Title of invention: Steel plate and enamel product
Technical field
[0001]
　The present invention relates to steel sheets and enamel products.
　The present application claims priority based on Japanese Patent Application No. 2018-095190 filed in Japan on May 17, 2018, the contents of which are incorporated herein by reference.
Background technology
[0002]
　Enamel products are made by baking glass on the surface of a steel plate. Since enamel products have heat resistance, weather resistance, chemical resistance, and water resistance, they have been widely used as materials for kitchen utensils such as pots and sinks and building materials. Such an enamel product is generally manufactured by processing a steel sheet into a predetermined shape, assembling it into a product shape by welding or the like, and then performing an enamel treatment (firing treatment).
[0003]
　Steel sheets used as materials for enamel products (steel sheets for enamel) have characteristics such as firing strain resistance, nail skipping resistance after enamel treatment, enamel adhesion, foam resistance and black spot defects after enamel treatment. Desired. In addition, in the production of enamel products, since press working is usually performed to obtain a product shape, the enamel steel sheet is required to have good formability.
[0004]
　In addition, since the enamel treatment improves the corrosion resistance in a harsh corrosive environment containing sulfuric acid and the like, the range of application of enamel products is expanding to the energy field such as power generation equipment. In such a field, there is a need for reliability against fatigue and the like during aged use, and further, for the purpose of reducing the weight of parts, it is required to increase the strength of the steel sheet used. Regarding the reliability against fatigue, etc., the change in the structure of the steel sheet in the manufacturing process from the processing of the steel sheet into the product shape to the brazing process, that is, the change in strength due to the difference in the structure inside the steel sheet. It is known to affect.
[0005]
　So far, with respect to the change in the structure of the steel sheet due to the enamel treatment, for example, Patent Document 1 describes a method for preventing deterioration of nail jump resistance due to coarsening of the crystal grain size. In Patent Document 1, it is necessary to optimize the composition, size, shape, ratio, and number of inclusions based on the known high oxygen steel, and at the same time, add a small amount of Ni, Cr, V, and Mo. It is described that by adding Nb, B, and Ti accordingly and optimizing the manufacturing conditions of the steel sheet, it is possible to reduce the decrease in nail skipping resistance even when repeated broom treatment is performed. ing.
[0006]
　Further, in Patent Document 2, the structure of the steel sheet for deflection, that is, ferrite, is solved in response to the problem that the dimensional accuracy is deteriorated due to the occurrence of bending during firing due to the decrease in strength due to the grain growth in the deflection treatment of high oxygen steel. It is described that it is effective to make the particle size uniform and reduce the particle size distribution. In Patent Document 2, Ni and Cr are added in order to make the structure of the hot-rolled steel sheet finer and to make the grain growth uniform by annealing in the steel sheet manufacturing process.
[0007]
　Further, Patent Document 3 defines an oxide precipitation state in order to suppress softening of high oxygen steel in enamel treatment. In Patent Document 3, fine oxides are left, and grain growth in the enamel firing step is suppressed by a pinning effect to suppress softening.
[0008]
　In both Patent Documents 1 and 2, it is considered that certain characteristics can be ensured in enamel products that have been enamel-treated with structural changes. However, in Patent Documents 1 and 2, the addition of Ni is indispensable in order to solve the problems related to the grain growth in the enamel treatment. That is, in order to solve the problem, it is necessary to add an expensive alloying element. Further, with respect to Patent Document 2, by coarsening the oxide by adding Cr to make it difficult to hinder the growth of ferrite grains, the uniformity of the grain size of ferrite is improved, abnormal grain growth is suppressed, and the grains are mixed. Is suppressed. However, this method, which does not use suppression of grain growth by pinning precipitates and inclusions, causes non-uniform particle size when the temperature in the member fluctuates during the enamel treatment, and the desired effect cannot be obtained. There is a possibility. In this case, the strength after the enamel treatment cannot be stably obtained.
[0009]
　Further, with respect to Patent Document 3, fine oxides are generated by controlling the manufacturing conditions in the steelmaking process after containing oxygen at a high concentration, and the pinning force of the oxides is used during enamel firing. It suppresses grain growth. This in itself is considered to be an excellent technique. In the first place, the reason why the oxygen content is increased in Patent Document 3 is to ensure the nail jump resistance, which is an important property of the enamel steel sheet.
[0010]
　Other methods for forming hydrogen trap sites by increasing the oxygen content for the purpose of improving nail jump resistance are described in Patent Documents 4 and 5. However, the method of increasing the oxygen content may cause oxide-induced defects such as baldness defects, which causes a problem that the steelmaking cost increases.
　Therefore, other than the utilization of oxides, it is desired to develop a technique capable of suppressing grain growth and ensuring nail skipping resistance.
[0011]
　As a technique for ensuring nail jump resistance other than the utilization of oxides, Patent Documents 4 and 5 disclose a method of utilizing BN as a trap site, and Patent Document 6 discloses TiS as a hydrogen trap site. The method of utilizing as is disclosed. However, in the method using TiS and BN, a large amount of elements such as S, B and N are added, so that a large amount of precipitates are generated. In this case, the ductility may decrease, and the addition of the element causes an increase in steelmaking cost. Further, when BN is utilized, a high oxygen component is often used, and the problem of using high oxygen steel remains.
[0012]
　As a technique for ensuring nail skipping resistance without using high oxygen steel and without utilizing BN and TiS, Patent Document 7 is generated by performing coarse MnS and decarburization annealing using low-carbon aluminum killed steel. The technology to utilize the void as a trap site is described. In the technique of Patent Document 7, low-carbon aluminum killed steel is used, so that the steelmaking cost is low, but there is a problem that the cost is high because decarburization annealing is performed.
Prior art literature
Patent documents
[0013]
