Steel sheet and component including same

Abstract

The present invention has as its object the provision of a steel sheet containing Ni, Cu, and Sn and having excellent painted corrosion resistance and a part containing the same. A steel sheet and a part containing thereof according to the present invention are characterized, in which a chemical composition of the steel sheet contains, by mass%, Si: 0.30 to 3.00%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000% and Sn: 0.003 to 1.000%, and satisfies Si / (Si+Mn)≥0.20, and an intensity of Sn measured by X-ray photoelectron spectroscopy in a thickness direction from a surface of the steel sheet satisfies (outermost surface I4 8 1 - 4 9 0 ) / (depth 50 nm I4 8 1 - 4 9 0 )≤ 0.5 and satisfies (depth 50 nm I4 8 4 . 9 ) / (depth 50 nm I4 8 6 . 5 )≤0.7.

Description

FIELD

[0001] The present invention relates to a steel sheet and a part containing the same.BACKGROUND

[0002] To improve the corrosion resistance of a steel sheet, raising the chemical convertibility of the surface of the steel sheet and making a conversion coating uniformly form on the surface of that steel sheet is effective.

[0003] In relation to this, for example, PTL 1 discloses a method of production of a high strength cold rolled steel sheet comprising using a continuous annealing furnace with a cooling zone including part or all of a 600 to 250°C steel sheet temperature range following heating for recrystallization and with a cooling system of one of more of gas cooling, dissipative cooling, and cooling tube cooling, or a facility for both a cold rolled steel sheet / hot dip galvanized steel sheet having a continuous annealing furnace, to continuously anneal a high strength cold rolled steel sheet, during which exposing a surface of the steel sheet to an iron oxidizing atmosphere in that steel sheet temperature range, pickling it at an annealing furnace exit side, then iron or Ni plating it to 1 to 50 mg / m2. Furthermore, this PTL 1 teaches that while usually the steel sheet surroundings are rendered an ultralow concentration oxygen and / or ultralow dew point inert atmospheric gas to prevent oxidation of the steel sheet, conversely to the above, by deliberately exposing the sheet to an oxidizing atmosphere, making Si, Mn, and even the iron of the steel sheet oxidize, and, in the pickling when exiting the annealing furnace, removing the iron oxide film of the steel sheet along with the Si, Mn, and other oxide film by the pickling, a high strength cold rolled steel sheet free from "bald spots" even if the contents of Si, Mn, etc. are high and excellent in chemical convertibility is obtained.

[0004] Further, PTL 2 discloses a steel sheet for automotive use containing copper (Cu) in 0.10 mass% or more and 0.50 mass% or less, having a number of particles of residual scale at the surface of 160,000 / mm 2< or less, and having a maximum size of copper compound particles exposed at the surface of 2 µm or less. Furthermore, PTL 2 teaches that by this constitution, since the size of the copper compound particles of the steel sheet serving as cathode points in chemical conversion is made 2 µm or less and the residual scale is made a predetermined amount or less, it is possible to provide a steel sheet excellent in chemical convertibility.[CITATION LIST][PATENT LITERATURE]

[0005] [PTL 1] Japanese Unexamined Patent Publication No. 2008-190030 [PTL 2] Japanese Unexamined Patent Publication No. 2020-084238 SUMMARY[TECHNICAL PROBLEM]

[0006] The above PTL 2 teaches that, in addition to copper (Cu), nickel (Ni), tin (Sn), and other elements cause a drop in the strength, shapeability, and other mechanical properties of a steel sheet for automobile use plus the corrosion resistance and other chemical stability, in particular that the copper compounds present on the surface of the steel sheet cause a drop in the chemical convertibility for improving the corrosion resistance. Further, in general, if the chemical convertibility drops, sometimes regions called "bald spots" where no conversion coating is formed are formed. As a result, sometimes the painted corrosion resistance falls.

[0007] Therefore, the present invention has as its object the provision of a steel sheet containing Ni, Cu, and Sn and having excellent painted corrosion resistance and a part containing the same.[SOLUTION TO PROBLEM]

[0008] The present invention includes at least the following aspects:(Aspect 1)

[0009] A steel sheet, in which steel sheet, a chemical composition of the steel sheet contains, by mass%, Si: 0.30 to 3.00%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000% and satisfies Si / (Si+Mn)≥0.20, and an intensity of Sn measured by X-ray photoelectron spectroscopy in a thickness direction from a surface of the steel sheet satisfies (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1 - 4 9 0 )≤ 0.5 and satisfies (depth 50 nm I 4 8 4. 9 ) / (depth 50 nm I 4 8 6 . 5 )≤0.7. (Aspect 2)

[0010] The steel sheet according to aspect 1 wherein an intensity of Sn measured by X-ray photoelectron spectroscopy in a thickness direction from a surface of the steel sheet satisfies (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1 - 4 9 0 )≤0.3.(Aspect 3)

[0011] The steel sheet according to aspect 1 or 2 wherein an intensity of Sn measured by X-ray photoelectron spectroscopy in a thickness direction from a surface of the steel sheet satisfies (depth 50 nm I 4 8 4 . 9 ) / (depth 50 nm I 4 8 6. 5 )≤ 0.5.(Aspect 4)

[0012] The steel sheet according to any of the aspects 1 to 3, wherein the chemical composition of the steel sheet contains, by mass%, Si: 0.50 to 3.00%.(Aspect 5)

[0013] The steel sheet according to any of the aspects 1 to 4, wherein the chemical composition of the steel sheet contains, by mass%, Ni: 0.040 to 1.000%, Cu: 0.040 to 1.000%, and Sn: 0.004 to 1.000%. (Aspect 6)

[0014] The steel sheet according to any one of aspects 1 to 5, having a conversion coating on the surface of the steel sheet.(Aspect 7)

[0015] A part including a steel sheet according to any of aspects 1 to 6.[ADVANTAGEOUS EFFECTS OF INVENTION]

[0016] According to the present invention, it is possible to provide a steel sheet containing Ni, Cu, and Sn which has excellent painted corrosion resistance and a part containing the same.DESCRIPTION OF EMBODIMENTS

