JP7125923B2 - High-carbon hot-rolled steel sheet for vacuum carburizing, its manufacturing method, and carburized steel parts - Google Patents

Description

The present invention relates to a vacuum carburizing high-carbon hot-rolled steel sheet to be subjected to vacuum carburizing, in particular, a high-carbon hot-rolled steel sheet for vacuum carburizing that is excellent in resistance to excessive carburization without excessive carburization during vacuum carburizing and is also excellent in cold workability, and The present invention relates to a manufacturing method thereof and a carburized steel part using the hot-rolled steel sheet.

At present, automotive drive system parts such as gears and dampers are used under high load conditions, so in addition to being required to have high wear resistance and high fatigue strength, they are also required to be manufactured at low cost. ing. In general, cutting, joining, and carburizing using hot forged materials have been performed as methods of manufacturing these parts. However, since there is a high need for low-cost production, hot-rolled steel sheets (high-carbon hot-rolled steel sheets), which are carbon steel materials for machine structures and alloy steel materials for machine structures specified in JIS G4051, are cold-worked. In order to secure the desired hardness, technology has been developed to perform hardening treatments such as carburizing oil hardening and induction hardening after processing into a desired shape. A technique has also been developed in which a soft steel sheet such as SPH270 and SAPH440 according to JIS is cold worked into a desired shape and then carbonitrided and shot peened. Therefore, the hot-rolled steel sheet used as the raw material is required to have excellent cold workability and hardenability, and various steel sheets have been proposed so far.

For example, in Patent Document 1, in terms of weight %, C: 0.15 to 0.9%, Si: 0.4% or less, Mn: 0.3 to 1.0%, and Cr: 1.2% or less , or Ti: 0.01 to 0.05%, B: 0.0005 to 0.005%, carbide having a spheroidization rate of 80% or more and an average particle size of 0.4 to 1.0 μm describes a high-carbon steel sheet for precision punching, which has a structure in which is dispersed in ferrite and has a notch tensile elongation of 20% or more.

In Patent Document 2, it is characterized by containing C: 0.2% or more, Ti: 0.01 to 0.05%, and B: 0.0003 to 0.005% in mass%, and the average grain size of carbide A high-carbon steel sheet with improved workability is described by setting the ratio of carbides having a diameter of 1.0 μm or less and 0.3 μm or less to 20% or less.

In Patent Document 3, in mass%, C: 0.10 to 1.2%, Si: 0.01 to 2.5%, Mn: 0.1 to 1.5%, S: 0.0005 to 0 0.05%, further Te: 0.0005 to 0.05%, further Se: 0.0005 to 0.05%, Sb: 0.001 to 0.05%, further Cr: 0.2 to 2.0% , B: 0.005%, consists of a structure mainly composed of ferrite and pearlite, and has a ferrite grain size of No. 11 or more, and has improved cold workability and decarburization. steel is described.

Patent Document 4 contains C: 0.20 to 0.40%, Si: 0.10% or less, Mn: 0.50% or less, and B: 0.0005 to 0.0050% by mass%. , and further contains 0.002 to 0.03% in total of one or more of Sb, Sn, Bi, Ge, Te, and Se, is composed of ferrite and cementite, and the cementite density in the ferrite grains is 0.10. /μm ² or less, a hardness of 75 or less in HRB, and a total elongation of 38% or more, and are characterized by excellent hardenability and workability. there is

Patent Document 5 contains C: 0.20 to 0.48%, Si: 0.10% or less, Mn: 0.50% or less, and B: 0.0005 to 0.0050% by mass%. , and further contains 0.002 to 0.03% in total of one or more of Sb, Sn, Bi, Ge, Te, and Se, is composed of ferrite and cementite, and the cementite density in the ferrite grains is 0.10 /μm ² or less, a hardness of 65 or less in HRB, and a total elongation of 40% or more. there is

Patent Document 6 contains C: 0.20 to 0.40%, Si: 0.10% or less, Mn: 0.50% or less, and B: 0.0005 to 0.0050% by mass%. , and further contains 0.002 to 0.03% in total of one or more of Sb, Sn, Bi, Ge, Te, and Se, and the proportion of solid solution B in the B content is 70% or more, It consists of ferrite and cementite, has a microstructure in which the cementite density in the ferrite grains is 0.08 pieces/μm ² or less, has a hardness of 73 or less in HRB, and has a total elongation of 39% or more. A high carbon hot rolled steel sheet is described.

In Patent Document 7, in mass%, C: 0.15 to 0.37%, Si: 1% or less, Mn: 2.5% or less, B: 0.0010 to 0.0050%, and Sb, Sn at least one of: 0.003 to 0.10% in total and satisfying the relationship of 0.50 ≤ (14 [B]) / (10.8 [N]), the balance being Fe and It has a composition consisting of inevitable impurities, consists of a ferrite phase and cementite, has a microstructure in which the average grain size of the ferrite phase is 10 μm or less, the cementite spheroidization rate is 90% or more, and the total elongation is 37% or more. A high carbon hot-rolled steel sheet is described which is characterized by:

In Patent Document 8, in mass %, C: 0.02 to less than 0.30%, Si: 0.005 to less than 0.5%, Mn: 0.01 to less than 3.0%, Cr: 0.01% to less than 0.5%. 005 or more and 3.0% or less, Ti: 0.010 or more and 0.150% or less, B: 0.0005 or more and 0.01% or less, the balance being Fe and impurities, {100 } <011> to {223} <110> orientation groups with an average X-ray random intensity ratio of 7.0 or less, an average equivalent circle diameter of the carbide of 5.0 μm or less, and an aspect ratio of 2.0 μm or less. A carburizing steel sheet is described in which the number ratio of carbides of 0 or less is 80% or more with respect to all carbides, and the number ratio of carbides existing in ferrite grains is 60% or more with respect to all carbides. there is

In Patent Document 9, in mass %, C: 0.02 to less than 0.30%, Si: 0.005 to less than 0.5%, Mn: 0.01 to less than 3.0%, Ti: 0.01% to less than 0.5%. 010 or more and 0.150% or less, Cr: 0.005 or more and 3.0% or less instead of part of Fe, and B: 0.0005 or more and 0.01% or less. The balance is composed of Fe and impurities, the number of carbides per 1000 μm 2 is 100 or less, and the number ratio of carbides having an aspect ratio of 2.0 or less is 10% or more of the total carbides. , a carburizing steel sheet in which the average equivalent circle diameter of carbide is 5.0 μm or less, and the average grain size of ferrite is 10 μm or less.

JP 2009-299189 A JP 2005-344194 A Japanese Patent No. 4012475 JP 2015-017283 A JP 2015-017284 A WO2015/146173 Japanese Patent No. 5458649 WO2019-044970 WO2019-044971

In recent years, for example, transmission damper parts and gears have been required to be used under high load conditions, so the conventional carburizing and quenching has had the problem of insufficient wear resistance and fatigue strength. In addition, there is a strong demand for further productivity improvement (shortening of production time). Therefore, in a carburizing atmosphere with an oxygen concentration lower than that of the atmosphere, a carburizing step of heating to austenitizing temperature or higher to form a carburized layer on the surface, that is, a step of applying vacuum carburizing and water quenching to obtain a desired hardness. A manufacturing method that combines the Compared to gas carburizing, vacuum carburizing does not cause intergranular oxidation, so it is said that a carburized product with high fatigue strength can be obtained. On the other hand, vacuum carburizing tends to increase the carbon concentration on the surface of the steel sheet, and during the diffusion period of carbon, which is essential for vacuum carburizing, overlap of diffusion fields occurs, especially in parts with sharp angles, so excessive carburizing is likely to occur. . For example, in the edge part such as the tooth end of a gear, the carbon concentration at the tip tends to be higher than in the flat part. acts as a huge defect, and there is a concern that the fatigue strength will decrease. Furthermore, in a steel sheet having a high Cr concentration before vacuum carburizing, the carbon concentration in the surface layer becomes high after vacuum carburizing, and cementite is likely to precipitate. This is because Cr tends to concentrate in cementite, but since the diffusion rate in steel is slow, once a high Cr concentration region exists, the high Cr concentration region remains after carburization, so cementite is likely to occur after carburization. . Therefore, controlling the maximum Cr concentration in cementite before heat treatment is an important method for suppressing excessive carburization.

Therefore, in order to obtain the desired fatigue strength, it is also important to suppress the formation of carbides at grain boundaries in vacuum carburizing. With respect to the above problems, it is difficult to solve the problems with the conventional technologies described in the patent documents for the following reasons.

The technique described in Patent Document 1 is a medium- and high-carbon steel sheet with a C content of 0.15 to 0.9% by weight. It was found that the chipping tensile elongation is closely correlated, and that they greatly depend on the dispersion form of carbides (spheroidization ratio, average particle size of carbides). Furthermore, by spheroidizing the carbides and controlling the dispersed form of the carbides even if the average particle size is increased, the precision punchability is sufficiently improved within a range that does not interfere with the heat treatment such as quenching and tempering that is performed after the parts are formed. This is what I proposed. However, in this patent, it is not possible to reduce the C concentration in the surface layer in a state where the diffusion fields of C tend to overlap with each other due to the acute angle shape in the vacuum carburizing. In addition, since the Cr concentration in the cementite of the steel sheet cannot be controlled, the C concentration on the surface increases after vacuum carburization, making it impossible to suppress cementite precipitation (excessive carburization prevention effect).