Patent Document 1: Japanese Patent Application Laid-Open No. 2001-316760
Patent Document 2: Japanese Patent Application Laid-Open No.
2000-636985 Patent Document 3: Japanese Patent
Application Laid-Open No. 6115691 Patent Document 4: Japanese Patent Application Laid-Open No. 8-27522
Patent Document 5: Japanese Patent Application Laid-Open No. 7-2492997
Patent Document 6: Japanese Patent Application Laid-Open No. 2-104640
Patent Document 7: Japanese Patent Application Laid-Open No. 6-192727
Outline of the invention
Problems to be solved by the invention
[0014]
　The present invention has developed the above-mentioned steel sheet technology and is excellent in formability, nail skipping resistance after enamel treatment, strength characteristics after enamel treatment and appearance after enamel treatment (generation of bubbles and black spots is suppressed). The subject is to provide steel sheets and enamel products.
Means to solve problems
[0015]
　The present invention has been made to solve the above problems, and the gist of the present invention is as follows.
[0016]
[1] The steel sheet according to one aspect of the present invention has a chemical composition of mass%, C: 0.0050 to 0.0700%, Si: 0.0010 to 0.0500%, Mn: 0.0500 to 1. .0000%, P: 0.0050 to 0.1000%, S: 0.0010 to 0.0500%, Al: 0.007 to 0.100%, O: 0.0005 to 0.0100%, B: 0.0003 to 0.0100%, N: 0.0010 to 0.0100%, Ti: 0 to 0.0100%, Nb, Zr, V, Mo, W, 1 type or 2 or more types in total 0 0020 to 0.0300%, Cu: 0 to 0.045%, Cr, Ni 1 or 2 in total 0 to 1.000%, As, Se, Ta, Sn, Sb, Ca, Mg, A total of 0 to 0.1000% of one or more of Y and REM is contained, and the balance is composed of Fe and impurities, satisfying the formulas (1) and (2), and as a metal structure, ferrite. , Cementite in the ferrite crystal grains and one or two types of cementite and pearlite at the grain boundaries of the ferrite are contained, and the particle size is 0.3 to 0.3 in the ferrite crystal grains. 1.5 μm cementite exists in the range of 1.00 × 10 -1 piece / μm 2 or less, and the average value of the major axis is 0.5 to 15 μm and the number density is at the grain boundaries of the ferrite. Is 1 or 2 of cementite and pearlite having a value of 5.00 × 10 -4 to 1.00 × 10-1 / μm 2 and is the N content contained in the BN [Nas BN]. A steel sheet in which the relationship between and the B content contained in the steel satisfies the formula (3).
　Ti <(N-0.0003) × 3.43 ・ ・ ・ Equation (1)
　C> 0.25 × Ti + 0.129 × Nb + 0.235 × V + 0.132 × Zr + 0.125 × Mo + 0.0652 × W + 0.0040 ・.・ ・ Equation (2)
　[Nas BN] / (1.27 × B) <0.95 ・ ・ ・ Equation (3)
　However, the element symbol in equations (1) to (3) is the mass% of the element. [Nas BN] in the formula (3) represents the N content in mass% contained in the BN.
[2] The steel sheet according to the above [1] may contain Cu: 0.010 to 0.045% in mass%.
[3] The steel sheet according to the above [1] or [2] may contain 0.005 to 1.000% in total of one or two types of Cr and Ni in mass%.
[4] In the steel sheet according to any one of [1] to [3] above, one or more of As, Se, Ta, Sn, Sb, Ca, Mg, Y, and REM are totaled in% by mass. May contain 0.0005 to 0.1000%.
[5] In the steel sheet according to any one of [1] to [4] above, the steel sheet may be a cold-rolled steel sheet.
[6] In the steel sheet according to any one of [1] to [5] above, the steel sheet may be an enamel steel sheet.
[7] The enamel product according to another aspect of the present invention includes the steel plate according to any one of the above [1] to [4].
The invention's effect
[0017]
　The steel sheet according to the above aspect of the present invention is excellent in moldability, nail skipping resistance after enamel treatment, and strength after enamel treatment. It also has excellent enamel adhesion and appearance after enamel treatment. Therefore, it is suitable as a steel sheet (steel sheet for enamel) which is a base material of enamel products applied to kitchen utensils, building materials, energy fields and the like.
A brief description of the drawing
[0018]
[Fig. 1] Fig. 1 is a diagram showing a measurement example of major diameters of cementite and pearlite existing on grain boundaries.
Mode for carrying out the invention
[0019]
　The steel sheet according to the present embodiment has been obtained through various studies in order to overcome the problems of the conventional steel sheet. Strength of characteristics Based on the findings obtained as a result of examining the effects of chemical composition and manufacturing conditions on the characteristics.
　That is, it is based on the following findings 1) to 4).
[0020]
1) Regarding the strength after the enamel treatment, by utilizing the solid solution C and the iron carbide by containing a certain amount or more of C, the grain growth during the enamel treatment can be suppressed and the decrease in strength can be suppressed. In particular, the effect of solid solution C and iron carbide on strain-induced grain growth when light processing is applied is large. Therefore, by utilizing solid solution C and iron carbide, the decrease in strength after broom treatment is suppressed. be able to. The mechanism is not clear, but it can be considered as follows. During the enamel treatment, solid solution C is present due to the dissolution of carbides. When the solid solution C is present, it may have an effect of suppressing grain boundary movement and an effect of transforming into austenite during the enamel treatment to pin the ferrite grain boundary and suppress grain growth. If iron carbide also remains, it is considered that the pinning effect suppresses grain growth. Further, by containing the carbide-forming elements of Nb, V, Zr, Mo, and W, the grain growth can be suppressed by the pinning effect of the generated carbide, and the decrease in strength can be suppressed. Further, when the decrease in strength after the enamel treatment is small, the decrease in fatigue strength is also suppressed.
[0021]
2) Further, by containing C, cementite and pearlite are produced. Since these act as hydrogen trap sites, sufficient nail jump resistance can be ensured even if the precipitation amount of iron oxides, TiS and BN in high oxygen steel is limited to some extent. Specifically, by controlling the size and number of cementites, sufficient nail jump resistance can be obtained.
[0022]
3) Of the above precipitates, BN has a high function as a hydrogen trap site. Therefore, if the Ti content is limited to reduce the amount of N precipitated as TiN and the BN remains, the nail jump resistance is improved. improves.
[0023]
4) Regarding moldability, C is an element that affects the formation of iron carbide, Si, Mn, P are solid solution strengthening elements, Nb, Zr, V, Mo, W, and inclusions are elements that contribute to precipitation strengthening. By containing an appropriate amount of O, which affects the formation of, ductility can be ensured by suppressing an excessive increase in strength.
[0024]
　Hereinafter, the steel sheet according to this embodiment will be described in detail. The steel sheet according to this embodiment is suitably used as a base material for an enamel product.
[0025]