[0017] As explained above, in general, if a steel sheet drops in chemical convertibility, regions called "bald spots" where no conversion coating is formed sometimes occur. As a result, the corrosion resistance sometimes falls. For example, if Ni, Cu, Sn, and other elements are present dissolved in a steel sheet, the potential of the steel sheet becomes more precious compared with the potential in the state where these elements are not dissolved and, at the time of chemical conversion, sometimes the etchability of Fe falls. In particular, if a steel sheet simultaneously contains the three elements of Ni, Cu, and Sn, due to the Ni and Cu, the Sn easily concentrates at the surface of the steel sheet during the annealing step. The Sn concentrating at the surface of the steel sheet obstructs chemical conversion, therefore causes etching defects at the time of chemical conversion of the surface of the steel sheet and makes formation of a conversion coating difficult. For this reason, in such a case, the chemical convertibility falls and as a result the painted corrosion resistance ends up falling. Therefore, if a steel sheet simultaneously contains the three elements of Ni, Cu, and Sn, such a fall in the painted corrosion resistance particularly becomes a problem.

[0018] As the method for producing steel, two types of methods are generally known: the method of using the natural resource of iron ore as the main raw material for obtaining molten iron at a blast furnace, then refining this at a converter etc. to produce molten st eel and the method of using the recycled resource of scrap metal as the main raw mater ial for producing molten steel at an electric furnace. The steel produced by the former method, i.e., the blast furnace material, can contain Ni, Cu, Sn, and other elements as a dded elements, therefore it is necessary to suitably deal with this issue if these elements are contained. On the other hand, in the steel produced by the latter method, i.e., the el ectric furnace material, since as explained above, scrap metal is used as the main raw m aterial, Ni, Cu, Sn, and other scrap derived elements (so-called "tramp elements") are co ntained in relatively large amounts. In addition, the three elements of Ni, Cu, and Sn ar e easily simultaneously contained. Therefore, in the electric furnace material, this issue p articularly becomes prominent.

[0019] Therefore, the inventors engaged in studies focusing in particular on the distribution of elements at the steel sheet surface layer so as to provide a steel sheet having excellent painted corrosion resistance even when the steel sheet simultaneously contains the three elements of Ni, Cu, and Sn. As a result, they learned that even among the three elements of Ni, Cu, and Sn, Sn in particular easily concentrates at the surface of a steel sheet in an annealing step due to the presence of Ni and Cu and the content of metal Sn at the surface of the steel sheet in the dissolved state (below, sometimes referred to as the "solute Sn") easily increases. Solute Sn present at the surface of a steel sheet obstructs chemical conversion, therefore causes etching defects at the time of chemical conversion of the surface of a steel sheet and becomes a cause of deterioration of the painted corrosion resistance.

[0020] The inventors discovered that by oxidizing the Sn in a steel sheet surface layer to form tin oxide (below, sometimes referred to as "Sn oxide"), it is possible to reduce the content of solute Sn at the surface of a steel sheet and that, further, by removing the Sn oxide at the surface of a steel sheet, it is possible to remarkably improve the chemical convertibility of the surface of a steel sheet and as a result obtain an excellent painted corrosion resistance. Specifically, the inventors discovered that even in a steel sheet containing the three elements of Ni, Cu, and Sn, by adopting the specific technique of making the chemical composition of the obtained steel sheet contain Si in 0.30 to 3.00% and satisfy Si / (Si+Mn)≥0.20, pickling, then brush grinding the hot rolled steel sheet, performing an annealing step at a high dew point, making the dwell time at the range of 600 to 700°C in the annealing step 12.5 seconds or less, and, furthermore, performing the pickling after the annealing step, it is possible to reduce the contents of solute Sn and Sn oxide at the surface of the steel sheet so that the intensity of Sn measured by X-ray photoelectron spectroscopy in the thickness direction from the surface of the steel sheet satisfies (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1 - 4 9 0 )≤0.5 and (depth 50 nm I 4 8 4 .9 ) / (depth 50 nm I 4 8 6 . 5 )≤0.7. Furthermore, they discovered that due to this, it is possible to remarkably improve the chemical convertibility of the surface of a steel sheet and as a result obtain an excellent painted corrosion resistance.

[0021] The present invention was completed based on the above such discoveries and includes the following aspects of an embodiment:

[0022] Below, a preferred embodiment of the steel sheet of the present invention will be explained in detail.<Steel Sheet>

[0023] A steel sheet according to one embodiment of the present invention has a specific chemical composition containing, by mass%, Si: 0.30 to 3.00%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000% and satisfying Si / (Si+Mn)≥0.20.

[0024] Further, the steel sheet of the present embodiment is provided with a distinctive constitution having an intensity of Sn measured by X-ray photoelectronic spectroscopy in a thickness direction from the surface of the steel sheet satisfying (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1 - 4 9 0 )≤0.5 and satisfying (depth 50 nm I 4 8 4 . 9 ) / (depth 50 nm I 4 8 6 . 5 )≤0.7.

[0025] In general, in chemical conversion, anodic dissolution of Fe (etching) occurs, whereupon electrons are generated. On the other hand, the electrons generated by the anodic dissolution of Fe cause a cathodic reaction (2H +< +2e -< →H 2 , 10H +< +NO 3 -< +8e -< →NH 4 +< +3H 2 O) to occur. In turn, the pH of the chemical conversion solution near the surface of the steel sheet rises. Along with this, zinc phosphate crystals and other compounds precipitate forming a conversion coating at the surface of the steel sheet.

[0026] In this regard, in a steel sheet with Ni, Cu, and Sn present dissolved in the steel sheet, compared with one where these elements are not dissolved, the potential of the steel sheet becomes precious and sometimes, at the time of chemical conversion, the etchability of Fe falls. Further, among the three elements of Ni, Cu, and Sn as well, Sn in particular easily concentrates at the surface of the steel sheet during the annealing step due to the presence of Ni and Cu, therefore the concentration of Sn at the surface of the steel sheet in the dissolved state easily increases. The solute Sn present at the surface of the steel sheet, as explained above, obstructs the chemical conversion of the surface of the steel sheet, therefore causes etching defects at the time of chemical conversion and becomes a cause of a drop in the painted corrosion resistance.

