the ratio of the number of carbides with an aspect ratio of 2.0 or less in the total carbide to 80% or more . In addition, when the heating temperature is higher than the Ac 1 point defined by the above formula (101), the ratio of the number of carbides formed in the iron crystal grains of the ferrite particles to the total carbides is less than 60%, and good bending cannot be obtained. Sex. In addition, the lower limit of the temperature range of the heating temperature is not particularly specified, but once the temperature range of the heating temperature is less than 600 ° C, the holding time during the annealing process becomes longer, and the manufacturing cost becomes disadvantageous. Therefore, the temperature range of the heating temperature should be set to 600 ° C or higher. In order to more appropriately control the state of carbides, the average heating rate in the annealing step of this embodiment is preferably set to 20 ° C / h or more. In addition, in order to more appropriately control the state of carbides, the average heating temperature in the annealing step of this embodiment is preferably set to 50 ° C./h or less. In order to more appropriately control the state of the carbide, the temperature range of the heating temperature in the annealing step of this embodiment is preferably set to 630 ° C or higher. In addition, in order to more appropriately control the state of carbides, the temperature range of the heating temperature in the annealing step of this embodiment is preferably set to 670 ° C or lower.

[保持時間：在Ac 1點以下之溫度區域，10h以上且100h以下]
本實施形態的退火步驟中，必須在如上所述的Ac 1點以下(宜為600℃以上且Ac 1點以下)之溫度區域，保持10h以上且100h以下。當保持時間小於10h時，便不會充分促進碳化物球狀化，而難以將總碳化物中縱橫比2.0以下之碳化物個數比率控制在80%以上。另一方面，當保持時間大於100h時，碳化物的平均等效圓直徑會大於5.0μm，而彎曲性會劣化。為了更適切控制碳化物的狀態，本實施形態退火步驟中的保持時間宜設為20h以上。又，為了更適切控制碳化物的狀態，本實施形態退火步驟中的保持時間宜設為80h以下。 [Holding time: in the temperature range below Ac 1 point, more than 10h and less than 100h]
The annealing step of the present embodiment, it is necessary at one point or less (preferably of above 600 ℃ and the Ac 1 point) above the temperature range of Ac, 10h and 100h holding more or less. When the holding time is less than 10 hours, the spheroidization of carbides is not sufficiently promoted, and it is difficult to control the ratio of the number of carbides with an aspect ratio of 2.0 or less in the total carbides to 80% or more. On the other hand, when the holding time is more than 100 hours, the average equivalent circle diameter of the carbide is larger than 5.0 μm, and the bendability is deteriorated. In order to more appropriately control the state of carbides, the holding time in the annealing step of this embodiment should preferably be set to 20 hours or more. In addition, in order to more appropriately control the state of carbides, the holding time in the annealing step of this embodiment is preferably set to 80 hours or less.

[冷卻條件：以5℃/h以上且100℃/h以下之平均冷卻速度來冷卻]
本實施形態的退火步驟中，如上所述的加熱保持後，將鋼板以5℃/h以上且100℃/h以下之平均冷卻速度進行冷卻。在此，所謂平均冷卻速度，是自加熱保持溫度(換言之，退火結束時之溫度)起至550℃為止的平均冷卻速度。當平均冷卻速度小於5℃/h時，碳化物會過度粗大化，而彎曲性會劣化。另一方面，當平均冷卻速度大於100℃/h時，便不會充分促進碳化物球狀化，而變得難以將總碳化物中縱橫比2.0以下之碳化物個數比率控制在80%以上。為了更適切控制碳化物的狀態，自加熱保持溫度起至550℃為止之平均冷卻速度宜設為20℃/h以上。又，為了更適切控制碳化物的狀態，本實施形態的退火步驟中，自加熱保持溫度起至550℃為止之平均冷卻速度宜設為50℃/h以下。 [Cooling conditions: cooling at an average cooling rate of 5 ° C / h or more and 100 ° C / h or less]
In the annealing step of this embodiment, after heating and holding as described above, the steel sheet is cooled at an average cooling rate of 5 ° C./h or more and 100 ° C./h or less. Here, the average cooling rate is an average cooling rate from the heating and holding temperature (in other words, the temperature at the end of annealing) to 550 ° C. When the average cooling rate is less than 5 ° C / h, carbides are excessively coarsened, and bendability is deteriorated. On the other hand, when the average cooling rate is greater than 100 ° C / h, the spheroidization of carbides is not sufficiently promoted, and it becomes difficult to control the ratio of the number of carbides with an aspect ratio of 2.0 or less in the total carbides to 80% or more . In order to more appropriately control the state of the carbide, the average cooling rate from the heating and holding temperature to 550 ° C should be set to 20 ° C / h or more. In addition, in order to more appropriately control the state of carbides, in the annealing step of this embodiment, the average cooling rate from the heating and holding temperature to 550 ° C is preferably set to 50 ° C / h or less.

