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 an Fe layer is formed on at least one surface of the steel sheet in an amount WFe that satisfies [Sn]×7500 ≤ WFe.

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. A typical chemical conversion treatment method is zinc phosphate conversion treatment. Zinc phosphate conversion treatment has been widely used for a long time 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 can be liquid or solid, and therefore, regardless of whether the surface preparation agent is liquid or solid, it is required to exhibit high zinc phosphate conversion treatment properties.

[0005] 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 for the purpose of improving the chemical conversion treatment properties of cold-rolled sheets.

[0006] Other methods for improving chemical conversion treatment include, for example, Patent Document 3, which describes applying 20 to 1500 mg / m² to the surface of a cold-rolled steel sheet containing Si: 0.1 to 1.5 wt.% and Mn: 0.2 to 3.0 wt.%. ２ A technique is described for improving chemical treatment properties by forming an iron coating layer of a certain amount. However, this technique does not adequately consider the influence of trump elements specific to electric furnace materials.

[0007] Japanese Patent Publication No. 2020-84238, Japanese Patent Publication No. 2020-84325, Japanese Patent Publication No. Hei 5-320952

[0008] 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 present invention aims to provide a steel sheet that can exhibit broadly excellent zinc phosphate conversion treatment properties, even when the steel sheet contains trump elements, regardless of whether the surface modifier used in the zinc phosphate conversion treatment is liquid or solid.

[0009] To solve the above problems, the inventors diligently researched the effect of trump elements on chemical conversion treatment properties. They revealed that the presence of Sn in particular among the trump elements degrades the chemical conversion treatment properties. Based on this, the inventors further diligently researched methods to improve the chemical conversion treatment properties of steel sheets containing Sn. As a result, they found that good chemical conversion treatment properties can be obtained by forming an Fe layer on the surface of the steel sheet in an amount that satisfies a predetermined relationship depending on the Sn content in the steel.

[0010] The present invention is based on the above findings, and its gist is as follows: [1] Having a component composition containing Sn: 0.005% by mass or more and 0.200% by mass or less, with an Fe layer formed on at least one of the surfaces, the amount of Fe elemental mass W of the Fe layer attached to one side Ｆｅ (Unit: mg / m ２ A steel plate characterized by satisfying the following formula (1): [Sn] × 7500 ≤ W Ｆｅ ... (1) Here, [Sn] is the Sn content in the steel.

[0011] [2] The amount of adhesion W Ｆｅ 1500 mg / m² ２ The steel plate described in [1] above.

[0012] [3] The steel sheet according to [1] or [2] above, wherein the Fe layer further comprises 10% by mass or less in total of one or more selected from the group consisting of B, C, P, N, O, Ni, Mn, Zn, and Co.

[0013] [4] The steel sheet according to any one of [1] to [3] above, wherein the component composition further contains one or more selected from the group consisting of Cu: 0.08% to 1.00% by mass, Cr: 0.010% to 1.000% and Mo: 0.01% to 0.50% and further contains one or more selected from the group consisting of 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 and Ni: 1.0% or less, with the remainder being Fe and unavoidable impurities.

[0014] [5] 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 [4] above, further containing one or more selected from the group consisting of REM: 0.005% or less, with the remainder being Fe and unavoidable impurities.

[0015] According to the present invention, it is possible to provide a steel sheet that has good chemical conversion treatment properties even when containing a predetermined amount of Sn. More specifically, even when containing the above-mentioned predetermined tramp element, it is possible to provide a steel sheet that exhibits excellent zinc phosphate chemical conversion treatment properties whether accompanied by a liquid surface modifier or a solid surface modifier.

[0016] Hereinafter, the present invention will be specifically described. The following description relates to examples of preferred embodiments of the present invention, and the present invention is not limited to the embodiments described below. In this specification, "%" represents "mass%" unless otherwise specified. Also, the numerical range represented by "~" in this specification means a range including the numerical values described before and after "~" as the lower limit value and the upper limit value, respectively. Also, when only one of the numerical values described before and after "~" is provided with a unit, the other is also provided with the same unit unless otherwise specified.