[0027] In the steel sheet of the present embodiment, even if a steel sheet containing the three elements of Ni, Cu, and Sn, due to the technique explained later, the contents of solute Sn and Sn oxide at the surface of the steel sheet are reduced so that the intensity of Sn measured by X-ray photoelectronic spectroscopy (XPS) in the thickness direction from the surface of the steel sheet satisfies (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1 - 4 9 0 )≤0.5 and satisfies (depth 50 nm I 4 8 4 . 9 ) / (depth 50 nm I 4 8 6 . 5 )≤0.7. Due to this, in the steel sheet of the present embodiment, the chemical convertibility of the surface of the steel sheet can be remarkably improved and as a result an excellent painted corrosion resistance can be obtained.

[0028] It should be noted that, the steel sheet of the present embodiment covers not only electric furnace materials unavoidably containing Ni, Cu, and Sn as tramp elements, but also blast furnace materials containing Ni, Cu, and Sn as essential elements or optional elements. Further, according to the steel sheet of the present embodiment, compared with a conventional steel sheet simultaneously containing the three elements of Ni, Cu, and Sn, it is possible to obtain excellent chemical convertibility and in turn obtain excellent painted corrosion resistance. Therefore, the steel sheet of the present embodiment is particularly useful in the field of automobiles where excellent painted corrosion resistance is demanded.

[0029] Below, the constituent elements of the steel sheet of the present embodiment will be explained in detail.[Chemical Composition]

[0030] In the present embodiment, the steel sheet has a specific chemical composition containing, by mass%, Si: 0.30 to 3.00%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000% and satisfying Si / (Si+Mn)≥0.20. The present invention has as its object, as explained above, the provision of a steel sheet having excellent painted corrosion resistance even if a steel sheet containing Ni, Cu, and Sn and achieves that object by making the contents of solute Sn and Sn oxide at the surface of the steel sheet decrease so that the intensity of Sn measured by XPS in the thickness direction from the surface of the steel sheet satisfies the above specific relationship to thereby achieve the above object.

[0031] Therefore, the chemical composition of the steel sheet is not particularly limited so long as containing, by mass%, Si: 0.30 to 3.00%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000% and satisfying Si / (Si+Mn)≥0.20.

[0032] It should be noted that, regarding Ni, Cu, and Sn, from the viewpoints of the chemical convertibility, strength, and corrosion resistance of the steel sheet, Ni is preferably, by mass%, 0.040% or more. Further, Ni is preferably, by mass%, 1.000% or less. Similarly, Cu is preferably, by mass%, 0.040% or more. Further, Cu is preferably, by mass%, 1.000% or less. Similarly, Sn is preferably, by mass%, 0.004% or more. Further, Sn is preferably, by mass%, 1.000% or less. Also, it should be noted that, the preferable contents etc. of these elements will be explained later.

[0033] In particular, in the present embodiment, the chemical composition of the steel sheet preferably contains, by mass%, Ni: 0.040 to 1.000%, Cu: 0.040 to 1.000%, and Sn: 0.004 to 1.000%.

[0034] The chemical composition of the steel sheet of the present embodiment can contain, in addition to Ni, Cu, and Sn, any alloying elements generally added in the technical field of the present invention in amounts within suitable ranges.

[0035] Below, the chemical composition able to be employed in the steel sheet of the present embodiment will be explained in detail. The following explanation is intended to simply illustrate the chemical composition of a steel sheet for application as a steel sheet for automobile use and is not intended to limit the present invention to a steel sheet having such a specific chemical composition.

[0036] For example, the steel sheet of the present embodiment may have a chemical composition containing, by mass%, C: 0.001 to 0.500%, Si: 0.30 to 3.00%, Mn: 0.10 to 3.00%, Al: 0.001 to 2.000%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, Sn: 0.003 to 1.000%, P: 0.100% or less, S: 0.100% or less, N: 0.0150% or less, O: 0.0100% or less, Ti: 0 to 0.150%, Nb: 0 to 0.150%, B: 0 to 0.0100%, Mo: 0 to 1.000%, Cr: 0 to 1.000%, V: 0 to 0.150%, W: 0 to 1.000%, Hf: 0 to 0.050%, Mg: 0 to 0.050%, Zr: 0 to 0.500%, Ca: 0 to 0.050%, REM: 0 to 0.100%, As: 0 to 0.100%, Ir: 0 to 1.000%, and a balance: Fe and impurities. It should be noted that this chemical composition satisfies Si / (Si+Mn)≥0.20.

[0037] Below, these elements will be explained in further detail.[C: 0.001 to 0.500%]

[0038] C is an element inexpensively making the strength increase and is an important element for controlling the strength of the steel. To sufficiently obtain such an effect, the C content is preferably 0.001% or more. The C content may also be 0.005% or more, 0.010% or more, 0.030% or more, 0.040% or more, 0.070% or more, 0.100% or more, 0.150% or more, or 0.200% or more. On the other hand, if excessively containing C, sometimes a drop in elongation is invited. For this reason, the C content is preferably 0.500% or less. The C content may also be 0.450% or less, 0.400% or less, 0.350% or less, 0.300% or less, or 0.250% or less.[Si: 0.30 to 3.00%]

[0039] Si is an element effective for raising strength as a solution strengthening element. Further, Si is formed surrounding Si oxides as nuclei. It is also an element contributing to the formation of oxides of Mn. To sufficiently obtain these effects, the Si content is 0.30% or more. The Si content may also be 0.35% or more, 0.40% or more, 0.45% or more, 0.50% or more, 0.55% or more, or 0.60% or more. On the other hand, if excessively containing Si, sometimes, along with an increase in steel strength, a drop in elongation is invited. For this reason, the Si content is 3.00% or less. The Si content may also be 2.50% or less, 2.00% or less, 1.50% or less, or 1.20% or less. In particular, the Si content is preferably 0.50 to 3.00%. It should be noted that the Si content, as explained later, has to satisfy the relationship of Si / (Si+Mn)≥0.20.[Mn: 0.10 to 3.00%]

