Steel sheet

Abstract

The present invention provides a steel sheet which exhibits excellent chemical conversion coating properties even when containing a tramp element. A steel sheet according to the present invention contains a specific amount of Sn, and Ni is adhered to at least one surface of the steel sheet in an amount WNi that satisfies [Sn]×200 ≤ WNi ≤ [Sn]×7500 and WNi ≤ 150.

Description

steel plate

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

[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 that improves the properties of steel sheets, such as corrosion resistance and paintability. Therefore, steel sheets are required to exhibit good chemical conversion treatment properties. One method of chemical conversion treatment is zinc phosphate conversion treatment. Zinc phosphate conversion treatment has been widely used conventionally because it imparts excellent corrosion resistance and other properties to the surface of steel sheets. In this zinc phosphate 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, high zinc phosphate conversion treatment properties are required 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 applying 0.3 to 10 mg / dm of metallic Ni to the surface of a cold-rolled steel sheet. ２ A method for adhering it is described. Furthermore, Patent Document 4 describes applying Ni oxide and / or Ni hydroxide to the surface of a cold-rolled steel sheet in an amount of Ni by weight of 1 to 150 mg / m². ２ Methods for adhesion are described. However, these techniques do not take into account the influence of the tramp element, which is specific to electric furnace materials. Furthermore, the effect on Zr conversion treatment is not considered.

[0008] Japanese Patent Publication No. 2020-84238, Japanese Patent Publication No. 2020-84325, Japanese Patent Publication No. 56-116883, Japanese Patent Publication No. 59-159987

[0009] The present invention was made to improve upon the above-mentioned problems and aims to provide a steel sheet that has excellent chemical conversion treatment properties even when it contains trump elements, such as electric furnace steel made from iron scrap. More specifically, the aim is to achieve both excellent zinc phosphate and Zr chemical conversion treatment properties even in steel sheets containing trump elements. Even more specifically, the aim is to ensure high levels of both zinc phosphate and Zr chemical conversion treatment properties, at least with liquid surface adjustment.

[0010] In order to solve the above problems, the inventors of the present invention have intensively studied the influence of the playing card element on the formation treatment property. As a result, it has been clarified that the formation treatment property deteriorates particularly due to the inclusion of Sn among the playing card elements. Furthermore, the inventors of the present invention have intensively studied a method for improving the formation treatment property of a steel sheet containing Sn. As a result, it has been found that good formation treatment property can be obtained by adhering an amount of Ni that satisfies a predetermined relationship according to the Sn content in the steel to the surface of the steel sheet.

[0011] The present invention has been made based on the above findings, and the 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 Ni is adhered to at least one of the surfaces, and the adhesion amount W of the Ni in terms of the mass of Ni element per side Ｎｉ (unit: mg / m ２ ) satisfies the following formulas (1) and (2). Steel sheet characterized by: [Sn] × 200 ≤ W Ｎｉ ≤ [Sn] × 7500... (1) W Ｎｉ ≤ 150... (2) Here, [Sn] is the Sn content in the steel.

[0012] [2] The steel sheet according to [1] above, wherein the adhesion amount W Ｎｉ further satisfies the following formula (3). [Sn] × 800 ≤ W Ｎｉ ≤ [Sn] × 7500... (3)

[0013] [3] The steel sheet according to [1] or [2] above, wherein the adhesion amount W Ｎｉ is 90 mg / m ２ or less.

[0014] [4] The steel sheet according to any one of [1] to [3] above, wherein the Ni is metallic Ni.

[0015] [5] The component composition is, by 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 [4] above.

[0016] [6] The component composition is, by 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 [5] above.

[0017] According to the present invention, even if Sn is contained in a predetermined amount, it is possible to provide a steel sheet having good chemical conversion treatment properties in both zinc phosphate chemical conversion treatment and Zr chemical conversion treatment. More specifically, even if the above-mentioned predetermined amount of Sn is contained, it is possible to provide a steel sheet capable of ensuring both high zinc phosphate chemical conversion treatment properties involving at least liquid surface adjustment and Zr chemical conversion treatment properties.

[0018] 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.