The technique described in Patent Document 2 is that by setting the average grain size of cementite to 1 μm or less, cementite is sufficiently melted during quenching to obtain a predetermined quenching hardness, and fine carbides of 0.3 μm or less are reduced to 20%. % or less, the carbides of different sizes are distributed almost randomly, and the workability is improved by sharing the strain evenly over the entire region. Even in this patent, the Cr concentration in the cementite cannot be controlled, and the carbon concentration on the surface increases after vacuum carburization, so the cementite precipitation cannot be suppressed (excessive carburization prevention effect).

In Patent Document 3, Te, Se, and S are used to solve the problem that a decarburized layer is formed depending on the steel type and the surface hardness of the final product cannot be secured in the method of softening by slow cooling after hot rolling instead of spheroidizing annealing. , Sb, etc., to suppress the decarburization reaction on the surface of the steel material. However, even in this patent, cementite precipitation cannot be suppressed by controlling the surface carbon concentration after vacuum carburizing.

In Patent Documents 4 to 6, in addition to reducing the steel strengthening elements Mn and Si as much as possible, by reducing the cementite density in the ferrite grains, the hardness of the steel sheet can be reduced and a predetermined elongation can be achieved. By adding Sb, Sn, Bi, Ge, Te, and Se, for example, even when annealing is performed in a nitrogen atmosphere, nitriding can be prevented, solid solution B can be secured, and high hardenability can be obtained. is suggesting. However, even in this patent, the Cr concentration in the cementite in the steel sheet before heat treatment cannot be controlled, and the carbon concentration on the surface increases after vacuum carburizing, making it impossible to suppress cementite precipitation (excessive carburization prevention effect).

In Patent Document 7, the contents of B and N [B] and [N] (% by mass) satisfy a predetermined relationship, and at least one of Sb and Sn is added in a predetermined amount, so that the quenching treatment Even if it is heated for a long time in an atmosphere in which air is sometimes mixed to control the carbon potential, B nitrides are not formed, and solid solution B can be secured, so hardenability can be secured. We have proposed a method to improve the cold workability by controlling the rate. However, even in this patent, the Cr concentration in the cementite in the steel sheet before heat treatment cannot be controlled, and the C concentration on the surface increases after vacuum carburization, so the cementite precipitation cannot be suppressed (excessive carburization prevention effect). .

Next, in Patent Document 8, by increasing the number of cementite in the grains present in the steel plate relative to the number of cementite in the grain boundaries, cracks propagating through the grain boundaries during working are suppressed from growing, and ferrite By controlling the average value of the X-ray random intensity ratio of the {100}<011> to {223}<110> orientation group of the crystal grains to a predetermined value or less, cracks are less likely to occur during hole expansion processing, and the hole It is proposed to improve spreadability. However, even in this patent, the Cr concentration in the cementite in the steel sheet before heat treatment cannot be controlled, and the C concentration on the surface increases after vacuum carburization, so the cementite precipitation cannot be suppressed (excessive carburization prevention effect). .

Patent Document 9 proposes to improve both the uniform elongation and the local elongation by reducing the number density of carbides in the steel sheet and refining the ferrite grains by adding Ti. However, even in this patent, the Cr concentration in the cementite in the steel sheet before heat treatment cannot be controlled, and the C concentration on the surface increases after vacuum carburization, so the cementite precipitation cannot be suppressed (excessive carburization prevention effect). .

In view of the above-mentioned conventional problems, the present invention improves the fatigue strength of members by applying heat treatment that combines excellent cold workability and vacuum carburizing and water quenching, and further suppresses excessive carburizing in vacuum carburizing. To provide a high-carbon hot-rolled steel sheet for vacuum carburizing, a method for manufacturing the same, and a carburized steel part manufactured by performing a vacuum carburizing treatment using the steel sheet.

The inventors of the present invention have made intensive studies in order to solve the above problems. That is, the relationship between the cold workability of the high-carbon hot-rolled steel sheet and the steel sheet structure, the suppression of excessive carburization in the vacuum carburizing of the steel sheet, the chemical composition for ensuring the desired hardness after quenching after vacuum carburizing, and the steel sheet structure The relationship and manufacturing conditions for obtaining a predetermined steel sheet structure were studied and the following findings were obtained.

(a) The hardness (hardness) and total elongation (hereinafter sometimes simply referred to as elongation) in the high-carbon hot-rolled steel sheet before quenching are such that the average grain size of ferrite is 5 μm or more and 25 μm or less, and the average grain size of all cementite is By setting the interval to 1 μm or more, it is possible to obtain a tensile strength TS of 420 MPa or less, a total elongation (El) of 39% or more, and a hole expansion ratio of 80% or more.

(b) By setting the maximum Cr concentration in the cementite in the steel sheet before carburizing to 23% by mass or less, precipitation of cementite can be suppressed even in sharp-angled parts when performing vacuum carburizing on the steel sheet, Sufficient fatigue strength can be achieved. That is, when the maximum Cr concentration in cementite exceeds 23% by mass, the C concentration in the surface layer tends to increase during vacuum carburization in parts having an acute angle of 60°, and cementite tends to precipitate. Furthermore, in the case of a component with an acute angle, for example, a component with an acute angle of 45°, the concentration of C on the surface side tends to increase during vacuum carburization, and cementite is more likely to precipitate. When a higher resistance to excessive carburization is required, setting the maximum Cr concentration in cementite to 17% by mass or less makes it possible to suppress precipitation of cementite even in parts having an acute angle of 45°. Furthermore, in the case of a part with an acute angle, for example, an acute angle of 30° or less, the C concentration in the surface layer tends to increase during vacuum carburization, and cementite is more likely to precipitate. When a higher resistance to excessive carburization is required, setting the maximum Cr concentration in cementite to 14% by mass or less makes it possible to suppress precipitation of cementite even in parts with an acute angle of 30° or less. Furthermore, by adding a predetermined amount of one of Sb and Sn to the steel, the increase in the C concentration in the surface layer can be suppressed, and cementite precipitation can be greatly reduced even in parts with an acute angle of less than 60° (especially 45° or less). can be reduced to

(c) As manufacturing conditions for securing a predetermined structure, the steel is held in a temperature range of 1050 to 1270 ° C. for 1 hour or more, and is rough rolled at a reduction ratio of 40% or more in a temperature range of 980 to 1080 ° C. Finishing temperature: finish rolling at Ar ₃ transformation point or higher, then primary cooling to 750°C at an average cooling rate of 20°C/s or higher, and secondary cooling to coiling temperature at an average cooling rate of 10°C/s or higher. , Winding temperature: over 580 ° C. to 700 ° C. After cooling to room temperature, as annealing,
(i) heating to the Ac ₁ transformation point or more and the Ac ₃ transformation point or less, holding in that temperature range for 0.5 hours or more, and then cooling to less than the Ar ₁ transformation point at an average cooling rate of 1 to 20 ° C./h, Hold for 10 hours or longer in the temperature range of 600 ° C. or higher and lower than the Ar ₁ transformation point, or (ii) hold for 1 h or higher and 40 hours or lower in the temperature range of 600 ° C. or higher and the Ac ₁ transformation point or lower.
By doing so, a predetermined structure can be secured.

The present invention has been made based on the above findings, and has the following gist.
[1] % by mass,
C: 0.10% or more and 0.30% or less,
Si: 0.20% or more and 0.80% or less,
Mn: 0.25% or more and 1.00% or less,
P: 0.03% or less,
S: 0.010% or less,
sol. Al: 0.10% or less,
N: 0.01% or less,
A microstructure containing Cr: 0.05% or more and 0.80% or less and B: 0.0005% or more and 0.0050% or less, the balance being Fe and inevitable impurities, and a microstructure containing ferrite and cementite, and the microstructure is
The area ratio of ferrite is 80% or more,
The average grain size of ferrite is 5 μm or more and 25 μm or less,
A high-carbon hot-rolled steel sheet for vacuum carburizing, wherein the maximum Cr concentration in all cementite is 23% by mass or less and the average spacing of all cementite is 1.0 μm or more.

[2] The high-carbon hot-rolled steel sheet for vacuum carburizing according to [1], wherein the ferrite has an average grain size of 10 μm or more and 25 μm or less, and the average spacing of the total cementite is 5.0 μm or more.

[3] The high-carbon hot-rolled steel sheet for vacuum carburizing according to [1], wherein the ferrite has an average grain size of 5 µm or more and less than 10 µm, and the average spacing of the all cementite is 1.0 µm or more and less than 5.0 µm.

[4] The high-carbon hot-rolled steel sheet for vacuum carburizing according to any one of [1] to [3], which has a tensile strength of 420 MPa or less and a total elongation of 39% or more.

[5] The high-carbon hot-rolled steel sheet for vacuum carburizing according to any one of [1] to [4], which has a hole expansion ratio of 80% or more.

[6] The high-carbon hot-rolled steel sheet for vacuum carburizing according to any one of [1] to [5], further containing Ti: 0.06% or less in mass %.