　The steel plate according to the present embodiment has C: 0.0050 to 0.0700%, Si: 0.0010 to 0.0500%, Mn: 0.0500 to 1.000%, P in mass%. : 0.0050 to 0.1000%, S: 0.0010 to 0.0500%, Al: 0.007 to 0.100%, O: 0.0005 to 0.0100%, B: 0.0003 to 0 .0100%, N: 0.0010 to 0.0100%, Ti: 0 to 0.0100%, one or more of Nb, Zr, V, Mo, W, 0.002 to 0. 0300%, Cu: 0 to 0.045%, Cr, Ni 1 or 2 in total 0 to 1.000%, As, Se, Ta, Sn, Sb, Ca, Mg, Y, REM 1 It contains 0 to 0.1000% of seeds or two or more seeds in total, and the balance is composed of Fe and impurities, satisfying the following formulas (1) and (2).
　Further, in the steel sheet according to the present embodiment, the relationship between [Nas BN], which is the N content contained in the BN, and the B content contained in the steel satisfies the formula (3).
[0026]
　Ti <(N-0.0003) × 3.43 ・ ・ ・ Equation (1)
　C> 0.25 × Ti + 0.129 × Nb + 0.235 × V + 0.132 × Zr + 0.125 × Mo + 0.0652 × W + 0.0040 ・.・ ・ Equation (2)
　[Nas BN] / (1.27 × B) <0.95 ・ ・ ・ Equation (3)
　However, the element symbols in equations (1) to (3) are the content of the element. It represents (mass%), and [Nas BN] in the formula (3) represents the amount of N (mass%) contained in the BN.
[0027]
　Further, the steel sheet according to the present embodiment may contain Cu: 0.010 to 0.045% in mass%.
　Further, the steel sheet according to the present embodiment may contain 1 or 2 types of Cr and Ni in a total amount of 0.005 to 1.000% in mass%.
　Further, the steel sheet according to the present embodiment further contains one or more of As, Se, Ta, Sn, Sb, Ca, Mg, Y, and REM in a total of 0.0005 to 0.1000 in mass%. % Or less may be contained.
[0028]
　The reasons for limiting the chemical composition of the steel sheet will be described below. Here, "%" means mass%.
[0029]
C: 0.0050 to 0.0700% The
　smaller the C content, the smaller the amount of cementite and pearlite formed. Therefore, the nail jump resistance is lowered, and the effect of suppressing grain growth during enamel treatment is lost, resulting in a decrease in strength. Further, when the C content exceeds 0.0700%, pinholes due to bubble defects are likely to occur. Also, due to the large amount of cementite or pearlite produced, ductility is reduced. Therefore, the C content is set to 0.0050 to 0.0700%. It is preferably in the range of 0.0100 to 0.0300%.
[0030]
Si: 0.0010 to 0.0500%
　Si is a solid solution strengthening element, and is also an element having an effect of suppressing a decrease in strength due to enamel treatment. However, if the Si content is excessive, the ductility is lowered and the manufacturing cost is increased. Therefore, the Si content is set to 0.0010 to 0.0500%. It is preferably in the range of 0.0040 to 0.0300%.
[0031]
Mn: 0.0500 to 1.000%
　Mn is an important component that affects the formation of MnS used as a precipitation site of BN, which is effective for nail jump resistance of steel sheets for enamel. MnS itself also has the effect of improving nail jump resistance. Further, Mn is an element that prevents hot brittleness caused by S during hot rolling. In order to obtain these effects, the Mn content is set to 0.0500% or more. However, if the Mn content is excessive, the ductility deteriorates. Therefore, the upper limit of the Mn content is set to 1.0000% or less. It is preferably in the range of 0.0800 to 0.5000%.
[0032]
P: 0.0050 to 0.1000%
　P is an element effective for increasing the strength of the steel sheet. Further, P is also an element having an effect of suppressing a decrease in strength due to enamel treatment. In order to obtain these effects, the P content is set to 0.0050% or more. On the other hand, if the P content is excessive, P may segregate at the grain boundaries of the steel sheet at a high concentration during the enamel treatment, which may cause bubbles, black spots, and the like. In addition, ductility may decrease. Therefore, the P content is set to 0.1000% or less. It is preferably 0.0500% or less.
[0033]
S: 0.0010 to 0.0500%
　S is an element forming MnS. This sulfide acts as a precipitation site of BN and contributes to the improvement of nail jump resistance. MnS itself also has the effect of improving nail jump resistance. In order to obtain these effects, the S content is set to 0.0010% or more. Desirably, it is 0.0030% or more. However, if the S content is excessive, defects caused by MnS may occur. Therefore, the S content is set to 0.0500% or less. It is preferably 0.0300% or less.
[0034]
Al: 0.007 to 0.100%
　Al is an element that acts as a deoxidizing element. When the Al content is low, the deoxidizing effect is low and the amount of inclusions increases. Therefore, the Al content is set to 0.007% or more. On the other hand, if the Al content is excessive, the ductility is lowered. Therefore, the Al content is set to 0.100% or less. It is preferably in the range of 0.010 to 0.060%.
[0035]
O: 0.0005 to 0.0100% When the
　O content is high, a large amount of iron oxide is generated, which causes a decrease in ductility and also causes a scab. From this point of view, the O content should be reduced as much as possible. However, if the O content is excessively lowered, the manufacturing cost increases. Therefore, the content of O is set to 0.0005 to 0.0100%. It is preferably in the range of 0.0010 to 0.0070%.
[0036]
B: 0.0003 to 0.0100%
　B is contained to generate a BN having an effect of improving the nail jump resistance of the steel sheet for enamel. In addition, B that did not become BN exists as a solid solution B and suppresses crystal grain growth during the enamel treatment. In order to obtain these effects, the B content needs to be 0.0003% or more. It is preferably 0.0005% or more. On the other hand, when the B content becomes excessive, the crystal grain growth is remarkably suppressed and the ductility is lowered. Therefore, the B content is set to 0.0100% or less. It is preferably 0.0030% or less.
[0037]
N: 0.0010 to 0.0100%
　N is an element necessary for producing BN having an effect of improving the nail jump resistance of the steel sheet for enamel. In order to obtain this effect, the N content is set to 0.0010% or more. On the other hand, when the N content becomes excessive, the ductility decreases. Therefore, the N content is set to 0.0100% or less. It is preferably 0.0070% or less.
[0038]
Ti: 0 to 0.0100%
　Ti is an element that easily forms a nitride, and is an element that inhibits the formation of BN, which exerts an effect on nail jump resistance. Therefore, it is desirable not to contain it as much as possible. Therefore, the Ti content is set in the range of 0 to 0.0100%. It is preferably 0.0050% or less. However, if the Ti content is 0.0003% or less, the manufacturing cost may increase. Therefore, the lower limit of actual production may be 0.0003%.
[0039]
One or more of Nb, Zr, V, Mo, and W Total: 0.0020 to 0.0300%
　These elements form fine carbides and suppress grain growth. Due to the inclusion of these elements, the grain growth during the enamel treatment is suppressed and the decrease in strength is suppressed. However, excessive inclusion of these elements reduces ductility. Therefore, the total content of one or more of these elements is 0.0020 to 0.0300%. It is preferably 0.0030 to 0.0200%.
[0040]
　In the present embodiment, the following elements can be contained as needed in addition to the above elements. Since these elements do not have to be contained, the lower limit is 0%.
[0041]
Cu: 0 to 0.045%
　Cu may be contained in order to control the reaction between the glassy substance and the steel during the enamel treatment. When the above effect is obtained, the Cu content is preferably 0.010% or more. Cu may be 0%. On the other hand, if the Cu content is excessive, not only the reaction between the vitreous material and the steel is hindered, but also the workability may be deteriorated. Therefore, in order to avoid such an adverse effect, the Cu content is preferably 0.045% or less.
[0042]
One or more types of Cr and Ni: 0 to 1.000% in total
　Cr and Ni may be contained because they have an effect of improving the adhesion between the steel sheet and the enamel layer. When the total content of Cr and Ni is 0.005% or more, the effect of improving the adhesion to the enamel layer becomes remarkable, which is preferable. More preferably, it is 0.010% or more. On the other hand, when the total content of Cr and Ni exceeds 1.000%, the effect of improving the adhesion is saturated and the mechanical properties are also deteriorated. When Cr and Ni are contained, the effect can be expected to some extent even if the content is 0.500% or less. Therefore, when Cr and Ni are contained, the total content should be 0.005 to 1.000%. It is preferably 0.010 to 0.500%.
[0043]
One or more of As, Se, Ta, Sn, Sb, Ca, Mg, Y, REM: 0 to 0.1000% in total
　These elements form oxides in a trace amount and improve nail jump resistance. Has the effect of causing. However, if it is excessively contained, a large amount of oxide is precipitated. Since this oxide becomes the starting point of fracture during deformation, the ductility is reduced. Therefore, the total content of one or more of these elements is preferably 0 to 0.1000%. More preferably, it is 0.0005 to 0.1000%. More preferably, it is 0.0025 to 0.0500%. REM refers to one or more of the lanthanoid elements having atomic numbers 57 to 71 in the periodic table.
[0044]
　Further, by satisfying the following formulas (1) to (3), the nail skipping resistance is further improved, and the decrease in strength during the enamel treatment is further suppressed.
　　Ti <(N-0.0003) × 3.43 ・ ・ ・ Formula (1)
　As described above, Ti is an element that easily forms a nitride, and even when Ti is contained, the nail jump resistance is improved. It is necessary to leave N for forming the BN. Therefore, the Ti content is limited to the range of the formula (1).
[0045]
　　C> 0.25 × Ti + 0.129 × Nb + 0.235 × V + 0.132 × Zr + 0.125 × Mo + 0.0652 × W + 0.0040 ・ ・ ・ Equation (2)
　As described above for suppressing the decrease in strength during enamel treatment. The presence of solid solution C or the presence of iron carbide is required. In order to obtain such an effect, it is necessary that C in a solid solution state remains even when an alloy carbide of Ti, Nb, V, Zr, Mo, and W is formed. Therefore, the C content is limited to the range of the formula (2).
[0046]
　　[
　Nas BN] / (1.27 × B) <0.95 ・ ・ ・ Formula (3) B is contained to form BN and improve nail jump resistance, but solid solution B remains. If this is the case, the effect of suppressing the grain growth during the enamel treatment and suppressing the decrease in strength is produced. Therefore, all the contained B is prevented from being precipitated as BN. [Nas BN], which indicates the N content contained in BN, can be quantified by chemical analysis. Therefore, this value is used to define the BN formation state, and the amount of BN precipitation that is effective in suppressing grain growth. The range is specified in equation (3). [Nas BN] is determined by the extraction residue of steel (Brommethanol method).
[0047]