又，就本實施形態的退火步驟而言，在小於550℃之溫度區域中的平均冷卻速度，並無特別規定，只要是以任意平均冷卻速度冷卻至預定溫度區域即可。另外，冷卻停止溫度的下限，並未特別規定。但是，要想冷卻至室溫以下，實際機械作業上是有困難的，故室溫為實質下限。In the annealing step of this embodiment, the average cooling rate in a temperature range of less than 550 ° C is not particularly limited, and it may be cooled to a predetermined temperature range at an arbitrary average cooling rate. The lower limit of the cooling stop temperature is not particularly specified. However, in order to cool below room temperature, actual mechanical operations are difficult, so room temperature is the practical lower limit.

以上，已詳細說明完本實施形態的退火步驟。
透過實施如以上說明之熱輥軋步驟及退火步驟，就能製造出如先前說明之本實施形態的滲碳用鋼板。 The annealing step of this embodiment has been described in detail.
By performing the hot rolling step and the annealing step as described above, the steel sheet for carburizing of this embodiment as described above can be manufactured.

另外，在實施如以上說明的退火步驟之前，亦可將熱輥軋後的鋼板在大氣中以40℃以上且70℃以下之溫度區域，保持72h以上且350h以下。透過施行如此之保持，能讓肥粒鐵結晶粒內固溶的碳凝集體形成。這種碳凝集體，是數個原子碳在肥粒鐵結晶粒內凝集而成者。讓這種碳凝集體形成，藉此更為促進後段退火步驟中的碳化物形成。結果，能更為提升退火後鋼板中差排的移動容易度，而更為提升退火後鋼板的成形性。In addition, before performing the annealing step as described above, the hot-rolled steel sheet may be maintained in the air at a temperature range of 40 ° C to 70 ° C for 72 hours to 350 hours. By carrying out such a maintenance, carbon aggregates that are solid-solved in the iron crystal grains of the fat particles can be formed. This carbon agglomerate is formed by aggregating several atomic carbons in the iron crystal grains of the fertilizer. By allowing such carbon aggregates to form, the carbide formation in the subsequent annealing step is further promoted. As a result, the ease of movement of the differential rows in the steel sheet after annealing can be further improved, and the formability of the steel sheet after annealing can be further improved.

又，對於以上方式所得到的滲碳用鋼板，可例如施行冷加工作為後續步驟。又，對於經冷加工過的上述滲碳用鋼板，可例如在碳勢為0.4~1.0質量%之範圍下施行滲碳熱處理。滲碳熱處理條件並未特別限定，可適宜調整成能獲得所欲特性之方式。例如，可將滲碳用鋼板加熱至沃斯田鐵單相區域溫度為止，並在滲碳處理後，直接冷卻至室溫，亦可先暫時冷卻至室溫後再次加熱並急速冷卻。此外，以調整強度為目的，亦可對構件的整體或部分，施行回火處理。又，以獲得防鏽效果為目的，可在鋼板表面施予鍍敷；以提升疲勞特性為目的，則亦可在鋼板表面施予珠擊。
[實施例] Further, the steel sheet for carburizing obtained in the above manner may be subjected to cold working, for example, as a subsequent step. Further, the above-mentioned carburized steel sheet which has been cold worked may be subjected to a carburizing heat treatment in a range of a carbon potential of 0.4 to 1.0% by mass, for example. The conditions of the carburizing heat treatment are not particularly limited, and can be appropriately adjusted so as to obtain desired characteristics. For example, the steel sheet for carburizing can be heated to the temperature of the single phase area of Vosstian iron, and after the carburizing treatment, it can be directly cooled to room temperature, or it can be temporarily cooled to room temperature and then heated again and rapidly cooled. In addition, for the purpose of adjusting the strength, the whole or part of the component may be tempered. In addition, for the purpose of obtaining the anti-rust effect, plating can be applied to the surface of the steel sheet; for the purpose of improving fatigue characteristics, beading can also be applied to the surface of the steel sheet.
[Example]

