Steel sheet

Abstract

The present invention provides a steel sheet which exhibits excellent chemical conversion coating properties and corrosion resistance after coating even when containing a tramp element. A steel sheet according to the present invention contains a specific amount of Sn, and Zn is adhered to at least one surface of the steel sheet in an amount WZn that satisfies [Sn]×400+18 ≤ WZn ≤ 800.

Description

steel plate

[0001] The present invention relates to steel sheets, and more particularly to steel sheets with excellent chemical treatment properties and corrosion resistance after painting.

[0002] There are two methods for manufacturing steel plates: one using blast furnace steel produced in a blast furnace using iron ore as the main raw material, and the other using electric arc furnace steel produced in an electric arc furnace using iron scrap as the main raw material. When steel is produced in a blast furnace, a large amount of coke is used, resulting in high emissions of carbon dioxide, a greenhouse gas, and raising concerns about its impact on global warming. On the other hand, when steel is produced in an electric arc furnace, although electrical energy is required to melt the iron scrap, it does not require the use of coke, which has the advantage of reducing carbon dioxide emissions.

[0003] However, the iron scrap used as raw material contains trump elements such as Sn, Cu, Ni, Cr, and Mo, making it difficult to produce high-quality steel sheets. These trump elements can negatively affect the mechanical properties of the manufactured steel sheets and may also reduce their chemical treatment properties.

[0004] Chemical conversion treatment is a chemical surface treatment used to improve the properties of steel sheets, such as corrosion resistance and paintability. If the chemical conversion treatment properties are reduced, the corrosion resistance of the treated steel sheet surface after painting will decrease. Therefore, in order to obtain good corrosion resistance after painting, the steel sheet must have excellent chemical conversion treatment properties. One method of chemical conversion treatment is zinc phosphate chemical conversion treatment. Zinc phosphate chemical conversion treatment has been widely used for a long time because it provides excellent corrosion resistance and other properties to the surface of steel sheets. In this zinc phosphate chemical conversion treatment, surface preparation is usually performed as a pretreatment to ensure good film formation. Surface preparation agents come in liquid and solid forms, but liquid surface preparation agents are the most common. Therefore, it is required that the zinc phosphate chemical conversion treatment properties are high after surface preparation using at least a liquid surface preparation agent (liquid surface preparation).

[0005] Furthermore, Zr chemical conversion treatment is also becoming more widespread. Zr chemical conversion treatment has environmental advantages such as being phosphoric acid-free and low-sludge, and its use is expected to expand in the future. Therefore, in addition to the zinc phosphate chemical conversion treatment properties mentioned above, good Zr chemical conversion treatment properties are also required for steel sheets.

[0006] As a method for improving the chemical conversion treatment properties of steel sheets manufactured from electric furnace materials containing trump elements, for example, Patent Document 1 describes an automotive steel sheet containing 0.10% to 0.50% Cu, in which the particle size of copper compounds exposed on the surface is set to 2 μm or less to improve chemical conversion treatment properties. Patent Document 2 also describes a hot-rolled sheet in which the chemical conversion treatment properties are improved by controlling the shape and density of metal oxide particles and copper compound particles on the surface. However, these techniques require control of the process of oxidizing the steel billet before hot rolling. Furthermore, since both techniques target hot-rolled sheets, they cannot be applied to improve the chemical conversion treatment properties of cold-rolled sheets. Moreover, no consideration is given to Zr chemical conversion treatment properties.

[0007] On the other hand, as a method for improving the chemical conversion treatment properties of steel sheets manufactured from blast furnace materials, for example, Patent Document 3 describes a method for improving the adhesion amount to the surface of the steel sheet to 10 to 2000 mg / m². ２ A technology is described that achieves both mold galling resistance and chemical conversion treatment properties by having Zn and providing a predetermined crystal orientation. Furthermore, Patent Document 4 describes a method for achieving an adhesion amount of 100 to 5000 mg / m² on the surface of a cold-rolled steel sheet. ２ A method for manufacturing cold-rolled steel sheets with excellent chemical conversion treatment properties and corrosion resistance after painting is described, which involves electroplating with Zn and then contacting the sheet with an aqueous solution containing P. However, these techniques aim to improve the chemical conversion treatment properties of steel sheets containing Si and Mn, and do not take into account the influence of tramp elements specific to electric furnace materials. Furthermore, the effect on Zr chemical conversion treatment is not considered.

[0008] Japanese Patent Publication No. 2020-84238, Japanese Patent Publication No. 2020-84325, Japanese Patent Publication No. 2006-299351, Japanese Patent Publication No. 2012-167362

[0009] The present invention has been made to improve the above problems, and an object thereof is to provide a steel sheet having excellent chemical conversion treatment properties and corrosion resistance after painting even when it contains Trump elements like electric furnace materials made from iron scrap. More specifically, even for a steel sheet containing Trump elements, it is an object to achieve both excellent zinc phosphate chemical conversion treatment properties and Zr chemical conversion treatment properties, and also to make the corrosion resistance (corrosion resistance after painting) of the steel sheet after being painted on these chemical conversion treatment films excellent. Even more specifically, an object is to provide a steel sheet that ensures high levels of both at least zinc phosphate chemical conversion treatment properties involving liquid surface conditioning and Zr chemical conversion treatment properties, and also has high corrosion resistance after painting through at least the zinc phosphate chemical conversion treatment involving liquid surface conditioning.

[0010] To solve the above problems, the inventors of the present invention intensively studied the influence of Trump elements on chemical conversion treatment properties. And it was revealed that especially the incorporation of Sn among Trump elements deteriorates the chemical conversion treatment properties. Furthermore, the inventors of the present invention further intensively studied methods for improving the chemical conversion treatment properties and corrosion resistance after painting of steel sheets containing Sn. As a result, it was found that good chemical conversion treatment properties and corrosion resistance after painting can be obtained by attaching an amount of Zn that satisfies a predetermined relationship according to the amount of Sn in the steel to the surface of the steel sheet.

[0011] The present invention has been made based on the above findings, and its main configuration is as follows. [1] It has a component composition containing Sn: 0.005% by mass or more and 0.200% by mass or less, and Zn is attached to at least one of the surfaces, and the attached amount W of the Zn in terms of the mass of Zn element per one side Ｚｎ (unit: mg / m ２ ) satisfies the following formula (1), and is characterized by a steel sheet. [Sn] × 400 + 18 ≤ W Ｚｎ ≤ 800... (1) Here, [Sn] is the Sn content in the steel.