[0017] (Steel sheet) The steel sheet of the present invention contains a predetermined amount of Sn and has a predetermined amount of Fe layer formed on its surface. By forming a predetermined amount of Fe layer on the surface of the steel sheet, even if it contains a predetermined amount of Sn, which is a typical Trump element, the steel sheet can exhibit excellent phosphating treatment properties. The steel sheet of the present invention is widely excellent in phosphating treatment properties regardless of whether the surface conditioner is liquid or solid, so it can be applied to a wide range of fields regardless of the surface conditioning method. Furthermore, since it can be applied to both hot-rolled sheets and cold-rolled sheets, it is particularly useful as a steel sheet for automobiles. Also, since the steel sheet of the present invention is excellent in chemical conversion treatment properties even when it contains Trump elements, it can be particularly usefully applied to electric furnace materials using iron scrap as a raw material, and has an outstanding industrial effect considering the environment. And the steel sheet of the present invention can be obtained, for example, according to the manufacturing method described later.

[0018] First, the influence of Sn on the chemical conversion treatment properties of the steel sheet studied by the present inventors will be explained. The mechanism of the chemical conversion treatment reaction is as follows. When the steel sheet comes into contact with the chemical conversion treatment liquid, iron dissolution occurs from the steel sheet, and as a counter reaction, reduction of hydrogen ions occurs in the chemical conversion treatment liquid near the surface of the steel sheet. This is the reaction mechanism in which the pH at the steel sheet interface rises and a chemical conversion treatment film is deposited on the surface of the steel sheet. Here, when Sn is contained in the steel sheet, when the steel sheet comes into contact with the chemical conversion treatment liquid, 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＋ A hydroxide is formed by the reaction, and since hydrogen ions are generated in this process, the increase in pH at the steel sheet interface is hindered. Thus, it is considered that the presence of Sn in the steel sheet deteriorates the chemical conversion treatment property.

[0019] As a result of further investigations by the present inventors, it has been found that the deterioration of the chemical conversion treatment property due to such trump element is particularly significant in the nucleation during zinc phosphate chemical conversion treatment among chemical conversion treatments, and the zinc phosphate chemical conversion treatment property deteriorates significantly by containing Sn. Further, as described above, the zinc phosphate chemical conversion treatment is roughly classified into a case where a liquid surface conditioner is used and a case where a solid surface conditioner is used. According to the investigations by the present inventors, although the presence of Sn has an adverse effect on the chemical conversion treatment property in any of the cases where the above surface conditioners are used, it has also been found that the degree thereof varies depending on the type of the surface conditioner. Therefore, it has been found that in order to improve the zinc phosphate chemical conversion treatment property widely in the steel sheet containing Sn, it is necessary to find appropriate conditions for each case where the surface conditioner is used.

[0020] [Component composition] The steel sheet contains at least Sn: 0.005% or more and 0.200% or less. This assumes a steel sheet made of an electric furnace material using iron scrap as a raw material, and means that the steel sheet contains so-called trump elements. In the present invention, since a predetermined amount of Fe layer is formed on the surface of the steel sheet, even if such trump element exists, a good zinc phosphate chemical conversion treatment property can be realized.

[0021] In addition to the above Sn, the steel sheet may further contain at least one selected from the group consisting of Cu: 0.08% or more and 1.00% or less, Cr: 0.010% or more and 1.000% or less, and Mo: 0.01% or more and 0.50% or less. That is, the steel sheet contains, as trump elements, Sn: 0.005% by mass or more and 0.200% by mass or less, and optionally, Cu: 0.08% or more and 1.00% or less, Cr: 0.010% or more and 1.000% or less, and Mo: 0.01% or more and 0.50% or less. One or more selected from the group consisting of these elements may also be contained in the steel sheet, particularly when using an electric furnace material, as trump elements. In the present invention, since a predetermined amount of Fe layer is formed on the surface of the steel sheet, even if such additional trump element is present, and even when the steel sheet contains trump elements of Sn, Cu, Cr, and Mo, good zinc phosphate chemical conversion treatment properties can be achieved.