[0040] Mn is an element raising the quenchability of the steel and is an element effective for raising the strength. The Mn content may also be 0%, but to sufficiently obtain such an effect, the Mn content is preferably 0.10% or more. The Mn content may also be 0.50% or more, 1.00% or more, 1.30% or more, 1.50% or more, or 1.80% or more. On the other hand, if excessively containing Mn, sometimes, along with an increase in steel strength, a drop in elongation is invited. Further, if excessively containing Mn, sometimes a large number of oxides of Mn are formed on the surface of the steel. In this case, a large amount of oxygen is consumed for formation of oxides of Mn at the surface of the steel, whereupon there is a possibility that the above-mentioned Sn oxides will not be sufficiently formed and the solute Sn will not be sufficiently decreased. Therefore, the Mn content is preferably 3.00% or less. The Mn content may also be 2.80% or less, 2.50% or less, or 2.00% or less. It should be noted that the Mn content, as explained later, has to satisfy the relationship of Si / (Si+Mn)≥0.20.[Al: 0.001 to 2.000%]

[0041] Al is an element acting as a deoxidizer of the steel and acting to make the steel sounder. To sufficiently obtain such an effect, the Al content is preferably 0.001% or more. The Al content may also be 0.005% or more, 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, if excessively containing Al, sometimes coarse Al oxides are formed and the elongation of steel sheet falls. For this reason, the Al content is preferably 2.000% or less. The Al content may also be 1.500% or less, 1.000% or less, 0.500% or less, 0.100% or less, or 0.050% or less.[Ni: 0.010 to 1.000%][Cu: 0.010 to 1.000%]

[0042] Ni and Cu are elements contributing to improvement of precipitation strengthening or solution strengthening. To sufficiently obtain such an effect, the Ni and Cu content are respectively 0.010% or more. The Ni and Cu contents may also be respectively 0.020% or more, 0.030% or more, 0.040% or more, 0.050% or more, 0.080% or more, 0.100% or more, 0.150% or more, or 0.200% or more. On the other hand, if excessively containing these elements, sometimes formation of oxides at the surface of the steel sheet, in particular Si-based surface oxides or iron oxides, is excessively promoted. Therefore, the Ni and Cu contents are respectively 1.000% or less. The Ni and Cu contents may also be respectively 0.800% or less, 0.600% or less, 0.400% or less, or 0.300% or less.[Sn: 0.003 to 1.000%]

[0043] Sn is an element effective for raising the corrosion resistance. To sufficiently obtain such an effect, the Sn content is 0.003% or more. The Sn content may also be 0.004% or more, 0.008% or more, 0.010% or more, 0.020% or more, 0.030% or more, 0.040% or more, 0.050% or more, 0.080% or more, or 0.100% or more. On the other hand, if excessively containing Sn, sometimes formation of oxides at the surface of the steel sheet, in particular Si-based surface oxides or iron oxides, is excessively promoted. Therefore, the Sn content is 1.000% or less. The Sn content may also be 0.800% or less, 0.600% or less, 0.400% or less, 0.300% or less, or 0.200% or less.[P: 0.100% or Less]

[0044] P is an element segregating at the grain boundaries and promoting embrittlement of the steel. The smaller the P content, the more preferable. Ideally, it is 0%. However, excessive reduction of the P content sometimes invites a great increase in the production costs. For this reason, the P content may also be 0.0001% or more and may also be 0.001% or more or 0.005% or more. On the other hand, if excessively containing P, as explained above, sometimes embrittlement of the steel is invited due to grain boundary segregation. Therefore, the P content is preferably 0.100% or less. The P content may also be 0.050% or less, 0.030% or less, 0.020% or less, or 0.010% or less.[S: 0.100% or Less]

[0045] S is an element forming MnS and other nonmetallic inclusions in the steel and inviting a drop in ductility of the steel material part. The smaller the S content, the more preferable. Ideally it is 0%. However, excessive reduction of the S content sometimes invites a large increase in the production costs. For this reason, the S content may be 0.0001% or more and may be 0.0005% or more, 0.001% or more, or 0.002% or more. On the other hand, if excessively containing S, sometimes occurrence of cracks starting from the nonmetallic inclusions is invited at the time of cold working. Therefore, the S content is preferably 0.100% or less. The S content may also be 0.050% or less, 0.020% or less, or 0.010% or less[N: 0.0150% or Less]

[0046] N is an element forming coarse nitrides in the steel sheet and causing the workability of the steel sheet to fall. The smaller than N content, the more preferable. Ideally it is 0%. However, excessive reduction of the N content sometimes invites a large increase in the production costs. For this reason, the N content may be 0.0001% or more and may be 0.0005% or more or 0.0010% or more. On the other hand, if excessively containing N, as explained above, sometimes coarse nitrides are formed and the workability of the steel sheet is made to fall. Therefore, the N content is preferably 0.0150% or less. The N content may also be 0.0080% or less, 0.0050% or less, or 0.0030% or less.[O: 0.0100% or Less]

[0047] O is an element entering in the production process to form coarse inclusions and causes the workability of the steel sheet to fall. The smaller the O content, the more preferable. Ideally it is 0%. However, excessive reduction of the O content sometimes invites a large increase in the production costs. For this reason, the O content may be 0.0001% or more and may be 0.0005% or more or 0.0010% or more. On the other hand, if excessively containing O, as explained above, sometimes coarse inclusions are formed and the workability of the steel sheet is made to decrease. Therefore, the O content is preferably 0.0100% or less. The O content may also be 0.0080% or less, 0.0060% or less, or 0.0040% or less.

[0048] The preferable basic chemical composition of the steel sheet of the present embodiment is as explained above. Furthermore, the steel sheet of the present embodiment may, in accordance with need, contain at least one of the following elements in place of part of the balance of Fe.

