CARBON STEEL SHEET HAVING EXCELLENT CARBURIZATION PROPERTIES, AND METHOD FOR PRODUCING SAME

Abstract

The invention provides a carbon steel sheet including C: 0.20% to 0.45% by mass, Si: 0.05% to 0.8% by mass, Mn: 0.85% to 2.0% by mass, P: 0.001% to 0.04% by mass, S: 0.0001% to 0.006% by mass, Al: 0.01% to 0.1% by mass, Ti: 0.005% to 0.3% by mass, B: 0.0005% to 0.01% by mass and N: 0.001% to 0.01% by mass, in which a K value that can be obtained from 3C+Mn+0.5Si is greater than or equal to 2.0; surface hardness is less than or equal to 77 on the Rockwell B Scale; and the average content of N in a zone from the surface to a depth of 100 μm is less than or equal to 100 ppm. This carbon steel sheet is configured to be carburized in a carburization atmosphere with a carbon potential of 0.6 or less.

Description

TECHNICAL FIELD

The present invention relates to a carbon steel sheet having excellent carburization properties and a method for producing the same.

The present application claims priority based on Japanese Patent Application No. 2009-079959 filed in Japan on Mar. 27, 2009, and its content is incorporated by reference herein.

BACKGROUND ART

In the past, industrial machine parts or automobile parts, such as chains, gears, or clutches, were produced by hardening the surfaces thereof with a thermal treatment, such as quenching, after a forming process.

However, in recent years, as the forms of parts have become more complex, abrasion resistance, fatigue characteristics, or the like have been in demand. Therefore, materials are required to satisfy not only workability that can withstand complicated processes while processing parts but also hardenability for surface hardening. The hardenability and workability of materials are opposing characteristics in terms of material design. In general, softening of materials is effective for the improvement of workability, but most of elements for enhancing hardenability increase the hardness of the steel sheet, and thus sacrifice workability.

On the other hand, if hardenability of parts after the processing is not adequate, abnormal layers in which structures such as perlite, sorbite or troostite are mixed are generated in the products.

In order to manufacture steel sheets having excellent workability and hardenability at a low cost, it is effective to add B in the steel sheets. However, due to the reactivity of B, changes, such as oxidation, deboronization, or nitrogenization, occur at the surfaces of the steel sheet, therefore it is difficult to secure hardenability at a surface layer portion.

In addition, in steel sheets in which B has been added (hereafter referred to as B-added steel sheets) and carburizing has been performed at a carbon potential (Cp) of about 0.8 (which is commonly used), carburized carbon increases hardenability and thus it becomes difficult for quenched abnormal layers to occur at a surface layer portion after quenching, therefore no serious problem occurs. However, in weakly carburized zones with a low carbon potential (for example, Cp≦0.6), B degrades hardenability due to the above reaction, and furthermore, hardenability by carbon (C) also cannot be secured, therefore the B-added steel sheets are not widely used.

The carbon potential mentioned herein refers to a value indicating the carburizing capability of atmospheres when carburizing steel materials. The carbon potential is equivalent to the carbon concentration at steel surfaces when reaching an equilibrium with a gaseous atmosphere at a carburizing temperature.

Therefore, in B-added steel sheets, material optimization is required throughout all the processes from materials to parts processing, such as the establishment of production conditions in which the effects of the addition of B can be sufficiently obtained and the securement of workability for severe processes, such as profile forming, and treatability of surface hardening, such as carburizing.

Regarding production conditions of B-added steel sheets, Patent Document 1 discloses annealing under a hydrogen atmosphere with nitrogen content suppressed to 10% or less by volume or an argon atmosphere, but nothing is described regarding the prior or subsequent processes. In addition, there is no technology disclosed regarding a carburizing treatment at a low carbon potential which is the subject of the invention.

RELATED ART DOCUMENT

Patent Document

• [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. H5-331534

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

An object of the invention is to provide a B-added steel sheet having excellent hardenability even in a carburization with a low carbon potential condition and furthermore equipped with workability, and to optimize a method for producing thereof, in order to solve the above-described problems.

Means for Solving the Problems

The invention adopts the following measures to solve the above described problems.

(1) A first aspect of the invention is a carbon steel sheet configured to be carburized in a carburization atmosphere with a carbon potential of 0.6 or less, including: C, 0.20% to 0.45% by mass, Si: 0.05% to 0.8% by mass, Mn: 0.85% to 2.0% by mass, P: 0.001% to 0.04% by mass, S: 0.0001% to 0.006% by mass, Al: 0.01% to 0.1% by mass, Ti: 0.005% to 0.3% by mass, B: 0.0005% to 0.01% by mass and N: 0.001% to 0.01% by mass with a balance including Fe and inevitable impurities, wherein K value that can be obtained from 3C+Mn+0.5Si is greater than or equal to 2.0; surface hardness is less than or equal to 77 on Rockwell B Scale; and an average content of N in a zone from a surface to a depth of 100 μm is less than or equal to 100 ppm.

(2) The carbon steel sheet in the above (1) may further include one or more components selected from Nb: 0.01% to 0.5% by mass, V: 0.01% to 0.5% by mass, Ta: 0.01% to 0.5% by mass, W: 0.01% to 0.5% by mass, Sn: 0.003% to 0.03% by mass, Sb: 0.003% to 0.03% by mass, and As: 0.003% to 0.03% by mass.

(3) A second aspect of the invention is a method for producing the carbon steel sheet according to claim 1 or 2, including: heating a slab to less than or equal to 1200° C.; hot-rolling the slab at a final rolling temperature of 800° C. to 940° C. so as to obtain a steel sheet; cooling the steel sheet at a cooling rate of 20° C./second or more until a temperature of the steel sheet becomes less than or equal to 650° C., as a first cooling; cooling the steel sheet at a cooling rate of 20° C./second or less, as a second cooling subsequent to the first cooling; coiling the steel sheet at a temperature of 400° C. to 650° C.; pickling the steel sheet; and annealing the steel sheet for 10 hours or more at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C., as a first annealing.

(4) In the method for producing the carbon steel sheet described in the above (3), the first annealing may be performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the first annealing, a cooling rate may be set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

(5) The method for producing the carbon steel sheet described in the above (4) may further include cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the pickling, as a first cold-rolling.

(6) The method for producing the carbon steel sheet described in the above (5) may further include cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the first annealing, as a second cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the second cold-rolling, as a second annealing.

(7) In the method for producing the carbon steel sheet described in the above (6), the second annealing may be performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the second annealing, a cooling rate may be set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

(8) The method for producing the carbon steel sheet described in the above (7) may further include cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.

(9) In the method for producing the carbon steel sheet described in the above (8), the third annealing may be performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate may be set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

(10) The method for producing the carbon steel sheet described in the above (6) may further include cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.

(11) In the method for producing the carbon steel sheet described in the above (10), the third annealing may be performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate may be set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

(12) The method for producing the carbon steel sheet described in the above (4) may further include cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the first annealing, as a second cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the second cold-rolling, as a second annealing.

(13) In the method for producing the carbon steel sheet described in the above (12), the second annealing may be performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the second annealing, a cooling rate may be set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

(14) The method for producing the carbon steel sheet described in the above (13) may further include cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.

(15) In the method for producing the carbon steel sheet described in the above (14), the third annealing may be performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Act1+50° C., and after the third annealing, a cooling rate may be set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

(16) The method for producing the carbon steel sheet described in the above (12) may further include cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.

(17) In the method for producing the carbon steel sheet described in the above (16), the third annealing may be performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate may be set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

(18) The method for producing the carbon steel sheet described in the above (3) may further include cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the pickling, as a first cold-rolling.

(19) The method for producing the carbon steel sheet described in the above (18) may further include cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the first annealing, as a second cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the second cold-rolling, as a second annealing.

(20) In the method for producing the carbon steel sheet described in the above (19), the second annealing may be performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the second annealing, a cooling rate may be set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

(21) The method for producing the carbon steel sheet described in the above (20) may further include cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.

(22) In the method for producing the carbon steel sheet described in the above (21), the third annealing may be performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Act1+50° C., and after the third annealing, a cooling rate may be set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

(23) The method for producing the carbon steel sheet described in the above (19) may further include cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.

(24) In the method for producing the carbon steel sheet described in the above (23), the third annealing may be performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate may be set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

(25) The method for producing the carbon steel sheet described in the above (3) may further include cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the first annealing, as a second cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the second cold-rolling, as a second annealing.

(26) In the method for producing the carbon steel sheet described in the above (25), the second annealing may be performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the second annealing, a cooling rate may be set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

(27) The method for producing the carbon steel sheet described in the above (26) may further include cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.

(28) In the method for producing the carbon steel sheet described in the above (27), the third annealing may be performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate may be set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

(29) The method for producing the carbon steel sheet described in the above (25) may further include cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.

(30) In the method for producing the carbon steel sheet described in the above (29), the third annealing may be performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate may be set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

(31) A third aspect of the invention is a′ carbon steel sheet configured to be carburized in a carburization atmosphere with a carbon potential of 0.6 or less, including C, 0.20% to 0.45% by mass, Si: 0.05% to 0.8% by mass, Mn: 0.85% to 2.0% by mass, P: 0.001% to 0.04% by mass, 0.0001% to 0.006% by mass, Al: 0.01% to 0.1% by mass, Ti: 0.005% to 0.3% by mass, B: 0.0005% to 0.01% by mass and N: 0.001% to 0.01% by mass, and further including one or more components selected from: Cr: 0.01% to 2.0% by mass, Ni: 0.01% to 1.0% by mass, Cu: 0.005% to 0.5% by mass and Mo: 0.01% to 1.0% by mass with a balance including Fe and inevitable impurities, wherein K′ value that can be obtained from 3C+Mn+0.5Si+Cr+Ni+Mo+Cu is greater than or equal to 2.0; surface hardness is less than or equal to 77 on Rockwell B Scale; and an average content of N in a zone from a surface to a depth of 100 μm is less than or equal to 100 ppm.

(32) The carbon steel sheet in the above (31) may further include one or more components selected from Nb: 0.01% to 0.5% by mass, V: 0.01% to 0.5% by mass, Ta: 0.01% to 0.5% by mass, W: 0.01% to 0.5% by mass, Sn: 0.003% to 0.03% by mass, Sb: 0.003% to 0.03% by mass, and As: 0.003% to 0.03% by mass.

(33) The carbon steel sheet described in the above (31) or (32) is the method for producing the carbon steel sheet described in the above (31) or (32) including: heating a slab to less than or equal to 1200° C.; hot-rolling the slab at a final rolling temperature of 800° C. to 940° C. so as to obtain a steel sheet; cooling the steel sheet at a cooling rate of 20° C./second or more until a temperature of the steel sheet becomes less than or equal to 650° C., as a first cooling; cooling the steel sheet at a cooling rate of less than or equal to 20° C./second, as a second cooling subsequent to the first cooling; coiling the steel sheet at a temperature of 400° C. to 650° C.; pickling the steel sheet; and annealing the steel sheet for more than or equal to 10 hours at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C., as a first annealing.

