JP2009299175A - Method for producing spheroidized carburized steel sheet, and steel strip - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a rational carburizing technique wherein a carburized layer is formed on the surface of a steel strip, and also, the spheroidizing of carbides is attained by utilizing an existing heat treatment stage performed in a steel strip production process. <P>SOLUTION: In a method for producing a spheroidized carburized steel sheet, heat treatment is performed, wherein the heat treatment has: a stage (the first step) in which a carburized coating film generating a CO gas in the temperature region of 720 to 850°C in an inert gas atmosphere is brought into contact with the surface of a steel sheet made of a steel having a C content of <0.8 mass%, under ≥0.2 MPa, so as to form a contact structure of the steel sheet/the carburized coating film, and the steel sheet and the carburized coating film composing the contact structure are held in the range of 720 to 850°C for ≥3 hr in an inert gas atmosphere; a step (cooling in the first step→the second stage) in which annealing is performed to a temperature of 660°C to less than an Ac<SB>1</SB>point at a cooling rate of ≤15°C/hr; and a step (the second step) for holding a temperature of 660°C to less than an Ac<SB>1</SB>point for ≥10 hr after the annealing. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for producing a spheroidized carburized steel sheet in which a carburized layer is already formed on the surface and the carbide is spheroidized in the state of a steel sheet material, and spheronization in which the carburized layer is formed on the surface and the carbide is spheroidized. Related to carburized steel strip.

  Steel carburizing methods include a fixed carburizing method, a liquid carburizing method, a gas carburizing method, and a vacuum carburizing method.

In the solid carburizing method, the steel material to be treated is put in a carburizing box, and a large amount of solid carburizing agent is filled around the steel material to be treated in order to sufficiently generate carburizing gas (mainly CO), and is sealed at about 850 ° C. or more. To be heated.
In the liquid carburizing method, the steel material to be treated is immersed in a salt bath containing a carburizing agent, and harmful substances in the bath may become a problem.
In the gas carburizing method, carburizing is performed by exposing a steel material to be treated to a modified gas (mixed gas of CO, H ₂ , N ₂ , CH ₄ , CO _2, etc.) near 900 to 1000 ° C. The carburizing method used.
In the vacuum carburizing method, a sealed container containing a steel material to be treated is evacuated and then carburized by directly charging a gas such as methane, propane, ethylene, or acetylene into the container.

  In order to carry out these carburizing methods, expensive carburizing equipment is required. In order to form a good carburized layer on the entire surface of the steel material to be treated, it is important to uniformly bring the carburizing gas into contact with the surface of the steel material to be treated. It is not always easy to increase the scale of carburizing boxes, salt tubs, heating furnaces, etc. in order to sufficiently control and control carburizing gas. For this reason, steel materials to which the carburizing method is applied are generally limited to small parts. In addition, since carburizing takes a long time, the conventional carburizing method has drawbacks in terms of energy cost and productivity.

  The main purpose of carburizing is to harden and harden the surface layer of parts, but many of these parts are formed by processing a steel plate material. In this case, if it becomes possible to employ a process of forming a carburized layer on the surface at the stage of the steel plate material and omitting the carburizing process after the parts processing and subjecting it to a quenching process, the economic merit is great. Thus, conventionally, a method of carburizing by applying a carburizing agent to the surface of a steel plate material has been proposed (Patent Documents 1 to 4).

JP 2000-119838 A JP 2001-107217 A JP 2001-115249 A JP 2007-100210 A

According to the carburizing method in which the carburizing agent is applied as described above, a carburized layer can be formed on the steel plate surface at the material stage. However, according to the methods disclosed in the above patent documents, it is necessary to heat the material to a high temperature of approximately 900 ° C. or higher. According to the study by the inventors, it has been clarified that carburization does not proceed sufficiently only by heating to a lower temperature range (for example, Ac ₁ point to 850 ° C.). Exposing the steel sheet to a high temperature of 900 ° C. or higher for the purpose of carburizing only is not always beneficial. Hardening of materials due to coarsening of crystal grains and pearlite transformation during cooling tends to be a problem. In particular, the coarsening of the crystal grains causes a decrease in fatigue characteristics and impact value. Furthermore, an increase in equipment costs and a decrease in productivity due to an increase in processes are also problematic.