[0049] [Ti: 0 to 0.150%] [Nb: 0 to 0.150%] [V: 0 to 0.150%]

[0050] Ti, Nb, and V form carbonitrides in the steel to give the effect of improving the strength of the steel sheet by precipitation strengthening. The Ti, Nb, and V contents may also be 0%, but to sufficiently obtain such effects, the Ti, Nb, and V contents are preferably respectively 0.001% or more. The Ti, Nb, and V contents may also be respectively 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, even if containing much more of these elements, the effect becomes saturated. Inclusion of more than necessary in the steel material invites a rise in production costs. Therefore, the Ti, Nb, and V contents are preferably respectively 0.150% or less. The Ti, Nb, and V contents may also be respectively 0.120% or less, 0.100% or less, 0.080% or less, 0.050% or less, 0.020% or less, or 0.015% or less.[B: 0 to 0.0100%]

[0051] B segregates at the grain boundaries to raise the grain boundary strength and thereby has the effect of improving the low temperature toughness. The B content may also be 0%, but to sufficiently obtain the above such effect, the B content is preferably 0.0001% or more. The B content may also be 0.0002% or more, 0.0005% or more, or 0.0010% or more. On the other hand, even if B is contained much more, the effect becomes saturated and a rise in production costs is liable to be invited. Therefore, the B content is preferably 0.0100% or less. The B content may also be 0.0050% or less, 0.0030% or less, 0.0020% or less, or 0.0015% or less.

[0052] [Mo: 0 to 1.000%] [Cr: 0 to 1.000%] [W: 0 to 1.000%]

[0053] Mo, Cr, and W are elements raising the quenchability of the steel and contributing to improvement of the strength. The Mo, Cr, and W contents may also be 0%, but to sufficiently obtain such effects, the Mo, Cr, and W contents are preferably respectively 0.001% or more. The Mo, Cr, and W contents may also be respectively 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, even if these elements are contained much more, the effects become saturated. Inclusion of more than necessary in the steel material invites a rise in production costs. Therefore, the Mo, Cr, and W contents are preferably respectively 1.000% or less. The Mo, Cr, and W contents may also be respectively 0.500% or less, 0.100% or less, 0.050% or less, or 0.040% or less.

[0054] [Hf: 0 to 0.050%] [Mg: 0 to 0.050%] [Zr: 0 to 0.500%] [Ca: 0 to 0.050%] [REM: 0 to 0.100%]

[0055] Hf, Mg, Zr, Ca, and REM are elements enabling control of the form of the nonmetallic inclusions. The Hf, Mg, Zr, Ca, and REM contents may also be 0%, but to sufficiently obtain such an effect, the Hf, Mg, Zr, Ca, and REM contents are preferably respectively 0.0001% or more. The Hf, Mg, Zr, Ca, and REM contents may also be respectively 0.0005% or more or 0.001% or more. On the other hand, even if these elements are contained much more, the effect becomes saturated. Inclusion of more than necessary in the steel material invites a rise in production costs. Therefore, the Hf and Mg contents are preferably respectively 0.050% or less, while the Zr content is preferably 0.500% or less. The Hf, Mg, and Zr contents may also be respectively 0.010% or less, 0.005% or less, or 0.003% or less. Similarly, the Ca content is preferably 0.050% or less and the REM content is preferably 0.100% or less. The Ca and REM contents may also be respectively 0.010% or less, 0.005% or less, or 0.003% or less.

[0056] It should be noted that "REM" is the general name of the 17 elements of atomic number 21 scandium (Sc), atomic number 39 yttrium (Y), and the lanthanoid atomic number 57 lanthanum (La) to atomic number 71 lutetium (Lu). The REM content is the total content of these elements.[As: 0 to 0.100%]

[0057] As is an element effective for improvement of the corrosion resistance. The As content may also be 0%, but to sufficiently obtain such an effect, the As content is preferably 0.001% or more. The As content may also be 0.002% or more or 0.003% or more. On the other hand, even if As is contained much more, the effect becomes saturated. Inclusion of more than necessary in the steel material invites a rise in production costs. Therefore, the As content is preferably 0.100% or less. The As content may also be 0.008% or less or 0.005% or less.[Ir: 0 to 1.000%]

[0058] Ir is an element segregating at the former austenite grain boundaries and making the strength of the grain boundaries rise. The Ir content may also be 0%, but to sufficiently obtain such an effect, the Ir content is preferably 0.001% or more. The Ir content may also be 0.003% or more, 0.005% or more, or 0.010% or more. On the other hand, even if containing much more Ir, the effect becomes saturated. Inclusion of more than necessary in the steel material invites a rise in production costs. Therefore, the Ir content is preferably 1.000% or less. The Ir content may also be 0.500% or less, 0.100% or less, 0.030% or less, or 0.015% or less.

[0059] In the steel sheet, the balance other than the above elements is comprised of Fe and impurities. The "impurities" are constituents entering due to various factors in the production process such as ore, scrap, and other starting materials when industrially producing the steel sheet.[Si / (Si+Mn)≥0.20]

[0060] As explained above, in the steel sheet of the present embodiment, the contents of Si and Mn have to satisfy the relationship of Si / (Si+Mn)≥0.20. It should be noted that, in this relationship, "Si" means the Si content in mass% units. "Mn" means the Mn content in mass% units.

[0061] It is known that the Si and Mn in a steel sheet form oxides during annealing. Further, at the time of high temperature annealing, internal oxides comprised of Si oxides as nuclei around which Mn oxides are formed tend to be formed. If the Si content is small, many oxides of Mn are formed at the surface of the steel sheet. In this case, much oxygen is consumed for formation of oxides of Mn at the surface of the steel sheet, therefore the above-mentioned Sn oxides are not sufficiently formed and there is a possibility the remaining content of metal Sn cannot be sufficiently reduced.

[0062] Therefore, in the steel sheet of the present embodiment, by sufficiently increasing the Si content to the extent that Si / (Si+Mn) becomes 0.20 or more, it is possible to suppress the formation of oxides of Mn at the surface of the steel sheet and sufficiently form the above-mentioned Sn oxides, i.e., it is possible to sufficiently reduce the remaining content of metal Sn.

