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

Abstract

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

Carburizing steel sheet and manufacturing method of carburizing steel sheet

FIELD OF THE INVENTION The present invention relates to a steel sheet for carburizing and a method for manufacturing a steel sheet for carburizing.

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.

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.

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.

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.

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

Summary of invention         

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.       

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.

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.

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.

The gist of the present invention completed based on this concept is as follows.

[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] 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] 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] 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.

[Mathematical formula 1]

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.

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.       

Mode for Carrying Out the Invention Hereinafter, preferred embodiments of the present invention will be described in detail.

(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.

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.

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.

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.

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.

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.

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.

(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.

＜ 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% 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.

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% 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% 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% 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% 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% 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% 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% 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% 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% 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% 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% 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% 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% 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% 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% 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% 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% 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.

[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.

The chemical components of the steel sheet for carburizing in this embodiment have been described in detail above.

＜ 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%.

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.

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.

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.

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.

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.

[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.

[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.

[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.

[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.

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.

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.

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.

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.

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).

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.

The microstructure of the steel sheet for carburizing according to this embodiment has been described in detail.

[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.

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.

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.

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.

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.

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.

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.

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.

＜ 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.

The steel sheet for carburizing of this embodiment has been described in detail above.

(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.

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.

＜ 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.

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.

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.

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.

[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.

[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.

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.

[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.

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.

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.

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.

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.

＜ 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.

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.

[Mathematical formula 2]

[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.

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.

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.

[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.

[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.

[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.

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.

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.

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.

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]

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.

(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.

[Table 1-1]

[Table 1-2]

[table 2-1]

[Table 2-2]

[Table 2-3]

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.

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.

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

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. .

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.

[Mathematical formula 3]

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".

The microstructure and characteristics of each of the obtained steel sheets for carburizing are summarized in Table 3 below.

[Table 3-1]

[Table 3-2]

[Table 3-3]

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.

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.

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)

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. 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. 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. 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). 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.