[0012] [2] The steel sheet according to [1] above, wherein the attached amount W Ｚｎ is 450 mg / m ２ or less.

[0013] [3] The above-mentioned attached amount WＺｎ is 300 mg / m ２ or less, the steel sheet according to [1] above.

[0014] [4] The component composition is in mass %, C: 0.8% or less, Si: 3.0% or less, Mn: 12.0% or less, P: 0.1% or less, S: 0.03% or less, N: 0.015% or less, sol.Al: 1.0% or less, Cu: 1.00% or less, Cr: 1.000% or less, Ni: 1.0% or less, and Mo: 1.0% or less, further containing one or more selected from the group consisting of these, and the balance being Fe and inevitable impurities, the steel sheet according to any one of [1] to [3] above.

[0015] [5] The component composition is in mass %, B: 0.005% or less, Ti: 0.2% or less, Nb: 0.2% or less, V: 0.5% or less, Sb: 0.2% or less, W: 0.5% or less, Zr: 0.1% or less, Ca: 0.005% or less, Mg: 0.005% or less, Co: 0.5% or less, Zn: 0.05% or less, Ta: 0.1% or less, Ce: 0.02% or less, Se: 0.02% or less, Te: 0.02% or less, Ge: 0.02% or less, As: 0.02% or less, Sr: 0.02% or less, Cs: 0.02% or less, Hf: 0.02% or less, Pb: 0.02% or less, Bi: 0.02% or less, and REM: 0.005% or less, further containing one or more selected from the group consisting of these, and the balance being Fe and inevitable impurities, the steel sheet according to [4] above.

[0016] According to the present invention, even when containing a predetermined amount of Sn, it is possible to provide a steel sheet that has good chemical conversion treatment properties in both zinc phosphate conversion treatment and Zr conversion treatment, as well as good corrosion resistance after painting. More specifically, even when containing a predetermined amount of Sn, it is possible to provide a steel sheet that ensures high levels of both zinc phosphate conversion treatment properties accompanied by liquid surface adjustment and Zr conversion treatment properties, and that exhibits high corrosion resistance after painting even after undergoing zinc phosphate conversion treatment accompanied by liquid surface adjustment. Preferably, it is possible to provide a steel sheet that is excellent in both zinc phosphate conversion treatment properties and Zr conversion treatment properties, regardless of whether the zinc phosphate conversion treatment is accompanied by solid surface adjustment or liquid surface adjustment, and that exhibits high corrosion resistance after painting even after undergoing the zinc phosphate conversion treatment. Furthermore, preferably, it is possible to provide a steel sheet that exhibits high corrosion resistance after painting even after undergoing Zr conversion treatment.

[0017] The present invention will be described in detail below. The following description is an example of a preferred embodiment of the present invention, and the present invention is not limited to the embodiments described below. In this specification, "%" represents "mass%" unless otherwise specified. Furthermore, any numerical range expressed using "~" in this specification includes the numbers before and after "~" as the lower and upper limits, respectively. Also, if a unit is attached to only one of the numbers before or after "~", the same unit shall be attached to the other number unless otherwise specified.

[0018] (Steel Sheet) The steel sheet of the present invention contains a predetermined amount of Sn and has a predetermined amount of Zn attached to its surface. By having a predetermined amount of Zn attached to the surface of the steel sheet, the steel sheet can exhibit both excellent zinc phosphate conversion treatment properties and Zr conversion treatment properties, even while containing a predetermined amount of Sn, a typical trump element. Furthermore, the steel sheet that has undergone the conversion treatment can also exhibit excellent corrosion resistance after painting. Since the steel sheet of the present invention is excellent in both zinc phosphate conversion treatment properties and Zr conversion treatment properties, it can be applied to a wide range of fields regardless of the conversion treatment method. Furthermore, it can be applied to both hot-rolled and cold-rolled sheets and has excellent corrosion resistance after painting, making it particularly useful as a steel sheet for automobiles. In addition, since the steel sheet of the present invention is excellent in conversion treatment properties and corrosion resistance after painting even while containing trump elements, it can be particularly useful as an electric furnace material made from iron scrap, and has exceptional industrial effects that are environmentally friendly. The steel sheet of the present invention can be obtained, for example, according to the manufacturing method described later.

[0019] First, we will explain the effect of Sn on the chemical conversion treatment properties of steel sheets, as investigated by the present inventors. The mechanism of the chemical conversion treatment reaction is as follows: When a steel sheet comes into contact with the chemical conversion treatment solution, iron dissolution occurs from the steel sheet, and as a counter-reaction, reduction of hydrogen ions occurs in the chemical conversion treatment solution near the surface of the steel sheet. This increases the pH at the steel sheet interface, and a chemical conversion treatment film is deposited on the surface of the steel sheet. This is a common mechanism for both zinc phosphate and Zr chemical conversion treatments. Here, if Sn is contained in the steel sheet, when the steel sheet comes into contact with the chemical conversion treatment solution, iron dissolution occurs, and the Sn contained in the steel sheet also dissolves to become Sn ions. The dissolved Sn ions are Sn ２＋ +2H ２ O → Sn(OH) ２ +2H ＋ This reaction forms hydroxides. Since hydrogen ions are generated during this process, the increase in pH at the steel sheet interface is prevented. Thus, the presence of Sn in the steel sheet is thought to degrade the chemical conversion treatment properties.

[0020] Based on the reaction mechanism described above, it would seem that this deterioration of chemical conversion treatment properties due to Sn would occur similarly in both zinc phosphate and Zr conversion treatments. However, further investigation by the inventors revealed that the presence of Sn has a significant effect on nucleation, particularly in zinc phosphate conversion treatment, and that the presence of these elements greatly deteriorates the zinc phosphate conversion treatment properties. On the other hand, the effect of Sn on Zr conversion treatment properties was not necessarily as great as in the case of zinc phosphate conversion treatment. This was presumed to be because, in the case of Zr conversion treatment, an amorphous film is formed without nucleation, which is a different film formation process than zinc phosphate conversion treatment. Therefore, it was found that in order to achieve good zinc phosphate conversion treatment properties and Zr conversion treatment properties, and consequently good corrosion resistance after painting, in steel sheets containing Sn, it is necessary to find appropriate conditions for each.

[0021] [Composition] The steel sheet contains Sn: 0.005% or more and 0.200% or less. This assumes a steel sheet made using electric furnace material derived from iron scrap, meaning that the steel sheet contains Sn derived from the scrap. In this invention, a predetermined amount of Zn is attached to the surface of the steel sheet, so even in the presence of Sn, good zinc phosphate conversion treatment properties, Zr conversion treatment properties, and corrosion resistance after painting can be achieved simultaneously.