[0063] It should be noted that Si / (Si+Mn) may also be 0.21 or more, 0.22 or more, 0.23 or more, 0.24 or more, or 0.25 or more. Further, the upper limit of Si / (Si+Mn) may also be 1.00, 0.80, 0.60, or 0.50.

[0064] The chemical composition of the steel sheet may be measured by a general analysis method. For example, the chemical composition of the steel sheet may be measured using ICP-AES (inductively coupled plasma-atomic emission spectrometry) on cutting scraps based on JIS G 1201: 2022. Specifically, for example, a 35 mm square test piece is taken from near the sheet thickness 1 / 4 position of the steel sheet and identified by measurement by an ICPS-8100 made by Shimadzu etc. (measurement device) based on a calibration curve prepared in advance. Of the elements which cannot be measured by ICP-AES, C and S can be measured using the combustion-infrared absorption method, and N using the inert gas melting-thermal conductivity method.[Intensity of Sn Measured by X-Ray Photoelectronic Spectroscopy in Thickness Direction From Surface of Steel Sheet]

[0065] Outermost surface I 481 − 490 / depth 50 nm I 481 − 490 ≤ 0.5 Depth 50 nm I 484.9 / depth 50 nm I 486.5 ≤ 0.7

[0066] The steel sheet of the present embodiment, as explained above, is provided with a characterizing configuration of the intensity of Sn measured by X-ray photoelectronic spectroscopy (XPS) in the thickness direction from the surface of the steel sheet satisfying (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1 - 4 9 0 )≤0.5 and satisfying (depth 50 nm I 4 8 4 . 9 ) / (depth 50 nm I 4 8 6 . 5 )≤0.7.

[0067] Here, in this Description, the "surface of the steel sheet" means the outermost surface of the steel sheet. If the steel sheet has a conversion coating, it means the interface of the conversion coating and the back iron. It should be noted that, in this Description, the "steel sheet surface layer" is the region near the surface of the steel sheet. Specifically, it means the region between the 0.1 µm depth position in the thickness direction from the surface of the steel sheet and the 2.0 µm depth position in the thickness direction from the surface of the steel sheet. It should also be noted that, in this Description, the "steel sheet surface layer" will sometimes be simply called the "surface layer".

[0068] Further, regarding the intensity of Sn measured by XPS in the thickness direction from the surface of the steel sheet, "satisfying (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1 - 4 9 0 )≤0.5" means the Sn content at the surface of the steel sheet, specifically the total content of the solute Sn and Sn oxide, is 0.5 time or less of the Sn content at the depth 50 nm position in the thickness direction from the surface of the steel sheet (i.e., the total content of the solute Sn and Sn oxide). In other words, this means the Sn content of the surface of the steel sheet is an amount low to the extent of becoming half or less of the Sn content of the inside of the steel sheet (steel sheet surface layer).

[0069] It should further be noted that, in this Description, I 4 8 1 - 4 9 0 is the integrated value of emitted photoelectronic intensity in the range of a binding energy of 481 to 490 eV and the integrated value of the emitted photoelectronic intensity derived from solute Sn and Sn oxide.

[0070] Similarly, I 4 84 . 9 is the emitted photoelectronic intensity appearing at a binding energy of 484.9 eV and the emitted photoelectronic intensity derived from solute Sn.

[0071] Further, I 4 8 6 . 5 is the emitted photoelectronic intensity appearing at a binding energy of 486.5 eV and the emitted photoelectronic intensity derived from Sn 2 +< , i.e., Sn oxide. It should be noted that, the method of measuring these will be explained later.

[0072] Furthermore, regarding the intensity of Sn measured by XPS in the thickness direction from the surface of the steel sheet, "satisfying (depth 50 nm I 4 8 4 . 9 ) / (depth 50 nm I 4 8 6 . 5 )≤0.7" means the content of solute Sn at a depth position of 50 nm in the thickness direction from the surface of the steel sheet is 0.7 time or less of the Sn 2 +< of the same position, i.e., 0.7 time or less of the content of Sn oxide. In other words, this means that due to the large amount of Sn oxide formed at the steel sheet surface layer, the content of the solute Sn in the steel sheet surface layer is a relatively small amount.

[0073] These relationships relating to the intensity of Sn at the steel sheet surface layer can be realized by the specific technique of making the chemical composition of the obtained steel sheet contain Si in 0.30 to 3.00% and satisfy Si / (Si+Mn)≥0.20, pickling, then brush grinding the hot rolled steel sheet, and performing an annealing step at a high dew point, making a dwell time between 600 to 700°C in the annealing step 12.5 seconds or less, and further pickling after the annealing step. It should be noted that the specific method of production of the steel sheet of the present embodiment will be explained later.

[0074] As explained above, if the contents of solute Sn and Sn oxide at the surface of the steel sheet are reduced so that the intensity of Sn measured by XPS in the thickness direction from the surface of the steel sheet satisfies (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1 - 4 9 0 )≤0.5 and satisfies (depth 50 nm I 4 8 4 . 9 ) / (depth 50 nm I 4 8 6 . 5 )≤0.7, it is possible to remarkably improve the chemical convertibility of the surface of the steel sheet and as a result obtain an excellent painted corrosion resistance.

[0075] In the present embodiment, from the viewpoint of a more excellent painted corrosion resistance being obtained, the intensity of Sn measured by XPS in the thickness direction from the surface of the steel sheet preferably satisfies (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1 - 4 9 0 )≤0.3. It should be noted that (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1-4 9 0 ) may also be 0.2 or less.

[0076] Further, in the present embodiment, from the viewpoint of a more excellent painted corrosion resistance being obtained, the intensity of Sn measured by XPS in the thickness direction from the surface of the steel sheet preferably satisfies (depth 50 nm I 4 8 4 . 9 ) / (depth 50 nm I 4 8 6 . 5 )≤0.5. It should be noted that (depth 50 nm I 4 8 4 . 9 ) / (depth 50 nm I 4 8 6 . 5 ) may also be 0.4 or less.(Method of Measurement of XPS)

[0077] The XPS may be measured in the following way.