[0022] Sn: 0.005 to 0.200% Sn (tin) is a typical trump element contained in iron scrap. Sn of 0.005% or more is an amount exceeding the content that can be mixed at an inevitable impurity level, and as described above, it reduces the chemical conversion treatment properties of the steel sheet. In particular, it greatly impairs the zinc phosphate chemical conversion treatment properties. On the other hand, when the Sn amount exceeds 0.200%, the toughness of the steel sheet decreases due to the segregation of Sn at the grain boundaries. Therefore, the Sn amount is set to 0.200% or less, preferably 0.050% or less. The excessively contained Sn amount can be adjusted, for example, by mixing with hot metal produced by the blast furnace method without scrap. However, in the present application, since a predetermined amount of Fe layer is formed, even if it contains Sn of, for example, 0.010% or more, more than 0.100%, 0.110% or more, and up to 0.200%, the chemical conversion treatment properties are good.

[0023] Cu: 0.08-1.00% Cu (copper) is one of the trump elements that can be further mixed in. Cu of 0.08% or more exceeds the amount that can be mixed in at an unavoidable impurity level, and is thought to further deteriorate the chemical conversion treatment properties by a mechanism similar to that of Sn. In particular, it is thought to further impair the zinc phosphate chemical conversion treatment properties. On the other hand, if the amount of Cu exceeds 1.00%, surface defects caused by red-hot brittleness, etc., may occur. Therefore, the amount of Cu is preferably 1.00% or less, more preferably less than 0.50%, and even more preferably 0.45% or less. The amount of Cu that is excessively included can be adjusted, for example, by a method similar to that for Sn. However, since a predetermined amount of Fe layer is formed in this application, even if Cu is further included in amounts of, for example, 0.10% or more, more than 0.50%, 0.55% or more, and up to 1.00%, the chemical conversion treatment properties can be kept good.

[0024] Cr: 0.010 to 1.000% Cr (chromium) is another element that can be further mixed in. Cr content of 0.010% or more exceeds the amount that can be mixed in at an unavoidable impurity level, and is thought to further degrade the chemical conversion treatment properties through a mechanism similar to that of Sn. In particular, it is thought to further impair the zinc phosphate chemical conversion treatment properties. On the other hand, if the Cr content exceeds 1.000%, the pitting corrosion resistance may deteriorate. Therefore, the Cr content is preferably 1.000% or less, more preferably less than 0.100%, and even more preferably 0.090% or less. Excessively contained Cr can be adjusted, for example, by a method similar to that used for Sn. However, since a predetermined amount of Fe layer is formed in this application, even if Cr is further included, for example, more than 0.20%, 0.25% or more, and up to 1.000%, the chemical conversion treatment properties can be kept good.

[0025] Mo: 0.01-0.50% Mo (molybdenum) is another potential impurity element that can be added. Mo content exceeding 0.01% is above the level of unavoidable impurity and is thought to further degrade the chemical conversion treatment properties through a mechanism similar to that of Sn. In particular, it is thought to further impair the zinc phosphate conversion treatment properties. On the other hand, if the Mo content exceeds 0.50%, the hot rolling properties may decrease. Therefore, the Mo content is preferably 0.50% or less, more preferably less than 0.20%, and even more preferably 0.15% or less. Excessively contained Mo content can be adjusted, for example, by a method similar to that used for Sn.