[0019] (Steel Sheet) The steel sheet of the present invention contains a predetermined amount of Sn and has a predetermined amount of Ni attached to its surface. By having a predetermined amount of Ni 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 when containing a predetermined amount of Sn, a typical trump element. Since the steel sheet of the present invention has excellent zinc phosphate conversion treatment properties and Zr conversion treatment properties, it can be applied to a wide range of fields regardless of the chemical treatment method. Furthermore, it can be applied to both hot-rolled and cold-rolled sheets, making it particularly useful as a steel sheet for automobiles. In addition, since the steel sheet of the present invention has excellent chemical conversion treatment properties even when containing trump elements, it can be particularly useful as an electric furnace material made from iron scrap, and it has exceptional industrial effects that are environmentally friendly. The steel sheet of the present invention can be obtained, for example, by the manufacturing method described later.

[0020] 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 ２O → 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.

[0021] Based on the reaction mechanism described above, it appears 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 Sn greatly deteriorates the zinc phosphate conversion treatment properties. On the other hand, the effect of Sn on Zr conversion treatment properties was not as significant as in the case of zinc phosphate conversion treatment. This was presumed to be because Zr conversion treatment involves a different film formation process than zinc phosphate conversion treatment, where an amorphous film is formed without nucleation. Therefore, it was concluded that in order to achieve good zinc phosphate and Zr conversion treatment properties in steel sheets containing Sn, it is necessary to find appropriate conditions for each.

[0022] [Composition] The steel sheet contains Sn: 0.005% or more and 0.200% by mass 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 Ni is attached to the surface of the steel sheet, so even in the presence of Sn, it is possible to achieve both good zinc phosphate conversion treatment properties and Zr conversion treatment properties.

[0023] 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 conversion treatment properties of steel sheets. In particular, it greatly impairs the zinc phosphate conversion treatment properties. On the other hand, if the amount of Sn exceeds 0.200%, excessive Sn ions are generated during the chemical conversion treatment, excessively raising the pH of the chemical conversion treatment solution, making the chemical conversion treatment difficult. Therefore, the amount of Sn should be 0.200% or less, preferably 0.050% or less. The amount of Sn contained in excess 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 Ni is attached, so even if Sn content is 0.010% or more, over 0.100%, 0.110% or more, or up to 0.200%, the chemical conversion treatment properties are good.

[0024] In addition to the Sn mentioned above, the steel sheet may contain other components without particular restrictions, 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 may suitably contain one or more of the following elements selected from the following: 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, each in an amount greater than 0%. The component composition of the steel sheet can then consist of Fe and unavoidable impurities as a remainder of the above elements.

[0025] Furthermore, the steel sheet may contain, in addition to or in place of the above-mentioned arbitrary elements, 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 material can suitably contain one or more elements selected from the following: 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, each in an amount greater than 0%. The remainder of the elements other than those mentioned above can be Fe and unavoidable impurities.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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 Ni 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] Ca: 0.005% or less. Ca can be included in amounts exceeding 0% because it has the 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] [Adhesion of Ni] The steel plate having the above-described component composition has Ni adhering to at least one of its surfaces. In this case, the amount of Ni adhering to each surface, calculated on a basis of Ni elemental mass W, Ｎｉ (Unit: mg / m ２ It is crucial that the following equations (1) and (2) are both satisfied: [Sn] × 200 ≤ W Ｎｉ ≦ [Sn]×7500...(1) W Ｎｉ ≤ 150 ... (2) Also, the amount of adhesion W on the surface of the steel plate ＮｉPreferably, the following formula (3) is further satisfied on at least one side, and more preferably, the following formula (3) is further satisfied on both sides: [Sn] × 800 ≤ W Ｎｉ ≦[Sn]×7500...(3)

[0053] Here, [Sn] is the Sn content in the steel (unit: mass%). Therefore, in the above equation (1), the W that encloses the symbol "≦" Ｎｉ However, [Sn] × 200 and [Sn] × 7500 have different units. However, the inventors have found that the unit area (m²) of the steel plate surface ２ The amount of Ni attached per unit (mg), i.e., W Ｎｉ (mg / m ２ We discovered that the effect of ) on the chemical treatment properties 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. The same applies to equation (3).