　The metal structure of the steel sheet according to the present embodiment contains ferrite, cementite and / or pearlite, and has a structure mainly composed of ferrite. More specifically, the metallographic structure of the steel plate according to the present embodiment contains ferrite, cementite in the grain of ferrite, and cementite and / or pearlite in the grain boundary of ferrite. Further, it may contain one or more of carbides, nitrides and oxides other than cementite. Since ferrite has excellent ductility, the steel sheet according to the present embodiment can realize excellent workability by using ferrite as the main phase. In addition, the presence of cementite or pearlite in the metal structure improves the nail jump resistance, which is a necessary property of the enamel steel sheet. It is considered that this is because hydrogen generated during the enamel treatment is trapped at the interface between ferrite and cementite. On the other hand, if cementite or pearlite is present, hydrogen generated during the enamel treatment may be released to the outside of the steel sheet as a hydrocarbon gas. In that case, it may cause bubble defects. Therefore, it is necessary to limit the size and number density of cementite and pearlite contained therein.
[0048]
　First, regarding cementite in the crystal grains of ferrite, the number density of cementite having a particle size of 0.3 to 1.5 μm is set to 1.00 × 10 -1 piece / μm 2 or less. Cementite finely precipitated in the ferrite crystal grains dissolves during the enamel treatment and is released as carbon monoxide or carbon dioxide gas, causing foam defects. Therefore, it is necessary to limit the number of fine intragranular carbides in the ferrite crystal grains to 1.00 × 10 -1 / μm 2 or less. Intragranular cementite having a particle size of more than 1.5 μm is harmless and is not specified. Further, cementite having a particle size of less than 0.3 μm has a small effect on nail jump resistance even if foam defects occur. Therefore, the number density is evaluated by measuring intragranular cementite having a particle size of 0.3 to 1.5 μm. The particle size of one cementite is the average of the major axis and the minor axis.
[0049]
　Next, cementite and / or pearlite existing on the grain boundaries of ferrite is present in the hydrogen diffusion path during the broom treatment, and therefore has the effect of trapping hydrogen and improving nail jump resistance. The mean value of the major axis of cementite and / or pearlite is limited to 0.5 to 15 μm, and the number density of cementite and pearlite is 5.00 × 10 -4 to 1.00 × 10 -1 piece / μm 2 . Restrict. When the average value of the major axis of cementite and pearlite is less than 0.5 μm, the effect of improving the nail jump resistance is small. In addition, it becomes easy to dissolve during the enamel treatment and is released as carbon monoxide or carbon dioxide gas, which causes foam defects. On the other hand, when the average value of the major axis exceeds 15 μm, it becomes a starting point of fracture during processing and the ductility is lowered. Therefore, the average value of the major axis is set to 0.5 to 15 μm.
　Further, when the number density is less than 5.00 × 10 -4 pieces / μm 2 , the effect of improving the nail skipping resistance is not seen, and when the number density is more than 1.00 × 10 -1 piece / μm 2 , It becomes the starting point of destruction at the time of deformation, and the ductility decreases. Therefore, the number density of cementite and / or pearlite existing on the grain boundaries of ferrite is 5.00 × 10 -4 to 1.00 × 10 -1 / μm 2.And. Either one of cementite and pearlite may be present, and both may be present. Further, the cementite referred to here is distinguished from the lamellar cementite contained in pearlite, and means cementite not contained in the pearlite tissue.
[0050]
　Cementite and pearlite undergo picral corrosion after polishing the rolling cross section of the steel sheet, and appear as black contrast when observed with an optical microscope. As a representative point of the steel plate structure, a portion at a position (1 / 4t) of 1/4 of the plate thickness t in the plate thickness direction from the surface is observed. Further, by adjusting the degree of picral corrosion, ferrite grain boundaries can also appear, so that the relationship between the observation positions of cementite and pearlite and the grain boundaries can be determined. Observation should be performed at a magnification of 400 to 1000 times. When the cementites precipitated at the grain boundaries are connected at the triple points of the grain boundaries, the lengths of the cementites precipitated at the sides of the grain boundaries are measured and added up. In the case of pearlite, it may be surrounded by a plurality of ferrite grains, but even in that case, the number is measured assuming that it exists at the ferrite grain boundary. A schematic diagram of a measurement example is shown in FIG. The number density of cementite and pearlite described above is a value obtained by dividing the number of observed pieces by the observed area, and the unit is piece / μm 2 .
[0051]
　For example, in FIG. 1, cementite a exists at one grain boundary between two ferrite crystal grains, and the major axis is the length La along the grain boundary. Cementite b exists along two grain boundaries formed by three ferrite crystal grains, and the sum of lengths Lb1 and Lb2 (Lb1 + Lb2) along each grain boundary is defined as the major axis. Cementite c exists along three grain boundaries formed by four ferrite crystal grains, and the sum of lengths Lc1 to Lc3 (Lc1 + Lc2 + Lc3) along each grain boundary is defined as the major axis. Cementite d exists along three grain boundaries formed by three ferrite crystal grains, and the total length (Ld1 + Ld2 + Ld3) of the lengths Ld1 to Ld3 along each grain boundary is defined as the major axis. The maximum major axis Le to Li is the major axis of each of the pearlites e to i.
[0052]
　Further, the average crystal grain size of ferrite in the steel sheet structure before the enamel treatment is preferably 30.0 μm or less at a position (1 / 4t) of the plate thickness t in the plate thickness direction from the surface. By setting the average crystal grain size to 30.0 μm or less, the strength of the steel sheet can be increased. It is preferably 20.0 μm or less, more preferably 15.0 μm or less. It is desirable that the average crystal grain size is small in order to increase the strength, but the processability deteriorates as the average crystal grain size becomes smaller. Therefore, it is necessary to determine the optimum crystal grain size for the desired product shape.
[0053]
　The average crystal grain size of ferrite is calculated as a circle-equivalent diameter by obtaining the average crystal area per crystal grain by the method using the square test line described in JIS G0551: 2013 Annex B. That is, assuming that the average crystal area is a, the average crystal grain size d is represented by the following formula (4).
[0054]
d = 2√ (a / π) ・ ・ ・ Equation (4)
[0055]

