CN112119174A

CN112119174A - Steel sheet and enamel product

Info

Publication number: CN112119174A
Application number: CN201980032420.9A
Authority: CN
Inventors: 楠见和久; 福里哲次
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2018-05-17
Filing date: 2019-05-17
Publication date: 2020-12-22
Anticipated expiration: 2039-05-17
Also published as: KR20210002639A; CN112119174B; WO2019221286A1; TWI704237B; KR102504491B1; JPWO2019221286A1; JP6683294B1; TW201947043A

Abstract

A steel sheet having a predetermined chemical composition satisfying Ti < (N-0.0003). times.3.43 and C > 0.25 XTi +0.129 XNb +0.235 XV +0.132 XZr +0.125 XMo +0.0652 XW +0.0040, and containing ferrite as a microstructure; cementite in the grains of ferrite; and 1 or 2 kinds of cementite and pearlite at the grain boundary of ferrite, and the number density in the crystal grain of ferrite is 1.00X 10^‑1Per mu m²Cementite with a particle size of 0.3 to 1.5 μm is present in the range belowThe average value of the long diameter of the ferrite grain boundary is 0.5 to 15 mu m, and the number density is 5.00 multiplied by 10^‑4～1.00×10^‑1Per mu m²1 or 2 kinds of cementite and pearlite, and the N content contained in BN, i.e., [ N as BN ]]The relation with the B content contained in the steel satisfies [ N as BN ]]/(1.27×B)＜0.95。

Description

Steel sheet and enamel product

Technical Field

The present invention relates to a steel sheet and an enamel product.

This application claims priority based on patent application No. 2018-095190, filed in japan on 17.05.2018, the contents of which are incorporated herein by reference.

Background

The enamel product is made by sintering glass on the surface of steel plate. Enamel products have been widely used as materials for kitchen utensils such as pots and sinks, building materials, and the like, because of their functions of heat resistance, weather resistance, chemical resistance, and water resistance. Such an enamel product is generally produced by processing a steel sheet into a predetermined shape, assembling the steel sheet into a product shape by welding or the like, and then performing an enamel treatment (firing treatment).

As characteristics of a steel sheet used as a material of an enamel product (steel sheet for enamel), burning deformation resistance, fishscaling resistance after enamel treatment, enamel adhesion, bubble resistance after enamel treatment, black spot defect resistance, and the like are required. In addition, in the production of enamel products, press working is generally performed to obtain the shape of the products, and therefore, good formability is required of steel sheets for enamel.

Further, by applying the enamel treatment, corrosion resistance under a severe corrosive environment containing sulfuric acid or the like is improved, so that the enamel product is also widely applied to the energy field of power generation equipment or the like. In such fields, reliability against fatigue and the like in the course of years of use is required, and further, high strength of the steel sheet used is required for the purpose of weight reduction of parts. It is known that: the reliability against fatigue and the like described above is affected by changes in the structure and form of the steel sheet, that is, strength changes due to differences in the structure and form in the steel sheet, in the manufacturing process from the processing of the steel sheet into the product shape to the enameling treatment.

Hitherto, with respect to a change in the texture of a steel sheet accompanying an enamel treatment, for example, patent document 1 describes a method for preventing deterioration of fishscale resistance due to coarsening of crystal grain size. Patent document 1 describes: the deterioration of the fishscale resistance can be reduced even in the case of repeated enameling treatments by optimizing the composition, size, shape, ratio and number of inclusions based on a known high-oxygen steel, adding a small amount of Ni, Cr, V and Mo, and if necessary, adding Nb, B and Ti, and optimizing the production conditions of the steel sheet.

Further, patent document 2 describes: in the case of the enamel treatment of high-oxygen steel, it is effective to make the grain size distribution smaller by making the ferrite grain diameter, which is the structural form of the steel sheet for enamel, uniform, in order to solve the problem that the dimensional accuracy deteriorates due to the deflection during firing caused by the decrease in strength accompanying the grain growth. In patent document 2, Ni and Cr are added to refine the structure of the hot-rolled steel sheet in the manufacturing process of the steel sheet and to make grain growth uniform during annealing.

In patent document 3, the state of oxide deposition is defined in order to suppress softening in the enamel treatment of high-oxygen steel. In patent document 3, softening is suppressed by keeping fine oxide and suppressing grain growth in the enamel firing step by pinning effect.

In both patent documents 1 and 2, it is considered that certain characteristics can be secured in an enamel product subjected to an enamel treatment involving a structural change, but in patent documents 1 and 2, in order to solve the problem concerning grain growth in the enamel treatment, Ni must be added. That is, in order to solve the problem, it is necessary to add expensive alloying elements. In patent document 2, Cr is added to coarsen oxides and thereby inhibit ferrite grain growth, thereby improving uniformity of ferrite grain size, inhibiting abnormal grain growth, and inhibiting mixed grains. However, in this method in which grain growth is not suppressed by pinning of precipitates and inclusions, it is also conceivable that: in the case of temperature fluctuations in the component during the enamel treatment, there is a possibility that the particle size is not uniform and the desired effect cannot be obtained. In this case, the strength after the enamel treatment cannot be stably obtained.

In addition, patent document 3 discloses that oxygen is contained at a high concentration, and the production conditions in the steel making process are controlled to generate fine oxides, and the grain growth during firing of the enamel is suppressed by the pinning force of the oxides. This technique itself is considered to be an excellent technique. The reason why the oxygen content is increased in patent document 3 is to ensure the fishscale resistance which is an important characteristic of the steel sheet for enamel.

A method of forming a trap site (trap site) of hydrogen by increasing the amount of oxygen contained for the purpose of improving the fish scaling resistance is also described in patent document 4 and patent document 5. However, in the method of increasing the oxygen content, defects due to oxides such as scaly fold defects may occur, and there is a problem that the steel manufacturing cost becomes high.

Therefore, it is desired to develop a technique capable of suppressing grain growth and ensuring the fishscale resistance, in addition to effective utilization of oxides.

As a technique for ensuring the fishscale resistance in addition to the effective use of oxides, patent documents 4 and 5 disclose a method for effectively using BN as a trap site, and patent document 6 discloses a method for effectively using TiS as a hydrogen trap site. However, in the method using TiS or BN, a large amount of element such as S, B, N is added, and therefore a large amount of precipitates are formed. In this case, it is conceivable that the ductility is reduced, and the addition of the element causes an increase in the steel manufacturing cost. In addition, when BN is effectively used, a high oxygen component is often used, and there remains a problem in the case of using high oxygen steel.