[0078] First, a sample for measuring the intensity of Sn is prepared. Here, the sample is obtained by cutting out the steel sheet to be measured from the product covered. The sample is cleaned of oil and dirt on its surface using a solvent etc. It should be noted that if the steel sheet has paint or a conversion coating or if the steel sheet is a steel sheet included in an auto part, the paint or conversion coating is removed by a later explained paint removal step or conversion coating removal step.

[0079] The sample prepared in the above way is, for example, measured for intensity at a predetermined binding energy (BE) down to a depth of 50 nm in the thickness direction from the surface of the sample using a photoelectronic spectrometer JPS-9200 made by JEOL. Specifically, it is measured in accordance with the next procedure.

[0080] First, the inside of the sample holder of the apparatus is rendered a vacuum. It should be noted that the pressure inside the holder at the time of measurement is, even at a maximum, 3.0×10 - 6< Pa, while the pressure inside the holder at the time of sputtering is, even at a maximum, 1.0×10 -5< Pa.

[0081] Before measuring the sample, the time taken for cutting by 20 nm sputter using a 20 nm thickness SiO 2 wafer is obtained.

[0082] Further, after measuring the outermost surface, the sample is sputtered over a time of 2.5 times the time obtained above and the intensity is measured. The sputtering is performed by 7.5×10 -2< Pa Ar gas. The thus obtained measured value is made the depth 50 nm measurement value.

[0083] For the intensity of the Sn, the BE values at 0.1 eV pitch are smoothed and the BE value at 484.9 eV of the obtained curve is employed.

[0084] Further, the I 4 8 1 - 4 9 0 at XPS measurement is made the integrated value of the emitted photoelectronic intensity at the 481 eV≤BE≤490 eV range.

[0085] It should be noted that "I 4 8 4 . 9 " is the emitted photoelectronic intensity at BE=484.9 eV. Similarly, "I 4 8 6 . 5 is the emitted photoelectronic intensity at BE=486.5 eV.

[0086] The specific measurement conditions of XPS are as follows: X-ray source: Mg ray source X-rag diameter: Φ1 mm X-ray output: 12 kV, 25 mA Acceleration voltage: 3 kV [Conversion coating]

[0087] As explained above, the steel sheet of the present embodiment may have a conversion coating on its surface. The steel sheet of the present embodiment is decreased in solute Sn and Sn oxides at the surface of the steel sheet and has excellent chemical convertibility, therefore can be formed with a close knit high structural uniformity conversion coating at the surface of the steel sheet. As a result, excellent painted corrosion resistance can be exhibited.

[0088] It should be noted that, as the chemical conversion used for formation of a conversion coating, for example, a known zinc phosphate-based chemical conversion solution or a zirconium-based chemical conversion solution may be mentioned.(Thickness of Steel Sheet)

[0089] In the present embodiment, the thickness of the steel sheet may be a general sheet thickness. As such a sheet thickness, for example, a 0.2 to 8.0 mm sheet thickness may be mentioned. Furthermore, the thickness of the steel sheet may also be 0.3 mm or more, 0.6 mm or more, 1.0 mm or more, 1.6 mm or more, or 2.0 mm or more. Similarly, the thickness of the steel sheet may also be 7.0 mm or less, 6.0 mm or less, 5.0 mm or less, or 4.0 mm or less.

[0090] It should be noted that if the steel sheet has a conversion coating, the overall thickness of the steel sheet is comprised of the above-mentioned sheet thickness plus the thickness of the conversion coating. The thickness of the conversion coating may be, for example, 1 µm or more, 5 µm or more, 10 µm or more, 15 µm or more, or 20 µm or more. Further, the thickness of the conversion coating may also be, for example, 60 µm or less, 55 µm or less, 50 µm or less, 45 µm or less, or 40 µm or less.(Mechanical Properties)[Vickers Hardness]

[0091] The steel sheet of the present embodiment may also have, as strength, for example, a 90 HV or more Vickers hardness. The Vickers hardness of the steel sheet may also be 150 HV or more, 190 HV or more, 200 HV or more, 250 HV or more, 300 HV or more, 350 HV or more, 400 HV or more, or 450 HV or more. Further, the Vickers hardness of the steel sheet may also be 650 HV or less, 600 HV or less, 550 HV or less, or 500 HV or less.

[0092] It should be noted that the Vickers hardness is determined as follows based on JIS Z 2244-1: 2024.

[0093] First, a test piece is cut out from any position of the steel sheet except the end parts so that a cross-section vertical to the surface (sheet thickness cross-section) can be observed. The sheet thickness cross-section of the test piece cut out is polished using #600 to #1500 silicon carbide paper.

[0094] Next, a solution comprised of particle size 1 to 6 µm diamond powder dispersed in alcohol or other diluent or pure water is used to polish the sheet thickness cross-section of the test piece to a mirror finish. The sheet thickness cross-section is made the measured surface.

[0095] Next, a micro-Vickers hardness tester is used to measure the Vickers hardness by a load of 1 kgf at intervals of 3 times or more of the indentations. Specifically, a total of 20 points are randomly measured near the 1 / 4 position of sheet thickness of the test piece. The arithmetic average of these values is determined as the Vickers hardness of the steel sheet.<Part>

[0096] The steel sheet of the present embodiment, as explained above, can realize excellent chemical convertibility compared with a conventional steel sheet simultaneously containing the three elements of Ni, Cu, and Sn and in turn can realize excellent painted corrosion resistance. Therefore, the steel sheet of the present embodiment is useful for use in parts etc. in technical fields in which excellent painted corrosion resistance is demanded. In particular, the steel sheet of the present embodiment is useful in parts etc. in the automobile industry.