[0026] In addition to the aforementioned Sn, Cu, Cr, and Mo, the steel sheet may contain other components without particular restriction, as long as they are elements commonly found in ordinary 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 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, and Ni: 1.0% or less, each in an amount greater than 0%. The remainder of the steel sheet's composition, other than the above elements, may consist of Fe and unavoidable impurities.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[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] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] [Formation of Fe layer] The steel sheet having the above-described component composition has an Fe layer formed on at least one of its surfaces. At this time, the amount of Fe elemental mass W of the Fe layer is Ｆｅ (Unit: mg / m ２ It is crucial that the following equation (1) is satisfied: [Sn] × 7500 ≤ W Ｆｅ ... (1) Here, [Sn] is the Sn content in the steel (unit: mass%). Therefore, in the above equation (1), the W that encloses the symbol "≦" Ｆｅ [Sn] × 7500 has different units. However, the inventors have determined that the unit area (m²) of the steel plate surface is different. ２ The amount of Fe layer attached per unit (mg), i.e., W Ｆｅ (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.

[0051] A predetermined amount W is deposited on the surface of the steel plate, depending on the amount of Sn in the steel. ＦｅBy forming an Fe layer, 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 chemical conversion treatment properties can be improved regardless of the type of surface modifier. The inventors speculate on the reason for this as follows: That is, by forming an Fe layer on the surface of the steel sheet satisfying formula (1), when the steel sheet comes into contact with the chemical conversion treatment solution, the Fe layer present on the surface of the steel sheet dissolves first. As a result, the pH at the interface between the steel sheet and the chemical conversion treatment solution rises without being affected by trump element elements in the steel sheet (base metal), and zinc phosphate nucleation can occur. After all of the Fe layer formed on the surface has dissolved, it is then affected by trump element elements contained in the base metal. However, since sufficient zinc phosphate nucleation has already occurred by this time, it is thought that the nucleation and zinc phosphate film formation reactions proceed rapidly.

[0052] Amount of Fe layer deposited on one side, calculated based on the elemental mass of Fe, W Ｆｅ (mg / m ２ The amount of Sn in the steel [Sn] (%) is controlled and must be ([Sn] × 7500) or more. The amount of Fe layer adhesion W Ｆｅ [Sn] × 7500 (mg / m²) ２ If the amount of Fe layer is less than ), zinc phosphate nucleation will not proceed sufficiently, and the improvement effect on chemical conversion treatment will not be obtained. Ｆｅ Preferably, the amount of Fe elemental mass is 1500 mg / m² per side. ２ The above is preferable, and more preferably 1500 mg / m² ２ It is greater than, and more preferably 2000 mg / m² ２ That concludes the explanation. By setting the limit above the lower limit, the entire surface of the steel plate can be more reliably coated with zinc phosphate before the Fe layer completely dissolves and the base metal is exposed during the chemical conversion treatment. Therefore, the chemical conversion treatment can be completed with less adverse effects from tramp element elements contained in the base metal, resulting in even better chemical conversion treatment performance. On the other hand, the amount of Fe layer W Ｆｅ The upper limit is not particularly limited from the standpoint of improving chemical treatment properties, but from the standpoint of treatment cost, it is 5000 mg / m². ２ The following is preferable.

[0053] The Fe layer may be formed on only one side of the steel plate, or on both sides. When the Fe layer is formed on both sides of the steel plate, the amount of Fe layer W on each side Ｆｅ Each of these must satisfy the above formula (1). Whether the Fe layer is formed on only one side of the steel plate surface or on both sides should be determined according to the need for post-processing treatments on the steel plate, such as painting or rust prevention treatment.

[0054] Here, W is the amount of Fe layer attached to the steel plate, calculated in terms of Fe elemental mass. Ｆｅ The Fe layer can be measured as follows: A 10 mm x 15 mm sample is taken from the steel plate after the Fe layer has been formed, embedded in resin so that the cross-section of the steel plate is exposed, and polished to create a sample for cross-sectional observation. Ten arbitrary locations on this cross-sectional observation sample are observed using a scanning electron microscope (SEM) at an acceleration voltage of 5 kV and a magnification of 2,000 to 50,000 times depending on the thickness of the Fe layer. The average thickness of the Fe layer for the ten observed fields is then multiplied by the specific gravity of iron to convert it to the amount per side of the Fe layer. If the Fe layer is an iron alloy layer (Fe-based layer) as described later, elemental analysis of the Fe-based layer is performed using an energy-dispersive X-ray spectrometer (EDX), and the value in terms of Fe elemental mass can be calculated by multiplying the obtained Fe mass ratio by the value obtained without considering the alloy elements as described above.