Patent document 7 describes, as a technique for ensuring the fishscaling resistance without using high-oxygen steel and without effectively utilizing BN and TiS: a technique of using low-carbon aluminum-killed steel, and effectively utilizing coarse MnS and voids (void) generated by decarburization annealing as trap sites. In the technique of patent document 7, the steel manufacturing cost is low because a low-carbon aluminum killed steel is used, but there is a problem that the cost is high because decarburization annealing is performed.

Prior art documents

Patent document

Patent document 1 Japanese laid-open patent application No. 2001-316760

Patent document 2 Japanese laid-open patent publication No. 2000-063985

Patent document 3 Japanese patent No. 6115691

Patent document 4 Japanese patent application laid-open No. Hei 8-27522

Patent document 5 Japanese patent application laid-open No. Hei 7-242997

Patent document 6 Japanese patent application laid-open No. Hei 2-104640

Patent document 7 Japanese patent application laid-open No. Hei 6-192727

Disclosure of Invention

The present invention has been made in view of the above-mentioned technical developments of the steel sheet, and an object of the present invention is to provide a steel sheet and an enamel product excellent in formability, fishscaling resistance after enamel treatment, strength characteristics after enamel treatment, and appearance after enamel treatment (suppression of formation of bubbles and black spots).

The present invention has been made to solve the above problems, and the gist of the present invention is as follows.

[1]The steel sheet according to one aspect of the present invention has a chemical composition containing, in mass%, C: 0.0050-0.0700%, Si: 0.0010 to 0.0500%, Mn: 0.0500-1.0000%, P: 0.0050 to 0.1000%, S: 0.0010 to 0.0500%, Al: 0.007-0.100%, O: 0.0005 to 0.0100%, B: 0.0003 to 0.0100%, N: 0.0010-0.0100%, Ti: 0 to 0.0100%, 1 or 2 or more of Nb, Zr, V, Mo, and W: 0.0020 to 0.0300% in total, Cu: 0 to 0.045%, 1 or 2 of Cr and Ni: 0 to 1.000% in total, 1 or 2 or more of As, Se, Ta, Sn, Sb, Ca, Mg, Y, and REM: 0 to 0.1000% in total, the balance comprising Fe and impurities, satisfying formula (1) and formula (2), and containing ferrite as a metal structure; cementite within the ferrite grains; and 1 or 2 kinds of cementite and pearlite at the grain boundary of the ferrite, wherein the number density in the grain of the ferrite is 1.00 x 10^-1Per mu m²Cementite with a grain diameter of 0.3 to 1.5 μm exists in the range below, the average value of the long diameter is 0.5 to 15 μm, and the number density is 5.00 x 10^-4～1.00×10^-1Per mu m²1 or 2 kinds of cementite and pearlite, and the N content contained in BN, i.e., [ N as BN ]]The relationship with the B content contained in the steel satisfies the formula (3).

Ti < (N-0.0003). times.3.43. formula (1)

C > 0.25 XTi +0.129 XNb +0.235 XV +0.132 XZr +0.125 XMo +0.0652 XW + 0.0040. formula (2)

[ N as BN ]/(1.27 XB) < 0.95. formula (3)

Wherein the element symbols in the formulae (1) to (3) represent the content of the element in mass%, and [ N as BN ] in the formula (3) represents the content of N in mass% contained in BN.

[2] The steel sheet according to the above [1], which may contain

Cu：0.010～0.045％。

[3] The steel sheet according to the above [1] or [2], which may contain, in mass%

1 or 2 of Cr and Ni: the total amount is 0.005-1.000%.

[4] The steel sheet according to any one of the above [1] to [3], which may contain, in mass%)

1 or more than 2 of As, Se, Ta, Sn, Sb, Ca, Mg, Y and REM: the total amount is 0.0005 to 0.1000%.

[5] The steel sheet according to any one of the above [1] to [4], wherein the steel sheet is a cold-rolled steel sheet.

[6] The steel sheet according to any one of the above [1] to [5], wherein the steel sheet is a steel sheet for enamel.

[7] An enamel product according to another aspect of the present invention includes the steel sheet according to any one of the above [1] to [4 ].

The steel sheet according to the above aspect of the present invention is excellent in formability, fishscale resistance after enameling, and strength after enameling. Further, the enamel adhesion and the appearance after enamel treatment are also excellent. Therefore, the steel sheet is suitable as a steel sheet (steel sheet for enameling) which is a base material of enamelware used in kitchen products, building materials, energy fields, and the like.

Drawings

Fig. 1 is a view showing an example of measuring the major diameters of cementite and pearlite present in grain boundaries.

Detailed Description

The steel sheet according to the present embodiment is obtained as a result of various studies to overcome the problems of the conventional steel sheet, and the effects of the chemical composition and the production conditions are studied on the formability of the steel sheet, the fishscale resistance after the enameling treatment, and the strength characteristics after the enameling treatment.

Namely, the following findings 1) to 4) were made.

1) In the strength after the enamel treatment, the solid-dissolved C and the iron carbide are effectively utilized by containing a certain amount or more of C, whereby grain growth at the time of the enamel treatment can be suppressed, and the strength can be suppressed from being lowered. In particular, since the influence of solid solution C and iron carbide is large for strain-induced grain growth in the case of light working, the strength reduction after the enamel treatment can be suppressed by effectively utilizing solid solution C and iron carbide. The mechanism is not clear, but can be considered as follows. Solid solution C is present due to the dissolution of carbides during the enamel treatment. When solid solution C is present, there is a possibility that an effect of suppressing grain boundary migration and an effect of suppressing grain growth by transforming into austenite to pin ferrite grain boundaries at the time of enamel treatment are generated. In addition, in the case where iron carbide also remains, an effect of suppressing grain growth by the pinning effect is conceivable. Further, by containing the carbide-forming elements Nb, V, Zr, Mo, and W, grain growth can be suppressed by the pinning effect of the generated carbide, and a decrease in strength can be suppressed. In addition, when the strength after the enamel treatment is less decreased, the decrease of the fatigue strength is also suppressed.

2) Further, by containing C, cementite and pearlite are produced. Since these function as hydrogen trap sites, sufficient fishscale resistance can be ensured even if the amount of precipitated iron oxides, TiS, and BN in the high-oxygen steel is limited to some extent. Specifically, by controlling the size and number of cementite, sufficient fishscale resistance can be obtained.