接著，針對本發明的實施例進行說明。另外，實施例中的條件，是用來確認本發明的可實施性及效果所採用的一條件例，本發明並不受此一條件例所限定。只要不脫離本發明的要旨，並達成本發明之目的，本發明能採用各種條件。Next, embodiments of the present invention will be described. In addition, the conditions in the examples are examples of conditions used to confirm the feasibility and effect of the present invention, and the present invention is not limited by such a condition example. As long as it does not deviate from the gist of the present invention and achieve the purpose of the present invention, the present invention can adopt various conditions.

(試驗例)
將具有以下表1所示化學組成的鋼材，依以下表2所示條件進行熱輥軋(及冷輥軋)後，施行退火而獲得滲碳用鋼板。依以下表2所示條件進行熱輥軋後，於大氣中55℃下保持105小時之外，還依以下表2所示條件進行退火。在此，就以下表2所示條件之一例而言，在用以獲得供給至熱輥軋之鋼材的連續鑄造步驟，透過控制每單位時間的熔融鋼鑄造量來施行鋼材健全化處理。另外，以下表1及表2中，底線表示本發明之範圍外。 (Test example)
A steel having a chemical composition shown in Table 1 below was subjected to hot rolling (and cold rolling) under the conditions shown in Table 2 below, and then annealed to obtain a steel sheet for carburizing. After hot rolling was performed under the conditions shown in Table 2 below, it was annealed under the conditions shown in Table 2 in addition to being held at 55 ° C in the atmosphere for 105 hours. Here, as one example of the conditions shown in Table 2 below, in the continuous casting step to obtain the steel material to be supplied to the hot rolling, the steel material sanitizing treatment is performed by controlling the amount of molten steel cast per unit time. In addition, in Tables 1 and 2 below, the underline indicates outside the scope of the present invention.

[表1-1]
[Table 1-1]

[表1-2]
[Table 1-2]

[表2-1]
[table 2-1]

[表2-2]
[Table 2-2]

[表2-3]
[Table 2-3]

就所得之各個滲碳用鋼板，透過先前說明的方法而測定：(1)總碳化物中縱橫比2.0以下之碳化物個數比率、(2)總碳化物中形成於肥粒鐵結晶粒內的碳化物個數比率、(3)碳化物的平均等效圓直徑、(4)鋼板表層的平均氮濃度、及(5)球狀化退火後的肥粒鐵平均結晶粒徑。另外，球狀化退火後的肥粒鐵平均結晶粒徑，是所獲得之滲碳用鋼板中肥粒鐵的平均結晶粒徑。Each of the obtained steel sheets for carburizing was measured by the method described previously: (1) the ratio of the number of carbides having an aspect ratio of 2.0 or less in the total carbides, and (2) the total carbides formed in the iron crystal grains of the ferrite grains. Ratio of the number of carbides, (3) the average equivalent circle diameter of carbides, (4) the average nitrogen concentration of the steel sheet surface, and (5) the average grain size of ferrous iron after spheroidizing annealing. The average grain size of the ferrous iron after spheroidizing annealing is the average grain size of the ferrous iron in the obtained steel sheet for carburizing.

又，為了評價所得之各個滲碳用鋼板的彎曲性，從滲碳用鋼板之任意位置採取試驗片，並根據德國汽車工業會所規定之VDA基準(VDA238-100)，以下述條件實施了彎曲性的測定。在本試驗例中，彎曲試驗所得到最大荷重時之位移以VDA基準而轉換成角度，並求出最大彎曲角度(單位：度)。In addition, in order to evaluate the bendability of each obtained carburizing steel sheet, test pieces were taken from any position of the carburizing steel sheet, and the bendability was performed under the following conditions in accordance with the VDA standard (VDA238-100) prescribed by the German Automobile Industry Association. Determination. In this test example, the displacement at the maximum load obtained in the bending test was converted into an angle based on the VDA standard, and the maximum bending angle (unit: degree) was obtained.