[0054] A predetermined amount W is deposited on the surface of the steel plate, depending on the amount of Sn in the steel. Ｎｉ By attaching Ni, high chemical conversion treatment properties can be imparted to the steel sheet even if a predetermined amount of Sn is contained. 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: By attaching Ni to the surface of the steel sheet satisfying formulas (1) and (2), a local galvanic cell is formed on the surface, and the dissolution of iron is promoted. As more iron is dissolved, the pH at the interface between the steel sheet and the chemical conversion treatment solution is promoted, and the adverse effect of pH reduction due to Sn can be canceled out.

[0055] Note that Ni may be attached to only one side of the steel plate, or to both sides of the steel plate. If Ni is attached to both sides of the steel plate, the amount of Ni attached to each side W Ｎｉ Each of these must satisfy the above equations (1) and (2). Whether Ni is attached to only one side of the steel plate surface or to both sides should be determined according to the need for post-processing treatments on the steel plate, such as painting or rust prevention treatment.

[0056] Amount of deposit W calculated based on the mass of Ni element per side Ｎｉ (mg / m ２ The amount of Ni attached to the steel is controlled according to the amount of Sn [Sn] (%) and must be ([Sn] × 200) or more. Ｎｉ [Sn] × 200 (mg / m ２ Below this level, sufficient improvement in the chemical conversion treatment performance cannot be obtained. In particular, the lower limit of the amount of Ni attached greatly affects the zinc phosphate chemical conversion treatment performance. If the amount of Ni attached is above the above lower limit, the zinc phosphate chemical conversion treatment performance, especially with liquid surface adjustment, can be greatly improved. The amount of Ni attached is preferably [Sn] × 750 (mg / m²). ２ ) or more, more preferably [Sn] × 800 (mg / m²) ２ ) or more, and more preferably [Sn] × 1250 (mg / m²) ２ This concludes the explanation. As a result, the performance of zinc phosphate conversion treatment, which involves solid surface preparation, can also be greatly improved.

[0057] On the other hand, the amount of Ni attached W Ｎｉ If [Sn] × 7500 exceeds this, the local galvanic effect by Ni becomes too strong, and a stable chemical conversion coating cannot be obtained. Also, W Ｎｉ 150 mg / m² ２ Even if the amount exceeds this, the exposed area of ​​the base steel plate decreases. As a result, the dissolution reaction of iron is suppressed, and the chemical conversion treatment performance decreases. Furthermore, if the amount of Ni attached is excessively high, the effect of suppressing the dissolution reaction of iron is stronger in Zr chemical conversion treatment, which has a higher pH of the treatment solution compared to zinc phosphate chemical conversion treatment, so the Zr chemical conversion treatment performance tends to decrease particularly. From this perspective, the amount of Ni attached is 150 mg / m². ２ The following, preferably 90 mg / m² ２ The following is more more than 80 mg / m² ２ The following, and more preferably 50 mg / m² ２ The following applies:

[0058] Here, the amount of Ni deposited on the steel plate, calculated in terms of the mass of Ni elements, is W. ＮｉThis can be measured using an X-ray fluorescence analyzer. By measuring with a test piece whose Ni content is known and creating a calibration curve showing the relationship between Ni content and X-ray intensity, the Ni content of the target steel plate can be determined by the calibration curve method. Furthermore, by using a steel plate with a Ni 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 the Ni attached to the surface of the steel plate separately from the Ni arbitrarily contained in the steel plate. As a specific example of such a test piece, a steel plate with a Ni content within ±0.03% of the Ni content of the steel plate under evaluation can be used.

[0059] The form of Ni adhering to the surface of the steel plate is not particularly limited, and includes metallic Ni; NiO and Ni ２ O ３ Ni oxides such as Ni(OH) ２ Examples include Ni hydroxides such as those listed above. Among these, metallic Ni is preferred. In the case of steel sheets that do not contain Sn, if metallic Ni is present on the surface, its local galvanic effect is strong, and the resulting coating may not be stable due to its sensitivity to the chemical conversion treatment conditions. However, in the case of steel sheets containing a predetermined amount of Sn, as in the present invention, the reaction becomes less sensitive to the chemical conversion treatment conditions due to the action of Sn. Therefore, even if metallic Ni is present on the surface of the steel sheet, good chemical conversion treatment properties can be obtained regardless of changes in the chemical conversion treatment conditions. Rather, in the case of steel sheets containing Sn to the extent that the chemical conversion treatment properties deteriorate, metallic Ni has a stronger effect of promoting the dissolution of iron than Ni oxides and hydroxides, thus providing a greater improvement effect on the chemical conversion treatment properties. For example, even with zinc phosphate chemical conversion treatment with solid surface modifiers, which generally tend to reduce chemical conversion treatment properties more than with liquid surface modifiers, attaching metallic Ni allows for an effective local galvanic effect, promoting the dissolution of iron and the precipitation of zinc phosphate. As a result, the improvement effect on the chemical conversion treatment properties can be further enhanced. Here, the morphology of the attached Ni can be confirmed using an X-ray photoelectron spectroscopy apparatus. Ni 2p ３ / ２ From the chemical shift values, metallic Ni, Ni oxide, and Ni hydroxide can be distinguished.