[0055] The type of Fe layer present on the surface of the steel sheet is not particularly limited and can be appropriately selected according to the required performance. For example, the Fe layer may be pure Fe. When the Fe layer is pure Fe, the Fe content in the Fe layer shall be 99% by mass or more, and the remainder other than Fe shall be unavoidable impurities. The Fe layer may also be an Fe-based layer containing any alloying element. Examples of alloys constituting the Fe-based layer include Fe-B alloy, Fe-C alloy, Fe-P alloy, Fe-N alloy, Fe-O alloy, Fe-Ni alloy, Fe-Mn alloy, Fe-Zn alloy, and Fe-Co alloy. These Fe-based layers can be obtained well by alloy plating. However, since the inclusion of Sn, Cu, Cr, and Mo may actually degrade the chemical conversion treatment properties, it is preferable that the Fe-based layer does not contain these elements.

[0056] Furthermore, the composition of the Fe-based layer is not particularly limited. For example, it can contain more than 0% in total alloying elements selected from the group consisting of B, C, P, N, O, Ni, Mn, Zn, and Co, and it is preferable that it contains 1% or more. It is also preferable that the total content of alloying elements in the Fe-based layer be 10% or less. By limiting the total amount of alloying elements other than Fe in the Fe-based layer to 10% or less, a decrease in electrolytic efficiency can be prevented, and the Fe-based layer can be formed at a lower cost. It is preferable that the remainder of the Fe-based layer, other than the alloying elements, consists of Fe and unavoidable impurities.

[0057] Here, the type and content of the formed Fe layer can be confirmed in the same way as the method for confirming the amount of Fe layer adhesion described above. Specifically, this can be confirmed by observing the cross-section of the steel plate using a SEM and performing elemental analysis of the Fe layer using EDX. Note that the elements such as Fe that make up the base steel plate and the elements such as Fe that make up the Fe layer can be distinguished and confirmed by analyzing only the outermost Fe layer from the cross-section described above.

[0058] It is preferable that the Fe layer be uniformly attached to the surface of the steel plate, and that the area ratio of exposed base metal where the Fe layer is not attached is 20% or less. If the area ratio of exposed base metal exceeds 20%, the effect of improving the chemical conversion treatment properties by the Fe layer 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 observing the cross-section of the steel plate using a SEM under conditions of a 1000x field of view and an acceleration voltage of 5kV, and calculating the ratio of the length where the Fe layer is not formed to the total length of the cross-section along the surface of the steel plate. The above ratio can be calculated for 10 fields of view, and the average value can be taken as the area ratio of exposed base metal (area %).

[0059] 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.

[0060] (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 subjecting the resulting hot-rolled sheet to an Fe layer formation treatment. 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 subjecting the resulting cold-rolled sheet to an Fe layer formation treatment. 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 subjecting the resulting cold-rolled and annealed sheet to an Fe layer formation treatment. In particular, as will be described later, a steel sheet can be suitably obtained by subjecting the cold-rolled and annealed sheet to an Fe layer formation treatment. 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.

[0061] [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%.

[0062] [Hot Rolling] An example of the hot rolling process is described below. In the hot rolling process, a steel slab having a composition containing at least 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.

[0063] [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.

[0064] [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 Fe layer formation treatments 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.

[0065] [Annealing] An example of the annealing process is described below. By performing annealing (recrystallization 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 of the steel sheet, 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 the subsequent Fe layer formation treatment 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.