  In view of such a current situation, the present invention uses a conventional heat treatment step performed in a steel plate manufacturing process to form a carburized layer on the surface of the steel plate and to make a carbide spheroidize into a reasonable carburizing technology. And a steel strip obtained by applying the technique.

  As a result of detailed research, the inventors have formed a carburized coating film on the surface of the steel sheet and applied a contact pressure of a certain level or more between the carburized coating film and the steel sheet surface. It was found that carburization proceeds at a relatively low temperature range. The present invention has been completed based on such new findings.

That is, in the present invention, a carburized coating having a property of generating CO gas in a temperature range of 720 to 850 ° C. in an inert gas atmosphere is applied to the surface of a steel plate made of steel having a C content of less than 0.8% by mass at 0.2 MPa. The contact structure of “steel plate / carburized coating” is formed by contacting with the above pressure, and 3 hours in the range of 720 to 850 ° C. in an inert gas atmosphere with respect to the steel plate and carburized coating film constituting the contact structure. Step of maintaining above (first step), then step of slow cooling at a cooling rate of 15 ° C./hr or less to a temperature in the range of 660 ° C. or more and less than Ac ₁ point (first step → second step cooling), Provided is a method for producing a spheroidized carburized steel sheet, which is subjected to a heat treatment having a step (second step) of maintaining at a temperature within a range of 660 ° C. or more and less than Ac ₁ point after slow cooling for 10 hours or more. In particular, by interposing the carburized coating between adjacent steel plate surfaces of a coiled steel strip, a “steel plate / carburized coating” contact structure is formed between the steel plate surfaces on both sides thereof. Can do.

  A carburized coating film is a coating film containing a carburizing agent (carbon source). When an aqueous carburized coating is used, it means a coating obtained by drying and removing most of the water in the coating after application. . As the carburized coating film, one containing 3 to 30% by mass of charcoal powder, graphite, or carbon black as a carburizing agent and 3 to 30% by mass of a carbonate such as sodium carbonate as a carburizing accelerator can be applied. The content of these components in the carburized coating film is the content of all the components excluding moisture in the coating film. This corresponds to the content of all components excluding water added when preparing the water-based paint.

  The inert gas is a non-reducing and non-oxidizing gas such as a nitrogen gas, a rare gas (a gas of a group 18 element such as argon), or an NX gas (a nitrogen-based metamorphic gas).

Further, in the present invention, an annealed steel strip (hot-rolled annealed material or cold-rolled annealed material) obtained by melting a hypoeutectoid steel having a C content of less than 0.8% by mass, and a carburized layer on the surface layer portion. The difference between the average carbon concentration C ₁ (mass%) from the surface to the depth of 50 μm and the average carbon concentration C ₀ (mass%) in the thickness direction region at the thickness center position ± 25 μm ΔC = C ₁ −C ₀ Is a spheroidized carburized steel strip in which the spheroidization rate of carbide is 90% or more in the region from the surface to a depth of 50 μm. A steel strip already having such a carburized layer has not been obtained.

Here, the average carbon concentrations C ₁ and C ₀ are obtained by performing line analysis in the plate thickness direction on a predetermined plate thickness region in a cross section parallel to the plate thickness direction of the steel plate sample cut out from the steel strip using an analysis method such as EPMA. You can ask for it. In the present specification, in the carbide observed in a cross section parallel to the plate thickness direction, the longest diameter is referred to as the major axis, and the maximum diameter perpendicular to the major axis is referred to as the minor axis. Those having an aspect ratio of 2 or less are defined as spherical carbides. The spheroidization rate of carbides in the region from the surface to a depth of 50 μm refers to the ratio of spherical carbides to the carbides present in the observation field of the region.

According to the present invention, the following advantages can be obtained.
(1) Carburizing at a relatively low temperature of approximately 850 ° C. or less is possible. For this reason, coarsening of crystal grains is prevented, leading to improvement in toughness of parts.
(2) The steel strip wound in a coil shape can be carburized and is suitable for mass production.
(3) Since carburization has already been completed at the stage of the steel plate material, it is possible to omit the carburizing treatment after parts processing, and the burden on the parts manufacturer is reduced.
(4) Carburizing treatment can be performed using a carbide spheroidizing annealing (TCA) process performed in the current steel sheet manufacturing process. In this case, a new heat treatment furnace is not required, and an increase in the heat treatment process is not involved, so that an increase in manufacturing cost is suppressed.
(5) Since an annealed steel sheet having a high surface carbon concentration can be obtained, the “sag amount” is reduced when punching into a product shape, and the accuracy and quality of the parts are improved.