Effects of the Invention

In the configurations described in the above (1) and (31), since it is defined that the K value and the K′ value are greater than or equal to 2.0, and the average amount of N in surface layers is less than or equal to 100 ppm, it is possible to develop high hardenability even in a carburization with a low carbon potential condition and thus obtain a B-added carbon steel sheet equipped with high workability.

According to the configurations described in the above (2) and (32), it is possible to obtain an effect of stabilizing precipitates or improving toughness or an effect of suppressing component variations in the surface layer portion of a steel sheet.

According to the methods described in the above (3) and (33), it is possible to stably produce a carbon steel sheet having excellent workability and post-processing carburization treatability.

According to the methods described in the above (4) to (30), it is possible to further improve the workability or softening of a carbon steel sheet.

As described above, according to the invention, it is possible to produce a steel material having not only excellent carburization properties so as to prevent the generation of abnormal layers due to inferior hardenability while carburizing a B-added steel but also excellent workability for producing parts or the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a relationship between the K value or the K′ value and the average amount of N in a surface layer relating to the generation of abnormal layers during carburization.

FIG. 2 is a diagram showing a relationship between a crack in the profile formed portions and material hardness during the profile forming process.

FIG. 3 is a flowchart to explain the production method.

EMBODIMENTS OF THE INVENTION

The inventors conducted hardness variation or structure investigation at a surface layer portion during carburized quenching with a variety of changes in the components of B-added steel sheets or production conditions during production processes, and clarified the relationship between the structures and components of the surface layers which affect hardenability of the surface layers. As a result, it was found that there were cases in which, instead of martensite, structures which are more softened than martensite, such as pearlite, sorbite, troostite, were generated, and, particularly, such structures were often observed in the outermost surface layers from the surface to a depth of about 100 μm.

FIG. 1 shows the generation of abnormal layers in a 0.22% C-based material which has been carburized quenched at a carbon potential of 0.3. It was found that the abnormal layers have a significant relationship with the content of nitrogen (N) present in a surface layer of the steel sheet to a depth of 100 μm (the average amount of N in the surface layer) and the K values (or the K′ values) obtained by steel sheet components.

Here, the average amount of N in the surface layer refers to a value obtained by analyzing the content of nitrogen (N) in shavings taken after planing the surface of the steel sheet to a depth of 100 μm before carburization.

In order to observe the effect of steel sheet components, the K value represented by Formula (1) and the K′ value represented by Formula (2) were introduced.

K value=3C+Mn+0.5Si  (1)

Wherein C, Mn and Si represent the content of each element (% by mass).

K′ value=3C+Mn+0.5Si+Cr+Ni+Mo+Cu  (2)

Wherein C, Mn, Si, Cr, Ni, Mo and Cu represent the content of each element (% by mass). Further, they will be zero if the corresponding elements are not included.

As shown in FIG. 1, it was found that abnormal layers are not observed in a zone where the K value (the K′ value in a case in which Cr, Ni, Mo and Cu are included) is greater than or equal to 2.0 and the average amount of N in the surface layer is less than or equal to 100 ppm so that carburization properties are excellent. The reason why such a favorable zone as above can be obtained is because, as the average amount of N in the surface layer increases, the amount of nitrogen (N) precipitated as nitrides during the production processes is increased, and the growth of autenite grains are delayed during carburization, thereby degrading hardenability. In particular, it is considered that, since B is nitrogenized by N so as to become BN, the amount of solute B in the steel is decreased and thereby, the hardenability of the steel sheet is impaired.

In addition, in terms of steel sheet hardenability, the steel sheet needs to include alloy elements to a certain level, and the hardenability could be clarified by indicating the amount of the alloy elements with the K value (or K′ value) shown above. A higher K value (or K′ value) is advantageous to secure higher hardenability, but, if the K value (or K′ value) is too high, there are cases in which problems occur, such as increased steel sheet hardness that degrades workability, or the occurrence of hardening cracks during quenching depending on the form of parts. The upper limit of the K value (or K′ value in a case in which Cr, Ni, Mo and Cu are included) is not particularly specified, but, if the value exceeds 3.6, hardenability becomes too high, and thus defects, such as the above hardening crack or the like, will occur, therefore the value is desirably less than or equal to 3.6.

In terms of steel sheet workability, the invention defined the surface hardness of the steel sheet as less than or equal to 77 on the Rockwell B Scale (HRB). The steel material according to the invention, which may be used for automobile parts or the like, may subject to severe processes such as tooth-shape forming (profile forming) of gear parts. Therefore, workability that can withstand such severe processes is required.

In the invention, as the evaluation of workability, it was investigated whether a crack was formed in an area in the base portion of a profile formed portion where shear deformation occurs after conducting a processing test which simulated profile forming process. Using a 0.22% C-based steel material, 3 mm-thick steel sheets were produced under varied conditions of hot-rolling, cold-rolling and annealing so as to prepare test specimens. As to the shape of the profile formed object, a rack-shaped die was produced at a module of 1.5 mm defined by JIS-B1703, and 3 mm-thick steel sheets were 2 mm-pressed, and then whether or not cracks occurred in the profile formed portions was evaluated.

FIG. 2 shows the results. Crack occurrence from severe processes, such as profile forming, shows a favorable correspondence with surface hardness, and it was found that it is effective to achieve softening with a surface hardness of HRB 77 or less to produce a material that can withstand profile forming.

Meanwhile, in the invention, in terms of securing hardenability as described above, the lower limit of the K value (or K′ value) is defined. A higher K makes a material harder and thus is advantageous for hardness during quenching, but degrades workability, therefore problems, such as the formation of cracks, occur during processes. As a result, it is necessary to carry out the production method defined in the invention and to carry out softening of steel sheets while controlling the annealing atmosphere.

Hereinafter, steel sheet components and production conditions will be described.

C is a basic element necessary to obtain the strength of a steel sheet. With a carbon content of less than 0.20%, it is not possible to obtain the strength demanded to produce products, and hardenability is also degraded at the core portion of the parts so that desired characteristics cannot be obtained. However, since if a large amount of C exceeding 0.45% is included, it is difficult to secure toughness and formability after thermal treatments, the content of C is specified in a range from 0.20% by mass to 0.45% by mass (hereinafter, unless otherwise described, contents will have a unit of % by mass). A more preferable range is from 0.20% to 0.40%.

Si is used as a deoxidizing agent of steel and is also effective in terms of hardenability. It is necessary to include 0.05% or more of Si. However, since as the content of Si increases, degradation of surface texture occurs due to scale defects or the like during hot-rolling, the upper limit was defined as 0.80%. A more preferable range is 0.05% to 0.50%.

Mn is used as a deoxidizing agent and is also effective in terms of hardenability. In terms of securing hardenability during carburization carried out at a low Cp, addition of 0.85% or more of Mn is required in the invention. An excessive content of Mn results in degradation or scattering (variation) of impact characteristics caused by segregation-induced structural variation after quenching and tempering, therefore the upper limit is defined as 2.0%. A more preferable range is from 0.90% to 1.80%.

In the steel of the invention, P is a harmful element in terms of toughness or workability, therefore a lower content of P is desirable and the upper limit is defined as 0.04%. In addition, the lower limit is desirably lower, but a decrease in the content below 0.001% significantly raises industrial costs, therefore the lower limit is defined as 0.001%. A more preferable range is 0.003% to 0.025%.

S accelerates the generation of non-metallic inclusions in steel so as to degrade forming workability, toughness after thermal treatments, or the like. As a result, a lower content of S is desirable, and the upper limit thereof is defined as 0.006%. The lower limit is desirably lower, but a decrease in the content below 0.0001% significantly raises industrial costs, therefore the lower limit is defined as 0.0001%. A more preferable range is from 0.0001% to 0.003%.

Al is used as a deoxidizing agent of steel, and therefore 0.01% or more of Al is required. However, even when more than 0.10% of Al is added, the effect is saturated, and scale defects are likely to occur. In addition, Al is also effectively bonded with N and accelerates nitrogen absorption during steel sheet production. However, if the content exceeds 0.10%, Al nitrides are stabilized so as to hinder grain growth during carburization thermal treatments and degrade hardenability. As a result, the content of Al is defined in a range from 0.01% to 0.10%. A more preferable range is from 0.01% to 0.06%.

Ti is effective as a deoxidizing agent of steel. In addition, Ti effectively bonds with N. Therefore, it is necessary to add 0.005% or more of Ti from the relationship with the amount of N. However, even when more than 0.30% of Ti is added, the effect is saturated, and the cost also rises. Furthermore, since the amount of precipitates induced by nitrogen absorption during production processes is increased, grain growth is hindered during carburization and hardenability is degraded. As a result, the content of Ti is defined in a range from 0.01% to 0.30%. A more preferable range is 0.01% to 0.10%.

B is an effective element to improve the hardenability of steel, and such an effect can be achieved with an extremely small amount. In order to obtain the effect of hardenability improvement, it is necessary to add 0.0005% or more of B. However, if a large amount of B exceeding 0.01% is included, castability is degraded and cracks occur during slab casting. Furthermore, B-based compounds are generated in steel so as to cause adverse effects, such as a decrease in toughness. As a result, the content of B is defined in a range from 0.0005% to 0.01%. A more preferable range is 0.0005% to 0.005%.

N is bonded with B so as to generate nitrides and degrades the hardenability improvement effect of B. Therefore, a lower content of N is preferable, but a decrease in the content below 0.001% leads to an increase in costs. In addition, if the content of N exceeds 0.01% as an average composition of steel, a large amount of elements that bond with N, such as Al or Ti, are required, and precipitates, such as AlN or TiN, hinder grain growth during carburization so as to degrade hardenability, which not only results in generation of abnormal layers but also degrades mechanical characteristics, such as toughness. As a result, the upper limit of N content is defined as 0.01%. A more preferable range is 0.001% to 0.006%.

In addition, N is likely to intrude into steel during production processes and is introduced from the atmosphere during hot-rolling and heating or annealing, and, in particular, is likely to be concentrated in the surface layer portion, therefore it is necessary to suppress such effects in order to prevent the degradation of hardenability of parts in the surface layer portion. If the amount of nitrogen intruded from atmosphere during heating or annealing exceeds 100 ppm, the amount of precipitated nitrides becomes large during coiling or annealing, and grain growth is delayed during heating before quenching, thereby degrading hardenability. As a result, it is important to define the content of N particularly in the surface layer portion (a zone from the surface to a depth of 100 μm) (the average amount of N in the surface layer) as less than or equal to 100 ppm. The amount of N in the surface layer portion is further preferably less than or equal to 70 ppm.