[7] % by mass, and
The high-carbon hot-rolled steel sheet for vacuum carburizing according to any one of [1] to [6], containing 0.002% or more and 0.030% or less in total of any one or two or more of Sb and Sn. .

[8] In a region within 10 nm in the plate thickness direction from the surface of the steel sheet, the total concentration of one or more of Sb and Sn is 10 times (% by mass) or more than the concentration of the base material [7] ] The high-carbon hot-rolled steel sheet for vacuum carburizing.

[9]% by mass, and
Ni: 0.0005% or more and 2.50% or less,
Mo: 0.0005% or more and 0.25% or less,
Ta: 0.0005% or more and 0.1% or less,
W: 0.0005% or more and 0.1% or less,
Cu: 0.0005% or more and 0.1% or less,
Nb: 0.0005% or more and 0.1% or less,
V: 0.0005% or more and 0.1% or less and Ca: any one or more of 0.0005% or more and 0.1% or less The vacuum carburizing according to any one of [1] to [8] High carbon hot-rolled steel sheet for

[10] A method for producing a high-carbon hot-rolled steel sheet for vacuum carburizing according to any one of [1] to [9],
A steel material having the chemical composition described in [1], [6], [7] or [9] is held in a temperature range of 1050 to 1270° C. for 1 hour or more, and is reduced in a temperature range of 980 to 1080° C. Rough rolling at a rate of 40% or more, finishing temperature: finish rolling at a transformation point of Ar ₃ or more, then primary cooling to 750°C at an average cooling rate of 20°C/s or more, average cooling rate: 10°C/s Secondary cooling to the coiling temperature above, coiling at a coiling temperature of over 580 ° C. to 700 ° C. After cooling to room temperature, the high carbon for vacuum carburizing is subjected to the treatment (i) or (ii) as annealing. A method for producing a hot-rolled steel sheet.
(i) heating to the Ac ₁ transformation point or more and the Ac ₃ transformation point or less, holding in that temperature range for 0.5 hours or more, and then cooling to less than the Ar ₁ transformation point at an average cooling rate of 1 to 20 ° C./h, The temperature is maintained in the temperature range of 600° C. or more and less than the Ar ₁ transformation point for 10 hours or more.
(ii) Hold for 1 hour or more and 40 hours or less in a temperature range of 600° C. or more and Ac ₁ transformation point or less.

[11] A method for producing a high-carbon hot-rolled steel sheet according to any one of [2], [4] to [9],
A steel material having the chemical composition described in [1], [6], [7] or [9] is held in a temperature range of 1050 to 1270° C. for 1 hour or more, and is reduced in a temperature range of 980 to 1080° C. Rough rolling at a rate of 40% or more, finishing temperature: finish rolling at a transformation point of Ar ₃ or more, then primary cooling to 750°C at an average cooling rate of 20°C/s or more, average cooling rate: 10°C/s Above, it is secondary cooled to the coiling temperature, coiled at a coiling temperature of more than 580 ° C. to 700 ° C. After cooling to normal temperature, it is heated to the Ac ₁ transformation point or more and Ac ₃ transformation point or less, and 0.00 in the temperature range. High carbon heat for vacuum carburizing, held for 5 hours or more, then cooled to less than the Ar ₁ transformation point at an average cooling rate of 1 to 20 ° C./h, and held in the temperature range of 600 ° C. or more and less than the Ar ₁ transformation point for 10 hours or more. A method for manufacturing a rolled steel sheet.

[12] A method for producing a high-carbon hot-rolled steel sheet according to any one of [3] to [9],
A steel material having the chemical composition described in [1], [6], [7] or [9] is held in a temperature range of 1050 to 1270° C. for 1 hour or more, and is reduced in a temperature range of 980 to 1080° C. Rough rolling at a rate of 40% or more, finishing temperature: finish rolling at a transformation point of Ar ₃ or higher, then primary cooling to 750°C at an average cooling rate of 20 to 65°C/s or less, average cooling rate: 10 to 750°C. Secondary cooling to the coiling temperature at 65 ° C./s or less, coiling temperature: over 580 ° C. to 700 ° C. After cooling to normal temperature, the temperature range of 600 ° C. or higher and Ac ₁ transformation point or lower for 1 h or more and 40 h or less. A method for producing a high-carbon hot-rolled steel sheet for vacuum carburizing, characterized by holding

[13] A steel part obtained by carburizing the hot-rolled steel sheet according to any one of [1] to [9],
A carburized steel part comprising a carburized layer and a non-carburized layer, both layers having a martensitic structure and free of cementite having a size of 10 μm or more.

According to the present invention, a high-carbon hot-rolled steel sheet that has both excellent elongation properties and hole expansibility to ensure cold workability, and is excellent in excessive carburization resistance and hardenability in vacuum carburization, and the steel sheet are used. It is possible to provide a carburized steel part that is By applying this part to automotive parts such as gears and dampers in transmissions, it is possible to secure high fatigue strength and to manufacture automotive parts that require stable quality. It can greatly contribute to

The high-carbon hot-rolled steel sheet of the present invention, the method for producing the same, and the carburized steel parts using the steel sheet will be described below in detail.
[Component composition]
First, the reason for limiting the amount of each element in the chemical composition of the high-carbon hot-rolled steel sheet of the present invention will be described. In addition, "%", which is a unit of content in the following component compositions, means "% by mass" unless otherwise specified.
C: 0.10% or more and 0.30% or less C is an important element for obtaining strength after quenching. If the C content is less than 0.10%, the desired hardness cannot be obtained by heat treatment after molding, so the C content must be 0.10% or more. However, if the amount of C is 0.30% or more, the steel becomes hard, and cold workability and toughness after heat treatment deteriorate. Therefore, the amount of C is set to 0.10% or more and 0.30% or less. When using for cold working of parts that are complicated in shape and difficult to press, the C content is preferably 0.25% or less, more preferably 0.20% or less.

Si: 0.20% or more and 0.80% or less Si is an element that enhances resistance to excessive carburization. When the Si content is less than 0.20%, the C concentration in the surface layer of the steel sheet tends to increase in the vacuum carburizing process. increases and cementite precipitates. Since this coarse cementite becomes a starting point of fatigue fracture, it causes deterioration of fatigue characteristics. Therefore, Si should be 0.20% or more. It is more preferably 0.25% or more and more than 0.30%. On the other hand, Si is an element that increases strength by solid solution strengthening. As the amount of Si increases, the steel sheet becomes harder and the elongation becomes insufficient, so the amount of Si is made 0.80% or less. More preferably, it is 0.75% or less and 0.65% or less.

Mn: 0.25% or more and 1.00% or less Mn is an element that enhances hardenability, and when added in excess, inhibits excessive carburization resistance. If the Mn content exceeds 1.00%, the carbon concentration in the surface layer of the steel sheet increases in the vacuum carburizing process. Coarse cementite precipitates on austenite grain boundaries. Since this cementite becomes a starting point of fatigue fracture, it causes deterioration of fatigue characteristics. Therefore, the Mn content is set to 1.00% or less. It is preferably 0.90% or less, 0.75% or less, more preferably 0.65% or less. On the other hand, if it is less than 0.25%, ferrite transformation may occur in a quenching process with a high cooling rate such as water quenching, and the predetermined hardness may not be obtained after quenching. do. Preferably it is 0.30% or more.

P: 0.03% or less P is an element that increases the strength by solid solution strengthening. When the P content exceeds 0.03%, the steel sheet is hardened, the elongation of the steel sheet is lowered, and the toughness after quenching is impaired due to grain boundary segregation. Therefore, the amount of P is set to 0.03% or less. To obtain higher elongation, the P content is preferably 0.02% or less. Since P reduces the elongation of the steel sheet, the smaller the amount of P, the better. In addition, since refining cost increases when P is excessively reduced, the amount of P is preferably 0.005% or more. More preferably, it is 0.007% or more.

S: 0.010% or less S is an element that must be reduced because it forms sulfides and reduces the elongation of the steel sheet and the toughness after heat treatment. If the S content exceeds 0.010%, the elongation of the steel sheet and the toughness after quenching are significantly degraded. Therefore, the amount of S is set to 0.010% or less. In order to obtain excellent cold workability and toughness after quenching, the S content is preferably 0.005% or less. Since S lowers cold workability and toughness after quenching, it is more preferably 0.001% or less. Note that excessive reduction of S increases the refining cost, so the S content is preferably 0.0005% or more.

sol. Al: 0.10% or less sol. If the amount of Al exceeds 0.10%, AlN is generated during heating in the quenching treatment, and the austenite grains become too fine. This promotes the formation of ferrite phase during cooling, the structure becomes ferrite and martensite, and the hardness after quenching decreases. Therefore, sol. The amount of Al is set to 0.10% or less. It is preferably 0.06% or less. In addition, sol. Al has a deoxidizing effect, and is preferably 0.005% or more for sufficient deoxidizing.