　A preferable manufacturing method of the steel sheet according to the present embodiment will be described.
　The steel sheet according to the present embodiment can be manufactured through the steps of melting, casting, hot rolling, cold rolling, annealing, and temper rolling. Each step may be set based on a conventional method except for the conditions shown below.
[0056]
　The points in manufacturing the steel sheet according to the present embodiment are the control of the precipitation state of cementite and pearlite of the steel sheet and the control of the precipitation state of BN. As explained above, while limiting the number density of fine cementite deposited in the ferrite grains, the nail jump resistance is improved by controlling the size and number density of cementite and pearlite generated at the ferrite grain boundaries. It is possible to suppress bubble defects. Further, by controlling the precipitation state of BN and leaving the solid solution B while precipitating the BN, the nail jumping resistance can be improved, and the grain growth during the enamel treatment can be suppressed to suppress the decrease in strength.
[0057]
　The slab heating temperature in hot rolling is 1000 to 1300 ° C, the finishing temperature of hot rolling is Ar3 to 1000 ° C, the rolling reduction rate of Ar3 + 100 ° C or less is over 25%, the rolling end temperature is Ar3 ° C or higher, and the winding temperature is 500 to 500. 800 ° C is preferable.
[0058]
　When the slab is heated at a temperature lower than 1000 ° C., BN is likely to be generated, and there is a concern that the B content remaining as a solid solution B may decrease. The upper limit of the slab heating temperature is not particularly specified, but it is desirable to set it to about 1300 ° C. for economic reasons.
[0059]
　If the finishing temperature of hot rolling is less than Ar3 ° C, ferrite is generated during rolling and transformation does not occur during cooling after rolling, so that part may become coarse grains and non-uniform crystal grains may occur. is there. Further, when the finishing temperature exceeds 1000 ° C., the temperature drop allowance to the winding temperature is large and it is not economical. Therefore, the finishing temperature is preferably in the range of Ar3 to 1000 ° C.
　When finish rolling is performed, Ar3 is estimated using a prediction formula from the steel components shown in the following formula (a). Rolling conditions are set based on Ar3 predicted by this method.
　Ar3 (° C.) = 901-325 × C-92 × Mn + 33 × Si + 287 × P + 40 × Al-30 (a)
　However, the element symbol (C, Mn, Si, P, Al) in the formula (a) contains the element. Represents the amount (% by mass).
　In addition, whether or not the finish rolling temperature was actually less than Ar3 ° C. is confirmed by changing the finish temperature in actual operation, performing hot rolling, observing the microstructure of the rolled plate, and checking for the presence or absence of coarse grains. be able to. Coarse grains are generated at a portion where the finishing temperature is lower than Ar3 ° C., and are mainly generated at the edge of the steel sheet and the surface layer. The average particle size is 1.5 times or more the average particle size at the center of the plate width and the center of the plate thickness.
[0060]
　The take-up temperature is not particularly limited, but if the take-up temperature is less than 500 ° C, the size of cementite and pearlite generated during hot rolling becomes small, which may affect the carbides after cold rolling annealing. .. Therefore, 500 ° C. or higher is desirable. Further, in the case of a line that is continuously annealed in a subsequent process and has no overaging process, the winding temperature is preferably 550 ° C. or higher. Further, when the winding temperature exceeds 800 ° C., the scale generated on the surface becomes thick, and the cost in pickling in the subsequent process increases. Therefore, 800 ° C. or lower is desirable.
[0061]
　The reduction rate (cumulative reduction rate) of Ar3 + 100 ° C. or less during hot rolling shall be more than 25%. When the rolling ratio in the temperature range of Ar3 + 100 ° C or lower is 25% or less, the effect of cumulative strain becomes small, and the γ grain boundaries that are the nucleation sites of ferrite transformation or ferrite pearlite transformation that occur after finish rolling are reduced, and cementite Alternatively, the density of pearlite produced becomes coarse and coarse. When such a hot-rolled steel sheet is used, it is considered that the density of cementite and / or pearlite precipitation at the grain boundaries after cold-rolling annealing is reduced. Further, when the reduction rate of Ar3 + 100 ° C. or lower is 25% or less, it is considered that the particle size of the hot-rolled steel sheet becomes coarse and the r value decreases. In order to ensure press formability, both the r value in the rolling direction after cold rolling and annealing or the r value in the direction orthogonal to the rolling direction and the rolling direction (hereinafter referred to as the orthogonal direction) are 0. It is preferably 8 or more, and in order to achieve this, it is necessary to set the rolling reduction rate of Ar3 + 100 ° C. or less to 25% or more.
　After hot rolling, pickling or the like is carried out to remove the scale generated on the surface, but the method and conditions are not particularly specified.
[0062]
　The hot-rolled steel sheet after hot-rolling is cold-rolled. The rolling reduction (cold rolling ratio) in cold rolling is not particularly specified, and rolling may be performed under conditions suitable for each cold rolling mill. Usually, a reduction rate of 50 to 90% is desirable.
[0063]
　Continuous annealing is performed on the cold-rolled steel sheet after cold rolling. The continuous annealing step is an important step that affects the formation of iron carbide. The annealing temperature is preferably in the range of 700 to 850 ° C. When annealed at a temperature of 700 ° C. or higher, the amount of fine cementite in the grains is dissolved and reduced, and the amount of precipitation can be controlled to such that foam defects do not occur. If the annealing temperature is less than 700 ° C., the dissolution of cementite becomes insufficient. On the other hand, if annealing is performed at a temperature higher than 850 ° C., iron carbides are dissolved too much, and cementite and pearlite having a size effective for nail jump resistance are less likely to remain.
　Regarding the rate of temperature rise, if the rate of temperature rise from 650 ° C. at which dissolution of iron carbide occurs to the annealing temperature is too large, the dissolution of iron carbide is small and a large amount of fine intragranular carbide remains, so that bubble defects are likely to occur. Therefore, the rate of temperature rise from 650 ° C. to the annealing temperature is preferably 50 ° C./s or less. Regarding continuous annealing, in the method for producing a steel sheet for enamel, OCA (Open Coil Annealing) may be used to perform decarburization annealing in which the dew point in the atmosphere is increased, but in this embodiment, decarburization annealing is not performed. The reason is that when decarburization annealing is performed, the carbon concentration in the steel decreases, and the carbide disappears, so that the desired carbide state of the steel sheet according to the present embodiment cannot be secured. In this case, the grain growth of ferrite cannot be suppressed, and sufficient strength may not be obtained. For example, annealing is performed in an atmosphere containing hydrogen having a volume concentration of 3%, the balance being nitrogen, and a dew point of −40 ° C.
[0064]
　When the overaging treatment is performed after continuous annealing, it is desirable to keep the temperature in the temperature range of 200 ° C. to 500 ° C. for 20 s (seconds) or more. In this case, cementite at the grain boundaries of the ferrite crystal grains grows, and the nail jump resistance is improved. As described above, the take-up temperature during hot rolling in the case of superaging treatment is preferably 500 ° C. or higher. When the temperature of the overaging treatment is less than 200 ° C., the effect of growing cementite at the grain boundaries is not sufficient, and when the temperature exceeds 500 ° C., the cementites at the grain boundaries grow large and the cementites at the grain boundaries become too large. When the overaging treatment is not performed, it is desirable that the winding temperature during hot rolling is 550 ° C. or higher.
[0065]
　After that, temper rolling is performed mainly for the purpose of shape control. In temper rolling, strain is introduced into the steel sheet by the temper rolling ratio at the same time as controlling the shape. At this time, if the temper rolling ratio is increased, that is, the amount of strain introduced into the steel sheet is increased, abnormal grain growth during welding or enamel processing is promoted. For this reason, it is not desirable to apply more strain than necessary to the temper rolling ratio up to the rolling ratio that allows shape control. From the viewpoint of shape control, the rolling ratio of temper rolling is preferably 2% or less.
[0066]
　From the above, a cold-rolled steel sheet having desired characteristics can be obtained. The obtained steel sheet can be used as a steel sheet for enamel as a base material for enamel products.
[0067]
　Further, the steel sheet according to the present embodiment is made into an enamel product by being processed into a predetermined shape, assembled into a product shape by welding or the like, and subjected to an enamel treatment (firing treatment). Regarding the enamel treatment, for example, the glassy material of the glaze and the steel sheet may be brought into close contact with each other by heating the steel sheet coated with the glaze to a predetermined temperature and holding it for a predetermined time. The preferred firing treatment conditions for the steel sheet according to the present embodiment are, for example, a firing temperature of 750 to 900 ° C. and a firing time of 1.5 to 10 minutes (in a furnace). Further, firing may be repeated several times for double coating and repair. By performing the firing treatment under such conditions, the solid solution C and the iron carbide can suppress the grain growth during the enamel treatment and suppress the decrease in strength. The conditions of the firing treatment shown here are merely examples, and do not limit the conditions of the enamel treatment of the steel sheet according to the present embodiment.
Example
[0068]
　Steels having the chemical compositions shown in Tables 1-1A to 1-3B and Tables 1-4A to 1-4B (the balance is Fe and impurities) were melted in a converter and made into slabs by continuous casting. Steel sheets were manufactured from these slabs under the conditions shown in Table 2. That is, after heating the slab, rough rolling and finish rolling were performed, and the slab was wound into a hot-rolled steel sheet. Then, after pickling the hot-rolled steel sheet, the rolling ratio of cold-rolling is changed to obtain a cold-rolled steel sheet, and further, in an atmosphere containing hydrogen having a volume concentration of 3%, the balance is nitrogen, and the dew point is −40 ° C. After continuous annealing, temper rolling was performed to obtain a steel sheet having a thickness of 0.8 mm. In order to keep the plate thickness after temper rolling constant, the plate thickness of the hot-rolled steel sheet was changed with respect to the rolling ratio of cold rolling. Some steel sheets were annealed and then overaged.
　Further, Ar3 was calculated by the above formula (a), and a reduction rate of Ar3 + 100 ° C. or lower (Ar3 or higher) was set using this value. Manufacturing method No. In C1 to C13, the aim of the reduction rate of Ar3 + 100 ° C. or less is 30% or more, and the production method No. In C14, the aim of the reduction rate was set to 25%. Actually, the reduction rate was as shown in Tables 3-1 to 3-4.
　In addition, the relationship with Ar3 points was confirmed by observing the microstructure of the hot-rolled steel sheet from the presence or absence of coarse grains. Specifically, those having an average particle size of 1.5 times or more the average particle size at the center of the plate width and the center of the plate thickness were judged to be coarse particles. The manufacturing method No. shown in Table 2. It is considered that the hot rolling finish temperatures of C1 to C14 were all in the range of Ar3 to 1000 ° C. The heating rate in Table 2 is the heating rate from 650 ° C. to the annealing temperature.
[0069]
　The characteristics of the steel sheet manufactured above were evaluated by various methods shown below.
[0070]
For
　mechanical properties, a tensile test was performed using a JIS No. 5 test piece in accordance with JIS Z2241: 2011, and tensile strength (Rm) and elongation at break (A) were measured. From the viewpoint of strength, those having a tensile strength of 300 MPa or more were judged to have sufficient strength, and from the viewpoint of moldability, those having a breaking elongation of 30% or more were judged to be excellent in moldability.
　Further, the r value (plastic strain ratio) when the test piece was taken parallel to the rolling direction and perpendicular to the rolling direction was measured according to JIS Z2254: 2008. As a result of the measurement, both the r value in the rolling direction and the r value in the orthogonal direction were 0.8 or more except for d38 described later.
[0071]