[0022] Sn: 0.005-0.200% Sn (tin) is a typical trump element contained in iron scrap. Sn content of 0.005% or more exceeds the amount that can be mixed in at an unavoidable impurity level, and as mentioned above, it reduces the chemical treatment properties of steel sheets. In particular, it greatly impairs the zinc phosphate chemical treatment properties. On the other hand, if the amount of Sn exceeds 0.200%, excessive Sn ions are generated during the chemical treatment, excessively raising the pH of the chemical treatment solution, making the chemical treatment difficult. Therefore, the amount of Sn should be 0.200% or less, preferably 0.050% or less. Excessively contained Sn can be adjusted, for example, by mixing it with molten iron produced by the blast furnace method that does not contain scrap. However, in this application, a predetermined amount of Zn is attached, so even if Sn content is, for example, 0.010% or more, over 0.100%, 0.110% or more, and up to 0.200%, the chemical treatment properties are good.

[0023] In addition to the Sn mentioned above, the steel sheet may contain other components without particular restriction, as long as they are elements that are normally present in steel sheets. For example, from the viewpoint of controlling material properties, the steel sheet can suitably contain, in amounts greater than 0%, one or more of the following arbitrary elements: C: 0.8% or less, Si: 3.0% or less, Mn: 12.0% or less, P: 0.1% or less, S: 0.03% or less, N: 0.015% or less, and sol. Al: 1.0% or less, Cu: 1.00% or less, Cr: 1.000% or less, Ni: 1.0% or less, and Mo: 1.0% or less. The remainder of the steel sheet's composition, other than the above elements, can be Fe and unavoidable impurities.

[0024] Furthermore, in addition to the above arbitrary elemental components, or in place of the above arbitrary elements, the steel sheet may contain the following arbitrary elements: B: 0.005% or less, Ti: 0.2% or less, Nb: 0.2% or less, V: 0.5% or less, Sb: 0.2% or less, W: 0.5% or less, Zr: 0.1% or less, Ca: 0.005% or less, Mg: 0.005% or less, Co: 0.5% or less, Zn: 0.05% or less, Ta: 0.1% or less, Ce The following elements can be suitably contained in amounts greater than 0%: 0.02% or less of each: Se: 0.02% or less, Te: 0.02% or less, Ge: 0.02% or less, As: 0.02% or less, Sr: 0.02% or less, Cs: 0.02% or less, Hf: 0.02% or less, Pb: 0.02% or less, Bi: 0.02% or less, and REM: 0.005% or less. The remainder of the elements other than those mentioned above can be Fe and unavoidable impurities.

[0025] C: 0.8% or less. From the viewpoint of controlling the steel structure and obtaining the desired strength, the amount of C can be greater than 0%. To obtain good weldability, it is preferable that the amount of C be 0.8% or less.

[0026] Si: 3.0% or less. Si can be included in amounts exceeding 0% to obtain the effect of increasing the strength of the steel through solid solution strengthening without significantly impairing workability. From the viewpoint of improving hot rolling and cold rolling properties, it is preferable that the Si content be 3.0% or less.

[0027] Mn: 12.0% or less. Mn can be included in amounts exceeding 0% for the purpose of increasing the strength of the steel through solid solution strengthening, as well as for improving hardenability and controlling the microstructure. However, if the amount of Mn exceeds 12.0%, the cost is high relative to the effect, so it is preferable that it be 12.0% or less.

[0028] P: 0.1% or less. Since P is an element that strengthens steel, it can be included in amounts greater than 0%. However, since a high P content reduces weldability, it is preferable that the P content be 0.1% or less.

[0029] S: 0.03% or less. S can be included in amounts exceeding 0% because it has the effect of improving scale detachability during hot rolling. However, S also reduces weldability and bendability, so it is preferable that the amount of S be 0.03% or less.

[0030] N: 0.015% or less. N can be included in amounts exceeding 0% because it forms nitrides in steel, refining the crystal grains and improving toughness. However, since N is also an element that forms nitrides in steel and reduces hot rolling properties, it is preferable that the amount of N be 0.015% or less.

[0031] Sol. Al: 1.0% or less. Sol. Al (acid-soluble aluminum) can be included in amounts exceeding 0% for deoxidation purposes. However, if the amount of sol. Al exceeds 1.0%, the strength of the material will decrease drastically, so it is preferable that the amount of sol. Al be 1.0% or less.

[0032] Cu: 1.00% or less. Cu (copper) can be included in amounts exceeding 0% because it has the effect of improving corrosion resistance. On the other hand, if the amount of Cu becomes too high, surface defects caused by red-hot brittleness, etc., may occur. Therefore, it is preferable that the amount of Cu be 1.00% or less. Cu is one of the elements that can be mixed into steel as a trump element, and together with Sn, it may further deteriorate the chemical conversion treatment properties of the steel sheet. In particular, if the amount of Cu is 0.08% or more, or even 0.10% or more, the chemical conversion treatment properties may decrease even further.

[0033] Cr: 1.000% or less. Cr (chromium) can be included in amounts exceeding 0% because it has the effect of improving the hardenability of steel. On the other hand, if the amount of Cr becomes too high, the pitting corrosion resistance may deteriorate. Therefore, it is preferable that the amount of Cr be 1.000% or less. Cr is also one of the elements that can be mixed into steel as a trump element, and together with Sn, it may further deteriorate the chemical conversion treatment properties of the steel sheet. In particular, if the amount of Cr is 0.010% or more, the chemical conversion treatment properties may further decrease.

[0034] Ni: 1.0% or less. Nickel (Ni) can be included in amounts exceeding 0% because it has the effect of improving the strength of steel. On the other hand, if the amount of Ni becomes too high, it will lead to an increase in cost, so it is preferable that the amount of Ni be 1.0% or less. Note that Ni is also one of the elements that can be mixed into steel as a trump element, and together with Sn, it may further deteriorate the chemical conversion treatment properties of steel sheets.

[0035] Mo: 1.0% or less. Mo (molybdenum) can be included in amounts exceeding 0% because it has the effect of improving the hardenability of steel. On the other hand, if the amount of Mo becomes too high, the hot rolling properties may decrease. For this reason, the amount of Mo is preferably 1.0% or less, and more preferably 0.50% or less. Mo is also one of the elements that can be mixed into steel as a trump element, and together with Sn, it may further deteriorate the chemical conversion treatment properties of the steel sheet. In particular, if the amount of Mo is 0.01% or more, the chemical conversion treatment properties may decrease even further.