In the present invention, a carburizing method for heat treatment is used after applying a paint containing a carburizing agent (carbon source) to the steel sheet surface to form a carburized coating film. However, the following two points are important in order for carburization to proceed efficiently in a relatively low temperature region of 850 ° C. or lower.
(I) The carburized coating film is brought into contact with the steel sheet surface at a pressure of 0.2 MPa or more.
(Ii) Carburizing treatment atmosphere should be an inert gas atmosphere.

The following reactions are thought to be involved in carburizing.
C + O ₂ → CO ₂ (1)
C + CO ₂ → 2CO (2)
2CO + Fe → [Fe + C] + CO ₂ (3)

Formula (1) is a process in which C derived from a carburizing agent and oxygen react to generate CO ₂ gas. In the present invention, oxygen is supplied from a carburizing accelerator (carbonate) blended in the carburized coating.
Formula (2) is a process in which the generated CO ₂ gas reacts with C derived from the carburizing agent to generate CO which is a carburizing gas.
Equation (3) is a process in which carburization occurs when CO produced in Equation (2) reacts with Fe on the steel sheet surface. C in the part of [Fe + C] means C that has penetrated into the steel.

By forming a “steel plate / carburized coating” contact structure, CO ₂ and CO generated from the carburized coating are difficult to escape, and a relatively high concentration of carburizing gas (mainly CO) exists in the vicinity of the steel plate surface. The environment is likely to be maintained. However, it is difficult to advance carburization in a temperature range of 850 ° C. or less simply by forming a “steel plate / carburized coating” contact structure.

However, according to the studies by the inventors, carburization proceeds at a temperature of 850 ° C. or lower when subjected to heat treatment in an inert gas with a contact pressure of a certain level or more applied between the steel plate and the carburized coating. It became clear. The mechanism is not clear at the present time, but when the contact pressure is applied, the partial pressure of the generated carburizing gas (CO) increases, and the reaction of formula (3) is likely to proceed. It is done.
Hereinafter, matters for specifying the present invention will be described.

[Material steel plate]
As the material steel plate (steel plate subjected to carburizing treatment), known hypoeutectoid steels having a C content of less than 0.8% by mass can be widely applied. For example, various steel types such as ordinary steel, steel for machine structure, and alloy steel for machine structure can be selected depending on applications. Specifically, in addition to standard steel types including SS400, SPHC, SPCC, SC, SK, SCr, SCM, and SNCM, newly developed hypoeutectoid steel types are widely applicable. A hot-rolled steel plate or a cold-rolled steel plate can be applied as the steel plate used for the carburizing treatment. The material steel plate is preferably in the state of a steel strip.

(Carburized coating)
The carburized coating film formed on the surface of the steel sheet has a property of generating CO gas in a temperature range of 720 to 850 ° C. in an inert gas atmosphere. If no CO gas is generated in this temperature range, carburization at a low temperature of 850 ° C. or lower cannot be realized using heat treatment that also serves as spheroidizing annealing. Moreover, since the carburizing process is performed in an inert gas atmosphere as will be described later, it is necessary to generate CO gas in the above temperature range in the inert gas atmosphere.
A coating film having such properties can be obtained by applying an aqueous paint containing the following components to the surface of the steel sheet and then drying.

<Carburizing agent>
As the carburizing agent (carbon source), solid carburizing agents such as charcoal powder, graphite, and carbon black can be used. The particle size of the solid carburizing agent is desirably 0.5 μm or less. In general, the smaller the particle size, the easier it is to disperse uniformly. However, when the particle size is 0.2 μm or less, the solid carburizing agents tend to aggregate, so it is preferable to use one having a particle size of 0.2 to 0.5 μm. The blending amount of the solid carburizing agent in the carburized coating film (that has undergone the drying process) is desirably 3% by mass or more. If the concentration of the carburizing agent is lower than that, it is difficult to stably perform effective carburizing such that the ΔC is 0.1% by mass or more. When ΔC is less than 0.1% by mass, fatigue characteristics and wear characteristics equivalent to those produced by ordinary gas carburizing cannot be sufficiently exhibited. More preferably, the content is 5% by mass or more. On the other hand, even if the content exceeds 30% by mass, the carburizing effect is almost saturated at an annealing temperature of about 720 to 850 ° C. Therefore, the content of the carburizing agent in the carburized coating is desirably 3 to 30% by mass, and usually 5 to 20% by mass is sufficient.