　Precipitates in steel are subjected to picral corrosion after polishing the cross section parallel to the direction of cold rolling, and by observing with an optical microscope, ferrite crystal grains. Measurements were made for cementite present in, cementite and / or pearlite present at grain boundaries. That is, picral corrosion was performed after polishing the rolling cross section of the steel sheet. As a representative point of the steel plate structure (metal structure), a portion at a position (1 / 4t) of 1/4 of the plate thickness t in the plate thickness direction from the surface was observed. Cementite and pearlite appear as black contrast when observed under a light microscope. In addition, by adjusting the degree of picral corrosion, ferrite grain boundaries were made to appear, and the relationship between the observation positions of cementite and pearlite and the grain boundaries was determined. The observation was performed at a magnification of 400 to 1000 times. When the cementites precipitated at the grain boundaries were connected at the triple points of the grain boundaries, the lengths of the cementites precipitated at the sides of the grain boundaries were measured and added up. In the case of pearlite, it may be surrounded by a plurality of ferrite grains, but even in that case, the number was measured assuming that it was present at the ferrite grain boundary. A schematic diagram of a measurement example is shown in FIG. The number density of cementite and pearlite is a value obtained by dividing the number of observed pieces by the observed area, and the unit is piece / μm 2 .
　All of D1 to D89 and d1 to d46 contained ferrite, cementite in the grain of ferrite, and cementite and / or pearlite in the grain boundary of ferrite as a metal structure.
[0072]
　The average crystal grain size of ferrite was calculated as a circle-equivalent diameter by determining the average crystal area per crystal grain by the method using the square test line described in JIS G0551: 2013 Annex B. That is, assuming that the average crystal area is a, the average crystal grain size d is a value represented by the following formula (5).
[0073]
d = 2√ (a / π) ・ ・ ・ Equation (5)
[0074]
In
　addition, the decrease in strength due to grain growth after enamel treatment was evaluated. Specifically, a steel sheet that has been cold-rolled with a rolling reduction of 10% in order to simulate press processing is subjected to a heat treatment that simulates broom processing for 4 minutes at a furnace temperature of 830 ° C. The tensile strength was determined by the above method, and the ratio of the strength after the heat treatment to the strength before the heat treatment was determined. When the tensile strength after the enamel treatment was 0.85 (85%) or more of the tensile strength before the enamel treatment, it was judged that the decrease in the strength after the enamel treatment was suppressed.
[0075]
　The enamel characteristics were investigated as follows.
The
　nail-skipping resistance is a dry method using a steel plate with a size of 100 x 150 mm by the powder electrostatic coating method, 100 μm of glaze is applied, and the temperature in the air is 830 ° C for 5 minutes. Evaluation was performed on the fired ones. An enameled steel sheet is placed in a constant temperature bath at 160 ° C for 10 hours to perform a nail skipping promotion test, and the condition of nail skipping is visually checked by A: excellent, B: slightly excellent, C: normal, D: problematic. If it is A, B, or C, it is judged that the predetermined nail jump resistance is secured, and the case of D evaluation is rejected. Specifically, A is when no nail skipping occurs, B is when 1 to 5 nail skipping occurs, C is when 6 to 15 nail skipping occurs, and D is 15 or more nail skipping. It was assumed that it occurred.
[0076]
The
　enamel adhesion is obtained by using a steel sheet that has been enamel-treated in the same manner as described above. E. I. Since there is no difference in adhesion in the adhesion test method (ASTM C313-59), the weight of a 2 kg ball head is dropped three times from a height of 1 m, and the delamination state of the deformed part is measured with 169 palpation needles. , The area ratio of the unpeeled portion was evaluated. When the area ratio of the unpeeled portion was 40% or more, it was judged that the method had sufficient adhesion.
[0077]