[0036] As described above, if the steel sheet contains one or more elements selected from Cu, Cr, and Mo in addition to Sn, the chemical treatment properties of the steel sheet may be further impaired by a mechanism similar to that of Sn. However, in this application, since a predetermined amount of Zn is attached to the surface, the chemical treatment properties of the steel sheet can be improved even if the steel sheet contains two or more trump elements, and even if the steel sheet contains trump elements of Sn, Cu, Cr, and Mo.

[0037] B: 0.005% or less. B has the effect of improving the hardenability of steel, so it can be included in amounts greater than 0%. On the other hand, if the B content is too high, it may cause a significant decrease in hot rolling properties. Therefore, it is preferable that the amount of B be 0.005% or less.

[0038] Ti: 0.2% or less. Since Ti is an element that contributes to increasing the strength of steel sheets, it can be included in amounts greater than 0%. On the other hand, if the Ti content is too high, it may lead to an increase in rolling load and a decrease in ductility. For this reason, it is preferable to keep the Ti content at 0.2% or less.

[0039] Nb: 0.2% or less. Since Nb is an element that contributes to increasing the strength of steel sheets, it can be included in amounts greater than 0%. On the other hand, if the Nb content is too high, it may lead to an increase in rolling load and a decrease in ductility. Therefore, it is preferable to keep the Nb content at 0.2% or less.

[0040] V: 0.5% or less. V has the effect of improving the hardenability of steel, so it can be included in amounts greater than 0%. On the other hand, if the V content is too high, it can cause a significant decrease in castability. Therefore, it is preferable that the V content be 0.5% or less.

[0041] Sb: 0.2% or less. Sb can be included in amounts exceeding 0% because it suppresses oxidation and nitriding of the steel surface and improves descaling properties. On the other hand, if the Sb content is too high, castability may decrease. Therefore, it is preferable that the Sb content be 0.2% or less.

[0042] Water (W): 0.5% or less. Water can be included in amounts exceeding 0% because it has the effect of improving the hardenability of steel. On the other hand, if the water content is too high, it may lead to a decrease in hot workability. Therefore, it is preferable that the water content be 0.5% or less.

[0043] Zr: 0.1% or less. Zr can be included in amounts exceeding 0% because it has the effect of improving the hardenability of steel. On the other hand, if the Zr content is too high, it may lead to a decrease in ductility. Therefore, it is preferable that the Zr content be 0.1% or less.

[0044] Ca: 0.005% or less. Ca can be included in amounts exceeding 0% because it has an effect of improving flexibility. On the other hand, if the Ca content is too high, it may cause deterioration of flexibility and a decrease in surface quality. Therefore, it is preferable that the Ca content be 0.005% or less.

[0045] Mg: 0.005% or less. Mg can be included in amounts exceeding 0% because it has an effect of improving ductility. On the other hand, if the Mg content is too high, it may lead to a decrease in ductility. Therefore, it is preferable that the Mg amount be 0.005% or less.

[0046] Co: 0.5% or less. Co improves the ultimate deformation capacity of steel plates and enhances tensile flange properties, so it can be included in amounts exceeding 0%. On the other hand, if the Co content is too high, its effect saturates, leading to increased costs. Therefore, it is preferable that the Co content be 0.5% or less.

[0047] Zn: 0.05% or less. Zn can be included in amounts exceeding 0% because it improves the ultimate deformability of steel plates and enhances their elongation flange properties. On the other hand, if the Zn content is too high, its effect saturates, leading to increased costs. Therefore, it is preferable that the Zn content be 0.05% or less.

[0048] Ta: 0.1% or less. Since Ta is an element that contributes to increasing the strength of steel plates, it can be included in amounts greater than 0%. On the other hand, if the Ta content is too high, its effect will saturate, leading to increased costs. Therefore, it is preferable that the amount of Ta be 0.1% or less.

[0049] Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi: 0.02% or less each. These elements improve the ultimate deformability of steel sheets and enhance tensile flange properties, so they can be included in amounts exceeding 0%. On the other hand, if the content of these elements is too high, their effect saturates, leading to increased costs. Therefore, it is preferable that the content of each of these elements be 0.02% or less.

[0050] REM: 0.005% or less. REM has the effect of improving ductility and flangeability, so it can be included in amounts greater than 0%. On the other hand, if the REM content is too high, it may lead to a decrease in ductility. Therefore, it is preferable that the REM content be 0.005% or less. REM refers to rare earth metals, and is a collective term for 17 elements, which are the 15 lanthanide elements plus Y and Sc. One or more of these elements may be included. The REM content refers to the total content of these elements.

[0051] [Adhesion of Zn] The steel plate having the above-described component composition has Zn adhering to at least one of its surfaces. At this time, the amount of Zn adhering to each surface, calculated in terms of the elemental mass of Zn, W Ｚｎ (Unit: mg / m ２ It is crucial that the following equation (1) is satisfied: [Sn] × 400 + 18 ≤ W Ｚｎ ≤ 800 ... (1) Here, [Sn] is the Sn content in the steel (unit: mass%). Therefore, in the above equation (1), the W enclosing the symbol "≤" Ｚｎ[Sn] × 400 + 18 have different units. However, the inventors have found that the unit area (m²) of the steel plate surface ２ ) Amount of Zn attached per unit (mg), i.e., W Ｚｎ (mg / m ２ We discovered that the effect of ) on chemical treatment properties and paint corrosion resistance correlates with the Sn concentration in the steel sheet, expressed as mass percent, and after diligent study, we completed the above specific relationship equation. Thus, the magnitude relationship in equation (1) represents the relationship between numerical values ​​without considering units.

[0052] A predetermined amount W is deposited on the surface of the steel plate, depending on the amount of Sn in the steel. Ｚｎ By attaching Zn, high chemical conversion treatment properties can be imparted to the steel sheet even if a predetermined amount of Sn is present. In particular, the zinc phosphate conversion treatment properties can be improved without impairing the good Zr conversion treatment properties. The inventors speculate on the reason for this as follows: That is, when Zn, which dissolves more easily than iron, is present on the surface of the steel sheet satisfying formula (1), a rapid dissolution reaction of Zn occurs when the steel sheet comes into contact with the zinc phosphate conversion treatment solution. Therefore, the reduction of hydrogen ions, which is the counterpart reaction, also occurs rapidly, promoting an increase in pH at the interface between the steel sheet and the chemical conversion treatment solution. As a result, the nucleation of zinc phosphate is promoted, and the adverse effect of pH reduction due to Sn can be canceled out.