N: 0.01% or less If the N content exceeds 0.01%, the austenite grains become too fine during heating for quenching due to the formation of AlN, promoting the formation of a ferrite phase during cooling and reducing the hardness after quenching. do. Therefore, the amount of N is set to 0.01% or less. More preferably, it is 0.008% or less. More preferably, it is 0.006% or less. Although the lower limit is not particularly specified, N forms AlN, Cr-based nitrides and BN, which moderately suppresses the growth of austenite grains during heating in the quenching process, and improves the toughness after quenching. is an element. Therefore, the N content is preferably 0.0005% or more.

Cr: 0.05% or more and 0.80% or less Cr is an element that enhances hardenability, and when added in excess, inhibits excessive carburization resistance. That is, when the amount of Cr exceeds 0.80%, the C concentration in the surface layer increases in the vacuum carburizing process. Therefore, coarse cementite precipitates on the prior austenite grain boundaries. Since this cementite becomes a starting point of fatigue fracture, it causes deterioration of fatigue characteristics. Therefore, the Cr content is set to 0.80% or less. In a shape with a sharper angle, the C diffusion field overlaps and the C concentration increases. Therefore, it is advantageous to further suppress cementite precipitation, and the C content is preferably 0.60% or less. More preferably, it is 0.50% or less, 0.40% or less, or 0.20% or less. On the other hand, if the Cr content in the steel is less than 0.05%, ferrite is likely to occur in the surface layer, especially during carburizing and quenching, and a completely quenched structure cannot be obtained, resulting in a decrease in hardness. . Preferably, it is 0.08% or more.

B: 0.0005% or more and 0.0050% or less In the present invention, B is an important element that enhances hardenability and suppresses grain boundary segregation of P. If the amount of B is less than 0.0005%, a sufficient effect is not recognized, so the amount of B must be 0.0005% or more. Preferably, it is 0.0010% or more. On the other hand, when the amount of B exceeds 0.0050%, elongation decreases due to the refinement of ferrite grains due to B. Therefore, the amount of B is set to 0.0050% or less. Preferably, it is 0.0040% or less.
The balance other than the above is Fe and unavoidable impurities.

With the above essential elements, the high-carbon hot-rolled steel sheet of the present invention can obtain the desired properties. The high-carbon hot-rolled steel sheet of the present invention may further contain the following elements, if necessary, for the purpose of further improving workability, hardenability, and excessive carburization resistance.

Ti: 0.06% or less Ti is an effective element for enhancing hardenability. When the hardenability is insufficient only by containing Cr and B, the hardenability can be improved by including Ti. If the amount of Ti is less than 0.005%, the effect is not recognized, so when Ti is contained, it is preferably 0.005% or more. More preferably, it is 0.007% or more. On the other hand, if the amount of Ti exceeds 0.06%, the steel sheet before quenching hardens and the elongation is impaired. Preferably, it is 0.04% or less.

0.002% or more and 0.030% or less in total of any one or both of Sb and Sn Sb and Sn are effective in suppressing excessive carburization from the surface layer of the steel sheet because they are concentrated in the surface layer of the steel sheet. element. In the case of a part with a shape sharper than 60° (especially 45° or less), the C concentration in the surface layer of the steel sheet tends to increase due to overlapping of C diffusion fields during vacuum carburizing, and cementite tends to precipitate. Therefore, it is effective to add a predetermined amount of at least one of Sb and Sn to steel even when higher resistance to excessive carburization is required. As a result, an increase in the C concentration in the surface layer of the steel sheet can be suppressed, and cementite precipitation can be greatly reduced even in parts with an acute angle of 45° or less. If the total content of one or two of these elements is less than 0.002%, the above effects are not sufficiently obtained. More preferably, it is 0.005% or more. On the other hand, even if the total content of one or two of these elements exceeds 0.030%, the effect of preventing excessive carburization is saturated. In addition, since these elements tend to segregate at grain boundaries, if the total of one or two elements exceeds 0.030%, the content becomes too high, which may cause grain boundary embrittlement. . Therefore, when at least one of Sb and Sn is contained, the total content of one or two of these elements is preferably 0.030% or less. More preferably, it is 0.020% or less.

Ni: 0.0005-2.50%
Ni is an element highly effective in improving toughness and hardenability. If less than 0.0005%, there is no addition effect, so the lower limit is made 0.0005%. If it exceeds 2.50%, the effect of addition is saturated and the cost increases, so the upper limit is made 2.50%. A more preferable range is 1.00% or less, and most preferably 0.10% or less.

Mo: 0.0005-0.25%
Mo is an element effective in improving hardenability and temper softening resistance. If less than 0.0005%, the addition effect is small, so the lower limit is made 0.0005%. Since the addition effect saturates when it exceeds 0.25%, the upper limit is made 0.25%. It is more preferably 0.10% or less, 0.05% or less, and most preferably 0.03% or less.

Ta: 0.0005-0.1%
Ta, like Nb, forms carbonitrides and is an element effective in preventing abnormal grain growth and grain coarsening during heating before quenching and improving temper softening resistance. If less than 0.0005%, the addition effect is small, so the lower limit is made 0.0005%. Further, if the content exceeds 0.1%, the effect of addition is saturated and the quenching hardness is lowered, so the upper limit is defined as 0.1%. More preferably 0.05% or less, most preferably 0.03% or less.

W: 0.0005 to 0.1%
Like Nb and V, W forms carbonitrides and is an element effective in preventing abnormal grain growth of austenite grains during heating before quenching and improving temper softening resistance. If less than 0.0005%, the addition effect is small, so the lower limit is defined as 0.0005%. If the content exceeds 0.1%, the effect of addition is saturated and the quenching hardness is lowered, so the upper limit is defined as 0.1%. More preferably 0.05% or less, most preferably 0.03% or less.

Cu: 0.0005-0.1%
Cu is an element effective for ensuring hardenability. If it is less than 0.0005%, the effect of addition cannot be sufficiently confirmed, so the lower limit is made 0.0005%. If it exceeds 0.1%, flaws during hot rolling are likely to occur, resulting in deterioration in manufacturability such as a drop in yield, so the upper limit is made 0.1%. A more preferable range is 0.05% or less.

Nb: 0.0005-0.1%
Nb is an element that forms carbonitrides and is effective in preventing abnormal grain growth of crystal grains during heating before quenching, improving toughness, and improving temper softening resistance. If it is less than 0.0005%, the effect of containing it is not sufficiently exhibited, so the lower limit is preferably made 0.0005%. On the other hand, when the content exceeds 0.1%, not only does the effect of the Nb content saturate, but the Nb carbide increases the tensile strength of the base material and reduces the elongation. Therefore, it is preferable to set the upper limit to 0.1%. More preferably 0.05% or less, most preferably 0.03% or less.

V: 0.0005 to 0.1%
Like Nb and Ta, V forms carbonitrides and is an element effective in preventing abnormal grain growth of crystal grains during heating before quenching, improving toughness, and improving temper softening resistance. If it is less than 0.0005%, the effect of addition is not sufficiently exhibited, so the lower limit is made 0.0005%. If the content exceeds 0.1%, not only the effect of addition is saturated, but also the V carbide increases the tensile strength of the steel sheet and reduces the elongation, so the upper limit is made 0.1%. More preferably 0.05% or less, most preferably 0.03% or less.

Ca: 0.0005-0.1%
Ca is effective for desulfurizing steel, and for that purpose, it is added at 0.0005% or more. On the other hand, if the amount of Ca added is too large, low-melting and coarse oxides of Al and Ca are formed, and complex oxysulfides tend to absorb Ca and become coarse. Since these coarse oxides are likely to become starting points of fatigue fracture, the upper limit is defined as 0.1%. More preferably 0.05% or less, most preferably 0.03% or less.

[Microstructure]
Next, the reasons for limiting the microstructure of the high-carbon hot-rolled steel sheet of the present invention will be explained.
In the present invention, the microstructure includes ferrite and cementite, the area ratio of ferrite is 80% or more, the average grain size of ferrite is 5 μm or more and 25 μm or less, the maximum Cr concentration in all cementite is 23% by mass or less, and all cementite is 1.0 μm or more.

The microstructure of the high-carbon hot-rolled steel sheet of the present invention contains ferrite and cementite, preferably ferrite and cementite. The structure of the high-carbon hot-rolled steel sheet according to the present invention may contain residual structures such as pearlite and bainite in addition to the ferrite and cementite described above. If the total area ratio of the residual structure is 5% or less, it does not impair the effects of the present invention, so it may be contained. Below, the reasons for limiting the requirements in the microstructure will be explained for each requirement.

[Ferrite area ratio: 80% or more]
If the area ratio of ferrite is less than 80%, the cold workability becomes inferior, and cold working may be difficult especially in parts with a high degree of workability. .

[Average grain size of ferrite: 5 μm or more and 25 μm or less]
If the average grain size of ferrite is less than 5 μm, the strength before cold working increases and the cold workability deteriorates, so the average grain size is made 5 μm or more. More preferably, it is 6 µm or more. On the other hand, if it exceeds 25 μm, the strength of the steel sheet decreases. In general, quenching is performed to increase the surface hardness of the product. Therefore, the ferrite average grain size is set to 25 μm or less. It is more preferably 20 μm or less, and still more preferably 15 μm or less.

Here, from the viewpoint of imparting a higher elongation (42% or more) to the steel sheet, it is preferable to set the average grain size of ferrite to 10 μm or more and 25 μm or less.