試驗片尺寸：30mm(輥軋方向)×60mm(與輥軋垂直之方向)
彎曲稜線：與輥軋平行之方向
試驗方法：輥支撐、衝頭施壓
輥徑：φ30mm
衝頭形狀：前端R＝0.4mm
輥間距：2.0×板厚(mm)+0.5mm
施壓速度：20mm/min
試驗機：SHIMADZU AUTOGRAPH(註冊商標) 20kN Test piece size: 30mm (rolling direction) x 60mm (direction perpendicular to rolling)
Bending ridge line: direction parallel to rolling test method: roll support, punch pressure roll diameter: φ30mm
Punch shape: front end R = 0.4mm
Roller spacing: 2.0 × plate thickness (mm) + 0.5mm
Pressure speed: 20mm / min
Testing machine: SHIMADZU AUTOGRAPH (registered trademark) 20kN

又，為了對於所得之各個滲碳用鋼板評價滲碳後之韌性，是分別對於所得之各個滲碳用鋼板，施行了以下的滲碳處理。也就是說，將各個滲碳用鋼板在碳勢為0.8質量%之氣體環境下，於900℃保持了2.5hr後，進一步於850℃保持0.5hr來施予滲碳處理，並以100℃進行了油淬火。之後，於160℃保持2.0hr來施予回火處理，並空冷至室溫。從滲碳熱處理後的鋼板任意位置，採取2mm-V凹口夏比試驗片，並依照JIS Z2242所規定之試驗方法，在室溫下實施夏比試驗而測定了衝撃值(J/cm 2)。 In addition, in order to evaluate the toughness after carburization of each of the obtained steel sheets for carburizing, each of the obtained steel sheets for carburizing was subjected to the following carburizing treatment. That is, each steel sheet for carburizing was maintained at 900 ° C. for 2.5 hr in a gas environment having a carbon potential of 0.8% by mass, and then was further maintained at 850 ° C. for 0.5 hr to be subjected to carburizing treatment. Oil quenched. After that, tempering was performed while maintaining at 160 ° C. for 2.0 hr, and air-cooled to room temperature. A 2mm-V notch Charpy test piece was taken from an arbitrary position of the steel plate after carburizing heat treatment, and the impact value (J / cm 2 ) was measured by performing a Charpy test at room temperature in accordance with the test method specified in JIS Z2242. .

另外，作為參考而算出顯示滲碳後淬火性之指標，即理想臨界直徑。理想臨界直徑D i是一種從鋼板成分所算出的指標，並可使用Grossmann/Hollomon, Jaffe的方法依照以下式(201)來算出。理想臨界直徑D i之數值越大，則顯示淬火性越優異。 In addition, as an index, an index showing the hardenability after carburization, that is, the ideal critical diameter was calculated. The ideal critical diameter Di is an index calculated from the composition of the steel sheet, and can be calculated according to the following formula (201) using the method of Grossmann / Hollomon, Jaffe. The larger the value of the ideal critical diameter D i , the more excellent the hardenability.

[數學式3]
[Mathematical formula 3]

本試驗例中，滲碳用鋼板之最大彎曲角為100°以上且滲碳後的衝撃值為60J/cm 2以上者，視為冷加工的彎曲性及滲碳後之韌性優異，並作為「實施例」。 In this test example, if the maximum bending angle of the steel sheet for carburizing is 100 ° or more and the impact value after carburizing is 60J / cm 2 or more, it is considered to be excellent in cold workability and toughness after carburizing. example".

所得之各個滲碳用鋼板的顯微組織及特性，統整列示於以下表3。The microstructure and characteristics of each of the obtained steel sheets for carburizing are summarized in Table 3 below.

[表3-1]
[Table 3-1]

[表3-2]
[Table 3-2]

[表3-3]
[Table 3-3]

由上述表3清楚可知，顯然，就符合本發明實施例的滲碳用鋼板而言，滲碳用鋼板之最大彎曲角達100°以上，並且，滲碳後的衝撃值達60J/cm 2以上，具有優異的成形性及滲碳後之韌性。又，作為參考所記載之理想臨界直徑也達5以上，可知符合本發明實施例的滲碳用鋼板也兼具優異的淬火性。 As is clear from the above Table 3, it is clear that, for the carburizing steel sheet according to the embodiment of the present invention, the maximum bending angle of the carburizing steel sheet is 100 ° or more, and the impact value after carburizing is 60J / cm 2 or more , Has excellent formability and toughness after carburizing. In addition, the ideal critical diameter described as a reference is 5 or more, and it is understood that the steel sheet for carburizing according to the embodiment of the present invention also has excellent hardenability.