[0060] It is preferable to deposit Ni discontinuously on the surface of the steel plate as island-shaped Ni deposits. Furthermore, the average diameter of the Ni deposit islands, as viewed from above the surface, is preferably 0.3 μm or more, more preferably 0.5 μm or more, and preferably 5 μm or less. If the average diameter is less than 0.3 μm, the effect of local galvanic formation by Ni is difficult to obtain. On the other hand, if the average diameter exceeds 5 μm, the Ni may inhibit the dissolution of iron, and the chemical conversion treatment performance may not improve. The shape and size of the Ni deposits can be confirmed by elemental analysis using a scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDX) at a 2000x magnification field and an acceleration voltage of 5 kV, identifying regions where Ni is 40 mass% or more. The average value of the major and minor axes of that region is determined as the diameter of each Ni deposit island. Then, the same analysis is performed for 10 fields of view to determine the diameter of all Ni deposit islands within the 10 fields of view, and the average value is calculated as the average diameter.

[0061] 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.

[0062] (Method for Manufacturing Steel Sheets) The steel sheet of the present invention is 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 Ni adhesion treatment to the resulting hot-rolled sheet. Alternatively, the steel sheet 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 Ni adhesion treatment to the resulting cold-rolled sheet. Or, the steel sheet 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 Ni 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 Ni 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.

[0063] [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%.

[0064] [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.

[0065] [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.

[0066] [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 Ni 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.

[0067] [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 Ni 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.

[0068] 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 sufficient improvement in the chemical conversion treatment properties due to subsequent Ni adhesion. On the other hand, if the annealing temperature exceeds 900°C, the material properties of the steel sheet may deteriorate.

[0069] 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.

[0070] [Pretreatment] As a pretreatment before depositing Ni, 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 Ni deposit treatment can be carried out. The methods of degreasing and rinsing are not particularly limited, and conventional methods can be used. The pickling method 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.

[0071] [Ni Adhesion] The method for adhering a predetermined amount of Ni to the surface of a steel sheet 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. In particular, adhering Ni in discontinuous island-like structures rather than uniform film-like structures makes it easier to obtain the effect of improving chemical conversion treatment properties by Ni. From this viewpoint, electroplating and electroless displacement plating, which are easy to adhere Ni to the surface of the steel sheet in island-like structures, are preferred. Furthermore, from the viewpoint of easily forming island-like Ni adhering areas having the above-mentioned average diameter by changing various conditions, electroplating and electroless displacement plating are also preferred. The electroplating conditions are an acidic solution containing Ni with a current density of 5 A / dm ２ It is preferable to perform the Ni deposition treatment as follows. More preferably, the current density is 1 A / dm ２ The following applies:

[0072] 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 Ni elemental mass per side deposited on at least one surface of the cold-rolled annealed sheet. Ｎｉ (Unit: mg / m ２ A method for manufacturing a steel sheet, comprising: a Ni adhesion step of adhering Ni to the steel sheet such that it satisfies the following formulas (1) and (2). [Sn] × 200 ≤ W Ｎｉ ≦ [Sn]×7500...(1) W Ｎｉ ≤ 150 ... (2) Here, [Sn] is the Sn content in the steel.

[0073] 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 Ni elemental mass W deposited on at least one side of the surface of the cold-rolled annealed sheet. Ｎｉ (Unit: mg / m ２ A method for manufacturing a steel sheet, comprising: a Ni adhesion step of adhering Ni to the steel sheet such that it satisfies the following formulas (1) and (2). [Sn] × 200 ≤ W Ｎｉ ≦ [Sn]×7500...(1) W Ｎｉ ≤ 150 ... (2) Here, [Sn] is the Sn content in the steel.