In addition, when the number of ferrite grain boundaries increases, cementite tends to preferentially precipitate at the grain boundaries, making it difficult to form coarse cementite. In particular, in order to suppress coarse cementite of 2 μm or more and ensure a hole expansion ratio of 90% or more, the average grain size of ferrite is preferably 5 μm or more and less than 10 μm.

[Maximum Cr concentration in all cementite in steel plate is 23% by mass or less]
During heating in the austenite region of vacuum carburizing, the cementite in the steel sheet melts. Since the diffusion of C is fast, the carburizing holding time is sufficient for the cementite to dissolve, but the diffusion of Cr is extremely slow. There are places where the Cr concentration is high even after the period. Since Cr is an element that easily dissolves into cementite, the presence of a high Cr concentration region becomes a site where cementite easily precipitates after carburizing. If cementite having a maximum Cr concentration of more than 23% by mass exists in the steel sheet before carburizing, the cementite tends to precipitate after vacuum carburizing. More preferably, it is 17% by mass or less, 14% by mass or less, 10% by mass or less, and even more preferably 6% by mass or less.

[Average spacing of all cementite of 1.0 μm or more]
It is necessary to increase local ductility to improve hole expansibility, and it is important to suppress the connection of voids. Voids are likely to occur at interfaces between ferrite and coarse cementite, and in order to suppress the connection of voids, it is necessary to set the interval between all cementites to 1.0 μm or more. As a result, a hole expanding ratio of 80% or more can be secured.
On the other hand, if excessively coarse cementite exists, the crack propagates in the plate thickness direction from the crack generated at the interface between cementite and ferrite. As a result, there is a possibility that the hole expansibility may deteriorate, so from the viewpoint of not generating coarse cementite, the average spacing is preferably 20.0 μm or less. The average interval of cementite is the free path of cementite.

Here, in order to reliably achieve a high elongation of 42% or more and a hole expansion ratio of 90% or more in a steel plate, the above ferrite crystal grain size must be 10 μm or more. It is preferable to set the interval to 5.0 μm or more.

In addition, if coarse cementite is present, cracks propagate in the sheet thickness direction and the hole expansibility is lowered. Therefore, coarse cementite must not be present. In particular, in order to prevent the presence of coarse cementite of 2 μm or more, the average spacing of all cementite is preferably 1.0 μm or more and less than 5.0 μm.

The average grain size of ferrite, the circle-equivalent diameter of cementite, the area ratio of ferrite, and the like can be measured by the methods described in the examples below.

Furthermore, satisfying the following favorable conditions in the microstructure is effective in improving the mechanical properties (ductility).
[The ratio of the number of cementite in the ferrite grain boundary to the number of cementite in the ferrite grain is 0.5 or more]
Usually, the diameter of cementite that exists before quenching is about 0.2 to 6.0 μm in equivalent circle diameter, and exists in ferrite grains and grain boundaries. When a large number of fine cementite particles are present in ferrite grains, the local ductility increases and the hole expansibility improves. On the other hand, if a large amount of cementite exists in the ferrite grains, the growth of the ferrite grains is suppressed in the annealing process, the ferrite grains become finer, the structure becomes non-uniform, and the elongation decreases. Therefore, in order to obtain excellent elongation while ensuring the expansibility, the ratio of the number of cementites in the ferrite grain boundaries to the number of cementites in the ferrite grains is preferably 0.5 or more. It is preferably 0.7 or more, more preferably 1.0 or more.

[Average particle size of cementite is 0.2 μm or more and 2.5 μm or less]
First, the average particle size of cementite means the average circle equivalent diameter. If the cementite has a large average particle size, cracking occurs during tensile deformation, and high elongation cannot be obtained. Therefore, the average particle size is preferably 2.5 μm or less. More preferably, it is 2.0 μm or less. On the other hand, if the average particle size is less than 0.2 μm, the amount of cementite of 0.1 μm or less, which is effective for precipitation strengthening, increases and the steel sheet hardens and elongation decreases. preferable. More preferably, the thickness is 0.3 μm or more.

[Concentration of Sb and Sn in the surface layer of the steel sheet]
Further, as described above, when one of Sb and Sn is added as necessary, in a region within 10 nm in the thickness direction from the surface of the steel sheet, one or two of Sb and Sn It is preferable that the total concentration of the above is 10 times or more the concentration of the base material.
Furthermore, Sb and Sn have the effect of suppressing excessive carburization from the steel plate surface layer by concentrating them in the steel plate surface layer. , has the effect of suppressing cementite precipitation. If the total concentration of one or more of Sb and Sn in a region within 10 nm from the steel plate surface is less than 10 times the concentration of the base material, the above effects are difficult to develop. In order to concentrate either Sb or Sn in the surface layer of the steel sheet, the slab heating temperature should be 1050 to 1270° C., for example.

[Mechanical properties]
The high-carbon hot-rolled steel sheet of the present invention is required to have excellent cold workability because it is cold-pressed to form automotive parts such as gears and transmission dampers. In addition, it is necessary to increase the hardness by quenching treatment, impart wear resistance, and exhibit high fatigue properties. Therefore, the high-carbon hot-rolled steel sheet of the present invention has a tensile strength of 420 MPa or less, a total elongation (El) of 39% or more, and a hole expansion ratio of 80% or more. It is possible to achieve both excellent hardenability (hardenability and resistance to excessive carburization).
In addition, the tensile strength, total elongation (El), and hole expansibility described above can be measured by the methods described in Examples described later.

[Production method]
The high-carbon hot-rolled steel sheet of the present invention uses steel having the chemical composition according to the above as a material, holds the steel material in a temperature range of 1050 to 1270 ° C. for 1 hour or more, and reduces the reduction in the temperature range of 980 to 1080 ° C.: Rough rolling at 40% or more, finishing temperature: finish rolling at Ar ₃ transformation point or more, then primary cooling to 750°C at an average cooling rate of 20°C/s or more, average cooling rate: 10°C/s or more It is manufactured by secondary cooling to the coiling temperature, coiling at a coiling temperature of over 580° C. to 700° C., cooling to room temperature, and then undergoing the step (i) or (ii) as annealing.
(i) heating to the Ac ₁ transformation point or more and the Ac ₃ transformation point or less, holding in that temperature range for 0.5 hours or more, and then cooling to less than the Ar ₁ transformation point at an average cooling rate of 1 to 20 ° C./h, The temperature is maintained in the temperature range of 600° C. or more and less than the Ar ₁ transformation point for 10 hours or longer.
(ii) Hold for 1 hour or more and 40 hours or less in a temperature range of 600° C. or more and Ac1 transformation point or less.

The reasons for limitations in the method for producing a high-carbon hot-rolled steel sheet according to the present invention will be described below. In the description, "°C" regarding temperature indicates the temperature on the surface of the steel plate or the surface of the steel material.

In the present invention, the method for manufacturing the steel material is not particularly limited. For example, both a converter and an electric furnace can be used to smelt the high carbon steel of the present invention. High-carbon steel melted by a known method such as a converter is made into a slab or the like (steel material) by ingot casting-slabbing rolling or continuous casting. A slab is usually hot rolled (rough hot rolling, finish rolling) after being heated.
For example, in the case of a slab manufactured by continuous casting, direct rolling may be applied in which the slab is rolled as it is or while being heat-retained for the purpose of suppressing a temperature drop. In the hot rolling, the material to be rolled may be heated by a heating means such as a bar heater in order to secure the finishing temperature of finish rolling.

[Slab heating temperature: held in the temperature range of 1050 to 1270°C for 1 hour or more]
During slab heating, it is necessary to make the Cr concentration in the steel uniform, and to concentrate Sb and Sn on the surface of the steel sheet when Sb and Sn are added. If the heating temperature is too high, the surface condition deteriorates due to scale, and if Sb and Sn are added, the concentrated layer of Sb and Sn on the surface will peel off, so the upper limit was set to 1270 ° C. . On the other hand, if the heating temperature is less than 1100° C., the diffusion rate of Cr decreases and the Cr concentration in the steel does not become uniform within a predetermined time. Also, when Sb and Sn are added, the concentration of Sb and Sn in the surface layer slows down, and the concentration does not reach a predetermined amount over the entire surface layer. If the holding time is less than 1 hour, Cr cannot diffuse sufficiently and the Cr concentration in the steel does not become uniform. Moreover, when Sb and Sn are added, the Sb and Sn are not concentrated by a predetermined amount over the entire surface layer, so the time was set to 1 hour or more. On the other hand, heating for 5 hours or more is preferable because the concentration of Sb and Sn in the surface layer becomes saturated when Sb and Sn are added, and the surface is oxidized to deteriorate the surface condition.

[Reduction rate in the temperature range of 980 to 1080 ° C.: Rough rolling at 40% or more]
In rough hot rolling, grains are refined by repeating recrystallization in the austenite region. If the crystal grains at this stage are too coarse, the number of crystal grain boundaries where the diffusion speed is high is reduced, and Cr cannot be diffused sufficiently. Therefore, by making the segregation of Cr as small as possible in the steel before finish rolling, cementite having a low Cr concentration can be obtained when undergoing pearlite transformation during cooling after finish rolling. Density can also be controlled below a predetermined value. For that purpose, it is necessary to set the rolling reduction in the temperature range of 980 to 1080° C. to 40% or more. The total rolling reduction in rough rolling is preferably 82 to 90%. More preferably, it is 83-88%.