3) Among the above precipitates, BN has a high function as a hydrogen trap site, and therefore, if the Ti content is limited to reduce the amount of N precipitated as TiN and leave BN, the fishscale resistance is improved.

4) Regarding formability, ductility can be ensured by suppressing an excessive increase in strength by appropriately containing C, which is an element that affects iron carbide formation, Si, Mn, and P, which are solid-solution strengthening elements, Nb, Zr, V, Mo, and W, which contribute to precipitation strengthening, and O, which affects the generation of inclusions.

The steel sheet according to the present embodiment will be described in detail below. The steel sheet according to the present embodiment can be suitably used as a base material for enamel products.

< chemical composition >

The steel sheet according to the present embodiment contains, in mass%

C: 0.0050-0.0700%, Si: 0.0010 to 0.0500%, Mn: 0.0500-1.0000%, P: 0.0050 to 0.1000%, S: 0.0010 to 0.0500%, Al: 0.007-0.100%, O: 0.0005 to 0.0100%, B: 0.0003 to 0.0100%, N: 0.0010-0.0100%, Ti: 0 to 0.0100%, 1 or 2 or more of Nb, Zr, V, Mo, and W: 0.002-0.0300% in total, Cu: 0 to 0.045%, 1 or 2 of Cr and Ni: 0 to 1.000% in total, 1 or 2 or more of As, Se, Ta, Sn, Sb, Ca, Mg, Y, and REM: 0 to 0.1000% in total, and the balance of Fe and impurities, and satisfies the following formulae (1) and (2).

In the steel sheet according to the present embodiment, the relationship between the N content [ N as BN ] contained in BN and the B content contained in steel satisfies expression (3).

Ti < (N-0.0003). times.3.43. formula (1)

[ N as BN ]/(1.27 XB) < 0.95. formula (3)

In the formulae (1) to (3), the element symbol indicates the content (mass%) of the element, and [ N as BN ] in the formula (3) indicates the amount (mass%) of N contained in BN.

The steel sheet according to the present embodiment may contain, in mass%, Cu: 0.010-0.045%.

The steel sheet according to the present embodiment may contain 1 or 2 of Cr and Ni in mass%: the total amount is 0.005-1.000%.

The steel sheet according to the present embodiment may further contain 1 or 2 or more of As, Se, Ta, Sn, Sb, Ca, Mg, Y, and REM in mass%: the total amount is 0.0005 to 0.1000%.

The reason for limiting the chemical composition of the steel sheet will be described below. Here, "%" means mass%. C: 0.0050-0.0700%

The smaller the C content, the less cementite and pearlite formation, and the less the antifishscale property, and the grain growth suppression effect in the enamel treatment is lost, and the strength is lowered. When the C content exceeds 0.0700%, voids due to bubble defects are likely to occur. Further, since cementite or pearlite is generated in a large amount, ductility is reduced. Therefore, the C content is set to 0.0050 to 0.0700%. Preferably 0.0100-0.0300%.

Si：0.0010～0.0500％

Si is a solid-solution strengthening element and 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 decreases and the manufacturing cost increases. Therefore, the content of Si is set to 0.0010 to 0.0500%. Preferably 0.0040 to 0.0300%. Mn: 0.0500-1.0000%

Mn is an important component that affects the formation of MnS used as BN precipitation sites that exhibit an effect on the fishscale resistance of the enamel steel sheet. In addition, MnS itself has an effect of improving the fishscale resistance. Further, Mn is an element for preventing hot brittleness caused by S at the time of hot rolling. In order to obtain these effects, the Mn content is set to 0.0500% or more. However, if the Mn content becomes excessive, ductility deteriorates. Therefore, the upper limit of the Mn content is set to 1.0000% or less. Preferably 0.0800-0.5000%.

P：0.0050～0.1000％

P is an element effective for increasing the strength of the steel sheet. In addition, 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 a high concentration in the grain boundaries of the steel sheet during the enameling treatment, which may cause the generation of bubbles, black spots, and the like. In addition, ductility may be reduced. Therefore, the P content is set to 0.1000% or less. Preferably 0.0500% or less.

S：0.0010～0.0500％

S is an element forming MnS. The sulfide acts as a precipitation site of BN, and contributes to improvement of the fishscale resistance. In addition, MnS itself has an effect of improving the fishscale resistance. In order to obtain these effects, the S content is set to 0.0010% or more. Preferably 0.0030% or more. However, when the S content becomes excessive, defects due to MnS may occur. Therefore, the S content is set to 0.0500% or less. Preferably 0.0300% or less.

Al：0.007～0.100％

Al is an element that functions as a deoxidizing element. If the Al content is small, the deoxidation 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, ductility is reduced. Therefore, the Al content is set to 0.100% or less. Preferably 0.010-0.060%.

O：0.0005～0.0100％

When the O content is increased, a large amount of iron oxide is produced, which causes a reduction in ductility and a scaly fold defect. From this viewpoint, it is preferable that the O content is as low as possible. However, if the O content is excessively reduced, the production cost increases. Therefore, the content of O is set to 0.0005 to 0.0100%. Preferably 0.0010 to 0.0070%.

B：0.0003～0.0100％

B is contained for the purpose of forming BN having an effect of improving the fishscale resistance of the enamel steel sheet. In addition, B not becoming BN is present as solid solution B, and grain growth in the enamel treatment is suppressed. In order to obtain these effects, the B content needs to be 0.0003% or more. Preferably 0.0005% or more. On the other hand, if the B content becomes excessive, grain growth is significantly suppressed, and ductility is reduced. Therefore, the B content is set to 0.0100% or less. Preferably 0.0030% or less.

N：0.0010～0.0100％

N is an element necessary for producing BN having an effect of improving the fishscale resistance of the enamel steel sheet. In order to obtain this effect, the N content is set to 0.0010% or more. On the other hand, if the N content becomes excessive, ductility decreases. Therefore, the N content is set to 0.0100% or less. Preferably 0.0070% or less.