[0053] Note that Zn may be attached to only one side of the steel plate, or to both sides of the steel plate. If Zn is attached to both sides of the steel plate, the amount of Zn attached to each side W Ｚｎ Each of these must satisfy the above equation (1). Whether to attach Zn to only one surface of the steel plate or to both surfaces should be determined according to the requirements for post-processing of the steel plate, such as painting.

[0054] Amount of deposit W calculated based on the mass of Zn element per side Ｚｎ (mg / m ２ The amount of Zn attached W is controlled according to the amount of Sn [Sn] (%) in the steel and must be ([Sn] × 400 + 18) or more. Ｚｎ [Sn] × 400 + 18 (mg / m³) ２Below this level, sufficient improvement in chemical conversion treatment performance cannot be obtained. In particular, the lower limit of the amount of Zn attached greatly affects the performance of zinc phosphate chemical conversion treatment and the corrosion resistance after painting. If the amount of Zn attached is above the above lower limit, the performance of zinc phosphate chemical conversion treatment, especially with liquid surface adjustment, can be greatly improved, and the corrosion resistance after painting can be made excellent. The amount of Zn attached is preferably 50 mg / m ２ The above is preferable, and more preferably 150 mg / m² ２ This concludes the explanation. This significantly improves the performance of zinc phosphate conversion treatment, which involves solid surface adjustment. Furthermore, it can further enhance the performance of Zr conversion treatment. In addition, it can further improve the corrosion resistance after painting following these conversion treatments.

[0055] On the other hand, the amount of Zn attached W Ｚｎ 800 mg / m² ２ Beyond a certain point, the corrosion resistance after painting deteriorates. This deterioration in corrosion resistance after painting occurs regardless of whether zinc phosphate conversion treatment or Zr conversion treatment is used. This is thought to be because Zn dissolves at a faster rate than Fe, and when the painted steel plate is exposed to a corrosive environment, the dissolution of metallic Zn occurs vigorously beneath the paint film. As a result, if the amount of Zn adhering is excessive, corrosion beneath the paint film is more likely to progress, and the corrosion resistance deteriorates. Furthermore, as will be discussed later, the corrosion resistance after painting is particularly likely to decrease after Zr conversion treatment. From this perspective, the amount of Zn adhering W Ｚｎ 800 mg / m² ２ The following, preferably 450 mg / m² ２ The following, and more preferably 300 mg / m² ２ The following applies:

[0056] Regarding the upper limit of Zn adhesion mentioned above, the following can be inferred in more detail: In zinc phosphate conversion treatment, the amount of Zn (e.g., metallic Zn layer) adhering to the surface of the steel plate is 800 mg / m². ２ It dissolves to a certain extent. Therefore, the amount of Zn attached is 800 mg / m². ２In the following cases, no Zn, such as a metallic Zn layer, remains on the steel sheet surface after zinc phosphate conversion treatment. Therefore, even if paint is applied, there is no Zn beneath the paint film, and corrosion due to Zn dissolution does not progress, resulting in good corrosion resistance after painting. Also, the more Zn that adheres to the steel sheet surface before zinc phosphate conversion treatment, the lower the P ratio (the ratio of phosphophyllite P to phopite H, P / (P+H)) of the zinc phosphate film formed during zinc phosphate conversion treatment. Since phopite has inferior acid and alkali resistance to phosphophyllite, a decrease in the P ratio leads to a decrease in corrosion resistance after painting. Zn adhesion amount: 450 mg / m² ２ In the following cases, the decrease in the P ratio can be effectively suppressed, which is expected to result in better corrosion resistance after painting. Furthermore, in the case of Zr conversion treatment, the amount of Zn dissolved is less than in zinc phosphate conversion treatment, at 300 mg / m². ２ This is the extent of the problem. Therefore, in order to further improve the corrosion resistance after painting following Zr chemical treatment, the amount of Zn adhering to the steel plate surface before Zr chemical treatment should be 300 mg / m². ２ The following is preferable. In a Zr coating, there is no decrease in corrosion resistance due to the P ratio as in a zinc phosphate coating. Therefore, if the Zn layer completely disappears from the steel plate surface after the Zr chemical treatment, the corrosion resistance after painting is expected to be even better.

[0057] Here, the amount of Zn deposited on the steel plate, calculated in terms of Zn elemental mass W. Ｚｎ This can be measured using an X-ray fluorescence analyzer. By measuring with a test piece whose Zn deposition amount is known and creating a calibration curve showing the relationship between Zn deposition amount and X-ray intensity, the Zn deposition amount of the target steel plate can be determined by the calibration curve method. Furthermore, by using a steel plate with a Zn content equivalent to that of the steel plate under evaluation as the test piece used to create the calibration curve, it is possible to measure Zn deposited on the surface of the steel plate separately from Zn arbitrarily contained in the steel plate. As a specific example of such a test piece, a steel plate with a Zn content within ±0.03% of the Zn content of the steel plate under evaluation can be used.

[0058] The form of Zn adhering to the surface of the steel plate is not particularly limited and may include metallic Zn; Zn oxides such as ZnO; and Zn(OH)２ Examples include Zn hydroxides such as the following.

[0059] It is preferable that Zn is uniformly attached to the surface of the steel plate, and that the area ratio of exposed base metal where Zn is not attached is 30% or less. If the area ratio of exposed base metal exceeds 30%, the effect of improving the chemical conversion treatment properties by Zn will not be obtained in that area, and partial defects may occur after the chemical conversion treatment. The area ratio of exposed base metal can be determined by elemental analysis using a scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDX) under conditions of a 1000x field of view and an acceleration voltage of 5kV, and by dividing the field of view of the steel plate surface into 5μm square regions, and determining the percentage of regions (area %) in which the mass % of Zn in each region is 20% or less.