[Finishing temperature: finish rolling at Ar ₃ transformation point or higher]
If the finish rolling finish temperature is lower than the Ar ₃ transformation point, coarse ferrite grains are formed after hot rolling and after annealing, resulting in a marked decrease in elongation. For this reason, the finish rolling end temperature is set to the Ar ₃ transformation point or higher. It is preferably (Ar ₃ transformation point + 20°C) or higher. Although the upper limit of the finishing temperature of finish rolling need not be specified, it is preferably 1000° C. or less in order to perform cooling smoothly after finish rolling.
Moreover, the above-mentioned Ar ₃ transformation point can be determined by actual measurement by thermal expansion measurement during cooling such as the Formaster test or electrical resistance measurement.

[Average cooling rate in primary cooling after finish rolling: cooling to 750 ° C. at 20 ° C./s or more]
The average cooling rate to 750°C after finish rolling affects the cementite grain size distribution after spheroidizing annealing. In order to obtain a desired cementite grain size distribution after the annealing step following coiling, it is necessary to have a uniform ferrite and pearlite structure before the annealing step. If the average cooling rate is less than 20°C/s, uniform ferrite and pearlite structures cannot be obtained. Further, in non-uniform pearlite, the Cr concentration is locally high in a region where cementite exists, so that the Cr concentration in the cementite after annealing exceeds a predetermined concentration. Therefore, the primary average cooling rate after finish rolling is set to 20° C./s or more. There is no need to specify the upper limit, but if it exceeds 150 ° C./s, the pearlite structure is too fine and tends to remain in the ferrite grains after annealing, and the number of carbides at the ferrite grain boundaries relative to the number of cementites in the ferrite grains. is preferably 150° C./s or less, because it becomes difficult to keep the ratio of Δ within the predetermined range. More preferably, it is 100° C./s or less.

When the above-described annealing (ii) is selected as the annealing, if the primary average cooling rate after finish rolling exceeds 65 ° C./s, the pearlite structure is too fine and tends to remain in the ferrite grains after annealing. Since it becomes difficult to keep the ratio of the number of carbides at the ferrite grain boundaries to the number of cementites in the ferrite grains within a predetermined range, it is preferably 65° C./s or less.

On the other hand, when the above-described annealing (i) is selected as the annealing, even if the cooling rate after finish rolling is high and the pearlite becomes fine, the fine pearlite is formed in the austenite phase generated during the first stage annealing. Since it dissolves, it hardly affects the distribution of cementite after annealing.

[Average cooling rate in secondary cooling: cooling to winding temperature at 10 ° C./s or more]
If the secondary average cooling rate is less than 10°C/s, a pearlite structure with coarse lamellae is formed, and a predetermined cementite grain size distribution cannot be obtained after the annealing process. Since the Cr concentration in the cementite after annealing becomes higher than the predetermined concentration, the secondary average cooling rate is set to 10° C./s or more. There is no need to specify the upper limit, but if it exceeds 150 ° C./s, the pearlite structure is too fine and tends to remain in the ferrite grains after annealing, and the ratio of the number of carbides at the ferrite grain boundary to the number of cementite in the ferrite grains. within a predetermined range, it is preferably 150° C./s or less. More preferably, it is 100° C./s or less.

When the annealing (ii) described above is selected as the annealing, if the secondary average cooling rate exceeds 65 ° C./s, the pearlite structure is too fine and tends to remain in the ferrite grains after annealing, and the predetermined ferrite grains Since the ratio of the number of carbides at ferrite grain boundaries to the number of cementites cannot be obtained, it is preferable to set the rate to 65° C./s or less.

On the other hand, when the above-described annealing (i) is selected as the annealing, even if the cooling rate after finish rolling is high and the pearlite becomes fine, the fine pearlite is formed in the austenite phase generated during the first stage annealing. Since it dissolves, it hardly affects the distribution of cementite after annealing.

[Winding temperature: over 580°C to 700°C]
The hot-rolled steel sheet after finish rolling is wound into a coil shape. If the coiling temperature is too high, a lamellar coarse pearlite structure is generated in the hot-rolled steel sheet, and the Cr concentration locally increases in the region where cementite exists after the annealing process, and the Cr concentration of the cementite after annealing reaches a predetermined concentration. Therefore, the upper limit of the winding temperature is set to 700°C. It is preferably 690° C. or less. On the other hand, if the coiling temperature is too low, the hot-rolled steel sheet is hardened, which is not preferable. Therefore, the lower limit of the winding temperature is made over 580°C. It is preferably 600° C. or higher.

After being wound into a coil as described above, it may be cooled to room temperature and pickled. After the pickling treatment, the following annealing (i) or (ii) is performed.

[annealing (i)]
The reasons for limiting each condition in annealing (i) in which annealing is performed in two stages will be described.
[Heating to Ac ₁ transformation point or more and Ac ₃ transformation point or less and holding in the temperature range for 0.5 h or more (first stage annealing)]
By heating the hot-rolled steel sheet to an annealing temperature equal to or higher than the Ac ₁ transformation point, part of the ferrite in the steel sheet structure is transformed into austenite, fine carbides precipitated in the ferrite are dissolved, and C is added to the austenite. Dissolve. On the other hand, the remaining ferrite that has not been transformed into austenite is annealed at a high temperature, so that the dislocation density is reduced and the ferrite is softened. In addition, relatively coarse carbides (undissolved carbides) that have not been dissolved remain in the ferrite, but become even coarser due to Ostwald growth. When the annealing temperature is lower than the Ac ₁ transformation point, austenite transformation does not occur, so carbides cannot be dissolved in austenite. In addition, in the present invention, if the holding time at the Ac ₁ transformation _point or higher is less than 0.5 hours, the fine carbide cannot be sufficiently dissolved. and hold for 0.5 hours or more. On the other hand, if the first-stage annealing temperature exceeds the _Ac3 transformation point _, a large amount of rod-shaped cementite is obtained after annealing, and the predetermined elongation cannot be obtained. Also, the holding time is preferably 10 hours or less. Any of nitrogen, hydrogen, and a mixed gas of nitrogen and hydrogen can be used as the atmosphere gas during annealing.

[Cooling to less than the Ar ₁ transformation point at an average cooling rate of 1 to 20 ° C./h]
After the first stage annealing, the steel is cooled at an average cooling rate of 1 to 20° C./h below the Ar ₁ transformation point, which is the temperature range for the second stage annealing. During cooling, C discharged from austenite accompanying the transformation from austenite to ferrite precipitates as relatively coarse spherical carbides at the interfaces between ferrite and austenite and undissolved carbides as nucleation sites. In this cooling, it is necessary to adjust the cooling rate so as not to generate pearlite. If the cooling rate from the first stage annealing to the second stage annealing is less than 1° C./h, the production efficiency is poor, so the cooling rate is set to 1° C./h or more. On the other hand, if the rate exceeds 20° C./h, pearlite precipitates and the hardness increases, so the rate is set to 20° C./h or less.

[Holding for 10 hours or longer in the temperature range of 600°C or higher and lower than the Ar ₁ transformation point (second stage annealing)]
After the first stage annealing described above, the steel is cooled at a predetermined cooling rate and maintained in a temperature range below the Ar ₁ transformation point, thereby further growing coarse spherical carbides and eliminating fine carbides by Ostwald growth. . If the holding time below the Ar ₁ transformation point is less than 10 hours, the carbide cannot be sufficiently grown, and the strength after annealing becomes too large. For this reason, the second stage annealing is performed at a temperature lower than the Ar ₁ transformation point and maintained for 20 hours or more. Note that the annealing temperature in the second stage is set to 600° C. or higher in order to sufficiently grow the carbide. The temperature is preferably 660° C. or higher, and the holding time is preferably 35 hours or shorter from the viewpoint of production efficiency.

Cooling after the second stage annealing is basically furnace cooling to room temperature. Here, in the case of a cooling rate exceeding 50 ° C./h, cementite spheroidization becomes insufficient, and cementite with a large aspect ratio remains and may impede the ductility of the steel plate. It is preferably 1 to 30°C/h, more preferably 1 to 30°C/h.

[Annealing (ii)]
The reasons for limiting each condition in annealing (ii) in which annealing is performed in one step will be described.
[Annealing temperature: maintained in a temperature range of 600° C. or more and less than the Ac ₁ transformation point]
The hot-rolled steel sheet obtained as described above is subjected to annealing (cementite spheroidization annealing). When the annealing temperature is equal to or higher than the Ac ₁ transformation point, austenite is precipitated and a coarse pearlite structure is formed in the cooling process after annealing, resulting in a non-uniform structure. Therefore, the annealing temperature should be less than the Ac ₁ transformation point. It is preferably (Ac ₁ transformation point -10°C) or less. The annealing temperature must be 600° C. or higher, preferably 700° C. or higher, in order to obtain a predetermined cementite dispersed state. Any of nitrogen, hydrogen, and a mixed gas of nitrogen and hydrogen can be used as the atmospheric gas. Further, the holding time in annealing is preferably 0.5 to 40 hours. If the holding time at the annealing temperature is less than 0.5 hours, the effect of annealing is poor and the target structure of the present invention cannot be obtained. As a result, the target hardness and elongation of the steel sheet cannot be obtained. do not have. Therefore, the holding time at the annealing temperature is preferably 1.0 hours or more. More preferably, it is 5 hours or longer. On the other hand, the holding time at the annealing temperature is preferably 40 hours or less. More preferably, it is 35 hours or less.