Cr is an effective element that can be added in terms of the hardenability of steel, and the effect becomes remarkable with a content of 0.01% or more, but even when more than 2% of Cr is added, the effect is saturated, and the cost also rises. As a result, the content is defined in a range from 0.01% to 2.0%. A more preferable range is 0.05% to 0.50%.

Ni is an effective element in terms of improvement in the hardenability or toughness of steel, and addition of 0.01% or more is effective, but addition of more than 1% of Ni merely results in an increase in costs and rarely changes the effect, therefore the content is defined in a range from 0.02% to 1.0%. A more preferable range is 0.05% to 0.50%.

Cu is an effective element in terms of improvement in the hardenability or toughness of steels, and addition of 0.01% or more is effective, but addition of more than 0.5% of Cu merely results in an increase in costs and rarely changes the effect, therefore the content is defined in a range from 0.005% to 0.5%. A more preferable range is 0.02% to 0.35%.

Mo is an effective element that improves the hardenability of steel and an effective element to increase resistance against softening by tempering. In order to obtain such effects, addition of 0.01% or more is required. However, even when more than 1.0% of Mo is included, the effect is saturated, and the cost also rises, therefore the content is defined in a range from 0.01% to 1.0%. A more preferable range is 0.01% to 0.40%.

0.01% or more of Nb has effects of forming carbonitrides, stabilizing precipitates or improving toughness, but addition of more than 0.5% of Nb merely results in an increase in costs and a decrease in hardenability by the formation of carbides, therefore the content is defined in a range from 0.01% to 0.5%. A more preferable range is 0.01% to 0.20%.

Similarly to Nb, 0.01% or more of V has effects of forming carbonitrides, stabilizing precipitates or improving toughness, but addition of more than 0.5% of V merely results in an increase in costs and rarely changes the effect, and also lowers hardenability by the formation of carbides. Therefore, the content is defined in a range from 0.01% to 0.5%. A more preferable range is 0.01% to 0.20%.

Similarly to Nb and V, 0.01% or more of Ta has effects of forming carbonitrides, stabilizing precipitates or improving toughness, but addition of more than 0.5% of Ta merely results in an increase in costs and rarely changes the effect, and also lowers hardenability by the formation of carbides. Therefore, the content is defined in a range from 0.01% to 0.5%. A more preferable range is 0.01% to 0.30%.

Similarly to Nb, V and Ta, 0.01% or more of W has effects of forming carbonitrides, stabilizing precipitates or improving toughness, but addition of more than 0.5% of W merely results in an increase in costs and rarely changes the effect, and also lowers hardenability by the formation of carbides. Therefore, the content is defined in a range from 0.01% to 0.5%. A more preferable range is 0.01% to 0.20%.

Furthermore, in addition to the above, in the invention, in order to suppress component variation in the surface layer portion of steel sheets, a certain amount of one or more components selected from Sn, Sb and As may be added.

Sn, Sb, As: 0.003% to 0.03%

Sn, Sb and As are elements having a high tendency of segregating at interfaces, surfaces or the like and a function that suppresses surface reaction, such as nitrogen absorption or decarburization, during production processes. Therefore, the addition thereof has an effect of preventing remarkable component variation by suppressing the reaction of elements which are liable to induce component variation, such as nitrogen or carbon, even in a state in which steel materials are exposed to a high-temperature atmosphere during heating or annealing in a hot-rolling process. Therefore, Sn, Sb and As may be optionally added. With regard to the each added amount, if the amount is less than 0.003%, the effect is small, and, addition of a large amount exceeding 0.03% not only saturates the effect but also results in a decrease in toughness and an increase in costs by extending carburization time. As a result, it is desirable to be added in a range from 0.003% to 0.03%.

In the steel sheet according to the invention, the content of oxygen (O) is not defined, but, if oxides are agglomerated and thus coarsened, ductility is lowered, therefore the content of oxygen is preferably less than or equal to 0.025%. A lower content of oxygen is preferable, but a content of less than 0.0001% is technically difficult to achieve, therefore the content is preferably more than or equal to 0.0001%.

In addition, the carbon steel sheet according to the invention may include impurities inevitably mixed during production processes in addition to the above elements, but it is preferable to prevent impurities from being mixed therein as much as possible.

Next, production conditions will be described with reference to the flowchart in FIG. 3.

In the invention considering the consistent optimization of steel material components and an annealing process thereafter, hot-rolling is important, and it is important to intensively suppress component variation in the surface layer portion of steel sheets, that is, the intrusion of N into or decarburization in the surface layer portion. Therefore, heating is conducted at 1200° C. or less without applying high-temperature heating which is commonly used and conducted at a temperature of more than 1200° C. (S1). Furthermore, in this case, as soaking time is extended, nitrogen intrusion into the surface layer portion is also increased, and hardening characteristics of the products are affected, therefore it is important not to conduct heating for a long time. Specifically, it is preferable to conduct heating for a retention time not exceeding 60 minutes at 1200° C. and 90 minutes at 1100° C.

Next, hot-rolling is conducted at a final rolling temperature of 800° C. to 940° C. (S2). If the final rolling temperature is lower than 800° C., many burn-in-induced defects occur, and, if the final rolling temperature is higher than 940° C., the generation frequency of scale-induced defects is increased, and thus the product yield ratio is decreased, thereby increasing costs.

After finishing the final hot-rolling, cooling is conducted to 650° C. or less at a cooling rate of 20° C./second or more (S3, first cooling). If the cooling to 650° C. after finishing the rolling is conducted at a rate slower than 20° C./second, structural variations called pearlite bands resulting from segregation occur, which leads to degradation of workability. Therefore, the cooling rate is controlled at 20° C./second or more to a temperature of 650° C. or less after finishing the rolling, and then at 20° C./second or less to a coiling temperature for slow cooling which is supposed to be conducted on homogeneous pearlite transformation, pearlite+bainite structure, bainite structure or the like (S4, second cooling). Thereby, it is possible to suppress the occurrence of structural heterogeneity in the coils. In addition, with respect to the coiling temperature, it is possible to reduce structural variation in the coils by conducting coiling at a temperature of 400° C. to 650° C. which is to achieve structural homogeneity as described above (S5). The hot-rolled steel sheets produced by the above processes are pickled (S6). After pickling, annealing or cold-rolling is conducted as necessary depending on product sheet thickness or necessary levels of softening, but the following is important as production conditions in this case.

With respect to annealing, since the steel sheet according to the invention has a high carbon content, it is not possible to obtain the characteristics by a continuous annealing process that is used for soft steel sheets. Basically, a process in which coils are annealed as they are, such as batch annealing or box annealing, is applied (S7, first annealing).

In this case, in terms of preventing nitrogen concentration in the surface layer portion, an annealing atmosphere majorly includes hydrogen and has a hydrogen concentration of 95% or more. In addition, in the case of performing annealing in a hydrogen atmosphere, in terms of safety, the inside of an annealing furnace is firstly substituted with nitrogen at room temperature so as to form a nitrogen atmosphere, and then substituted with hydrogen. In this case, it is desirable to raise the temperature after substituting with hydrogen in terms of preventing nitrogenization, but the atmosphere may be substituted with hydrogen while raising temperature from a nitrogen atmosphere, and it is necessary to have a hydrogen concentration of 95% or more at a possible low temperature. In addition, in terms of preventing component variation in the surface layer portion, it is important to have, particularly, a dew point of more than or equal to −20° C. up to 400° C. and a dew point of less than or equal to −40° C. during retention at a temperature of more than or equal to 400° C. (retention time depends on materials, but 10 hours or more of retention at a temperature of 660° C. or more is desirable to soften the steel sheet according to the invention), and, if a dew point is high, deboronization, decarburization or the like occurs, and poorly-quenched abnormal layers are generated in a ease of performing carburization at a low carbon potential. By completing the above series of processes (hot-rolling+thermal treatments), the steel sheet according to the invention having excellent workability and, furthermore, excellent carburization properties during a carburization treatment after processing can be obtained.

In terms of softening, high-temperature annealing at a temperature of Ac1 or more is also effective. It is preferable to conduct annealing in a temperature range of “Ac1” to “Ac1+50° C.”, and then set a cooling rate of 5° C./hour so as to cool it to “Ac1−30° C.” after the annealing. Thereby, ferrite phases generated during cooling with a cooling rate of 5° C./hour or less are likely to be coarsened and softening is accelerated by austenite phases generated at Ac1 or more due to scavenging action by the fine carbides. If annealing is conducted at a temperature greater than “Ac1+50° C.”, in the components of the steel according to the invention, the phase ratio of austenite phases becomes too high and pearlite is generated at some places during cooling which hardens the steel, therefore, the temperature of high-temperature annealing in the present invention is preferably less than or equal to “Ac1+50° C.”. In addition, in the steel according to the invention, even when slow cooling is conducted after the temperature reached “Ac1−30° C.”, the effect is saturated and an extended annealing time results in an increase in costs, therefore the end-point temperature of slow cooling is preferably “Ac1−30° C.”.

Here, Ac1 represents a temperature at which austenite phases appear in the temperature-raising process, and, in the invention, A1 transformation points were obtained by taking samples from hot-rolled steel sheets and measuring expansion curves with a Formaster tester when raising the temperature at 0.3° C./second. In addition, written references also disclose simpler methods obtaining Ac1 from components, and an example thereof is Ac1 (° C.)=723−10.7×% Mn−16.9×% Ni+29.1×% Si+16.9×% Cr+290×% As+6.38×% W disclosed in “The Physical metallurgy of Steel” written by William C. Leslie, and such empirical formulae can be used.

Furthermore, the cold-rolling process is used to complete sheet product thickness with a high accuracy and to efficiently conduct softening in combination with annealing. Therefore, in the above series of processes, cold-rolling (S6-2, first cold-rolling) may be conducted after conducting the hot-rolling and coiling (S5) and the pickling (S6). Particularly, by cold-rolling with a rolling ratio of 5% or more, carbides are accelerated to be spherical, and recrystallization not accompanied by nuclei generation or softening in which grain diameters are relatively large when completing recrystallization and grain growth-induced coarsening is likely to occur is accelerated.

The upper limit is not particularly specified, but, if rolling is conducted with a rolling ratio exceeding 60%, homogeneity of the metallic structure of the steel sheet is further increased by cold-rolling, but a higher cold-rolling ratio makes grains recrystallized during annealing smaller, and thus annealing time needs to be extended for softening, therefore, the cold-rolling ratio can be determined in terms of costs and product homogenization.

In the production method according to the invention, it is possible to conduct another cold-rolling with a rolling ratio of 5% or more (S7-2, second cold-rolling) on the steel sheet and then conduct annealing in an atmosphere including 95% or more of hydrogen (S7-3, second annealing) after the above annealing. By undergoing the processes of the cold-rolling (S7-2, second cold-rolling) and an annealing (S7-3, second cold-rolling) after the above annealing (S7-1, first annealing), structural homogenization or crystal grain coarsening can be achieved, and it is possible to further proceed with workability improvement or softening.