[0060] During the manufacturing process, sn in steel sheets may become concentrated toward the surface of the sheet. When sn is concentrated near the surface of the steel sheet, the adverse effect of sn on the chemical conversion treatment properties increases, making the steel sheet more susceptible to deterioration. Therefore, in order to better suppress the adverse effect of sn on the chemical conversion treatment properties of the steel sheet, it is preferable to avoid the concentration of this element toward the surface of the steel sheet. From this viewpoint, it is preferable that the sn concentration in the region from the surface of the steel sheet to a depth of 0.3 μm in the thickness direction is three times or less than the sn concentration in the center of the thickness. It is more preferable that this preferred sn concentration condition is achieved on both sides of the steel sheet. The sn concentration in the surface region from the surface of the steel sheet to a depth of 0.3 μm can be determined by analyzing the elemental distribution in the depth direction using a glow discharge emission spectrometer. Furthermore, the Sn concentration in the center of the plate thickness may be determined by analyzing the central part of the plate thickness following the analysis of the surface layer, or, if the plate thickness is thick and continuous analysis takes time, the surface may be ground up to near the center of the plate thickness before analyzing the area near the center. The Sn concentration state can be well controlled, for example, by adjusting the manufacturing conditions described later, particularly the cold rolling conditions and / or subsequent annealing conditions.

[0061] (Method for Manufacturing Steel Sheets) The steel sheets of the present invention are not particularly limited, and can be obtained, for example, by hot-rolling a steel slab having the above-described component composition and then applying a Zn adhesion treatment to the resulting hot-rolled sheet. Alternatively, the steel sheets of the present invention may be obtained, for example, by sequentially hot-rolling, pickling, and cold-rolling a steel slab having the above-described component composition and then applying a Zn adhesion treatment to the resulting cold-rolled sheet. Or, the steel sheets of the present invention may be obtained, for example, by sequentially hot-rolling, pickling, cold-rolling, and annealing a steel slab having the above-described component composition and then applying a Zn adhesion treatment to the resulting cold-rolled and annealed sheet. In particular, as will be described later, a steel sheet can be suitably obtained by applying a Zn adhesion treatment to the cold-rolled and annealed sheet. In other words, the steel sheet can be a cold-rolled steel sheet, or a cold-rolled steel sheet that has undergone cold-rolling and subsequent annealing.

[0062] [Preparation of Steel Slabs] An example of the steel slab preparation process is described below. For the preparation of steel slabs, molten steel derived from electric furnace materials using iron scrap as a raw material may be used, or molten steel derived from electric furnace materials may be mixed with molten steel derived from blast furnace materials. Alternatively, the composition of the molten steel derived from blast furnace materials can be adjusted by conventional methods to achieve a predetermined composition. In this way, a steel slab can be obtained by conventional methods using molten steel with a composition containing at least Sn: 0.005% to 0.200%.

[0063] [Hot Rolling] An example of the hot rolling process is described below. In the hot rolling process, a steel slab having a composition containing Sn: 0.005% to 0.200% is heated and then rolled to obtain a hot-rolled sheet. The heating and rolling conditions of the steel slab can follow conventional methods.

[0064] [Pickling] An example of the pickling process is described below. In the pickling process, the obtained hot-rolled sheet is pickled to remove the oxide scale present on the surface of the hot-rolled sheet. The pickling conditions can follow conventional methods.

[0065] [Cold Rolling] An example of the cold rolling process is described below. In the cold rolling process, a cold-rolled sheet with a desired thickness is obtained by further rolling the hot-rolled sheet. In cold rolling, the cumulative rolling rate is preferably 30% or more, and more preferably 40% or more. During hot rolling and subsequent pickling, Sn may become concentrated on the surface of the steel sheet, and the chemical conversion treatment properties are further impaired in steel sheets where Sn is concentrated on the surface. Therefore, by thinning the concentrated layer through cold rolling, the adverse effect of Sn on the chemical conversion treatment properties can be further reduced. As a result, the improvement effect of the chemical conversion treatment properties by subsequent annealing and Zn adhesion treatment can be further enhanced. If the cumulative rolling rate is less than 30%, a relatively large amount of the concentrated layer remains, and the improvement effect of the chemical conversion treatment properties may not be enhanced. On the other hand, from the viewpoint of improving chemical conversion treatment properties, there is no upper limit to the cumulative rolling rate, but from the viewpoint of cold rolling load and material properties, it is preferable to set the cumulative rolling rate to 85% or less. Alternatively, the manufacturing method can follow that of ordinary cold-rolled sheets.

[0066] [Annealing] An example of the annealing process is described below. By performing annealing after the cold rolling described above, the Sn that is still concentrated on the surface can be diffused into the interior of the steel sheet in the thickness direction. By diffusing Sn into the interior of the steel sheet and reducing the amount remaining near the surface, the adverse effect of Sn on the chemical conversion treatment properties can be further reduced. As a result, the improvement effect of the subsequent Zn adhesion treatment on the chemical conversion treatment properties can be further enhanced. Annealing is preferably performed in a non-oxidizing atmosphere for iron in order to prevent oxidation of the steel sheet surface.

[0067] Furthermore, the annealing temperature is preferably 700°C or higher, and preferably 900°C or lower. If the annealing temperature is below 700°C, the diffusion of Sn is insufficient, and a concentrated layer tends to remain, which may prevent the improvement effect of subsequent Zn adhesion on the chemical conversion treatment from being fully obtained. On the other hand, if the annealing temperature exceeds 900°C, the material properties of the steel sheet may deteriorate.

[0068] When the annealing temperature is between 700°C and 900°C, the holding time is preferably 20 seconds or more. If the holding time is less than 20 seconds, the diffusion of Sn into the steel sheet may be insufficient, and the chemical conversion treatment properties may not be improved. There is no particular upper limit to the holding time, but from the viewpoint of productivity, it can be 600 seconds or less.

[0069] [Pretreatment] As a pretreatment before attaching Zn, degreasing and rinsing may be performed to clean the surface of the steel sheet, such as a hot-rolled or cold-rolled sheet. Furthermore, pickling and rinsing may be performed to activate the surface of the steel sheet. Following these pretreatments, the Zn attachment treatment can be carried out. The methods of degreasing and rinsing are not particularly limited, and conventional methods can be used. The method of pickling is also not limited, but various acids such as sulfuric acid, hydrochloric acid, nitric acid, and mixtures thereof can be suitably used. Among these, the use of sulfuric acid, hydrochloric acid, or mixtures thereof is more preferable. In addition, the concentration of the acid used in the pickling treatment is not particularly specified, but considering the ability to remove oxide films and the prevention of surface roughness (surface defects) due to over-pickling, it is preferably about 1 to 20% by mass. The pickling solution may also contain an antifoaming agent, a pickling accelerator, a pickling inhibitor, etc.