<Carburizing accelerator>
In order to promote the carburization by increasing the pressure of CO ₂ or the like in order to form a carburized layer on the steel sheet surface layer, it is common to add a carburizing accelerator to the carburized paint. The carburizing accelerator is not particularly limited as long as it does not generate a gas such as CO ₂ when it is added to the base aqueous paint and generates a gas only when heated to the carburizing temperature. A carbonate such as sodium carbonate or barium carbonate is preferred, although it is selected according to the type of aqueous medium used and the method of forming the film. When carbonate is used, the content in the carburized coating film should be 5% by mass or more. If it is less than that, the carburizing effect may be insufficient. On the other hand, even if it is added in excess of 30% by mass, the carburization promoting effect is hardly changed. Accordingly, the content of the carburizing accelerator in the carburized coating film is desirably 5 to 30% by mass, and usually 5 to 20% by mass is sufficient.

<Dispersant>
Water-soluble polymers used in paints include natural polymers, polyacrylic acid, styrene-maleic acid copolymers, and styrene-acrylic that are commonly used as dispersants for water-soluble resins or water-soluble resins. An acid copolymer, polyvinyl alcohol, polyethylene glycol, or the like can be applied. Taking into consideration the properties as a water-based paint, the formation of a film on a non-treated material and the workability, it is preferable to use a powder having an average particle size of about 1 to 3 μm. Moreover, it is preferable to mix | blend these resin in the quantity of 10-40 mass parts with respect to 100 mass parts of water. If the blending amount of the water-soluble polymer with respect to water is less than this, adhesion at the time of application may be insufficient. Moreover, in order to improve the wettability of a coating material and a steel plate, you may add a small amount of organic solvents, an antifoamer, etc.

〔Preprocessing〕
It is preferable to wash the surface of the steel plate in advance for the purpose of obtaining a uniform coating layer. The cleaning method is not particularly limited as long as a uniform film is formed after the coating composition is applied and baked, and then a uniform carburized layer can be formed when carburizing treatment is performed. For example, alkaline degreasing, organic Examples include degreasing with a solvent.

[Application]
The application method includes dipping method, roll coating method, spray method, bar coating method and the like, and the adhesion method is not particular. Moreover, you may form the film | membrane of a carburizing composition on the single side | surface of a steel strip, and it will be in the form where it intervened in the interface of a steel strip and a steel strip, and it applied to both surfaces by winding up in a coil shape. In this case, if the carburized paint is dried to 40 to 80 g / m ^{2 on} one side, the carburizing effect is sufficiently exhibited. Of course, both sides can be applied. If the coating amount is too small, the carburizing effect becomes unstable. If the coating amount is too large, the carburizing effect is saturated and the manufacturing cost increases.

[Dry]
The purpose is to evaporate the water content of the paint, and it may be about 100 to 130 ° C. × 1 min. Any heating conditions may be used as long as the water-soluble polymer does not boil or melt.

[Contact pressure]
Contact pressure is most important for the stability of carburizing effect. A contact pressure of 0.2 MPa or more is secured between “steel plate / carburized coating”. As a result, it was found that effective carburization such that ΔC in the temperature range of 850 ° C. or less is 0.1 mass% or more proceeds. As a result of detailed examination, carburization proceeds sufficiently even if the contact pressure is considerably high, but if the pressure becomes too high, a solid carburizing agent such as charcoal may be pushed into the surface of the steel sheet, and eventually it may become a soot. Usually, it may be in a range of 0.2 to 50 MPa, but in order to maintain a stable pressure without applying an excessive load to the equipment, the contact pressure is preferably about 1 to 25 MPa, and preferably 2 to 10 MPa. It is more preferable. When interposing a carburized coating between adjacent steel plate surfaces of a steel strip wound in a coil shape, by controlling the winding tension using a line that can be wound while applying tension to the steel strip It can be adjusted to various contact pressures. After being wound, the contact pressure can be prevented from lowering by tightening the entire coil using an iron coil band or the like. The relationship between the contact pressure and the winding tension may be confirmed in advance using an intermediate pressure paper, a load cell, a strain gauge, or the like.