In the production method according to the invention, it is possible to conduct additional cold-rolling with a rolling ratio of 5% or more (S7-4, third cold-rolling) on the steel sheet and then conduct annealing in an atmosphere including 95% of hydrogen (S7-5, third annealing) after the above annealing (S7-3, second annealing), and the annealing conditions for this case are as described above.

In addition, in the production method according to the invention, in terms of softening, it is possible to conduct the above annealing process in combination with cold-rolling more than three times, and, even in this case, the process needs to be carried out within the above production conditions.

The carbon steel sheet according to the first embodiment of the invention can be described in the following manner, that is, a carbon steel sheet which includes, by mass %, C: more than or equal to 0.20% and less than or equal to 0.45%, Si: more than or equal to 0.05% and less than or equal to 0.8%, Mn: more than or equal to 0.85% and less than or equal to 2.0%, P: more than or equal to 0.001% and less than or equal to 0.04%, S: more than or equal to 0.0001% and less than or equal to 0.006%, Al: more than or equal to 0.01% and less than or equal to 0.1%, Ti: more than or equal to 0.005% and less than or equal to 0.3%, B: more than or equal to 0.0005% and less than or equal to 0.01% and N: more than or equal to 0.001% and less than or equal to 0.01% with a balance including Fe and inevitable impurities; has a value represented by 3C+Mn+0.5Si+Cr+Ni+Mo+Cu of 2.0 or more and surface hardness of the steel sheet of less than or equal to 77 on the Rockwell B Scale (HRB); has an average content of nitrogen (N) in a zone from the surface to a depth of 100 μm of 100 ppm or less; is used in a weak carburization atmosphere with a carbon potential (Cp) of 0.6 or less; and has excellent carburization properties. Here, C, Mn, Si, Cr, N, Mo and Cu represent the content of each element (% by mass) and are zero when the corresponding elements are not included.

The above carbon steel sheet may further include, by mass %, one or more components selected from Cr: 0.01% to 2.0%, Ni: 0.01% to 1.0%, Cu: 0.005% to 0.5% and Mo: 0.01% to 1.0%; and has a value represented by 3C+Mn+0.5Si+Cr+Ni+Mo+Cu which is greater than or equal to 2.0.

The above carbon steel sheet may further include, by mass %, one kind or two or more kinds of Nb: from 0.01% to 0.5%, V: from 0.01% to 0.5%, Ta: from 0.01% to 0.5% and W: from 0.01% to 0.5%.

The above carbon steel sheet may further include, by mass %, one kind or two or more kinds of Sn: from 0.003% to 0.03%, Sb: from 0.003% to 0.03%, and As: from 0.003% to 0.03%.

When hot-rolling a slab including the above components, a carbon steel sheet having excellent carburization properties may be produced by conducting heating at less than or equal to 1200° C.; having a final rolling temperature of hot-rolling of 800° C. to 940° C.; after completion of the final rolling, conducting cooling at a cooling rate of 20° C./second or more to 650° C.; subsequently, conducting cooling at a cooling rate of 20° C./second or less; conducting coiling at a coiling temperature of 400° C. to 650° C.; then conducting pickling; and then conducting annealing for more than or equal to 10 hours at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of 400° C. or more.

It is also possible to conduct the above annealing after conducting cold-rolling with a rolling ratio of 5% to 60% after the above pickling.

It is also possible to conduct another annealing at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of up to 400° C. and of less than or equal to −40° C. at a temperature of 400° C. or more after conducting cold-rolling with a rolling ratio of 5% to 60% after the above annealing.

It is also possible to conduct another annealing at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of up to 400° C. and of less than or equal to −40° C. at a temperature of 400° C. or more after conducting cold-rolling with a rolling ratio of 5% to 60% after the above second annealing.

With respect to annealing conducted on the above hot-rolled steel sheet or cold-rolled steel sheet, it is possible to conduct annealing in an atmosphere having a hydrogen content of 95% or more and at an annealing temperature in a range from “Ac1” to “Ac1+50° C.” and to conduct slow cooling at a cooling rate of 5° C./hour or less to “Ac1−30° C.” after the annealing.

Examples

The invention will be described based on examples.

Steel specimens obtained by casting steel including components shown in Tables 1 to 6 into 50 kg steel ingots by vacuum melting were hot-rolled under the conditions described in Tables 7 to 12. Heating for hot-rolling was conducted in the air atmosphere, and the thickness of hot-rolled steel sheets was 3 mm in the case of conducting no cold-rolling. In the case of conducting cold-rolling, the thickness of the hot-rolled steel sheets were controlled so that the cold-rolled steel sheets will become 3 mm. The hot-rolled steel sheets were pickled by hydrochloric acid and then subjected to annealing or cold-rolling so as to produce 3 mm-thick steel sheets for evaluation. The details on the production conditions and evaluation results are shown in Tables 7 to 12. After that, under the conditions described in Tables 7 to 12, a single annealing case, a cold-rolling and annealing case, a case in which a first annealing was followed by cold-rolling and then annealing again (annealing twice), and those repetition case (annealing three times) were carried out as shown in Tables 7 to 12 according to each treatment condition. With respect to the annealing atmosphere, the inside of a furnace was first substituted with nitrogen at room temperature, and then hydrogen was introduced until a predetermined amount of hydrogen was attained, and then the temperature was raised. In addition, dew points were measured using a dew-point meter with a thin film aluminum oxide moisture sensor.

Surface hardness of the obtained steel sheets were measured on the Rockwell B Scale (HRB), and the average amount of N in the surface layer was obtained by analyzing the content of nitrogen (N) in shavings taken by planing the surface layer portion of the steel sheet at a depth of 100 μm before carburization. Then, specimens which had been subjected to a profile forming process were carburized, and whether abnormal layers were present on the surface was investigated.

Meanwhile, the carburization treatment was conducted by the gas carburization method, and carbon potentials were measured by the CO₂ amount controlling method using an infrared gas analyzer.

The numeric parts in the “No.” column in Tables 7 to 12 are equivalent to the “No.” in Tables 1 to 6, therefore it is possible to distinguish materials with which components have been subjected to which conditions.

As shown in Tables 7 to 12, in conditions departing from the conditions of the invention (underlined) and in the comparative steel, lack of product hardness, cracks during the profiling forming process or abnormal layers in the surface layer portion during carburization were observed, which clarified the effects of the invention.

[Table 1]

[Table 2]

[Table 3]

[Table 4]

[Table 5]

[Table 6]

[Table 7]

[Table 8]

[Table 9]

[Table 10]

[Table 11]

[Table 12]

In general, since workability is degraded with an increase in surface hardness, it is preferable to maintain the surface hardness of steel sheets before a carburization treatment at less than or equal to a certain value in terms of securing product workability. The surface hardness HRB (Rockwell B Scale) of the steel sheets produced according to the conditions of the invention were all less than or equal to HRB 77, and it was confirmed from the results of the profile forming test (Tables 7 to 12) that, if the HRB is less than or equal to HRB 77, no cracks occur. That is, it was confirmed that the steel sheet according to the invention was excellent in terms of workability.

In addition, from the results shown in Tables 7 to 12, it was confirmed that the steel sheets according to the invention show sufficient performance even at a low carbon potential (Cp≦0.6), thereby being excellent in terms of not only carburization properties but also workability.

From the evaluation results of carburization properties, it was confirmed that none of the steel sheets produced according to the conditions of the invention included abnormal layers. That is, it was confirmed that the steel sheet according to the invention was excellent in terms of carburization properties.

INDUSTRIAL APPLICABILITY

As described above, according to the invention, it is possible to obtain a steel sheet which has excellent workability and can secure hardenability at the surface layer portion during carburization and a production method thereof. Since this steel sheet can be applied not only to automobile parts or a variety of industrial machine parts, but also to a wide range of tools or blades, the steel has a broad range of applications and can be used throughout many industries, therefore it is needless to say that this steel sheet is highly valuable in an industrial sense.

	TABLE 1
	Chemical composition (%)
No	C	Si	Mn	P	S	Al	Ti	B	N	Cr	Ni	Cu	Mo
1	0.24	0.12	1.26	0.023	0.0022	0.022	0.015	0.0035	0.0024
2	0.28	0.2	1.08	0.035	0.0055	0.032	0.022	0.0023	0.0035
3	0.35	0.24	0.87	0.018	0.0032	0.034	0.023	0.0039	0.0028
4	0.34	0.25	0.85	0.025	0.0022	0.057	0.014	0.0017	0.0033	0.15
5	0.22	0.33	1.13	0.013	0.0041	0.044	0.034	0.0033	0.0022		0.15	0.02
	Chemical composition (%)	K value	Ac1
	No	Nb	V	Ta	W	Sn	Sb	As	(K′ value)	(° C.)	Note
	1								2.04	713	Invention steel
	2								2.02	717	Invention steel
	3								2.04	721	Invention steel
	4								(2.15)	724	Invention steel
	5								(2.11)	720	Invention steel

	TABLE 2
	Chemical composition (%)
No	C	Si	Mn	P	S	Al	Ti	B	N	Cr	Ni	Cu	Mo
6	0.21	0.05	1.22	0.009	0.0013	0.023	0.012	0.0019	0.0029	0.03	0.5
7	0.24	0.45	1.34	0.015	0.0025	0.046	0.023	0.0036	0.0035		0.12	0.06	0.08
8	0.22	0.22	1.25	0.022	0.0015	0.032	0.021	0.0028	0.0036	0.21	0.015	0.01
9	0.28	0.22	1.44	0.018	0.0054	0.026	0.028	0.0028	0.0019
10	0.33	0.42	0.88	0.027	0.005	0.033	0.033	0.0033	0.0032
	Chemical composition (%)	K value	Ac1
No	Nb	V	Ta	W	Sn	Sb	As	(K′ value)	(° C.)	Note
6								(2.41)	703	Invention steel
7								(2.49)	720	Invention steel
8								(2.25)	719	Invention steel
9	0.03							2.39	719	Invention steel
10		0.21						2.08	726	Invention steel

	TABLE 3
	Chemical composition (%)
No	C	Si	Mn	P	S	Al	Ti	B	N	Cr	Ni	Cu	Mo
11	0.23	0.28	1.25	0.027	0.0023	0.031	0.015	0.0023	0.0037
12	0.29	0.23	1.11	0.017	0.0044	0.028	0.033	0.0028	0.0022
13	0.31	0.23	0.96	0.025	0.0033	0.045	0.027	0.0042	0.0027
14	0.22	0.42	1.82	0.033	0.0048	0.019	0.027	0.0028	0.0031
15	0.34	0.5	1.55	0.031	0.0021	0.033	0.049	0.0038	0.0028
	Chemical composition (%)	K value	Ac1
No	Nb	V	Ta	W	Sn	Sb	As	(K′ value)	(° C.)	Note
11			0.28					2.08	719	Invention steel
12				0.08				2.10	718	Invention steel
13	0.015	0.3						2.01	719	Invention steel
14		0.03	0.28					2.69	716	Invention steel
15	0.023			0.08				2.82	721	Invention steel