Ti：0～0.0100％

Ti is an element which easily forms nitrides, and is an element which inhibits the generation of BN which exerts an effect on the fishscale resistance. Therefore, it is desired to be contained as little as possible. Therefore, the content of Ti is set to be in the range of 0 to 0.0100%. Preferably 0.0050% or less. However, if the Ti content is 0.0003% or less, the production cost may increase. Therefore, the lower limit value in actual production may be set to 0.0003%. 1 or a total of 2 or more of Nb, Zr, V, Mo and W: 0.0020 to 0.0300%

These elements form fine carbides and inhibit grain growth. By containing these elements, grain growth at the time of enamel treatment is suppressed, and a decrease in strength is suppressed. However, if these elements are contained excessively, ductility is reduced. Therefore, the total content of 1 or 2 or more of these elements is 0.0020 to 0.0300%. Preferably 0.0030 to 0.0200%.

In the present embodiment, the following elements may be contained as necessary in addition to the above elements. Since these elements may not be contained, the lower limit is 0%.

Cu：0～0.045％

Cu may be contained for controlling the reaction of the glass and the steel in the enamel treatment. In order to obtain the above-described effects, 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 glass and the steel is inhibited, but also the workability may be deteriorated. Therefore, in order to avoid such adverse effects, the Cu content is preferably set to 0.045% or less.

1 or more of Cr and Ni: the total content is 0-1.000%

Cr and Ni have an effect of improving the adhesion between the steel sheet and the enamel layer, and therefore they may be contained. When the total content of Cr and Ni is 0.005% or more, the effect of improving the adhesion between the steel and the enamel layer becomes remarkable, and therefore, this is preferable. More preferably 0.010% or more. On the other hand, if the total content of Cr and Ni exceeds 1.000%, the effect of improving the adhesion is saturated, and the mechanical properties are also degraded. When Cr and Ni are contained, the effect can be expected to some extent even if 0.500% or less is contained. Therefore, when Cr and Ni are contained, the total content is set to 0.005 to 1.000%. Preferably 0.010 to 0.500%.

More than 1 of As, Se, Ta, Sn, Sb, Ca, Mg, Y and REM: the total is 0-0.1000%

These elements form oxides when contained in a trace amount, and have an effect of improving the fishscale resistance. However, if the content is excessive, a large amount of oxide precipitates. This oxide becomes a starting point of fracture during deformation, and therefore ductility is reduced. Therefore, the content of 1 or more of these elements is preferably 0 to 0.1000% by total amount. More preferably 0.0005 to 0.1000%. More preferably 0.0025 to 0.0500%. REM is 1 or 2 or more of lanthanoid elements having atomic numbers of 57 to 71 in the periodic table.

Further, by satisfying the following formulae (1) to (3), the fishscale resistance is further improved, and the strength reduction at the time of enamel treatment is further suppressed.

Ti < (N-0.0003). times.3.43. formula (1)

As described above, Ti is an element that easily forms nitrides, and even when Ti is contained, N for forming BN that improves the fishscale resistance needs to remain. Therefore, the Ti content is limited to the range of formula (1).

In order to suppress the decrease in strength during the enamel treatment, it is necessary to have solid solution C or iron carbide as described above. In order to obtain such effects, it is necessary that solid solution state C remains even when alloy carbides of Ti, Nb, V, Zr, Mo, and W are formed. Therefore, the C content is limited to the range of formula (2).

[ N as BN ]/(1.27 XB) < 0.95. formula (3)

B is contained for forming BN and improving the fishscale resistance, but if solid solution B remains, it produces an effect of suppressing grain growth during the enameling treatment and suppressing a decrease in strength. Therefore, all B contained is not precipitated as BN. Since [ N as BN ] indicating the N content contained in BN can be quantified by chemical analysis, the state of BN production is defined using this value, and the range of BN precipitation amount effective for inhibiting grain growth is defined in formula (3). [ N as BN ] was determined from the steel extraction residue (bromomethanol method).

< Metal texture >

The microstructure of the steel sheet according to the present embodiment contains ferrite, cementite, and/or pearlite, and the microstructure mainly contains ferrite. More specifically, the steel sheet according to the present embodiment has a metal structure including: ferrite; cementite within the grains of ferrite; and cementite and/or pearlite at the grain boundaries of ferrite. Further, the alloy may further contain 1 or more of carbide, nitride, and oxide other than cementite. Since ferrite has excellent ductility, the steel sheet according to the present embodiment can realize excellent workability by using ferrite as a main phase. Further, if cementite or pearlite is present in the metal structure, the fishscale resistance, which is a necessary characteristic of the steel sheet for enamel, is improved. This is considered to be due to the trapping of hydrogen generated in the enamel treatment by the interface of ferrite and cementite. On the other hand, if cementite or pearlite is present, hydrogen generated in the enamel treatment is conceivably released as hydrocarbon gas to the outside of the steel sheet. In this case, the bubble defect is also caused. Therefore, the size and number density of the cementite and pearlite contained therein need to be limited.

First, the number density of cementite having a grain size of 0.3 to 1.5 μm is 1.00X 10^-1Per mu m²The following. Cementite precipitated finely in ferrite grains is dissolved in the enamel treatment and released as carbon monoxide or carbon dioxide gas, thereby generating a bubble defect. Therefore, it is necessary to limit the number of fine intragranular carbides existing in ferrite grains to 1.00 × 10^-1Per mu m²The following. The intraparticle cementite having a particle diameter of more than 1.5 μm is not particularly limited since it is harmless. In addition, cementite having a particle size of less than 0.3 μm has little effect on the fishscale resistance even if a bubble defect occurs. Therefore, the number density was evaluated by measuring the intraparticle cementite having a particle diameter of 0.3 to 1.5 μm. The particle size of one cementite is the average of the major and minor diameters.

Next, cementite and/or pearlite present at the ferrite grain boundaries is present in the diffusion path of hydrogen during the enamel treatment, and therefore, hydrogen is trapped, and the fishscale resistance is improved. The average value of the major diameters of these cementite and/or pearlite is limited to 0.5 to 15 [ mu ] m, and the number density of cementite and pearlite is limited to 5.00X 10^-4～1.00×10^-1Per mu m². When the average value of the major diameters of cementite and pearlite is less than 0.5 μm, the effect of improving the fishscale resistance is small. In addition, the glass is easily dissolved in the enamel treatment and is released as carbon monoxide or carbon dioxide gas, which causes bubble defects. On the other hand, when the average major axis exceeds 15 μm, this becomes a starting point of fracture during processing, and ductility is lowered. Therefore, the average of the major axis is set to 0.5 to 15 μm.