Cooling after the annealing is basically furnace cooling to room temperature. Here, in the case of a cooling rate exceeding 50 ° C./h, cementite spheroidization becomes insufficient, and cementite with a large aspect ratio remains and may impede the ductility of the steel plate. It is preferably 1 to 30°C/h, more preferably 1 to 30°C/h.

The above-mentioned Ac ₁ transformation point and Ac ₃ transformation point are the start temperature and end temperature of transformation from ferrite to austenite during heating, respectively. The Ar ₃ transformation point and the Ar ₁ transformation point are the start and finish temperatures of austenite to ferrite transformation upon cooling, respectively. Although these are not particularly limited, they can be determined by actually measuring thermal expansion and electrical resistance during heating and cooling by a Formaster test or the like.

Next, the reason why carburized steel parts are specified will be described.
[Both the carburized layer and the non-carburized layer have a martensitic structure]
For example, when the damper of a gear or transmission is used under high surface pressure, high fatigue properties and wear resistance are required. The presence of the carburized layer provides the desired toughness.

[No cementite of 10 µm or more]
Cementite with a size of 10 μm or more is required to be absent because it becomes a starting point of fracture in fatigue.

Although there is no particular description of the hardness distribution of the carburized steel part, it is preferable to have the hardness distribution and structure shown below. A carburizing step of heating a carburized steel part made of a steel plate to which B is added by adjusting the amount of Cr added to a carburizing atmosphere having an oxygen concentration lower than that of the air to a temperature equal to or higher than the austenitizing temperature to form a carburized layer on the surface, and the carburizing step. following the step, cooling the steel part at a cooling rate lower than the cooling rate for martensitic transformation and cooling the steel part to a temperature below the temperature at which structural transformation due to cooling is completed; A carburized layer having a surface hardness of 800 HV or more is formed from the surface layer to the inner side by performing a quenching process in which the desired part is heated to the austenite region and then cooled at a cooling rate equal to or higher than the cooling rate for martensite transformation. It has a thickness of 100 μm (first layer), has a carburized layer (second layer) of 400 μm or more with a hardness of 450 HV or more and less than 800 HV inside it, and further has an internal hardness of 380 HV or more and less than 600 HV inside the carburized layer. The structure of both the carburized layer and the non-carburized layer is martensite, and the carburized steel part has no cementite with an equivalent circle diameter of 10 μm or more.

Steels having chemical compositions of Steel Nos. A to X shown in Table 1 were melted and then hot-rolled according to the production conditions shown in Tables 2 and 3. Then, it was pickled and annealed at the annealing temperature and annealing time shown in Tables 2 and 3 to produce hot-rolled and annealed sheets with a thickness of 3.0 mm.
A test piece was taken from the hot-rolled steel sheet thus obtained, and the microstructure, tensile strength, elongation, hole expansibility, resistance to excessive carburization after vacuum carburization, and hardness distribution were determined as follows. The Ac ₃ transformation point, Ac ₁ transformation point, Ar ₁ transformation point and Ar ₃ transformation point shown in Table 1 were determined by the Formaster test.

(I) Microstructure The microstructure of the hot-rolled steel sheet is obtained by cutting and polishing a test piece (size: 3 mmt × 10 mm × 10 mm) taken from the center of the width of the steel plate, and then applying nital corrosion, using a scanning electron microscope (SEM). Then, images were taken at 1000 times magnification at three locations of 1/4 plate thickness. Each phase (ferrite, cementite, pearlite, etc.) was identified by image processing of the photographed structure photograph.
In addition, the area ratio of ferrite was obtained by binarizing ferrite and regions other than ferrite from the SEM image using image analysis software.

In addition, the average grain size of ferrite was obtained from the photographs of the structure by using the grain size evaluation method (cutting method) specified in JIS G 0551.

Furthermore, the average cementite diameter was evaluated for the photographed tissue photographs.
The average diameter of cementite was obtained by measuring the area of each cementite and averaging the equivalent circle diameters d according to the following formula (1).
d=(4S/π) ^1/2 (1)
Here, S: area of each cementite

In addition, the number of cementite present in the ferrite grains and the number of cementite present in the ferrite grain boundaries were measured from the structure photograph, and the number of cementite present in the ferrite grain boundaries was divided by the number of cementite present in the ferrite grains to obtain the number of cementite in the ferrite grains. The ratio of the number of cementite present at the ferrite grain boundary to the cementite present at the ferrite grain boundary was obtained.
The average interval of all cementite is obtained by binarizing the cementite and the area other than cementite from the SEM image using image analysis software, obtaining the area ratio of cementite, and calculating the area ratio (volume ratio) and the cementite average obtained above. Based on the diameter, the mean free path was obtained from the following formula (2) ^{ref 1)} .
Mean free path={1.81(f ^-1/2 )−1.63}dp/2 (2)
f: volume ratio (area ratio) of cementite,
dp: average particle diameter of cementite
ref 1) Metal Handbook 6th Revised Edition, p.319

The Cr concentration of cementite in the steel sheet was determined as follows.
Using FE-EPMA, a characteristic X-ray intensity mapping was performed in a region of 50 μm×50 μm in the cross section of the steel plate on which cementite was precipitated, and the elemental distribution of C and Cr was determined. Next, a region with a high C content in the mapping was determined to be cementite, and the Cr count obtained in the region was defined as the Cr concentration in the cementite. EPMA measurement conditions were acceleration voltage: 9 kV, irradiation current: 5×10 ⁻⁸ A, and ZAF correction calculation method was used for quantitative analysis. That is, the characteristic X-ray intensity measured from the analysis sample and the relative intensity, which is the ratio of the characteristic X-ray intensity of the standard sample having a Cr content of 99% or more, are obtained, and atomic number correction ^ref2) , absorption correction ^ref2) and Fluorescence excitation correction ^ref2) was performed and quantitative analysis was performed.
ref2) Surface Analysis Technology Selection Electron Probe Microanalyzer, Chapter 6, p.167

X-ray photoelectron spectroscopy (XPS) was used to evaluate the Sb concentration of the surface layer of the steel sheet. First, the entire binding energy range (wide scan) is measured to detect C, O, and other elements in iron and iron. Since the 3d _5/2 peak of Sb may overlap with the O _1s peak depending on the state of Sb, it was separated as follows.
(1) First, the areas of the main peaks of Fe, O, and other elements in iron were calculated, and the areas were divided by the relative sensitivity coefficients to calculate the atomic concentrations of all the elements. At this time, the peak area of O determines the energy range (527 to 538 ev) so as to include Sb. Strictly speaking, it is not the peak of O, but the peak of O+Sb.
(2) Re-measure the O and Sb peaks in the binding energy range centered on the O peak with an energy range of 525 to 540 ev (narrow scan). The O peak is separated into oxide (528.9 ev) and hydroxide (531.9 ev), and metal Sb 3d _5/2 (528.2 ev) and 3d _3/2 (537.6 ev), The O atom concentration determined in the above (1) is separated into O and Sb by the ratio of the peak of metal Sb and the peak of other peaks.
(3) Normalize so that the total concentration of additive elements in Sb, Fe, and Fe determined in (1) and (2) above is 100%.
(4) Convert the Sb concentration obtained by the methods (1) to (3) above to % by mass.
If a large amount of carbon contamination is detected on the sample surface and peaks of Fe and Sb are not observed, the surface is cleaned with an ion gun to remove this contamination. The conditions of the ion gun are not defined in the present application, but Ar may be used, and cleaning can be performed for 40s to 110s under the conditions of 10 mA and 3 kV, for example. Although the XPS measurement conditions are not defined in the present application, for example, the XPS can be measured under the measurement conditions (voltage of 10 kV, photoelectron extraction angle of 35°) using Al X-rays.

(II) Tensile strength and elongation of steel sheet A tensile test was performed at 10 mm / min using a JIS No. 5 tensile test piece cut out from a hot-rolled steel sheet in the direction of 0 ° (L direction) with respect to the rolling direction, and the nominal stress was A nominal strain curve was obtained, and the maximum stress was taken as the tensile strength. Also, the total elongation was determined by matching the broken samples. The result was defined as elongation (El).