	TABLE 4
	Chemical composition (%)
No	C	Si	Mn	P	S	Al	Ti	B	N	Cr	Ni	Cu	Mo
16	0.44	0.23	0.87	0.029	0.0025	0.048	0.029	0.0042	0.0025
17	0.22	0.21	1.18	0.023	0.0046	0.048	0.022	0.0048	0.0034	0.05			0.05
18	0.24	0.77	1.11	0.022	0.0028	0.076	0.028	0.0013	0.0036	0.35
19	0.29	0.13	1.05	0.008	0.0018	0.056	0.019	0.0017	0.0023	0.12	0.06
20	0.35	0.08	1.28	0.006	0.002	0.034	0.033	0.0022	0.0045		0.08	0.04
	Chemical composition (%)	K value	Ac1
No	Nb	V	Ta	W	Sn	Sb	As	(K′ value)	(° C.)	Note
16	0.08		0.02	0.012				2.31	723	Invention steel
17	0.03							(2.05)	717	Invention steel
18	0.012							(2.57)	739	Invention steel
19		0.02	0.013					(2.17)	717	Invention steel
20				0.015				(2.45)	710	Invention steel

	TABLE 5
	Chemical composition (%)
No	C	Si	Mn	P	S	Al	Ti	B	N	Cr	Ni	Cu	Mo
21	0.44	0.23	1.47	0.012	0.0034	0.029	0.041	0.0016	0.0039	0.014	0.02		0.12
22	0.28	0.35	1.82	0.013	0.0029	0.041	0.015	0.0027	0.0022			0.08
23	0.33	0.18	1.56	0.026	0.0043	0.019	0.06	0.0037	0.0037	0.023	0.015		0.011
24	0.23	0.15	0.94	0.029	0.0038	0.09	0.21	0.0051	0.0018	0.45	0.73	0.021
25	0.34	0.21	0.87	0.037	0.0029	0.017	0.011	0.0044	0.0015	0.34	0.035	0.025
26	0.28	0.24	0.87	0.026	0.0039	0.047	0.023	0.0023	0.0024		0.12	0.024	0.125
27	0.23	0.15	1.38	0.014	0.0022	0.036	0.022	0.0036	0.0033
28	0.29	0.35	1.28	0.027	0.0027	0.028	0.024	0.0029	0.0025	0.12	0.06	0.11
29	0.35	0.34	1.34	0.019	0.0015	0.042	0.029	0.0026	0.0024	0.22	0.12	0.23
	Chemical composition (%)	K value	Ac1
	No	Nb	V	Ta	W	Sn	Sb	As	(K′ value)	(° C.)	Note
	21		0.04		0.11				(3.06)	714	Invention steel
	22								(2.84)	714	Invention steel
	23			0.016					(2.69)	712	Invention steel
	24	0.011							(2.89)	719	Invention steel
	25		0.025		0.023				(2.37)	727	Invention steel
	26								(2.08)	721	Invention steel
	27					0.012			2.15	713	Invention steel
	28						0.015		(2.56)	722	Invention steel
	29	0.035				0.008		0.005	(3.13)	720	Invention steel

	TABLE 6
	Chemical composition (%)
No	C	Si	Mn	P	S	Al	Ti	B	N	Cr	Ni	Cu	Mo
30	0.23	0.1	0.59	0.025	0.0045	0.033	0.015	0.0022	0.0029	0.2	0.014	0.02
31	0.28	0.09	0.64	0.024	0.0033	0.043	0.021	0.0029	0.0032	0.23	0.014	0.02
32	0.35	0.07	0.52	0.022	0.0023	0.042	0.018	0.0022	0.0026	0.09		0.033
33	0.43	0.05	0.47	0.019	0.0034	0.029	0.017	0.0033	0.0035	0.09			0.04
34	0.24	0.83	1.25	0.025	0.0045	0.033	0.015	0.0025	0.0034	0.45	0.25	0.02
35	0.29	0.35	2.31	0.024	0.0057	0.037	0.028	0.0039	0.0033	0.23	0.014
36	0.44	0.19	2.13	0.019	0.0033	0.028	0.041	0.0018	0.0037	0.09			0.04
37	0.22	0.21	1.23	0.025	0.0034	0.035	0.022	0.0003	0.0035	0.21	0.014
	Chemical composition (%)	K value	Ac1
No	Nb	V	Ta	W	Sn	Sb	As	(K′ value)	(° C.)	Note
30								(1.54)	723	Comparative Example
31								(1.77)	722	Comparative Example
32								(1.70)	721	Comparative Example
33	0.015							(1.92)	721	Comparative Example
34								(3.09)	737	Comparative Example
35	0.013							(3.60)	712	Comparative Example
36	0.015							(3.68)	707	Comparative Example
37								(2.22)	719	Comparative Example

	TABLE 7
	Cold rolling and annealing processes
	Hot rolling conditions		(First) annealing
				Cooling	Cooling				Dew	Dew		Cooling
	Heating	Reten-	Final	rate	rate	Coiling	Cold		point	point at		rate when
	temper-	tion	temper-	to	before	temper-	rolling		thru	400° C. or		annealing
	ature	time	ature	650° C.	coiling	ature	ratio	Atmo-	400° C.	higher	Annealing	to Ac1 or
No	(° C.)	(min)	(° C.)	(° C./s)	(° C./s)	(° C.)	(%)	sphere	(° C.)	(° C.)	condition	higher
1A	1150	30	860	40	15	620	—	Hydrogen	−30	−45	700° C. ×	—
								100%			36 h
1B	1150	30	860	40	15	620	—	Hydrogen	−35	−50	700° C. ×	—
								100%			36 h
1C	1150	30	860	40	15	620	—	Hydrogen	−30	−60	700° C. ×	—
								100%			24 h
2A	1180	30	900	35	20	630	—	Hydrogen	−40	−60	710° C. ×	—
								95% +			36 h
								nitrogen 5%
2B	1180	30	900	35	20	630	—	Hydrogen	−45	−60	640° C. ×	—
								95% +			10 h
								nitrogen 5%
2C	1180	30	900	35	20	630	—	Hydrogen	−10	−40	710° C. ×	—
								95% +			36 h
								nitrogen 5%
3A	1200	35	860	40	10	600	—	Hydrogen	−30	−60	710° C. ×	—
								100%			48 h
3B	1280	40	900	30	15	620	—	Hydrogen	−35	−55	710° C. ×	—
								100%			48 h
3C	1200	60	860	40	10	580	—	Hydrogen	−35	−60	660° C. ×	—
								100%			6 h
4A	1150	40	880	25	10	600	—	Hydrogen	−40	−65	710° C. ×	—
								95% +			48 h
								nitrogen 5%
4B	1150	40	880	25	10	600	—	Hydrogen	−30	−35	710° C. ×	—
								95% +			48 h
								nitrogen 5%
5A	1150	40	880	20	5	580	—	Hydrogen	−30	−50	700° C. ×	—
								100%			48 h
5B	1150	40	880	20	5	580	30	Hydrogen	−25	−55	700° C. ×	—
								100%			36 h
	Cold rolling and annealing processes
	(Second) annealing		(Third) annealing
				Dew	Dew		Cooling			Dew	Dew
		Cold		point	point at		rate when	Cold		point	point at
		rolling		thru	400° C. or		annealing	rolling		thru	400° C. or
		ratio	Atmo-	400° C.	higher	Annealing	to Ac1 or	ratio	Atmo-	400° C.	higher
No	No.	(%)	sphere	(° C.)	(° C.)	condition	higher	(%)	sphere	(° C.)	(° C.)
1A	1A	—	—	—	—	—	—	—	—	—	—
1B	1B	—	—	—	—	—	—	—	—	—	—
1C	1C	20	Hydrogen	−35	−55	725° C. ×	1° C./hr	—	—	—	—
			100%			10 h
2A	2A	—	—	—	—	—	—	—	—	—	—
2B	2B	—	—	—	—	—	—	—	—	—	—
2C	2C	—	—	—	—	—	—	—	—	—	—
3A	3A	—	—	—	—	—	—	—	—	—	—
3B	3B	—	—	—	—	—	—	—	—	—	—
3C	3C	—	—	—	—	—	—	—	—	—	—
4A	4A	—	—	—	—	—	—	—	—	—	—
4B	4B	—	—	—	—	—	—	—	—	—	—
5A	5A	—	—	—	—	—	—	—	—	—	—
5B	5B	—	—	—	—	—	—	—	—	—	—
		Cold rolling and annealing processes
		(Third) annealing
	Cooling	Sheet product characteristics
			rate when	Amount of N in	Product	crack	Carburized
			annealing	sheet products	hard-	during	material
	Annealing	to Ac1 or	thru a depth of	ness	profile		abnormal
	No	condition	higher	100 μm (ppm)	(HRB)	forming	Cp	layers	Note
	1A	—	—	51	71	—	0.3	Not	Present
								present	invention
	1B	—	—	43	71	—	0.38	Not	Present
								present	invention
	1C	—	—	74	68	—	0.45	Not	Present
								present	invention
	2A	—	—	67	74	—	0.3	Not	Present
								present	invention
	2B	—	—	58	82	Crack	0.3	Not	Comparative
						occurred		present	Example
	2C	—	—	104	73	—	0.35	Present	Comparative
									Example
	3A	—	—	39	74	—	0.45	Not	Present
								present	invention
	3B	—	—	120	75	—	0.45	Present	Comparative
									Example
	3C	—	—	36	83	Crack	0.45	Not	Comparative
						occurred		present	Example
	4A	—	—	84	75	—	0.45	Not	Present
								present	invention
	4B	—	—	144	74	—	0.45	Present	Comparative
									Example
	5A	—	—	35	75	—	0.3	Not	Present
								present	invention
	5B	—	—	33	72	—	0.3	Not	Present
								present	invention