In addition, the number density is less than 5.00 multiplied by 10^-4Per mu m²In the case of (2), the effect of improving the fish scaling resistance is not observed, and the number density exceeds 1.00X 10^-1Per mu m²In the case of (2), the fracture starts at the time of deformation, and ductilityAnd decreases. Therefore, the number density of cementite and/or pearlite existing in the ferrite grain boundary is set to 5.00 × 10^-4～1.00×10^-1Per mu m². Either or both of cementite and pearlite may be present. The cementite as used herein means cementite which is not contained in the pearlite structure, unlike the lamellar cementite contained in pearlite.

Cementite and pearlite appeared as black contrasts when the steel sheet was subjected to picral alcohol solution (picral) etching after grinding the rolling direction cross section and observed with an optical microscope. As a representative point of the steel sheet structure, a portion (1/4t) distant from 1/4 whose surface has a sheet thickness t in the sheet thickness direction was observed. In addition, since ferrite grain boundaries can appear by adjusting the degree of bitter alcoholic solution corrosion, the relationship between the observed positions of cementite and pearlite and the grain boundaries can be determined. The observation is preferably performed at a magnification of 400 to 1000 times. When cementite precipitated at the grain boundaries is connected at the grain boundary triple point, the lengths of cementite precipitated at the sides of each grain boundary are measured and added. In the case of pearlite, the pearlite may be surrounded by a plurality of ferrite grains, but in this case, the number of pearlite is measured assuming that the pearlite exists in ferrite grain boundaries. FIG. 1 is a schematic view showing an example of the measurement. The number density of cementite and pearlite described above is a value obtained by dividing the observed number by the observed area, and the unit is unit/μm²。

For example, in fig. 1, cementite a exists in 1 grain boundary between two ferrite grains, and the length La along the grain boundary is defined as the major axis. Cementite b exists along two grain boundaries formed by 3 ferrite grains, and the total value (Lb1+ L b2) of lengths Lb1 and Lb2 along each grain boundary is defined as the major axis. The cementite c exists along 3 grain boundaries consisting of 4 ferrite grains, and the total of the lengths Lc1 to Lc3 (Lc1+ Lc2+ Lc3) along each grain boundary is defined as the major axis. The cementite d is present along 3 grain boundaries formed by 3 ferrite grains, and the total length (Ld1+ Ld2+ Ld3) of the lengths Ld1 to Ld3 along each grain boundary is defined as the major axis. Pearlite e to i, the maximum major axis Le to Li are defined as major axes.

The ferrite average crystal grain size in the steel sheet structure before the enamel treatment may be 30.0 μm or less at a position (1/4t) apart from 1/4 where the surface has a sheet thickness t in the sheet thickness direction. By setting the average crystal grain size to 30.0 μm or less, the strength of the steel sheet can be increased. Preferably 20.0 μm or less, and more preferably 15.0 μm or less. In order to achieve higher strength, it is desirable that the average crystal grain size is small, but workability deteriorates as the average crystal grain size becomes smaller. Therefore, it is necessary to determine the most suitable crystal particle size for a desired shape of an article.

The average crystal grain size of ferrite was determined by JIS G0551: 2013 appendix B, the average crystal area per 1 crystal grain was determined by the method using a square test line, and the average crystal area was calculated as the equivalent circle diameter (circle-equivalent diameter). That is, when the average crystal area is represented as a, the average crystal particle diameter d is represented by the following formula (4).

d＝2(a/π)^1/2The type (4)

< manufacturing method >

A preferred method for producing 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.

The key point in the production of the steel sheet according to the present embodiment is the control of the precipitation state of cementite and pearlite and the control of the precipitation state of BN in the steel sheet. As described above, by limiting the number density of fine cementite precipitated in ferrite grains, while controlling the size and number density of cementite and pearlite formed in ferrite grain boundaries, the fishscale resistance can be improved and the blister defects can be suppressed. Further, precipitation of BN and remaining of solid solution B are achieved by controlling the precipitation state of BN, whereby the fishscale resistance is improved, and the decrease in strength is suppressed by suppressing grain growth during the enameling treatment.

The slab heating temperature during hot rolling is preferably 1000 to 1300 ℃, the final working temperature of hot rolling is preferably Ar3 to 1000 ℃, the reduction ratio of Ar3+100 ℃ or less is preferably more than 25%, the rolling finishing temperature is preferably Ar3 ℃ or more, and the coiling temperature is preferably 500 to 800 ℃.

When the slab is heated at a temperature of less than 1000 ℃, BN is easily produced, and the content of B remaining as solid solution B may decrease. The upper limit of the slab heating temperature is not particularly limited, but for economic reasons, it is desirable to set the temperature to about 1300 ℃.

When the finish working temperature of hot rolling is less than Ar3 ℃, ferrite is generated during rolling, and phase transformation does not occur during cooling after rolling, so that the portions thereof become coarse grains, and grain unevenness may occur. In addition, when the final processing temperature exceeds 1000 ℃, the cost for reducing the temperature up to the coiling temperature is large and uneconomical, so that the final processing temperature is preferably in the range of Ar3 to 1000 ℃.

When finish rolling is performed, Ar3 is estimated using a prediction formula based on the steel composition shown in the following formula (a). The rolling conditions are set based on Ar3 predicted by this method.

Ar3(℃)＝901-325×C-92×Mn+33×Si+287×P+40×Al-30(a)

Wherein the symbol of the element (C, Mn, Si, P, Al) in the formula (a) represents the content (mass%) of the element.

Whether or not the finish rolling temperature is actually less than Ar3 ℃ can be confirmed by observing the microstructure of the rolled sheet and checking the presence or absence of coarse grains by hot rolling while changing the finish rolling temperature in actual operation. Coarse particles occur at a portion where the final processing temperature is less than Ar3 ℃, mainly at the end portion and the surface layer of the steel sheet. The average grain size is 1.5 times or more the average grain size at the center of the plate width and at the center of the plate thickness.

The coiling temperature is not particularly limited, but when the coiling temperature is less than 500 ℃, the size of cementite and pearlite generated during hot rolling may be reduced, and carbide after cold rolling and annealing may be affected. Therefore, it is preferably 500 ℃ or higher. Further, when a production line without an overaging step is used in the continuous annealing in the subsequent step, the coiling temperature is desirably 550 ℃ or higher. Further, when the coiling temperature exceeds 800 ℃, the scale formed on the surface becomes thick, and the cost in the pickling in the subsequent step increases. Therefore, 800 ℃ or lower is desirable.