另一方面，由上述表3清楚可知，顯然，吻合本發明比較例的滲碳用鋼板，其最大彎曲角或滲碳後的衝撃值之至少一者是小於基準值，而無法兼具成形性及滲碳後之韌性。On the other hand, as is clear from the above Table 3, it is clear that at least one of the maximum bending angle or the impact value after carburizing of the steel sheet for carburizing that meets the comparative example of the present invention is smaller than the reference value, and cannot have both formability. And toughness after carburizing.

以上，就本發明適宜的實施形態進行了詳細說明，不過本發明並不受限於上述例子。應當瞭解的是，只要是本發明所屬技術領域中具有通常知識者，在申請專利範圍所記載之技術思想範疇內，自然是能想到各種變更例或修正例，此等也當然是屬於本發明的技術範圍內。As mentioned above, although suitable embodiment of this invention was described in detail, this invention is not limited to the said example. It should be understood that, as long as those who have ordinary knowledge in the technical field to which the present invention belongs, within the scope of the technical ideas described in the scope of the patent application, it is naturally possible to think of various changes or amendments, which of course belong to the present invention. Within technology.

Claims (5)

一種滲碳用鋼板，其以質量%計含有：C：0.02%以上且小於0.30%、Si：0.005%以上且0.5%以下、Mn：0.01%以上且3.0%以下、P：0.1%以下、S：0.1%以下、sol.Al：0.0002%以上且3.0%以下、N：0.0001%以上且0.035%以下，剩餘部分由Fe及不純物所構成；肥粒鐵的平均結晶粒徑小於10μm；碳化物的平均等效圓直徑為5.0μm以下；相對於總碳化物，縱橫比2.0以下之碳化物個數比率為80%以上；相對於總碳化物，存在於肥粒鐵結晶粒內之碳化物個數比率為60%以上；從鋼板最外表面起算在深度方向上至50μm為止的區域中，平均氮濃度為0.040質量%以上且0.200質量%以下。A steel sheet for carburizing, which includes, in mass%, C: 0.02% or more and less than 0.30%, Si: 0.005% or more and 0.5% or less, Mn: 0.01% or more and 3.0% or less, P: 0.1% or less, S : 0.1% or less, sol.Al: 0.0002% or more and 3.0% or less, N: 0.0001% or more and 0.035% or less, the remainder is composed of Fe and impurities; the average grain size of the ferrous iron is less than 10 μm; The average equivalent circle diameter is less than 5.0 μm; the ratio of the number of carbides with an aspect ratio of 2.0 or less is 80% or more with respect to the total carbides; the number of carbides present in the iron crystal grains of the fertilizer with respect to the total carbides The ratio is 60% or more; in the region from the outermost surface of the steel plate in the depth direction to 50 μm, the average nitrogen concentration is 0.040% by mass or more and 0.200% by mass or less. 如請求項1之滲碳用鋼板，其以質量%計更含有下述之1種或2種以上來取代剩餘部分之Fe的一部分：Cr：0.005%以上且3.0%以下、Mo：0.005%以上且1.0%以下、Ni：0.010%以上且3.0%以下、Cu：0.001%以上且2.0%以下、Co：0.001%以上且2.0%以下、Nb：0.010%以上且0.150%以下、Ti：0.010%以上且0.150%以下、V：0.0005%以上且1.0%以下、B：0.0005%以上且0.01%以下。For example, the steel sheet for carburizing according to claim 1 further contains one or two or more of the following Fe to replace part of the remaining Fe: Cr: 0.005% or more and 3.0% or less, Mo: 0.005% or more 1.0% or less, Ni: 0.010% or more and 3.0% or less, Cu: 0.001% or more and 2.0% or less, Co: 0.001% or more and 2.0% or less, Nb: 0.010% or more and 0.150% or less, Ti: 0.010% or more 0.150% or less, V: 0.0005% or more and 1.0% or less, and B: 0.0005% or more and 0.01% or less. 如請求項1或2之滲碳用鋼板，其以質量%計更含有下述之至少一者來取代剩餘部分之Fe的一部分：W：1.0%以下、Ca：0.01%以下。For example, the steel sheet for carburizing according to claim 1 or 2 further includes at least one of the following to replace a part of Fe in the remaining portion: W: 1.0% or less, Ca: 0.01% or less. 