	TABLE 8
	Cold rolling and annealing processes
	Hot rolling conditions		(First) annealing
				Cooling	Cooling				Dew	Dew		Cooling
	Heating	Reten-	Final	rate	rate	Coiling	Cold		point	point at		rate when
	temper-	tion	temper-	to	before	temper-	rolling		thru	400° C. or		annealing
	ature	time	ature	650° C.	coiling	ature	ratio	Atmo-	400° C.	higher	Annealing	to Ac1 or
No	(° C.)	(min)	(° C.)	(° C./s)	(° C./s)	(° C.)	(%)	sphere	(° C.)	(° C.)	condition	higher
6A	1180	40	880	35	20	550	—	Hydrogen	−40	−60	690° C. ×	—
								100%			36 h
6B	1180	40	880	35	20	550	—	Hydrogen	−30	−60	690° C. ×	—
								90% +			48 h
								Nitrogen 10%
7A	1180	30	840	40	15	620	—	Hydrogen	−30	−50	700° C. ×	—
								100%			36 h
7B	1180	30	840	40	15	620	—	Hydrogen	−30	−50	700° C. ×	—
								100%			36 h
8A	1180	30	840	40	15	630	—	Hydrogen	−25	−45	690° C. ×	—
								100%			24 h
8B	1260	30	850	30	20	600	—	Hydrogen	−35	−60	690° C. ×	—
								100%			48 h
9A	1180	30	830	35	20	580	—	Hydrogen	−50	−60	710° C. ×	—
								100%			36 h
9B	1200	40	860	30	15	640	—	Hydrogen	−35	−60	735° C. ×	10° C./hr
								100%			12 h
9C	1200	60	860	30	15	640	—	Hydrogen	−40	−60	690° C. ×	—
								90% +			48 h
								Nitrogen 10%
9D	1200	90	840	30	15	620	—	Hydrogen	−35	−60	690° C. ×	—
								95% +			36 h
								nitrogen 5%
10A	1100	60	840	40	20	580	—	Hydrogen	−30	−55	710° C. ×	—
								100%			36 h
10B	1100	60	840	40	20	580	—	Hydrogen	−15	−45	710° C. ×	—
								100%			48 h
	Cold rolling and annealing processes
	(Second) annealing		(Third) annealing
				Dew	Dew		Cooling			Dew	Dew
		Cold		point	point at		rate when	Cold		point	point at
		rolling		thru	400° C. or		annealing	rolling		thru	400° C. or
		ratio	Atmo-	400° C.	higher	Annealing	to Ac1 or	ratio	Atmo-	400° C.	higher
No	No.	(%)	sphere	(° C.)	(° C.)	condition	higher	(%)	sphere	(° C.)	(° C.)
6A	6A	—	—	—	—	—	—	—	—	—	—
6B	6B	—	—	—	—	—	—	—	—	—	—
7A	7A	15	Hydrogen	−40	−60	700° C. ×	—	—	—	—	—
			100%			36 h
7B	7B	15	Hydrogen	−20	−35	700° C. ×	—	—	—	—	—
			100%			36 h
8A	8A	20	Hydrogen	−40	−60	690° C. ×	—	—	—	—	—
			100%			24 h
8B	8B	—	—	—	—	—	—	—	—	—	—
9A	9A	—	—	—	—	—	—	—	—	—	—
9B	9B	—	—	—	—	—	—	—	—	—	—
9C	9C	—	—	—	—	—	—	—	—	—	—
9D	9D	—	—	—	—	—	—	—	—	—	—
10A	10A	—	—	—	—	—	—	—	—	—	—
10B	10B	—	—	—	—	—	—	—	—	—	—
		Cold rolling and annealing processes
		(Third) annealing
	Cooling	Sheet product characteristics
			rate when	Amount of N in	Product	crack	Carburized
			annealing	sheet products	hard-	during	material
	Annealing	to Ac1 or	thru a depth of	ness	profile		abnormal
	No	condition	higher	100 μm (ppm)	(HRB)	forming	Cp	layers	Note
	6A	—	—	48	75	—	0.3	Not	Present
								present	invention
	6B	—	—	108	74	—	0.3	Present	Comparative
									Example
	7A	—	—	66	74	—	0.3	Not	Present
								present	invention
	7B	—	—	66	74	—	0.3	Present	Comparative
									Example
	8A	—	—	89	73	—	0.3	Not	Present
								present	invention
	8B	—	—	143	74	—	0.3	Present	Comparative
									Example
	9A	—	—	29	74	—	0.3	Not	Present
								present	invention
	9B	—	—	36	80	Crack	0.3	Not	Comparative
						occurred		present	Example
	9C	—	—	110	74	—	0.3	Present	Comparative
									Example
	9D	—	—	103	75	—	0.3	Present	Comparative
									Example
	10A	—	—	47	74	—	0.45	Not	Present
								present	invention
	10B	—	—	58	72	—	0.45	Present	Comparative
									Example

	TABLE 9
	Cold rolling and annealing processes
	Hot rolling conditions		(First) annealing
				Cooling	Cooling				Dew	Dew		Cooling
	Heating	Reten-	Final	rate	rate	Coiling	Cold		point	point at		rate when
	temper-	tion	temper-	to	before	temper-	rolling		thru	400° C. or		annealing
	ature	time	ature	650° C.	coiling	ature	ratio	Atmo-	400° C.	higher	Annealing	to Ac1 or
No	(° C.)	(min)	(° C.)	(° C./s)	(° C./s)	(° C.)	(%)	sphere	(° C.)	(° C.)	condition	higher
11A	1100	50	880	25	15	600	—	Hydrogen	−35	−50	690° C. ×	—
								100%			36 h
11B	1100	50	880	25	15	600	—	Hydrogen 5% +	−30	−45	690° C. ×	—
								Nitrogen 95%			24 h
11C	1100	50	880	25	15	600	—	Hydrogen	−40	−65	650° C. ×	—
								100%			12 h
11D	1100	150	880	25	15	600	—	Hydrogen	−40	−65	690° C. ×	—
								100%			36 h
12A	1100	45	900	30	10	570	—	Hydrogen	−40	−60	710° C. ×	—
								100%			36 h
12B	1100	45	900	30	10	570	—	Hydrogen	−40	−65	630° C. ×	—
								100%			10 h
12C	1100	45	900	30	10	570	—	Hydrogen	−35	−60	730° C. ×	7° C./hr
								100%			10 h
12D	1100	120	880	25	10	600	—	Hydrogen	−35	−45	890° C. ×	—
								100%			36 h
13A	1200	30	820	30	10	540	—	Hydrogen	−45	−60	710° C. ×	—
								100%			36 h
13B	1200	30	820	30	10	540	30	Hydrogen	−40	−60	710° C. ×	—
								100%			36 h
14A	1200	30	800	30	8	590	—	Hydrogen	−35	−55	690° C. ×	—
								100%			24 h
14B	1200	30	800	30	8	590	—	Hydrogen	−35	−55	690° C. ×	—
								100%			24 h
15A	1200	30	840	35	13	600	—	Hydrogen	−30	−50	710° C. ×	—
								100%			36 h
15B	1200	30	840	35	13	600	20	Hydrogen	−30	−50	690° C. ×	—
								100%			12 h
15C	1200	30	840	35	13	600	20	Hydrogen	−30	−50	690° C. ×	—
								100%			12 h
	Cold rolling and annealing processes
	(Second) annealing		(Third) annealing
				Dew	Dew		Cooling			Dew	Dew
		Cold		point	point at		rate when	Cold		point	point at
		rolling		thru	400° C. or		annealing	rolling		thru	400° C. or
		ratio	Atmo-	400° C.	higher	Annealing	to Ac1 or	ratio	Atmo-	400° C.	higher
No	No.	(%)	sphere	(° C.)	(° C.)	condition	higher	(%)	sphere	(° C.)	(° C.)
11A	11A	—	—	—	—	—	—	—	—	—	—
11B	11B	—	—	—	—	—	—	—	—	—	—
11C	11C	—	—	—	—		—	—	—	—	—
11D	11C	—	—	—	—		—	—	—	—	—
12A	12A	—	—	—	—	—	—	—	—	—	—
12B	12B	—	—	—	—	—	—	—	—	—	—
12C	12C	—	—	—	—	—	—	—	—	—	—
12D	12B	—	—	—	—	—	—	—	—	—	—
13A	13A	—	—	—	—	—	—	—	—	—	—
13B	13B	—	—	—	—	—	—	—	—	—	—
14A	14A	25	Hydrogen	−35	−55	690° C. ×	—	30	Hydrogen	−35	−55
			100%			36 h			100%
14B	14B	25	Hydrogen	−35	−55	690° C. ×	—	30	Hydrogen	−35	−55
			100%			36 h			80% +
									Nitrogen
									20%
15A	15A	20	Hydrogen	−20	−50	710° C. ×	—	25	Hydrogen	−45	−60
			100%			24 h			100%
15B	15B	25		−20	−50	680° C. ×	—	—	—	—	—
						6 h
15C		25	Hydrogen	−30	−60	690° C. ×	—	—	—	—	—
			92% +			36 h
			Nitrogen
		Cold rolling and annealing processes
		(Third) annealing
	Cooling	Sheet product characteristics
			rate when	Amount of N in	Product	crack	Carburized
			annealing	sheet products	hard-	during	material
	Annealing	to Ac1 or	thru a depth of	ness	profile		abnormal
	No	condition	higher	100 μm (ppm)	(HRB)	forming	Cp	layers	Note
	11A	—	—	52	73	—	0.45	Not	Present
								present	invention
	11B	—	—	323	75	—	0.45	Present	Comparative
									Example
	11C	—	—	45	83	Crack	0.45	Not	Comparative
						occurred		present	Example
	11D	—	—	125	74	—	0.3	Present	Comparative
									Example
	12A	—	—	41	75	—	0.33	Not	Present
								present	invention
	12B	—	—	34	81	Crack	0.33	Not	Comparative
						occurred		present	Example
	12C	—	—	46	79	Crack	0.33	Not	Comparative
						occurred		present	Example
	12D	—	—	108	76	—	0.33	Present	Comparative
									Example
	13A	—	—	45	77	—	0.45	Not	Present
								present	invention
	13B	—	—	40	73	—	0.45	Not	Present
								present	invention
	14A	680° C. ×	—	61	69	—	0.6	Not	Present
		12 h						present	invention
	14B	680° C. ×	—	129	69	—	0.3	Present	Comparative
		12 h							Example
	15A	680° C. ×	—	67	71	—	0.35	Not	Present
		12 h						present	invention
	15B	—	—	60	79	Crack	0.35	Not	Comparative
						occurred		present	Example
	15C	—	—	108	72	—	0.35	Present	Comparative
									Example