The reduction (cumulative reduction) of Ar3+100 ℃ or lower during hot rolling is set to more than 25%. When the rolling reduction in the temperature range of Ar3+100 ℃ or lower is 25% or lower, the effect of accumulated strain becomes small, the γ grain boundaries that become nucleation sites for ferrite transformation and ferrite-pearlite transformation that occur after finish rolling become small, and the density of cementite and pearlite formation becomes sparse and large. When such a hot-rolled steel sheet is used, it is considered that the density of cementite and/or pearlite precipitates in the grain boundaries after cold rolling annealing is reduced. When the reduction ratio of Ar3+100 ℃ or lower is 25% or lower, it is considered that the grain size of the hot-rolled steel sheet becomes coarse and the r value decreases. In order to ensure the rolling formability, it is preferable that the r value in the rolling direction after the cold rolling and annealing or both the r value in the rolling direction and the r value in the direction perpendicular to the rolling direction (hereinafter referred to as perpendicular direction) be 0.8 or more, and in order to achieve this, it is necessary to set the reduction ratio of Ar3+100 ℃ or less to more than 25%.

After hot rolling, pickling or the like is performed to remove scale formed on the surface, but the method and conditions are not particularly limited.

The hot-rolled steel sheet after hot rolling is cold-rolled. The reduction ratio (cold rolling ratio) in the cold rolling is not particularly limited, and rolling may be performed under conditions suitable for each cold rolling mill. The rolling reduction is usually preferably 50 to 90%.

The cold-rolled steel sheet is continuously annealed. 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 ℃. When annealing is performed at a temperature of 700 ℃ or higher, the amount of fine cementite in the grains is dissolved and reduced, and the amount of precipitation can be controlled to such an extent that no bubble defect occurs. If the annealing temperature is less than 700 ℃, dissolution of cementite becomes insufficient. On the other hand, when annealing is performed at a temperature exceeding 850 ℃, iron carbide is excessively dissolved, and cementite and pearlite having a size effective for the fishscaling resistance are less likely to remain.

As for the temperature increase rate, if the temperature increase rate from 650 ℃ at which the dissolution of the iron carbide occurs to the annealing temperature is too large, the dissolution of the iron carbide is small, and a large amount of fine intra-granular carbides remain, so that the bubble defect is likely to occur. Therefore, the temperature increase rate from 650 ℃ to the annealing temperature is preferably 50 ℃/s or less. In the continuous annealing, although decarburization annealing in which the dew point in the atmosphere is raised is sometimes performed by OCA (Open Coil annealing) in the method of manufacturing a steel sheet for enamel, decarburization annealing is not performed in the present embodiment. This is because the decarburization annealing reduces the carbon concentration in the steel and eliminates carbide, so that the steel sheet according to the present embodiment cannot maintain the desired carbide state. In this case, the grain growth of ferrite cannot be suppressed, and a sufficient strength may not be obtained. For example, annealing is performed in an atmosphere containing 3% hydrogen by volume and the remainder being nitrogen, and having a dew point of-40 ℃.

When the overaging treatment is performed after the continuous annealing, it is desirable to keep the temperature in the range of 200 to 500 ℃ for 20 seconds or more. In this case, cementite at the grain boundary of ferrite grains grows, and the fishscale resistance can be improved. The coiling temperature in hot rolling in the case of overaging is preferably 500 ℃ or higher as described above. When the temperature of the overaging treatment is less than 200 ℃, the effect of growth of cementite at the grain boundary is insufficient, and when it exceeds 500 ℃, the growth of cementite at the grain boundary becomes large, and the growth of cementite at the grain boundary becomes too large. When the overaging treatment is not performed, it is desirable that the coiling temperature during hot rolling is 550 ℃ or higher.

Then, temper rolling is performed for the main purpose of shape control. In temper rolling, strain is introduced into a steel sheet by the temper rolling rate while controlling the shape. In this case, if the temper rolling reduction is increased, that is, if the amount of strain introduced into the steel sheet is increased, abnormal grain growth during welding or enameling is promoted. Therefore, the temper rolling reduction is not desirable to apply strain more than necessary, with the upper limit being a rolling reduction at which shape control can be achieved. From the viewpoint of shape control, the reduction ratio in temper rolling is preferably 2% or less.

As described above, a cold-rolled steel sheet having desired characteristics can be obtained. The obtained steel sheet can be used as a steel sheet for enameling which is a substrate for enameling.

The steel sheet according to the present embodiment is processed into a predetermined shape, assembled into a product shape by welding or the like, and subjected to an enameling treatment (firing treatment) to produce an enameled product. The enamel treatment may be performed, for example, by heating the steel sheet coated with the enamel to a predetermined temperature and holding the temperature for a predetermined time to cause the vitreous material of the enamel to adhere to the steel sheet. Preferable conditions for the firing treatment of the steel sheet according to the present embodiment include, for example: the firing temperature is 750-900 ℃, and the firing time is 1.5-10 minutes (in a furnace). Further, the firing may be repeated several times for the purpose of secondary coating and repair. By performing the firing treatment under such conditions, grain growth in the enamel treatment can be suppressed by solid-solution C and iron carbide, and the strength can be suppressed from decreasing. The conditions of the firing treatment described here are merely examples, and are not intended to limit the conditions of the enamel treatment of the steel sheet according to the present embodiment.

Examples

Steels having chemical compositions (balance Fe and impurities) shown in tables 1-1A to 1-3B and tables 1-4A to 1-4B were melted in a converter and formed into slabs by continuous casting. These slabs were manufactured into steel sheets under the conditions described in table 2. That is, the slab is heated, rough rolled, finish rolled, and coiled to produce a hot-rolled steel sheet. Then, the hot-rolled steel sheet was pickled, and cold-rolled to a rolling reduction, to thereby prepare a cold-rolled steel sheet, and further, after continuous annealing was performed in an atmosphere containing 3% by volume of hydrogen and the balance of nitrogen and having a dew point of-40 ℃, temper rolling was performed, to thereby prepare a steel sheet having a thickness of 0.8 mm. In order to make the thickness after temper rolling constant, the thickness of the hot-rolled steel sheet is changed with respect to the rolling reduction of cold rolling. Some of the steel sheets were subjected to overaging treatment after annealing.

Further, Ar3 was calculated by the above formula (a), and the reduction ratio of Ar3+100 ℃ or lower (Ar3 or higher) was set using this value. In the production methods No. C1 to C13, the reduction ratio of Ar3+100 ℃ or lower was targeted at 30% or more, and in the production method No. C14, the reduction ratio was targeted at 25%. The reduction ratios were substantially as shown in tables 3-1 to 3-4.