[Annealing atmosphere]
When carburizing, it is necessary to anneal in an inert gas atmosphere. NX gas is preferred industrially as the inert gas. In a reducing gas containing hydrogen, hydrogen and the carburizing agent C may react to generate CH ₄ and H ₂ O, which may cause decarburization. In addition, the carburizing agent may burn and not carburize with oxidizing gas such as the atmosphere. In addition, the steel plate is oxidized and decarburized. The dew point of the inert atmosphere is preferably −40 ° C. or lower.

[Annealing conditions]
Annealing is composed of a first step of carburizing and a second step of spheroidizing cementite in the surface layer portion having a high carbon concentration.
In the first step, heating at 720 to 850 ° C. for 3 hours or more is necessary. At 720 ° C. or lower, the carbon diffusion rate is extremely slow, and therefore carburization is unlikely to proceed effectively. For example, the decomposition reaction of general charcoal is assumed to be 700 ° C. or higher, and according to the study by the inventors, it has been found that it is desirable to set the annealing temperature to 720 ° C. or higher. In ordinary gas carburizing, heating exceeding 850 ° C. is essential. However, in the present invention, carburization is possible in a relatively low temperature range of 720 to 850 ° C.

  Carburization is possible even when the temperature exceeds 850 ° C., but carburizing gas may not be secured at the interface due to deformation of the steel strip at a high temperature. It is preferable to stabilize the carburizing property at 850 ° C. If the carburization holding time in the first step is 3 hours or more, the carbon concentration in the surface layer is higher than that of the base material (steel plate before carburizing), but holding for 10 hours or more is preferable in consideration of the heat uniformity in the coil. If the holding time exceeds 30 hours, decarburization may occur at the extreme surface layer due to the disappearance of the carburizing agent. Also, holding for a long time increases the manufacturing cost. For this reason, the holding time in the first step is desirably 3 to 30 hours, and more preferably 10 to 20 hours.

  The cooling rate from the first step to the second step is 15 ° C./hr or less. When the annealing temperature is 720 to 850 ° C., the surface layer portion having a high carbon concentration partially becomes austenite, and when the cooling rate exceeds 15 ° C./hr, rod-like cementite may be generated, which causes deterioration of workability.

Next, in the second step aimed at spheroidizing annealing of cementite, it is necessary to hold for 10 hours or more in the range of 660 ° C. to Ac ₁ point. This is for spheroidizing and softening the precipitated cementite. If the cooling rate from the first step satisfies 15 ° C./h or less, then it may be gradually cooled to 660 ° C. for 10 hours or more. If the holding temperature in the second step is lower than 660 ° C., spheroidization is delayed, and there is a possibility that a spheroidization ratio of 90% or more that satisfies normal molding processability cannot be secured.

[Steel strip after annealing]
By the above process, having a carburized layer in a surface portion, the average mean carbon concentration C ₁ from the surface to 50μm depth and (mass%) in the thickness direction region of the plate thickness center position ± 25 [mu] m carbon concentration C ₀ (mass% ) Difference ΔC = C ₁ -C ₀ is 0.1% by mass or more, and a spheroidized carburized steel strip having a carbide spheroidization ratio of 90% or more in the region from the surface to a depth of 50 μm is obtained. it can. When ΔC is less than 0.1% by mass, the fatigue and wear characteristics equivalent to those produced by ordinary gas carburizing are sufficiently exhibited when the steel strip produced in the present invention is used for quenching and tempering. Not.

  A steel plate obtained by carburizing is processed into a part shape through press punching and various forming processes. The steel material having a part shape is subjected to a quenching and tempering process by omitting the carburizing process required in the conventional process, and becomes a carburized part subjected to a surface hardening process.

  Table 1 shows chemical components of the test materials.