	TABLE 10
	Cold rolling and annealing processes
	Hot rolling conditions		(First) annealing
				Cooling	Cooling				Dew	Dew		Cooling
	Heating	Reten-	Final	rate	rate	Coiling	Cold		point	point at		rate when
	temper-	tion	temper-	to	before	temper-	rolling		thru	400° C. or		annealing
	ature	time	ature	650° C.	coiling	ature	ratio	Atmo-	400° C.	higher	Annealing	to Ac1 or
No	(° C.)	(min)	(° C.)	(° C./s)	(° C./s)	(° C.)	(%)	sphere	(° C.)	(° C.)	condition	higher
16A	1180	40	860	25	20	550	—	Hydrogen	−35	−60	710° C. ×	—
								100%			36 h
16B	1180	40	860	25	20	550	—	Hydrogen	−20	−35	700° C. ×	—
								100%			48 h
16C	1180	40	860	25	20	550	—	Hydrogen	−35	−60	700° C. ×	—
								100%			12 h
17A	1180	50	860	30	20	500	—	Hydrogen	−30	−50	690° C. ×	—
								100%			36 h
17B	1180	50	860	30	20	500	20	Hydrogen	−30	−55	690° C. ×	—
								100%			48 h
17C	1220	80	840	40	15	600	—	Hydrogen	−25	−55	690° C. ×	—
								100%			24 h
17D	1180	50	860	30	20	500	—	Hydrogen	−25	−55	690° C. ×	—
								100%			24 h
18A	1180	40	880	20	10	500	—	Hydrogen	−20	−45	710° C. ×	—
								100%			48 h
18B	1180	40	880	20	10	500	50	Hydrogen	−25	−40	710° C. ×	—
								100%			36 h
18C	1180	40	880	20	10	500	50	Hydrogen	−25	−40	710° C. ×	—
								100%			36 h
19A	1180	40	880	40	15	560	—	Hydrogen	−30	−45	710° C. ×	—
								100%			36 h
19B	1180	40	880	40	15	560	40	Hydrogen	−30	−40	730° C. ×	2° C./hr
								100%			10 h
20A	1170	30	850	35	10	480	—	Hydrogen	−40	−60	700° C. ×	—
								100%			12 h
20B	1170	30	850	35	10	480	—	Hydrogen	−40	−60	700° C. ×	—
								100%			12 h
	Cold rolling and annealing processes
	(Second) annealing		(Third) annealing
				Dew	Dew		Cooling			Dew	Dew
		Cold		point	point at		rate when	Cold		point	point at
		rolling		thru	400° C. or		annealing	rolling		thru	400° C. or
		ratio	Atmo-	400° C.	higher	Annealing	to Ac1 or	ratio	Atmo-	400° C.	higher
No	No.	(%)	sphere	(° C.)	(° C.)	condition	higher	(%)	sphere	(° C.)	(° C.)
16A	16A	—	—	—	—	—	—	—	—	—	—
16B	16B	—	—	—	—	—	—	—	—	—	—
16C	16C	15	Hydrogen	−25	−60	700° C. ×	—	5	Hydrogen	−35	−55
			100%			36 h			100%
17A	17A	—	—	—	—	—	—	—	—	—	—
17B	17B	—	—	—	—	—	—	—	—	—	—
17C	17C	20	Hydrogen	−40	−60	700° C. ×	—	—	—	—	—
			100%			24 h
17D	17D	20	Hydrogen	−15	−45	700° C. ×	—	—	—	—	—
			100%			36 h
18A	18A	25	Hydrogen	−35	−60	700° C. ×	—	30	Hydrogen	−40	−65
			100%			36 h			100%
18B	18B	10	Hydrogen	−20	−50	700° C. ×	—	—	—	—	—
			100%			24 h
18C	18C	10	Hydrogen	−10	−40	700° C. ×	—	—	—	—	—
			100%			36 h
19A	19A	—	—	—	—	—	—	—	—	—	—
19B	19B	—	—	—	—	—	—	—	—	—	—
20A	20A	30	Hydrogen	−30	−60	725° C. ×	2° C./hr	—	—	—	—
			100%			12 h
20B	20B	30		−30	−60	690° C. ×	—	—	—	—	—
						36 h
		Cold rolling and annealing processes
		(Third) annealing
	Cooling	Sheet product characteristics
			rate when	Amount of N in	Product	crack	Carburized
			annealing	sheet products	hard-	during	material
	Annealing	to Ac1 or	thru a depth of	ness	profile		abnormal
	No	condition	higher	100 μm (ppm)	(HRB)	forming	Cp	layers	Note
	16A	—	—	43	74	—	0.6	Not	Present
								present	invention
	16B	—	—	49	73	—	0.48	Present	Comparative
									Example
	16C	730° C. ×	1° C./hr	51	71	—	0.48	Not	Present
		12 h						present	invention
	17A	—	—	51	71	—	0.3	Not	Present
								present	invention
	17B	—	—	58	69	—	0.3	Not	Present
								present	invention
	17C	—	—	135	71	—	0.3	Present	Comparative
									Example
	17D	—	—	65	70	—	0.3	Present	Comparative
									Example
	18A	690° C. ×	—	78	74	—	0.3	Not	Present
		12 h						present	invention
	18B	—	—	59	73	—	0.3	Not	Present
								present	invention
	18C	—	—	64	72	—	0.3	Present	Comparative
									Example
	19A	—	—	73	74	—	0.45	Not	Present
								present	invention
	19B	—	—	92	73	—	0.45	Not	Present
								present	invention
	20A	—	—	63	71	—	0.45	Not	Present
								present	invention
	20B	—	—	111	73	—	0.4	Present	Comparative
									Example

	TABLE 11
	Cold rolling and annealing processes
	Hot rolling conditions		(First) annealing
				Cooling	Cooling				Dew	Dew		Cooling
	Heating	Reten-	Final	rate	rate	Coiling	Cold		point	point at		rate when
	temper-	tion	temper-	to	before	temper-	rolling		thru	400° C. or		annealing
	ature	time	ature	650° C.	coiling	ature	ratio	Atmo-	400° C.	higher	Annealing	to Ac1 or
No	(° C.)	(min)	(° C.)	(° C./s)	(° C./s)	(° C.)	(%)	sphere	(° C.)	(° C.)	condition	higher
21A	1170	30	890	45	8	520	—	Hydrogen	−35	−55	710° C. ×	—
								100%			36 h
21B	1170	30	890	45	8	520	30	Hydrogen	−30	−65	730° C. ×	1° C./hr
								100%			10 h
21C	1170	30	890	45	8	520	—	Hydrogen	−40	−65	700° C. ×	—
								100%			36 h
21D	1180	40	880	35	10	580	—	Hydrogen	−40	−65	700° C. ×	—
								100%			36 h
22A	1120	30	910	50	10	550	—	Hydrogen	−35	−60	710° C. ×	—
								100%			36 h
22B	1120	30	910	50	10	550	—	Hydrogen	−15	−35	710° C. ×	—
								100%			36 h
23	1120	50	910	35	15	580	—	Hydrogen	−30	−50	700° C. ×	—
								100%			36 h
24	1100	60	830	20	15	600	25	Hydrogen	−30	−45	710° C. ×	—
								100%			24 h
25	1100	60	880	25	20	600	—	Hydrogen	−40	−60	710° C. ×	—
								100%			36 h
26	1100	60	860	35	15	540	—	Hydrogen	−35	−55	710° C. ×	—
								100%			36 h
27	1180	40	870	30	10	600	—	Hydrogen	−30	−60	690° C. ×	—
								95% +			72 h
								nitrogen 5%
28	1180	40	870	25	20	600	—	Hydrogen	−25	−50	690° C. ×	—
								95% +			72 h
								nitrogen 5%
29	1180	40	860	30	15	600	30	Hydrogen	−40	−55	690° C. ×	—
								95% +			36 h
								nitrogen 5%
	Cold rolling and annealing processes
	(Second) annealing		(Third) annealing
				Dew	Dew		Cooling			Dew	Dew
		Cold		point	point at		rate when	Cold		point	point at
		rolling		thru	400° C. or		annealing	rolling		thru	400° C. or
		ratio	Atmo-	400° C.	higher	Annealing	to Ac1 or	ratio	Atmo-	400° C.	higher
No	No.	(%)	sphere	(° C.)	(° C.)	condition	higher	(%)	sphere	(° C.)	(° C.)
21A	21A	25	Hydrogen	−35	−50	710° C. ×	—	—	—	—	—
			100%			48 h
21B	21B	—	—	—	—	—	—	—	—	—	—
21C	21C	30	Hydrogen	−30	−55	730° C. ×	10° C./hr	—	—	—	—
			100%			10 h
21D	21D	10	Hydrogen	−30	−55	700° C. ×		5	Hydrogen	−35	−55
			100%			12 h			100%
22A	22A	15	Hydrogen	−25	−60	710° C. ×	—	15	Hydrogen	−35	−55
			100%			36 h			100%
22B	22B	15	Hydrogen	−40	−65	710° C. ×	—	15	Hydrogen	−40	−60
			100%			36 h			100%
23	23	30	Hydrogen	−45	−60	700° C. ×	—	20	Hydrogen	−40	−55
			100%			24 h			100%
24	24	—	—	—	—	—	—	—	—	—	—
25	25	—	—	—	—	—	—	—	—	—	—
26	26	—	—	—	—	—	—	—	—	—	—
27	27	—	—	—	—	—	—	—	—	—	—
28	28	—	—	—	—	—	—	—	—	—	—
29	29	5	Hydrogen	−40	−55	690° C. ×	—	—	—	—	—
			95% +			12 h
			nitrogen 5%
		Cold rolling and annealing processes
		(Third) annealing
	Cooling	Sheet product characteristics
			rate when	Amount of N in	Product	crack	Carburized
			annealing	sheet products	hard-	during	material
	Annealing	to Ac1 or	thru a depth of	ness	profile		abnormal
	No	condition	higher	100 μm (ppm)	(HRB)	forming	Cp	layers	Note
	21A	—	—	76	72	—	0.6	Not	Present
								present	invention
	21B	—	—	55	72	—	0.6	Not	Present
								present	invention
	21C	—	—	64	79	Crack	0.6	Not	Comparative
						occurred		present	Example
	21D	730° C. ×	2° C./hr	64	69	—	0.6	Not	Present
		12 h						present	invention
	22A	700° C. ×	—	48	68	—	0.3	Not	Present
		24 h						present	invention
	22B	700° C. ×	—	116	68	—	0.3	Present	Comparative
		24 h							Example
	23	690° C. ×	—	72	70	—	0.45	Not	Present
		12 h						present	invention
	24	—	—	44	71	—	0.3	Not	Present
								present	invention
	25	—	—	32	74	—	0.45	Not	Present
								present	invention
	26	—	—	56	72	—	0.3	Not	Present
								present	invention
	27	—	—	61	73	—	0.3	Not	Present
								present	invention
	28	—	—	53	74	—	0.35	Not	Present
								present	invention
	29	—	—	48	74	—	0.4	Not	Present
								present	invention