[0074] The present invention will be described in more detail below based on examples. However, the present invention is not limited to these examples.

[0075] 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, Ni was deposited on both sides by electroplating to obtain steel sheets. For the Ni plating bath, a solution containing 240 g / L of nickel sulfate hexahydrate and 30 g / L of boric acid, with the pH adjusted to 1.5 using sulfuric acid, was used at a temperature of 50°C. The current density during plating was 0 to 1 A / dm². ２ By varying the time between 1 and 6 seconds, the amount of Ni deposited W can be increased. Ｎｉ The following was controlled. In addition, a steel sheet was obtained in which Ni hydroxide was attached by electroplating with 40 g / L of hydrogen peroxide added to the aforementioned Ni plating bath, under otherwise the same conditions as above.

[0076]

[0077] The morphology of Ni adhering to the surface of the steel sheets manufactured as described above was confirmed according to the method described above. Furthermore, the chemical conversion treatment properties of the steel sheets manufactured as described above were evaluated. The chemical conversion treatment properties were evaluated under the following three conditions.

[0078] [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.

[0079] [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 considered good. ２ The above was further evaluated as good. These results are shown in Table 2.

[0080] [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 coverage rate on the surface of the steel plate after conversion treatment was measured using the same method as in Condition 1. A zinc phosphate coverage rate of less than 80% was evaluated as poor, 80% or more and less than 100% as good, and 100% as even better. These results are shown in Table 2.

[0081]

[0082] As shown in Table 2, the zinc phosphate coating rate is the amount of Ni adhering to the surface of the steel plate W, whether a liquid surface modifier is used or a solid surface modifier is used. Ｎｉ When the amount was too little or too much, the amount decreased, and a tendency was observed for the zinc phosphate conversion treatment to decline. On the other hand, the amount of Zr attached was the amount of Ni attached to the surface of the steel sheet W Ｎｉ As the amount of Ni adhering to the steel sheet surface increased, the Zr conversion treatment properties tended to decrease. Thus, the zinc phosphate conversion treatment properties and Zr conversion treatment properties are related to the amount of Ni adhering to the steel sheet surface W. Ｎｉ It was found that it behaves differently in response to this.

[0083] Furthermore, the results in Table 2 show that even steel plates containing a predetermined amount of Sn that degrades the chemical conversion treatment properties still have a predetermined amount of W adhering to the surface. Ｎｉ It was found that by attaching Ni, it is possible to significantly improve the performance of zinc phosphate conversion treatment, which involves at least liquid surface adjustment, while ensuring good Zr conversion treatment performance. Furthermore, the amount of attachment W Ｎｉ By further controlling this, it was found that it is possible to significantly improve the performance of zinc phosphate conversion treatment, which involves solid surface adjustment, while ensuring good Zr conversion treatment performance.

[0084] 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.

Claims

1. Having a component composition containing Sn: 0.005% by mass or more and 0.200% by mass or less, with Ni attached to at least one of its surfaces, and the amount of Ni attached on one side in terms of Ni elemental mass W Ｎｉ (Unit: mg / m ２ A steel plate characterized in that it satisfies the following equations (1) and (2): [Sn] × 200 ≤ W Ｎｉ ≦ [Sn]×7500...(1) W Ｎｉ ≤ 150 ... (2) Here, [Sn] is the Sn content in the steel.

2. The amount of adhesion W Ｎｉ The steel plate according to claim 1, wherein the following formula (3) is further satisfied. [Sn] × 800 ≤ W Ｎｉ ≦[Sn]×7500...(3) 3. The amount of adhesion W Ｎｉ 90 mg / m² ２ The steel plate according to claim 1 or 2, which is as follows:

4. The steel plate according to any one of claims 1 to 3, wherein the Ni is metallic Ni.

5. The steel sheet according to any one of claims 1 to 4, wherein the component composition further contains one or more elements selected from the group consisting of, 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, with the remainder being Fe and unavoidable impurities.

6. 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 The steel sheet according to claim 5, further containing one or more selected from the group consisting of REM: 0.005% or less, with the remainder being Fe and unavoidable impurities.

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