一種滲碳用鋼板的製造方法，是製造如請求項1至3中任1項之滲碳用鋼板的方法，包含：熱輥軋步驟，是將具有如請求項1至3中任1項之化學組成的鋼材進行加熱，在800℃以上且小於920℃之溫度區域結束熱精輥軋，並在700℃以下之溫度進行捲取；及退火步驟，是將經前述熱輥軋步驟而得的鋼板、或將前述熱輥軋步驟後施行過冷輥軋的鋼板，透過已將氮濃度控制在體積分率計25%以上之氣體環境，以5℃/h以上且100℃/h以下之平均加熱速度，加熱至下述式(1)所定義的Ac1點以下之溫度區域為止，並施行在該Ac1點以下之溫度區域保持10h以上且100h以下的退火處理後，施行下述冷卻，該冷卻是自退火結束時之溫度起至550℃為止之溫度區域的平均冷卻速度設為5℃/h以上且100℃/h以下；在前述熱輥軋步驟中，是自前述熱精輥軋結束時起算1秒以內，開始進行平均冷卻速度大於50℃/s的冷卻；將前述退火處理後的肥粒鐵平均粒徑控制在小於10μm；此處，下述式(1)中，[X]之標記是表示元素X的含量(單位：質量%)，不含該元素時則代入零；[數學式1]Ac1=750.8-26.6[C]+17.6[Si]-11.6[Mn]-22.9[Cu]-23[Ni]+24.1[Cr]+22.5[Mo]-39.7[V]-5.7[Ti]+232.4[Nb]-169.4[Al]-894.7[B]‧‧‧式(1)。A method for manufacturing a steel sheet for carburizing, which is a method for manufacturing a steel sheet for carburizing as in any one of claims 1 to 3, including a hot rolling step, The steel with chemical composition is heated, the hot finishing rolling is finished in a temperature range of 800 ° C to less than 920 ° C, and the coiling is performed at a temperature of 700 ° C or lower; and the annealing step is obtained by the aforementioned hot rolling step Steel plates, or steel plates subjected to super-cold rolling after the aforementioned hot-rolling step, pass through a gas environment in which the nitrogen concentration has been controlled to 25% or more by volume fraction, at an average of 5 ° C / h and 100 ° C / h The heating rate is heated to a temperature range below the Ac 1 point defined by the following formula (1), and an annealing treatment is performed to maintain the temperature range below the Ac 1 point for 10 hours or more and 100 hours or less, and then the following cooling is performed. In this cooling, the average cooling rate in the temperature range from the temperature at the end of the annealing to 550 ° C is set to 5 ° C / h or more and 100 ° C / h or less; in the hot rolling step, the hot rolling Within 1 second from the end, the average cooling rate starts Cooling at 50 ° C / s; controlling the average grain size of the ferrous iron after the annealing treatment to less than 10 μm; here, in the following formula (1), the mark of [X] indicates the content of the element X (unit: Mass%), and substitute zero for this element; [Mathematical formula 1] Ac 1 = 750.8-26.6 [C] +17.6 [Si] -11.6 [Mn] -22.9 [Cu] -23 [Ni] +24.1 [ Cr] +22.5 [Mo] -39.7 [V] -5.7 [Ti] +232.4 [Nb] -169.4 [Al] -894.7 [B] ‧‧‧ Formula (1). 如請求項4之滲碳用鋼板的製造方法，其在用以獲得供給至前述熱輥軋步驟之前述鋼材的連續鑄造步驟中，施行鋼材健全化處理；該鋼材健全化處理是生成預定夾雜物、或降低預定元素中心偏析的處理之至少一者。The method for manufacturing a steel sheet for carburizing according to claim 4, wherein in the continuous casting step for obtaining the aforementioned steel material supplied to the aforementioned hot-rolling step, a steel material curing treatment is performed; the steel steel curing treatment is to generate a predetermined inclusion Or at least one of processes for reducing the center segregation of a predetermined element.
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CN102149839A (en) * 2009-03-27 2011-08-10 新日本制铁株式会社 Carbon steel sheet having excellent carburization properties, and method for producing same
CN104411846A (en) * 2012-06-28 2015-03-11 杰富意钢铁株式会社 High carbon steel pipe having excellent cold workability, machinability, and quenching properties, and method for manufacturing same
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