Further, the relationship with the Ar3 point was confirmed by observing the microstructure of the hot-rolled steel sheet, based on the presence or absence of coarse particles. Specifically, a crystal grain having an average grain size of 1.5 times or more the average grain size at the center of the plate thickness and having an average grain size at the center of the plate width is determined as a coarse grain. The hot-rolling finish temperatures of the production methods No. C1 to C14 shown in Table 2 are all considered to be in the range of Ar3 to 1000 ℃. The heating rate in table 2 is a heating rate from 650 ℃ to the annealing temperature.

The steel sheets manufactured as described above were subjected to characteristic evaluation by various methods as described below.

< mechanical Property >

Mechanical properties, according to JIS Z2241: 2011, tensile test was carried out using JIS5 test specimens, and tensile strength (Rm) and elongation at break (a) were measured. From the viewpoint of strength, a sample having a tensile strength of 300MPa or more is judged to have sufficient strength, and from the viewpoint of moldability, a sample having an elongation at break of 30% or more is judged to be excellent in moldability.

In addition, according to JIS Z2254: 2008, the r-value (plastic strain ratio) in the case where the sample was prepared in parallel to the rolling direction and in orthogonal to the rolling direction was measured. As a result of the measurement, r values in both the rolling direction and the perpendicular direction were 0.8 or more, except for d38 described later.

< Observation of metallic Structure (ferrite, cementite, pearlite) >

Precipitates in steel were measured for cementite present in ferrite grains, cementite present in grain boundaries, and/or pearlite present in grain boundaries by grinding a cross section parallel to the cold rolling direction, followed by picric alcohol solution etching and observation with an optical microscope. That is, the rolling direction cross section of the steel sheet is polished and then subjected to alcohol solution etching with bitter taste. As a representative point of the steel sheet structure (metal structure), the steel sheet structure is oriented in the sheet thickness directionThe portion at the position (1/4t) where the upper distance surface was 1/4 of the plate thickness t was observed. Cementite and pearlite appear as a black contrast when observed with an optical microscope. Further, the degree of bitter alcoholic solution corrosion was adjusted to cause ferrite grain boundaries to appear, and the relationship between the observed positions of cementite and pearlite and the grain boundaries was determined. The observation is performed at a magnification of 400 to 1000 times. When cementite precipitated at the grain boundaries is connected at the grain boundary triple point, the lengths of cementite precipitated at the sides of each grain boundary are measured and added. In the case of pearlite, the pearlite may be surrounded by a plurality of ferrite grains, but in this case, the number of pearlite is measured assuming that the pearlite exists in ferrite grain boundaries. FIG. 1 is a schematic view showing an example of the measurement. The number density of cementite and pearlite is a value obtained by dividing the observed number by the observed area, and the unit is unit/μm²。

D1-D89 and D1-D46 both containing ferrite as a metal structure; cementite in the grains of ferrite; and cementite and/or pearlite at the grain boundaries of ferrite.

The average crystal grain size of ferrite was determined by JIS G0551: 2013 the average crystal area per 1 crystal grain was obtained by the method using the square test line described in appendix B, and calculated as the equivalent circle diameter. That is, when the average crystal area is represented as a, the average crystal particle diameter d is a value represented by the following formula (5).

d＝2(a/π)^1/2The type (5)

< Strength characteristics after enameling >

In addition, the decrease in strength caused by grain growth after the enamel treatment was evaluated. Specifically, a steel sheet subjected to cold rolling with a reduction of 10% for the pseudo press working was subjected to a heat treatment simulating an enamel treatment at a furnace temperature of 830 ℃ for 4 minutes, and the tensile strength was determined by a tensile test in the same manner as described above, 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 strength decrease after the enamel treatment was suppressed.

Further, the enamel characteristics were examined as follows.

< resistance to fishscaling >

Regarding the fishscale resistance, 100 μm glaze was dry-coated by a powder electrostatic coating method using a steel sheet having a size of 100X 150mm, and the obtained sample was subjected to firing at an oven temperature of 830 ℃ for 5 minutes in the air, and evaluated. A fish scaling acceleration test was performed in which the enameled steel sheet was left in a constant temperature bath at 160 ℃ for 10 hours, in which a: excellent, B: excellent, C: in general, D: the scale explosion occurrence was visually judged on the 4-point scale with problems, and if A, B, C, it was judged that a predetermined scale explosion resistance was secured, and the evaluation D was judged to be a failure. Specifically, A is the case where no scale explosion occurs, B is the case where 1-5 scale explosions occur, C is the case where 6-15 scale explosions occur, and D is the case where 15 or more scale explosions occur.

Sealing of enamel

The enamel adhesion was evaluated by dropping a 2kg ball weight from a height of 1m 3 times, measuring the enamel peeling state of the deformed portion with 169 contact pins, and using the area ratio of the non-peeled portion, so that there was no difference in adhesion. When the area ratio of the non-peeled portion was 40% or more, it was judged that the enamel adhesion was sufficient.

< appearance >

Appearance after the enamel treatment, the state of bubbles and black spots was observed by visual observation of the enamel-treated steel sheet in the same manner as described above, and the ratio of a: very excellent, B: excellent, C: in general, D: slightly poor, E: the evaluation was performed in 5 steps of the significant difference, and if A, B, C, D is the result, it was judged that a predetermined appearance was obtained, and the case of the significant difference E evaluation was judged to be a failure.

The evaluation results are shown in tables 3-1 to 3-4. In nos. D1 to D89, the steel composition, the precipitated state of carbide, and the precipitated state of BN were within the ranges of the present invention, and exhibited good characteristics.

No. d1 has a low C content, and No. d2 has an excessive C content, and therefore, the mechanical properties are inferior.

No. d3 has a low Si content, and No. d4 has an excessive Si content, and therefore, the mechanical properties are inferior.

No. d5, the Mn content of the steel sheet is small, and thus the fishscale resistance is lowered.

No. d6, the Mn content of the steel sheet was excessive, and thus the mechanical properties were inferior.

No. d7 shows a low P content in the steel sheet, and No. d8 shows an excessive P content, resulting in poor mechanical properties.

In No. d9, the steel sheet had a low S content, and thus the fishscale resistance was low.

No. d10 has a low Al content in the steel sheet, and No. d11 has an excessive Al content, and therefore, the mechanical properties are inferior.