(III) Hole expanding property of steel plate A hole expanding test was performed according to the method described in JISZ2256. A hole of 10mmφ is punched in the center of a 125mm square steel plate with a clearance of 12±1%. The diameter D of the hole when the crack penetrates in the thickness direction is measured, and the hole expansion ratio is obtained by the following formula (3). In addition, it is set as the average value when the test was performed 3 times.
Hole expansion rate (%) = (D-10)/10 x 100 (3)

(IV) Excessive carburization resistance Excessive carburization resistance is determined in a carburizing process in which a carburizing layer is formed on the surface by heating above the austenitizing temperature in a carburizing atmosphere with a lower oxygen concentration than the atmosphere. was 26 minutes, the diffusion period was 930° C. for 95 minutes, and the carburization was carried out so that the surface C concentration of the flat specimen was 0.6%. Under the conditions shown in Tables 4 and 5, the presence or absence of cementite precipitation at the tip portions of the test pieces having shapes of 60°, 45°, and 30° was evaluated. As for the evaluation method, the tip portion of the test piece was cut out from the plate surface, polished and subjected to nital corrosion, and photographed at a magnification of 500 using a scanning electron microscope (SEM). In a region with a radius of 50 μm centered on the tip of each test piece, ⊙ indicates no cementite precipitation, ◯ indicates no cementite precipitation of 10 μm or more, and × indicates cementite precipitation of 10 μm or more. Those with ◯ and ⊚ in the shape of 60° are regarded as acceptable. More preferably, the shape of 45° is ◯ and ⊚, and more preferably the shape of 30° is ◯ and ⊚. In addition, the cementite diameter here evaluated the individual cementite diameter about the structure|tissue photograph which was image|photographed. Assuming that the cementite diameter is an ellipse, the area was obtained using the major axis and the minor axis, and converted to the equivalent circle diameter d by the following formula (4).
d=2(r _a ×r _b ) ^1/2 (4)
Here, r _a : major axis of each cementite
r _b : Minor diameter of each cementite

(V) Steel plate hardness distribution after water quenching after carburizing (carburizing hardenability)
In a carburizing process in which a hot-rolled steel sheet is heated to a temperature equal to or higher than the austenitizing temperature in a carburizing atmosphere having a lower oxygen concentration than the atmosphere to form a carburized layer on the surface, the carburizing period is 930°C for 26 minutes and the diffusion period is 930°C. Carburizing and quenching treatment was performed for 95 minutes so that the surface C concentration of the flat test piece was 0.6%, held at 860° C. for 30 minutes, and then slowly cooled. After that, it was heated to 900° C. and held for 10 seconds, and then cooled at a cooling rate equal to or higher than the cooling rate for martensite transformation. The hardness is measured at 0.1 mm intervals from the steel plate surface to a depth of 0.1 mm and a depth of 1.5 mm under the condition of a load of 1 kgf, and the hardness (HV) of the surface layer 0.1 mm at the time of carburizing and quenching. , the distance of the second layer (mm) having a hardness of 450 HV or more and less than 800 HV, and the hardness of the parent phase located inside it.

Hardenability was evaluated under the conditions shown in Tables 2 and 3 from the results obtained from (V) above. If the hardness at a depth of 0.1 mm from the steel plate surface is 800 HV or more, the distance of the second layer is 0.4 mm, and the hardness of the matrix is 380 HV or more, it can be evaluated that the hardenability is sufficient (hardenability ○). Other than that, the hardenability was determined to be unsatisfactory (x), and evaluated as poor in hardenability.

Claims (13)

in % by mass,
C: 0.10% or more and 0.30% or less,
Si: 0.20% or more and 0.80% or less,
Mn: 0.30 % or more and 1.00% or less,
P: 0.03% or less,
S: 0.010% or less,
sol. Al: 0.10% or less,
N: 0.01% or less,
A microstructure containing Cr: 0.05% or more and 0.80% or less and B: 0.0005% or more and 0.0050% or less, the balance being Fe and inevitable impurities, and a microstructure containing ferrite and cementite, and the microstructure is
The area ratio of ferrite is 90 % or more,
The average grain size of ferrite is 5 μm or more and 20 μm or less,
A high-carbon hot-rolled steel sheet for vacuum carburizing, wherein the maximum Cr concentration in all cementite is 23% by mass or less and the average spacing of all cementite is 1.0 μm or more. The high carbon hot-rolled steel sheet for vacuum carburizing according to claim 1, wherein the ferrite has an average grain size of 10 µm or more and 25 µm or less, and the average spacing of all cementite is 5.0 µm or more. The high carbon hot-rolled steel sheet for vacuum carburizing according to claim 1, wherein the ferrite has an average grain size of 5 µm or more and less than 10 µm, and the average spacing of the all cementite is 1.0 µm or more and less than 5.0 µm. The high carbon hot rolled steel sheet for vacuum carburizing according to any one of claims 1 to 3, which has a tensile strength of 420 MPa or less and a total elongation of 39% or more. The high-carbon hot-rolled steel sheet for vacuum carburizing according to any one of claims 1 to 4, which has a hole expansion ratio of 80% or more. The high carbon hot-rolled steel sheet for vacuum carburizing according to any one of claims 1 to 5, further containing Ti: 0.06% or less in mass%. % by mass, and
The high carbon hot-rolled steel sheet for vacuum carburizing according to any one of claims 1 to 6, containing 0.002% or more and 0.030% or less in total of one or more of Sb and Sn. The total concentration of one or more of Sb and Sn in a region within 10 nm in the plate thickness direction from the surface of the steel sheet is 10 times (mass%) or more than the concentration of the base material according to claim 7 High carbon hot rolled steel sheet for vacuum carburizing. % by mass, and
Ni: 0.0005% or more and 2.50% or less,
Mo: 0.0005% or more and 0.25% or less,
Ta: 0.0005% or more and 0.001 % or less,
W: 0.0005% or more and 0.001 % or less,
Cu: 0.0005% or more and 0.001 % or less,
Nb: 0.0005% or more and 0.001 % or less,
V: 0.0005% or more and 0.001 % or less and Ca: 0.0005% or more and 0.001 % or less The high carbon for vacuum carburizing according to any one of claims 1 to 8 Hot-rolled steel plate. A method for producing a high-carbon hot-rolled steel sheet for vacuum carburizing according to any one of claims 1 to 9,
The steel material having the chemical composition according to claim 1, 6, 7 or 9 is held in a temperature range of 1050 to 1270 ° C. for 1 hour or more, and the reduction ratio in the temperature range of 980 to 1080 ° C. is 40% or more. Rolling, finishing temperature: finish rolling at Ar ₃ transformation point or higher, then primary cooling to 750 ° C. at average cooling rate: 20 ° C./s or higher and 65 ° C./s or lower , average cooling rate: 10 ° C./s or higher 65 Secondary cooling to the coiling temperature at ℃/s or less , coiling at a coiling temperature of over 580 ° C to 700 ° C, cooling to room temperature, and annealing (i) or (ii), vacuum carburizing. Manufacturing method of high carbon hot-rolled steel sheet for
(i) Heating to the Ac ₁ transformation point or more and the Ac ₃ transformation point or less, holding in the temperature range for 4 hours or more, and then cooling to less than the Ar ₁ transformation point at an average cooling rate of 1 to 20 ° C./h to obtain 600 C. or more and less than the Ar ₁ transformation point for 10 hours or more.
(ii) Hold for 20 hours or more and 40 hours or less in a temperature range of 670 ° C. or more and Ac ₁ transformation point or less. A method for manufacturing a high-carbon hot-rolled steel sheet according to claim 2,
The steel material having the chemical composition according to claim 1, 6, 7 or 9 is held in a temperature range of 1050 to 1270 ° C. for 1 hour or more, and the reduction ratio in the temperature range of 980 to 1080 ° C. is 40% or more. Rolling, finishing temperature: Ar ₃ transformation point or higher, then primary cooling to 750 ° C. at an average cooling rate of 20 ° C./s or higher, average cooling rate: 10 ° C./s or higher to coiling temperature 2 Next, cool, coil at a coiling temperature of over 580°C to 700°C, cool to room temperature, heat to Ac ₁ transformation point or more and Ac ₃ transformation point or less, hold in the temperature range for 4 hours or more, and then 1 to 1 to 3 hours. A method for producing a high-carbon hot-rolled steel sheet for vacuum carburizing, comprising cooling to less than the Ar ₁ transformation point at an average cooling rate of 20°C/h and maintaining the temperature in a temperature range of 600°C or more and less than the Ar ₁ transformation point for 10 hours or longer. A method for manufacturing a high-carbon hot-rolled steel sheet according to claim 3,
The steel material having the chemical composition according to claim 1, 6, 7 or 9 is held in a temperature range of 1050 to 1270 ° C. for 1 hour or more, and the reduction ratio in the temperature range of 980 to 1080 ° C. is 40% or more. Rolling, finish rolling at finish temperature: Ar ₃ transformation point or higher, then primary cooling to 750°C at an average cooling rate of 20 to 65°C/s or less, winding at an average cooling rate of 10 to 65°C/s or less Secondary cooling to the take-up temperature, coiling at a coiling temperature of over 580°C to 700°C, cooling to room temperature, and then holding in the temperature range of 670 °C to Ac ₁ transformation point for 20 hours to 40 hours, for vacuum carburizing. A method for producing a high-carbon hot-rolled steel sheet. A steel part obtained by carburizing the hot-rolled steel sheet according to any one of claims 1 to 9,
A carburized steel part comprising a carburized layer and a non-carburized layer, both layers having a martensitic structure and free of cementite having a size of 10 μm or more.