	TABLE 12
	Cold rolling and annealing processes
	Hot rolling conditions		(First) annealing
				Cooling	Cooling				Dew	Dew		Cooling
	Heating	Reten-	Final	rate	rate	Coiling	Cold		point	point at		rate when
	temper-	tion	temper-	to	before	temper-	rolling		thru	400° C. or		annealing
	ature	time	ature	650° C.	coiling	ature	ratio	Atmo-	400° C.	higher	Annealing	to Ac1 or
No	(° C.)	(min)	(° C.)	(° C./s)	(° C./s)	(° C.)	(%)	sphere	(° C.)	(° C.)	condition	higher
30	1200	40	860	25	15	550	—	Hydrogen	−30	−45	710° C. ×	—
								100%			24 h
31	1200	40	880	30	10	600	—	Hydrogen	−40	−55	710° C. ×	—
								100%			36 h
32	1180	30	670	35	10	620	—	Hydrogen	−40	−60	710° C. ×	—
								100%			36 h
33	1180	30	830	30	20	590	—	Hydrogen	−35	−55	710° C. ×	—
								100%			36 h
34	1150	60	840	40	20	540	—	Hydrogen	−35	−50	710° C. ×	—
								100%			24 h
35	1150	60	900	25	15	580	—	Hydrogen	−25	−45	710° C. ×	—
								100%			36 h
36	1150	40	870	30	5	450	—	Hydrogen	−30	−55	700° C. ×	—
								100%			36 h
37	1180	30	850	30	15	600	—	Hydrogen	−25	−40	690° C. ×	—
								100%			24 h
	Cold rolling and annealing processes
	(Second) annealing		(Third) annealing
				Dew	Dew		Cooling			Dew	Dew
		Cold		point	point at		rate when	Cold		point	point at
		rolling		thru	400° C. or		annealing	rolling		thru	400° C. or
		ratio	Atmo-	400° C.	higher	Annealing	to Ac1 or	ratio	Atmo-	400° C.	higher
No	No.	(%)	sphere	(° C.)	(° C.)	condition	higher	(%)	sphere	(° C.)	(° C.)
30	30	—	—	—	—	—	—	—	—	—	—
31	31	—	—	—	—	—	—	—	—	—	—
32	32	—	—	—	—	—	—	—	—	—	—
33	33	15	Hydrogen	−35	−50	710° C. ×	—	—	—	—	—
			100%			12 h
34	34	20	Hydrogen	−40	−55	710° C. ×	—	25	Hydrogen	−30	−45
			100%			12 h			100%
35	35	15	Hydrogen	−30	−50	710° C. ×	—	20	Hydrogen	−35	−55
			100%			24 h			100%
36	36	20	Hydrogen	−30	−50	700° C. ×	—	15	Hydrogen	−45	−55
			100%			12 h			100%
37	37	20	Hydrogen	−45	−60	690° C. ×	—	—	—	—	—
			100%			24 h
		Cold rolling and annealing processes
		(Third) annealing
	Cooling	Sheet product characteristics
			rate when	Amount of N in	Product	crack	Carburized
			annealing	sheet products	hard-	during	material
	Annealing	to Ac1 or	thru a depth of	ness	profile		abnormal
	No	condition	higher	100 μm (ppm)	(HRB)	forming	Cp	layers	Note
	30	—	—	39	73	—	0.3	Present	Comparative
									Example
	31	—	—	46	72	—	0.3	Present	Comparative
									Example
	32	—	—	39	71	—	0.45	Present	Comparative
									Example
	33	—	—	48	69	—	0.45	Present	Comparative
									Example
	34	680° C. ×	—	51	80	Crack	0.3	Not	Comparative
		12 h				occurred		present	Example
	35	680° C. ×	—	49	81	Crack	0.45	Not	Comparative
		12 h				occurred		present	Example
	36	680° C. ×	—	53	82	Crack	0.6	Not	Comparative
		12 h				occurred		present	Example
	37	—	—	70	69	—	0.3	Present	Comparative
									Example

Claims

1. A carbon steel sheet configured to be carburized in a carburization atmosphere with a carbon potential of 0.6 or less, comprising:

C: 0.20% to 0.45% by mass,

Si: 0.05% to 0.8% by mass,

Mn: 0.85% to 2.0% by mass,

P: 0.001% to 0.04% by mass,

S: 0.0001% to 0.006% by mass,

Al: 0.01% to 0.1% by mass,

Ti: 0.005% to 0.3% by mass,

B: 0.0005% to 0.01% by mass and

N: 0.001% to 0.01% by mass

with a balance including Fe and inevitable impurities,

wherein K value that can be obtained from 3C+Mn+0.5Si is greater than or equal to 2.0;

surface hardness is less than or equal to 77 on Rockwell B Scale; and

an average content of N in a zone from a surface to a depth of 100 μm is less than or equal to 100 ppm.

2. The carbon steel sheet according to claim 1, further comprising one or more components selected from

Nb: 0.01% to 0.5% by mass,

V: 0.01% to 0.5% by mass,

Ta: 0.01% to 0.5% by mass,

W: 0.01% to 0.5% by mass,

Sn: 0.003% to 0.03% by mass,

Sb: 0.003% to 0.03% by mass, and

As: 0.003% to 0.03% by mass.

3. A method for producing the carbon steel sheet according to claim 1, comprising:

heating a slab to less than or equal to 1200° C.;

hot-rolling the slab at a final rolling temperature of 800° C. to 940° C. so as to obtain a steel sheet;

cooling the steel sheet at a cooling rate of 20° C./second or more until a temperature of the steel sheet becomes less than or equal to 650° C., as a first cooling;

cooling the steel sheet at a cooling rate of 20° C./second or less, as a second cooling subsequent to the first cooling;

coiling the steel sheet at a temperature of 400° C. to 650° C.;

pickling the steel sheet; and

annealing the steel sheet for 10 hours or more at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C., as a first annealing.

4. The method for producing the carbon steel sheet according to claim 3, wherein the first annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the first annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

5. The method for producing the carbon steel sheet according to claim 4, further comprising:

cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the pickling, as a first cold-rolling.

6. The method for producing the carbon steel sheet according to claim 5, further comprising:

cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the first annealing, as a second cold-rolling; and

annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the second cold-rolling, as a second annealing.

7. The method for producing the carbon steel sheet according to claim 6, wherein

the second annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the second annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

8. The method for producing the carbon steel sheet according to claim 7, further comprising:

cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and

annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.

9. The method for producing the carbon steel sheet according to claim 8, wherein the third annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

10. The method for producing the carbon steel sheet according to claim 6, further comprising:

cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and

annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.

11. The method for producing the carbon steel sheet according to claim 10, wherein the third annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

12. The method for producing the carbon steel sheet according to claim 4, further comprising:

cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the first annealing, as a second cold-rolling; and

annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the second cold-rolling, as a second annealing.

13. The method for producing the carbon steel sheet according to claim 12, wherein the second annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the second annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

14. The method for producing the carbon steel sheet according to claim 13, further comprising:

cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and

annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.

15. The method for producing the carbon steel sheet according to claim 14, wherein the third annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

16. The method for producing the carbon steel sheet according to claim 12, further comprising:

cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and

annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.

17. The method for producing the carbon steel sheet according to claim 16, wherein the third annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

18. The method for producing the carbon steel sheet according to claim 3, further comprising:

cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the pickling, as a first cold-rolling.

19. The method for producing the carbon steel sheet according to claim 18, further comprising:

cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the first annealing, as a second cold-rolling; and

annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the second cold-rolling, as a second annealing.

20. The method for producing the carbon steel sheet according to claim 19, wherein the second annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the second annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

21. The method for producing the carbon steel sheet according to claim 20, further comprising:

cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and

annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.

22. The method for producing the carbon steel sheet according to claim 21, wherein the third annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

23. The method for producing the carbon steel sheet according to claim 19, further comprising:

cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and

annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.

24. The method for producing the carbon steel sheet according to claim 23, wherein the third annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

25. The method for producing the carbon steel sheet according to claim 3, further comprising:

cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the first annealing, as a second cold-rolling; and

annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the second cold-rolling, as a second annealing.

26. The method for producing the carbon steel sheet according to claim 25, wherein the second annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the second annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

27. The method for producing the carbon steel sheet according to claim 26, further comprising:

cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and

annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.

28. The method for producing the carbon steel sheet according to claim 27, wherein the third annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

29. The method for producing the carbon steel sheet according to claim 25, further comprising:

cold-rolling the steel sheet with a rolling ratio of 5% to 60% after the second annealing, as a third cold-rolling; and

annealing the steel sheet at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C. after the third cold-rolling, as a third annealing.

30. The method for producing the carbon steel sheet according to claim 29, wherein the third annealing is performed in an atmosphere with a hydrogen content of 95% or more with an annealing temperature range from Ac1 to Ac1+50° C., and after the third annealing, a cooling rate is set to 5° C./hour or less until a temperature becomes Ac1−30° C. after annealing.

31. A carbon steel sheet configured to be carburized in a carburization atmosphere with a carbon potential of 0.6 or less, comprising

C: 0.20% to 0.45% by mass,

Si: 0.05% to 0.8% by mass,

Mn: 0.85% to 2.0% by mass,

P: 0.001% to 0.04% by mass,

S: 0.0001% to 0.006% by mass,

Al: 0.01% to 0.1% by mass,

Ti: 0.005% to 0.3% by mass,

B: 0.0005% to 0.01% by mass and

N: 0.001% to 0.01% by mass, and further comprising one or more components selected from:

Cr: 0.01% to 2.0% by mass,

Ni: 0.01% to 1.0% by mass,

Cu: 0.005% to 0.5% by mass and

Mo: 0.01% to 1.0% by mass

with a balance including Fe and inevitable impurities,

wherein K′ value that can be obtained from 3C+Mn+0.5Si+Cr+Ni+Mo+Cu is greater than or equal to 2.0;

surface hardness is less than or equal to 77 on Rockwell B Scale; and

an average content of N in a zone from a surface to a depth of 100 μM is less than or equal to 100 ppm.

32. The carbon steel sheet according to claim 31, further comprising one or more components selected from:

Nb: 0.01% to 0.5% by mass,

V: 0.01% to 0.5% by mass,

Ta: 0.01% to 0.5% by mass,

W: 0.01% to 0.5% by mass,

Sn: 0.003% to 0.03% by mass,

Sb: 0.003% to 0.03% by mass, and

As: 0.003% to 0.03% by mass.

33. The method for producing the carbon steel sheet according to claim 31, comprising:

heating a slab to less than or equal to 1200° C.;

hot-rolling the slab at a final rolling temperature of 800° C. to 940° C. so as to obtain a steel sheet;

cooling the steel sheet at a cooling rate of 20° C./second or more until a temperature of the steel sheet becomes less than or equal to 650° C., as a first cooling;

cooling the steel sheet at a cooling rate of less than or equal to 20° C./second, as a second cooling subsequent to the first cooling;

coiling the steel sheet at a temperature of 400° C. to 650° C.;

pickling the steel sheet; and

annealing the steel sheet for more than or equal to 10 hours at a temperature of 660° C. or more in an atmosphere with a hydrogen content of 95% or more and a dew point of less than or equal to −20° C. at a temperature of less than 400° C. and of less than or equal to −40° C. at a temperature of more than or equal to 400° C., as a first annealing.