[0070] [Zn Adhesion] The method for adhering a predetermined amount of Zn to the surface of a steel plate is not particularly limited, but examples include electroplating, electroless plating, and PVD (Physical Vapor Deposition). All of these can be carried out according to conventional methods. Among these, electroplating is preferred from the viewpoint of low cost. Furthermore, electroplating is also preferred from the viewpoint of easily achieving the uniform adhesion of Zn described above by changing various conditions. From the viewpoint of uniformly adhering Zn, it is preferable that the relative flow velocity between the steel plate and the plating solution during electroplating be slow, specifically preferably 100 mpm (meters per minute) or less, and more preferably 30 mpm or less. The relative flow velocity can be 0 mpm or more.

[0071] Based on the above, one preferred manufacturing method for obtaining the steel sheet of the present invention is as follows: A hot rolling step of heating and rolling a steel slab having a component composition containing Sn: 0.005% by mass or more and 0.200% by mass or less to obtain a hot-rolled sheet; a pickling step of pickling the hot-rolled sheet; a cold rolling step of rolling the hot-rolled sheet after pickling to obtain a cold-rolled sheet; an annealing step of annealing the cold-rolled sheet to obtain a cold-rolled annealed sheet; and an amount W of Zn element per side attached to at least one surface of the cold-rolled annealed sheet. Ｚｎ (Unit: mg / m ２ A method for manufacturing a steel sheet, comprising: a Zn attachment step of attaching Zn to the steel sheet such that it satisfies the following formula (1); [Sn] × 400 + 18 ≤ W Ｚｎ ≤ 800 ... (1) Here, [Sn] is the Sn content in the steel.

[0072] Furthermore, one more preferred manufacturing method for obtaining the steel sheet of the present invention is as follows: A hot rolling step of heating and rolling a steel slab having a component composition containing Sn: 0.005% by mass or more and 0.200% by mass or less to obtain a hot-rolled sheet; a pickling step of pickling the hot-rolled sheet; a cold rolling step of rolling the hot-rolled sheet after pickling at a cumulative rolling rate of 30% or more to obtain a cold-rolled sheet; an annealing step of annealing the cold-rolled sheet in a non-oxidizing atmosphere at a temperature of 700°C or higher for a holding time of 20 seconds or more to obtain a cold-rolled annealed sheet; and the amount of Zn elemental mass W per side attached to at least one surface of the cold-rolled annealed sheet. Ｚｎ (Unit: mg / m ２ A method for manufacturing a steel sheet, comprising: a Zn attachment step of attaching Zn to the steel sheet such that it satisfies the following formula (1); [Sn] × 400 + 18 ≤ W Ｚｎ ≤ 800 ... (1) Here, [Sn] is the Sn content in the steel.

[0073] The present invention will be described in more detail below based on examples. However, the present invention is not limited to these examples.

[0074] Steel slabs having the component composition shown in Table 1 were heated and hot-rolled, and the resulting hot-rolled sheets were pickled. The remainder other than the elements shown in Table 1 consisted of Fe and unavoidable impurities. In Table 1, "-" indicates that the element was not intentionally added and was at an unavoidable impurity level. Next, the hot-rolled sheets after pickling were cold-rolled at a cumulative rolling rate of 70% to obtain cold-rolled sheets with a thickness of 0.8 mm. Then, the obtained cold-rolled sheets were annealed in a nitrogen atmosphere containing 5 vol% hydrogen (non-oxidizing atmosphere) at an annealing temperature of 800°C for a holding time of 100 seconds to obtain cold-rolled annealed sheets. The obtained cold-rolled annealed sheets were then cleaned on the surface by alkaline electrolytic degreasing and sulfuric acid pickling as a pretreatment for plating. After that, Zn was deposited on both sides by electroplating to obtain steel sheets. For the Zn plating bath, a solution containing 288 g / L of zinc sulfate heptahydrate, with the pH adjusted to 2.0 using sulfuric acid, was used at a temperature of 50°C. During plating, the current density was set to 1 to 10 A / dm². ２ By varying the time between 0.5 and 5 seconds, the amount of Zn deposited W can be determined. Ｚｎ The following was controlled. In addition, the relative flow rate between the steel plate and the plating solution during plating was adjusted to 30 mph.

[0075]

[0076] The steel plates manufactured as described above were evaluated for their chemical conversion treatment properties and corrosion resistance after painting. The chemical conversion treatment properties were evaluated under the following three conditions.

[0077] [Zinc Phosphate Conversion Treatment (Condition 1)] Commercially available zinc phosphate conversion treatment agents (surface modifier: Preparen X, conversion treatment agent: Palbond SX35, both manufactured by Nippon Parkerizing Co., Ltd.) were used. Zinc phosphate conversion treatment was performed at a treatment temperature of 35°C and a treatment time of 90 seconds to coat the surface of the steel plate with zinc phosphate. These conditions use a zinc phosphate-based liquid surface modifier as the surface modifier. The zinc phosphate coverage rate on the steel plate surface after conversion treatment was calculated as follows. The surface of the steel plate after conversion treatment was observed using an SEM at a 1000x field of view, and the area of ​​the zinc phosphate crystal coating area and the exposed base metal area were determined separately, and the area ratio of the zinc phosphate crystal coating area to the total area was calculated. The same measurement was performed for 10 fields of view, and the average value of the area ratio of the zinc phosphate crystal coating area obtained for each is shown in Table 2 as the zinc phosphate coverage rate (area %). The zinc phosphate coating rate was evaluated as follows: less than 80% was considered poor, 80% to less than 100% was considered good, and 100% was considered even better.

[0078] [Zr Conversion Treatment (Condition 2)] A commercially available Zr conversion treatment agent (Palceed 1500, manufactured by Nippon Parkerizing Co., Ltd.) was used. Zr conversion treatment was performed at a treatment temperature of 40°C for a treatment time of 90 seconds to coat the surface of the steel plate with zirconium. The amount of Zr deposited on the surface of the steel plate after conversion treatment was measured using an X-ray fluorescence analyzer. A calibration curve showing the relationship between the amount of Zr deposited and X-ray intensity was created by measuring using test pieces with a known amount of Zr deposited, and the amount of Zr deposited on the surface of the target steel plate was determined using the calibration curve method. The amount of Zr deposited was 10 mg / m². ２ Less than 10 mg / m² is considered poor quality. ２ 20mg / m or more ２ Less than 20 mg / m² is good, 20 mg / m² ２ The above was further evaluated as good. The results are shown in Table 2.