[0066] 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 may be insufficient, and a concentrated layer may remain, which may prevent the improvement in chemical conversion treatment properties due to the subsequent formation of the Fe layer from being fully achieved. On the other hand, if the annealing temperature exceeds 900°C, the material properties of the steel sheet may deteriorate.

[0067] 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.

[0068] [Pretreatment] As a pretreatment before forming the Fe layer, 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, an Fe layer formation treatment, more preferably an Fe-based electroplating treatment, can be carried out. The methods for 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. 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.

[0069] [Formation of Fe layer] At least one side of the hot-rolled or cold-rolled sheet obtained in this way is coated with Fe W per side in terms of Fe elemental mass. Ｆｅ (mg / m ２ ) is controlled according to the amount of Sn in the steel [Sn] (%), and [Sn] × 7500 ≤ W Ｆｅ The Fe layer is formed in such a manner. The method for forming the Fe layer is not particularly limited, but it can be preferably done by electroplating because it is easy to form a uniform Fe layer.

[0070] The electroplating method for forming the Fe layer is not particularly limited. For example, a sulfuric acid bath, a hydrochloric acid bath, or a mixture of both can be used as the Fe-based electroplating bath. The Fe ion content in the Fe-based electroplating bath before the start of current application is Fe ２＋ Preferably, the Fe ion content in the Fe-based electroplating bath is 1.0 mol / L or more, and more preferably 1.5 mol / L or more. ２＋ If the above lower limit is met, it is easier to obtain a sufficient amount of Fe deposition. On the other hand, the upper limit of the Fe ion content in the Fe-based electroplating bath is, from the viewpoint of preventing the deposition of Fe compounds (sludge) in the plating bath, ２＋ It can be set to 2.0 mol / L or less.

[0071] Furthermore, the Fe-based electroplating bath may also contain Fe ions and alloying elements such as B, C, P, N, O, Ni, Mn, Zn, and Co. The content of these alloying elements in the bath should match the content in the Fe-based layer described above. In addition, conductivity enhancers such as sodium sulfate and potassium sulfate may be included as additives or impurities. The metal elements mentioned above may be included as metal ions, and nonmetal elements may be included as part of boric acid, phosphoric acid, nitric acid, organic acids, etc. For example, if the Fe-based electroplating bath is an iron sulfate plating bath, the iron sulfate plating solution may contain conductivity enhancers such as sodium sulfate and potassium sulfate, chelating agents, and pH buffers.

[0072] The bath temperature of the Fe-based electroplating bath is not particularly limited, but considering the ability to maintain a constant temperature, it is preferably 30°C or higher, more preferably over 35°C, and even more preferably 40°C or higher. Setting the bath temperature above the above lower limit makes it easier to form a uniform Fe layer than at lower temperatures. The bath temperature can be 60°C or lower from the viewpoint of not increasing the rate of Fe compound (sludge) formation. The pH of the Fe-based electroplating bath is not particularly limited, but considering the electrical conductivity of the plating bath, it is preferably 3.0 or lower. The pH of the plating bath can be 1.5 or higher from the viewpoint of electrolytic efficiency. The current density is also not particularly limited, but from the viewpoint of forming a uniform Fe layer, it is 10 A / dm ２ The above is preferable, and 100 A / dm ２ The following is preferable: 10 to 100 A / dm ２ A more favorable degree is preferred. The plate feeding speed during plating is preferably 5 mpm (meters per minute) or higher, and preferably 300 mpm or lower. Productivity tends to be poor at plate feeding speeds below 5 mpm. On the other hand, at plate feeding speeds of 300 mpm or higher, the equipment length required to obtain a sufficient amount of plating adhesion increases, which tends to increase costs.

[0073] 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 Fe element per side attached to at least one surface of the cold-rolled annealed sheet. Ｆｅ (Unit: mg / m ２ A method for manufacturing a steel sheet, comprising: an Fe layer forming step, in which an Fe layer is formed such that the following formula (1) is satisfied. [Sn] × 7500 ≤ W Ｆｅ ... (1) Here, [Sn] is the Sn content in the steel.