No. d12, the steel sheet had a low B content and the fishscale resistance was low. In addition, No. d13, B content is excessive, and thus mechanical properties are inferior.

No. d14, the N content of the steel sheet is small, and thus the fishscale resistance is reduced.

No. d15, the N content of the steel sheet was excessive, and thus the mechanical properties were inferior.

No. d16, the Ti content of the steel sheet was excessive, and thus the fishscale resistance was lowered.

In nos. d17 to d20, the content of group a elements (Nb, Zr, V, Mo, W) did not satisfy the scope of the invention, and the content of group B elements (Cr, Ni) of d21 steel sheets did not satisfy the scope of the invention, and therefore, the mechanical properties were inferior.

In nos. d22 and d23, the chemical composition of the steel sheet does not satisfy the formula (1), and thus the fishscale resistance is lowered.

No. d24 and d25, the steel sheet had poor mechanical properties because the chemical composition did not satisfy the formula (2).

No. d26 to d37 show that, although the steel composition is within the range of the present invention, the production conditions are out of the preferable range, and therefore the precipitated state of carbide and the precipitated state of BN are out of the range of the present invention, and good mechanical properties and enamel properties are not obtained.

In nos. d26 and d29, the heating temperature of the slab was low, BN was easily formed, the content of B remaining as solid solution B was low, and the formula (3) was not satisfied, and the mechanical properties were poor.

In nos. d27 and d30, the coiling temperature after hot rolling was low, the sizes of cementite and pearlite formed during hot rolling were small, the number density of cementite in ferrite grains became excessive, and the appearance was poor.

In No. d28, the overaging temperature was high, the cementite at the grain boundary grew large, and the cementite at the grain boundary became too large, so that the number density of cementite and pearlite at the ferrite grain boundary was insufficient, and the fishscale resistance was lowered.

In No. d31, the heating rate during annealing exceeded the upper limit, and d32, the annealing temperature was too low, so the number density of cementite in ferrite grains became excessive, and the appearance was poor.

In nos. d33 and d36, the coiling temperature was high, and the annealing temperature was too high for d34, so that the number density of cementite and pearlite at the ferrite grain boundary was insufficient, and the fishscale resistance was lowered.

No. d35 had a low coiling temperature, and the sizes of cementite and pearlite formed during hot rolling were reduced, and the number density of cementite in ferrite grains was excessive, resulting in poor appearance.

In No. d37, the overaging temperature was low, cementite at the grain boundary did not grow, the number density of cementite and pearlite in the predetermined range was not more than the lower limit, and the fishscale resistance was poor.

In No. d38, the reduction ratio in the temperature range from (Ar3+ 100). degree.C.to Ar3 was insufficient, and the grain boundary number density of cementite and pearlite became small. The r value in the rolling direction is less than 0.8 and is low.

In Nos. d39 to d46, the mechanical properties were inferior because the content of the group C elements (As, Se, Ta, Sn, Sb, Ca, Mg, Y, REM) did not satisfy the range of the invention.

From the results of tables 3-1 to 3-4, it was confirmed that: within the scope of the steel of the present invention, it is possible to provide a steel sheet for enameling excellent in enamel adhesion, appearance such as bubble generation, and fishscale resistance, and capable of suppressing a decrease in tensile strength after the enameling treatment.

TABLE 1-1A

Tables 1-1B

Underlining is outside the scope of the present invention.

Tables 1-2A

*1: total amount of Nb, Zr, V, Mo and W

*2: total amount of Cr and Ni

Tables 1-2B

*1: total amount of Nb, Zr, V, Mo and W

*2: total amount of Cr and Ni

Underlining is outside the scope of the present invention.

Tables 1 to 3A

Tables 1 to 3B

Underlining is outside the scope of the present invention.

Industrial applicability

The steel sheet according to the above aspect of the present invention is excellent in formability, fishscale resistance after enamel treatment, and strength characteristics when applied to kitchen supplies, building materials, energy fields, and the like after enamel treatment. Therefore, the steel sheet is suitable for enamel steel sheets and has high industrial applicability.

Claims

1. A steel sheet characterized by having a chemical composition containing, in mass%

C：0.0050～0.0700％、

Si：0.0010～0.0500％、

Mn：0.0500～1.0000％、

P：0.0050～0.1000％、

S：0.0010～0.0500％、

Al：0.007～0.100％、

O：0.0005～0.0100％、

B：0.0003～0.0100％、

N：0.0010～0.0100％、

Ti：0～0.0100％、

1 or 2 or more of Nb, Zr, V, Mo, W: 0.0020-0.0300% in total,

Cu：0～0.045％、

1 or 2 of Cr and Ni: 0 to 1.000 percent in total,

1 or more than 2 of As, Se, Ta, Sn, Sb, Ca, Mg, Y and REM: 0 to 0.1000% in total,

the balance of the Fe and impurities are contained,

satisfies the formula (1) and the formula (2),

as the metal structure, contains

Ferrite;

cementite within the grains of ferrite; and

1 or 2 kinds selected from among cementite and pearlite at grain boundaries of the ferrite,

in the grains of the ferrite, the number density is 1.00 x 10^-1Per mu m²Cementite having a particle diameter of 0.3 to 1.5 μm is present in the range below,

the ferrite grain boundary has an average length of 0.5 to 15 [ mu ] m and a number density of 5.00 x 10^-4～1.00×10^-1Per mu m²1 or 2 kinds selected from among cementite and pearlite,

the relation between the N content [ N as BN ] contained in BN and the B content contained in steel satisfies the formula (3),

ti < (N-0.0003) × 3.43 … formula (1)

C > 0.25 XTi +0.129 XNb +0.235 XV +0.132 XZr +0.125 XMo +0.0652 XW +0.0040 … formula (2)

[ N as BN ]/(1.27 XB) < 0.95 … formula (3)

2. The steel sheet according to claim 1, characterized by comprising, in mass%

Cu：0.010～0.045％。

3. The steel sheet according to claim 1 or 2, characterized by comprising, in mass%

1 or 2 of Cr and Ni: the total amount is 0.005-1.000%.

4. The steel sheet according to any one of claims 1 to 3, characterized by comprising, in mass%

5. The steel sheet according to any one of claims 1 to 4, wherein the steel sheet is a cold-rolled steel sheet.

6. The steel sheet according to any one of claims 1 to 5, wherein the steel sheet is a steel sheet for enameling.

7. An enamel product comprising the steel sheet according to any one of claims 1 to 4.