[0079] [Zinc Phosphate Conversion Treatment (Condition 3)] Commercially available zinc phosphate conversion treatment agents (surface modifier: Surffine 5N-10, conversion treatment agent: Surfdyne EC1000, both manufactured by Nippon Paint Surf Chemicals Co., Ltd.) were used. Zinc phosphate conversion treatment was performed at a treatment temperature of 40°C and a treatment time of 90 seconds to coat the surface of the steel plate with zinc phosphate. This condition uses a Ti colloid-based solid surface modifier as the surface modifier. The zinc phosphate coating rate on the surface of the steel plate after the conversion treatment was measured using the same method as in Condition 1. A zinc phosphate coating rate of less than 80% was evaluated as poor, 80% or more and less than 100% as good, and 100% as even better. The results are shown in Table 2.

[0080] The evaluation method for corrosion resistance after painting is as follows. Steel plates cut to a size of 150 mm x 70 mm were subjected to zinc phosphate conversion treatment (conditions 1 and 3) and Zr conversion treatment (condition 2) according to the above conditions 1 to 3. The zinc phosphate and Zr conversion treated plates obtained in this way were electrodeposited using commercially available electrodeposition paint (GT-150, manufactured by Kansai Paint Co., Ltd.) and baked in a 170°C furnace for 20 minutes to form an electrodeposited film with a thickness of 15 μm. Subsequently, cross-shaped cuts were made using a utility knife until they reached the steel plate, and a 120-cycle test was performed using a 0.5 mass% NaCl aqueous solution in accordance with the combined cycle test specified in SAE J2334. After that, the maximum bulge width on one side from the cut area was measured using a magnifying glass with a scale. If this maximum bulge width (simply shown as "bulge width" in the table) was 4.0 mm or less, the corrosion resistance after painting was evaluated as good. These results are shown in Table 2.

[0081]

[0082] As shown in Table 2, the zinc phosphate coating rate is the amount of Zn deposited on the surface of the steel sheet, W, whether a liquid surface modifier is used or a solid surface modifier is used. Ｚｎ When the amount was too small, it decreased, and a tendency was observed for the zinc phosphate conversion treatment performance to be significantly reduced. In addition, the amount of Zr attached was also affected by the amount of Zn attached to the surface of the steel sheet W. ＺｎThe amount of Zn W decreased as the amount of Zn decreased, and a tendency for Zr conversion treatment to decline was observed, but the degree of decline was not as great as in the case of zinc phosphate conversion treatment. Thus, zinc phosphate conversion treatment and Zr conversion treatment are related to the amount of Zn adhering to the steel sheet surface. Ｚｎ It was found that it behaves differently in response to this.

[0083] Furthermore, as shown in Table 2, the corrosion resistance after painting following zinc phosphate conversion treatment is determined by the amount of Zn (W) adhering to the surface of the steel plate, whether a liquid surface modifier is used or a solid surface modifier is used. Ｚｎ The condition tended to worsen both when the amount was too little and when it was too much. In particular, the amount of Zn attached W Ｚｎ When the amount was too small, there was a tendency for the corrosion resistance after painting to deteriorate significantly. On the other hand, the corrosion resistance after painting after Zr chemical treatment was related to the amount of Zn W. Ｚｎ A tendency for deterioration was observed when the amount of Zn was high. Thus, the corrosion resistance after painting following zinc phosphate conversion treatment and the corrosion resistance after painting following Zr conversion treatment are related to the amount of Zn W on the surface of the steel plate. Ｚｎ It was found that it behaves differently in response to this.

[0084] Furthermore, the results in Table 2 show that even steel plates containing a predetermined amount of Sn that degrades the chemical conversion treatment properties can still have a predetermined amount of W deposited on the surface. Ｚｎ It was found that by attaching Zn, excellent chemical conversion treatment properties and corrosion resistance after painting were achieved in both zinc phosphate and Zr chemical conversion treatments. Specifically, it was found that both the chemical conversion treatment properties with liquid surface adjustment and the Zr chemical conversion treatment properties, as well as the corrosion resistance after painting following the zinc phosphate treatment, could be greatly improved. Also, the amount of Zn attached W Ｚｎ 50 mg / m² ２ It was found that by further controlling the above, the performance of zinc phosphate conversion treatment with solid surface adjustment and subsequent corrosion resistance after painting can be greatly improved. Furthermore, the amount of Zn W Ｚｎ 300 mg / m² ２ By further controlling the process as described below, the maximum blister width in the post-paint corrosion resistance evaluation was 4.0 mm or less under all chemical treatment conditions, including Zr chemical treatment, indicating particularly good post-paint corrosion resistance.

[0085] Thus, despite containing Sn, the steel sheet of the present invention exhibits excellent chemical conversion treatment properties in both zinc phosphate conversion treatment and Zr conversion treatment, and also exhibits excellent subsequent paint corrosion resistance.

Claims

It has a component composition containing Sn: 0.005% by mass or more and 0.200% by mass or less. Zn is attached to at least one of the surfaces, and the amount of Zn attached on one side, calculated as the elemental mass of Zn, W Ｚｎ (Unit: mg / m ２ A steel plate characterized in that it satisfies the following formula (1). [Sn]×400+18 ≦ W Ｚｎ ≦ 800 ・・・(1) Here, [Sn] is the Sn content in the steel. The aforementioned amount of adhesion W Ｚｎ 450 mg / m² ２ The steel plate according to claim 1, which is as follows: 　 The aforementioned amount of adhesion W Ｚｎ 300 mg / m² ２ The steel plate according to claim 1, which is as follows: 　 The above component composition is, in mass%, C: 0.8% or less, Si: 3.0% or less, Mn: 12.0% or less, P: 0.1% or less, S: 0.03% or less, N: 0.015% or less, Sol. Al: 1.0% or less Cu: 1.00% or less, Cr: 1.000% or less, Ni: 1.0% or less and Mo: 1.0% or less A steel sheet according to any one of claims 1 to 3, further containing one or more selected from the group consisting of, with the remainder being Fe and unavoidable impurities. 　 The above component composition is, in mass%, B: 0.005% or less, Ti: 0.2% or less, Nb: 0.2% or less, V: 0.5% or less, Sb: 0.2% or less, W: 0.5% or less, Zr: 0.1% or less, Ca: 0.005% or less, Mg: 0.005% or less, Co: 0.5% or less, Zn: 0.05% or less, Ta: 0.1% or less, Ce: 0.02% or less, Se: 0.02% or less, Te: 0.02% or less, Ge: 0.02% or less, As: 0.02% or less, Sr: 0.02% or less, Cs: 0.02% or less, Hf: 0.02% or less, Pb: 0.02% or less, Bi: 0.02% or less and REM: 0.005% or less The steel sheet according to claim 4, further containing one or more selected from the group consisting of, with the remainder being Fe and unavoidable impurities.

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