[0074] 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 electroplating the amount of Fe elemental mass W per side on at least one surface of the cold-rolled annealed sheet. Ｆｅ (Unit: mg / m ２ A method for manufacturing a steel sheet, comprising: an Fe layer forming step, in which an Fe layer is formed such that the following formula (1) is satisfied. [Sn] × 7500 ≤ W Ｆｅ ... (1) Here, [Sn] is the Sn content in the steel.

[0075] The present invention will be described in more detail below based on examples. However, the present invention is not limited to these examples.

[0076] 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 consists 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 (non-oxidizing atmosphere) containing 5 vol% hydrogen 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. Subsequently, Fe layers were formed on both sides by electroplating to obtain steel sheets. As the Fe-based electroplating bath, Fe ２＋ A solution containing 1.2 mol / L of [substance name] and 0.2 mol / L of sodium sulfate, with the pH adjusted to 2.0 using sulfuric acid, was used after adjusting the temperature to 50°C. The current density during plating was 10 to 100 A / dm².２ By varying the immersion time between 0.5 and 5 seconds, the amount of Fe layer formed W Ｆｅ It was controlled.

[0077]

[0078] The steel sheets manufactured as described above were evaluated for their chemical conversion treatment properties. The evaluation method for chemical conversion treatment properties was carried out under the following two conditions.

[0079] [Zinc Phosphate Conversion Treatment (Condition 1)] 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. These conditions use a Ti colloid-based solid 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, 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.

[0080] [Zinc Phosphate Conversion Treatment (Condition 2)] 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. This condition uses a zinc phosphate-based liquid surface modifier as the surface modifier. The zinc phosphate coating rate on the surface of the steel plate after 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. These results are shown in Table 2.

[0081]

[0082] As shown in Table 2, the zinc phosphate coating rate is the amount of Fe layer W attached to the surface of the steel sheet, regardless of whether a solid surface modifier or a liquid surface modifier is used. Ｆｅ When the amount of Fe layer W was too small, it decreased, and a tendency was observed for the zinc phosphate conversion treatment to decline. However, the amount of Fe layer W Ｆｅ It was found that the degree to which the zinc phosphate conversion treatment performance deteriorates in response to a decrease in [a certain value] is greater when using a solid surface modifier than when using a liquid surface modifier, indicating a significant difference in the results.

[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 can still have a predetermined amount of W deposited on the surface. Ｆｅ It was found that by forming an Fe layer, the zinc phosphate conversion treatment properties can be greatly improved, whether liquid surface treatment or solid surface treatment is performed. Thus, despite containing tramp element elements, the steel sheet of the present invention exhibits excellent zinc phosphate conversion treatment properties under both solid surface treatment and liquid surface treatment conditions.

Claims

1. Having a component composition containing Sn: 0.005% by mass or more and 0.200% by mass or less, with an Fe layer formed on at least one of the surfaces, and the amount of Fe elemental mass of the Fe layer deposited on one side W Ｆｅ (Unit: mg / m ２ A steel plate characterized by satisfying the following formula (1): [Sn] × 7500 ≤ W Ｆｅ ... (1) Here, [Sn] is the Sn content in the steel.

2. The amount of adhesion W Ｆｅ 1500 mg / m² ２ The steel plate according to claim 1.

3. The steel sheet according to claim 1 or 2, wherein the Fe layer further comprises 10% by mass or less in total of one or more elements selected from the group consisting of B, C, P, N, O, Ni, Mn, Zn, and Co.

4. The steel sheet according to any one of claims 1 to 3, wherein the component composition further contains one or more selected from the group consisting of Cu: 0.08% to 1.00% by mass, Cr: 0.010% to 1.000% and Mo: 0.01% to 0.50% and further contains one or more selected from the group consisting of 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 and Ni: 1.0% or less, with the remainder being Fe and unavoidable impurities.

5. 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 4, 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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