　The appearance after enamelling is the same as above. Visually observe the enameled steel sheet and observe the condition of bubbles and black spots. A: Very good, B: Excellent, C: Normal, D: Slightly inferior. , E: Remarkably inferior, evaluated on a five-point scale. If A, B, C, and D, it was judged that a predetermined appearance was obtained, and the case of remarkably inferior E evaluation was rejected.
[0078]
　The evaluation results are shown in Tables 3-1 to 3-4. No. In D1 to D89, the steel component, the precipitation state of carbides, and the precipitation state of BN were within the range of the present invention, and showed good characteristics.
[0079]
　No. Since d1 has a low C content in the steel sheet, No. Since the C content of d2 was excessive, the mechanical properties became inferior.
　No. Since d3 has a low Si content in the steel sheet, No. Since the Si content of d4 was excessive, the mechanical properties became inferior.
　No. Since d5 has a low Mn content of the steel sheet, the nail jump resistance is lowered.
　No. Since the Mn content of the steel sheet of d6 was excessive, the mechanical properties were inferior.
　No. Since d7 has a low P content in the steel sheet, No. Since the P content of d8 was excessive, the mechanical properties became inferior.
　No. Since d9 has a low S content in the steel sheet, the nail jump resistance is lowered.
　No. Since d10 has a low Al content in the steel sheet, No. Since the Al content of d11 was excessive, the mechanical properties became inferior.
　No. Since the B content of the steel sheet of d12 is small, the nail jump resistance is lowered. In addition, No. Since the B content of d13 was excessive, the mechanical properties became inferior.
　No. Since d14 has a low N content in the steel sheet, the nail jump resistance is lowered.
　No. Since the N content of the steel sheet of d15 was excessive, the mechanical properties were inferior.
　No. Since the Ti content of the steel sheet of d16 was excessive, the nail jump resistance was lowered.
　No. In d17 to d20, the content of group A elements (Nb, Zr, V, Mo, W) does not meet the scope of the invention, and in d21, the content of group B elements (Cr, Ni) of the steel sheet is within the scope of the invention. The mechanical properties became inferior because the above conditions were not satisfied.
　No. In d22 and d23, the chemical composition of the steel sheet did not satisfy the equation (1), so that the nail jump resistance was lowered.
　No. Since the chemical composition of the steel sheet of d24 ​​and d25 does not satisfy the equation (2), the mechanical properties of d24 ​​and d25 are inferior.
[0080]
　No. In d26 to d37, although the steel component was within the range of the present invention, the production conditions were out of the preferable range, so that the precipitation state of carbides and the precipitation state of BN were out of the range of the present invention, and good mechanical properties and enamel This is an example in which the characteristics could not be obtained.
　No. In d26 and d29, the heating temperature of the slab was low, BN was easily generated, the content of B remaining as a solid solution B decreased, the equation (3) did not hold, and the mechanical properties became inferior.
　No. In d27 and d30, the take-up temperature after hot rolling was low, the size of cementite and pearlite generated during hot rolling was small, the number density of cementite in the ferrite grains was excessive, and the appearance was inferior. ..
　No. In d28, the overaging temperature is high, the cementite at the grain boundary grows large, and the cementite at the grain boundary becomes too large. As a result, the number density of cementite and pearlite at the ferrite grain boundary becomes insufficient, and the nail jump resistance is lowered. did.
　No. In d31, the heating rate at the time of annealing exceeded the upper limit, and in d32, the annealing temperature was too low, so that the number density of cementite in the ferrite grains became excessive and the appearance became inferior.
　No. Since the winding temperature of d33 and d36 was high and the annealing temperature of d34 was too high, the number densities of cementite and pearlite at the ferrite grain boundaries were insufficient, and the nail jump resistance was lowered.
　No. In d35, the winding temperature was low, the size of cementite and pearlite produced during hot rolling was small, the number density of cementite in the ferrite grains was excessive, and the appearance was inferior.
　No. In d37, the overaging temperature was low, cementite at the grain boundaries did not grow, the number density of cementite and pearlite in the specified range was below the lower limit, and the nail skipping resistance was inferior.
　No. In d38, the grain boundary number densities of cementite and pearlite became small because the reduction rate in the temperature range from (Ar3 + 100) ° C. to Ar3 was not sufficient. In addition, the r value in the rolling direction was as low as less than 0.8.
[0081]
　In addition, No. Since the contents of group C elements (As, Se, Ta, Sn, Sb, Ca, Mg, Y, REM) of d39 to d46 do not meet the invention range, the mechanical properties are inferior.
[0082]
　From the results shown in Tables 3-1 to 3-4, the steel of the present invention has excellent enamel adhesion, appearance such as foam generation, and nail jump resistance, and further suppresses a decrease in tensile strength after enamel treatment. It was confirmed that it is possible to provide steel sheets for enamel.
[0083]
[Table 1-1A]