First, the carburizing agent content and coating amount in the carburized coating and the influence of the annealing atmosphere were investigated.
A steel sheet of Table 1 (Ac ₁ point 726 ° C) was used to manufacture a hot-rolled steel sheet (width 920 mm) with a thickness of 4 mm, and after pickling, a cold-rolled steel strip with a thickness of 2.0 mm with a cold-rolling rate of 50% Finished.
On the other hand, the carburized paint uses a general water-soluble polymer (manufactured by Nippon Pure Chemicals Co., Ltd., acrylic acid copolymer; Junron), and charcoal powder (manufactured by Parker Heat Treatment; KG6, crushed KG6) has a particle size of 0 Various water-based paints containing sodium carbonate as a carburizing accelerator in substantially the same amount as the carburizing agent and the balance being distilled water were prepared. The content of the carburizing agent and the carburizing accelerator is a ratio of all components excluding distilled water.

After degreasing the cold-rolled steel strip with alkali, the carburized paint is dried and then continuously applied by a roll coating method so that the coating amount is 20 to 80 g / m ² , and then dried with hot air at 100 ° C. for 2 minutes to perform carburizing. After film formation, the steel strip was continuously wound up. In winding, the contact pressure at the interface between the steel sheet and the steel sheet was 4 MPa by tension control. Then, it tightened so that the contact pressure of a coil might not fall with an iron coil band, and it annealed so that a coil end surface (edge part of a steel strip) might become up and down.

The first step of the heat treatment was heated to 750 ° C. and held for 10 hours, and subsequently cooled at a cooling rate of 10 ° C./hr, and in the second step, held at 720 ° C. for 20 hours. The influence on the carburizing property was investigated using argon, nitrogen, NX (N ₂ -based 3% H ₂ ), hydrogen, and air as the annealing atmosphere gas. A sample capable of observing a cross section perpendicular to the sheet thickness direction from the obtained steel strip after annealing was prepared, and the average carbon concentration C ₁ from the surface to a depth of 50 μm was measured by measuring the C content with EPMA by the method described above. The difference ΔC between the average carbon concentration C ₀ (mass%) in the thickness direction region of (mass%) and the thickness center position ± 25 μm was determined.
The results are shown in Table 2.

In all of the examples of the present invention, good carburization with ΔC of 0.1% by mass or more was realized.
On the other hand, since No. 1 had a low carburizing agent content, ΔC was less than 0.1% by mass. Decarburization occurred because No. 10 was in a hydrogen atmosphere and No. 11 was in an air atmosphere.

Next, the influence of the contact pressure and the annealing conditions on the carburizability and workability of the materials having different carbon concentrations (steel A and steel B in Table 1) was investigated.
A hot-rolled steel sheet having a thickness of 4 mm (width: 920 mm) was produced, and after pickling, a cold-rolled steel strip having a thickness of 2.0 mm was finished with a cold-rolling rate of 50%.
On the other hand, the carburizing paint used the same component as in Example 1. However, here, the content of the carburizing agent (charcoal) was uniformly 5% by mass (appropriate), and the content of the carburizing accelerator (sodium carbonate) was uniformly 5% by mass (appropriate).

After degreasing the cold-rolled steel strip with alkali, the carburized paint is dried and then continuously applied by a roll coating method so that the coating amount is 20 to 80 g / m ² , and then dried with hot air at 100 ° C. for 2 minutes to perform carburizing. After film formation, the steel strip was continuously wound up. In the winding, the winding was performed such that the contact pressure at the interface between the steel plates was in a range of 0.1 to 24 MPa by tension control. Then, it tightened so that the contact pressure of a coil might not fall with an iron coil band, and it annealed so that a coil end surface (edge part of a steel strip) might become up and down.

In the first step of the heat treatment, the holding temperature is maintained at various temperatures in the range of 700 to 850 ° C. for 2 to 20 hours, followed by cooling at a cooling rate of 7 to 50 ° C./hr, and in the second step, 600 to 800 ° C. In the range of 6 to 30 hours. The annealing atmosphere gas was NX (N ₂ -based 3% H ₂ ). About the obtained steel strip after annealing, the average carbon concentration C ₀ (mass%) from the surface to the depth of 50 μm and the average carbon concentration C ₀ in the plate thickness direction region at the plate thickness center position of ± 25 μm in the same manner as in Example 1. The difference ΔC of (mass%) was determined. The case where ΔC was 0.1% by mass or more was evaluated as ○ (good carburizing property), and the others were evaluated as × (poor carburizing property), and the evaluation was evaluated as pass.