[0084]
[Table 1-1B]

[0085]
[Table 1-2A]

[0086]
[Table 1-2B]

[0087]
[Table 1-3A]

[0088]
[Table 1-3B]

[0089]
[Table 1-4A]

[0090]
[Table 1-4B]

[0091]
[Table 2]

[0092]
[Table 3-1]

[0093]
[Table 3-2]

[0094]
[Table 3-3]

[0095]
[Table 3-4]

Industrial applicability
[0096]
　The steel sheet according to the above aspect of the present invention is excellent in moldability, nail skipping resistance after enamel treatment, and strength characteristics when applied to kitchen utensils, building materials, energy fields, etc. after enamel treatment. Therefore, it is suitable as a steel sheet for enamel and has high industrial applicability.
The scope of the claims
[Claim 1]
　The chemical composition is mass%,
C: 0.0050 to 0.0700%,
Si: 0.0010 to 0.0500%,
Mn: 0.0500 to 1.0000%,
P: 0.0050 to 0.1000. %,
S: 0.0010 to 0.0500%,
Al: 0.007 to 0.100%,
O: 0.0005 to 0.0100%,
B: 0.0003 to 0.0100%,
N: 0. 0010 to 0.0100%,
Ti: 0 to 0.0100%,
one or two or more of Nb, Zr, V, Mo, W in total 0.0020 to 0.0300%,
Cu: 0 to 0 .045%,
1 or 2 types of Cr, Ni in total 0 to 1.000%,
1 type or 2 or more types of As, Se, Ta, Sn, Sb, Ca, Mg, Y, REM in total It
contains 0 to 0.1000%, and the balance is 　composed of Fe and impurities
　, satisfying the formulas (1) and (2), and as a
metal structure, ferrite and cementite in the crystal grains of the ferrite. Containing one or two types of cementite and pearlite at the grain boundaries of the ferrite,
　Cementite having a particle size of 0.3 to 1.5 μm is present in the 　crystal grains of the ferrite in a range of 1.00 × 10 -1 piece / μm 2 or less, and the
crystal grains of the ferrite. There are one or two types of cementite and pearlite with an average major axis of 0.5 to 15 μm and a number density of 5.00 × 10 -4 to 1.00 × 10 -1 / μm 2 . A steel plate characterized
　in
that the relationship between [Nas BN], which is the N content contained in the BN, and the B content contained in the steel satisfies the formula (3) .
　Ti <(N-0.0003) × 3.43 ・ ・ ・ Equation (1)
　C> 0.25 × Ti + 0.129 × Nb + 0.235 × V + 0.132 × Zr + 0.125 × Mo + 0.0652 × W + 0.0040 ・.・ ・ Equation (2)
　[Nas BN] / (1.27 × B) <0.95 ・ ・ ・ Equation (3)
　However, the element symbol in equations (1) to (3) is the mass% of the element. [Nas BN] in the formula (3) represents the N content in mass% contained in the BN.
[Claim 2]
　The steel sheet according to claim 1, wherein the steel sheet contains 0.010 to 0.045% of Cu in mass%.
[Claim 3]
　The steel sheet according to claim 1 or 2, wherein the steel sheet contains one or two types of Cr and Ni in a total amount of 0.005 to 1.000% by mass.
[Claim 4]
　Claims 1 to 1, which contain a total of 0.0005 to 0.1000% of one or more of As, Se, Ta, Sn, Sb, Ca, Mg, Y, and REM in mass%. The steel plate according to any one of claims 3.
[Claim 5]
　The steel sheet according to any one of claims 1 to 4, wherein the steel sheet is a cold-rolled steel sheet.
[Claim 6]
　The steel sheet according to any one of claims 1 to 5, wherein the steel sheet is an enamel steel sheet.
[Claim 7]
　An enamel product comprising the steel plate according to any one of claims 1 to 4.