In addition, from the SEM photograph of 1000 times including the area from the surface to the depth of 50 μm with respect to the cross section parallel to the plate thickness direction, the aspect ratio (described above) of the carbide existing in the area is measured, and the carbide spheres in the surface layer part are measured. The conversion rate was calculated.
Furthermore, the punched steel sheet was punched with a punching punch diameter of 10 mm and the clearance one side was 5%. did.
A spheroidization rate of 90% or more and a shear rate of 20% or more was evaluated as ◯ (good workability), and the others were evaluated as x (defective workability).
The results are shown in Table 3.

In all of the examples of the present invention, ΔC was 0.1% by mass or more and the carburizing property was good, and the workability was also judged good.
In contrast, No. 12 has a low contact pressure, the temperature of the first step of No. 13 is too low, and No. 17 has a short holding time of the first step. It was. Nos. 16 and 24 have a cooling rate from the first step to the second step that is too fast, No. 18 has an annealing temperature of the second step that is too low, No. 19 has a holding time of the second step that is too short, In .22, since the annealing temperature in the second step was too high, these produced rod-like cementite that deteriorated workability, the spheroidization rate was lowered, and the shear surface rate was also lowered.

  For reference, FIG. 1 shows a representative metal structure of a comparative material that has not been carburized and a material of the present invention that has been carburized.

The optical micrograph of the typical metal structure about the comparative material which has not carburized, and this invention material which carburized.

Claims (5)

The surface of a steel plate made of steel having a C content of less than 0.8% by mass is contacted with a carburized coating film that generates CO gas in an inert gas atmosphere at a temperature range of 720 to 850 ° C. at a pressure of 0.2 MPa or more. Forming a contact structure of “steel plate / carburized coating film” and maintaining the steel sheet and the carburized coating film constituting the contact structure in a range of 720 to 850 ° C. in an inert gas atmosphere for 3 hours or more ( 1st step), then, a step of gradually cooling at a cooling rate of 15 ° C./hr or less to a temperature in the range of 660 ° C. or more and less than Ac ₁ point (first step → second step cooling), 660 ° C. after the slow cooling The manufacturing method of the spheroidization carburized steel sheet which performs the heat processing which has the process (2nd step) hold | maintained at the temperature in the range below Ac ₁ point more than 10 hr above. A carburized coating film containing 3 to 30% by mass of powder of a carburizing agent (carbon source) and 3 to 30% by mass of carbonate on the surface of a steel plate made of steel having a C content of less than 0.8% by mass is 0.5. A contact structure of “steel plate / carburized coating” is formed by contact at a pressure of 2 MPa or more, and the steel plate and carburized coating constituting the contact structure are in a range of 720 to 850 ° C. in an inert gas atmosphere. Step of maintaining for 3 hours or more (first step), then gradually cooling to a temperature within the range of 660 ° C. or more and less than Ac ₁ point at a cooling rate of 15 ° C./hr or less (first step → second step cooling), A method for producing a spheroidized carburized steel sheet, which is subjected to a heat treatment having a step (second step) of maintaining at a temperature within a range of 660 ° C. or higher and less than Ac ₁ after the slow cooling for 10 hours or more.   The said carburizing agent (carbon source) is a manufacturing method of the spheroidization carburized steel plate of Claim 2 comprised by 1 or more types of charcoal powder, graphite, and carbon black.   The method for producing a spheroidized carburized steel sheet according to any one of claims 1 to 3, wherein the carburized coating film is interposed between adjacent steel sheet surfaces of a steel strip wound in a coil shape. An annealed steel strip obtained by melting a hypoeutectoid steel having a C content of less than 0.8% by mass, having a carburized layer in the surface layer, and having an average carbon concentration C ₁ (mass from the surface to a depth of 50 μm) %) And the average carbon concentration C ₀ (mass%) difference ΔC = C ₁ -C ₀ in the sheet thickness direction region at the sheet thickness center position ± 25 μm is 0.1 mass% or more, from the surface to the depth of 50 μm A spheroidized carburized steel strip having a spheroidization rate of carbide in the region of 90% or more.