[0097] In a preferred embodiment, an auto part including the steel sheet according to an embodiment of the present invention is provided. As examples of auto parts, frame parts, bumpers, and other structural parts and reinforcement parts etc. where strength is required may be mentioned. Further, as other examples of auto parts, roofs, hoods, fenders, doors, and other external panel parts where high beauty is demanded may be mentioned. These parts need only contain a steel sheet according to an embodiment of the present invention at least in part. For this reason, these parts at least partially satisfy the features of the steel sheet of the above embodiment. At portions of the steel sheet which do not directly contact the dies in press-forming and other shaping or which directly contact the dies, but where the degree of work is relatively low, the features of the steel sheet do not particularly change before and after shaping. For example, in a part including a steel sheet according to an embodiment of the present invention, the portion of the location where the sample was taken, explained later (i.e., a portion avoiding the locations of (i) to (iv) explained later) can be recognized as a nonworked portion. This portion maintains the features of the steel sheet of the above-mentioned embodiment before and after shaping as a part.

[0098] The steel sheet according to an embodiment of the present invention, for example, can be employed for the various auto parts explained earlier after being formed with any conversion coating or paint on the surface. Whether an auto part having paint or a conversion coating contains a steel sheet according to an embodiment of the present invention can be judged by removing the paint or conversion coating of the sample obtained from that auto part. The location from which the sample is taken in this case, the paint removal step, and the conversion coating removal step are as follows:(Location From Which Sample is Taken)

[0099] The sample used when performing the various measurements and analyses is taken from the auto part avoiding the locations of the following (i) to (iv). (i) Welded parts: locations within 20 mm from toes of spot welded parts and locations within 20 mm from toes of arc / laser welded parts. (ii) Worked parts: worked parts with radii of curvature of less than 15 mm and locations within 5 mm from the worked parts. (iii) End parts: end parts within 5 mm from cut end parts of a part. (iv) Red rust: locations within 5 mm from locations at which red rust is visually observed. (Paint Removal Step)

[0100] The surface of the sample cut out from the auto part or steel sheet is coated with a paint remover (Neorever ®< #160, made by Sansaikako) at room temperature and allowed to stand for 5 minutes. After that, the surface of the sample coated with the paint remover is rubbed using a hard sponge (for example, Kanefil ®< , made by Aion Co., Ltd.) to peel off the paint from the surface of the sample.

[0101] Next, the surface of the sample after peeling off the paint is rinsed and dried. At this time, the residual state of the paint is confirmed by measuring the surface of the rinsed and dried sample (100 µm square, 5 fields) by SEM-EPMA.

[0102] In the elemental distribution map obtained by EPMA, the regions where the C concentration becomes 10 mass% or more are identified and if the area ratio of those regions is 5% or more, it is judged that the paint has insufficiently been removed.

[0103] The area ratio of the regions where the C concentration becomes 10 mass% or more is measured by first obtaining an elemental distribution map of C in EPMA setting the C concentration range to 10 to 30%. The specific measurement conditions of the EPMA are as follows: Apparatus: JXA-8230 electron probe microanalyzer made by JEOL Acceleration voltage: 15 kV Emitted current: 0.05 µA Surface analysis: WDS Interval of analysis: 300 µm or more Area ratio: average value of 5 fields

[0104] Next, the obtained elemental distribution map of C is image processed to measure the area ratios. For the image processing, the image analysis software "ImageJ" is used. Specifically, the elemental distribution map of C is read into ImageJ, then the function "Make Binary" in "Binary" of "Process" is used to binarize the map so that regions with a C concentration of 10 mass% or more are displayed as black and regions with a C concentration of less than 10 mass% are displayed as white. After binarization, the function "Measure" of "Analyze" is used to read the numerical value of the "Area fraction" in the "Results". The read value is determined as the area ratio of the regions with a C concentration of 10 mass% or more.

[0105] If the paint is insufficiently peeled off, the paint is repeatedly removed until the area ratio of the regions with a C concentration of 10 mass% or more becomes less than 5%.(Conversion coating Removal Step)

[0106] A sample cut out from an auto part or steel sheet and stripped of paint is stripped of its conversion coating by a suitable method when removal of the conversion coating is necessary for XPS and other various measurements. For example, when the conversion coating is a zinc phosphate coating, the method based on JIS K 3151: 1996 is performed so as to remove the conversion coating from the surface of the sample. Specifically, the sample after removal of the paint is dipped in a 5% chromic acid aqueous solution heated to 75°C for 15 minutes to thereby remove the conversion coating from the surface of the sample.

[0107] Next, the surface of the sample after removal of the conversion coating is rinsed and dried. At this time, the residual state of the chemical conversion crystals is confirmed by measuring the surface of the rinsed and dried sample (100 µm square, 5 fields) by SEM-EPMA.

[0108] In the elemental distribution map obtained by EPMA, the regions where the P concentration becomes 5 mass% or more are identified and if the area ratio of those regions is 5% or more, it is judged that the conversion coating has insufficiently been peeled off.

[0109] The area ratio of the regions where the P concentration becomes 5 mass% or more is measured by first obtaining an elemental distribution map of P in EPMA setting the P concentration range to 5 to 10%. Next, the obtained elemental distribution map of P is image processed to measure the area ratios. For the image processing, the image analysis software "ImageJ" is used. Specifically, the elemental distribution map of P is read into ImageJ, then the function "Make Binary" in "Binary" of "Process" is used to binarize the map so that regions with a P concentration of 5 mass% or more are displayed as black and regions with a P concentration of less than 5 mass% are displayed as white. After binarization, the function "Measure" of "Analyze" is used to read the numerical value of the "Area fraction" in the "Results". The read value is determined as the area ratio of the regions with a P concentration of 5 mass% or more.

[0110] If the conversion coating is insufficiently peeled off, the conversion coating is repeatedly removed until the area ratio of the regions with a P concentration of 5 mass% or more becomes less than 5%.<Method of Production of Steel Sheet>

[0111] Next, a preferable method of production of a steel sheet according to one embodiment of the present invention will be explained. The following explanation is intended to illustrate the characteristic method for producing a steel sheet according to one embodiment of the present invention and is not intended to limit the steel sheet to one produced by the method of production such as explained below.

[0112] The steel sheet of the present embodiment, for example, can be produced by a method of production including a casting step of casting molten steel, adjusted so that the chemical composition of the finally obtained steel sheet becomes the specific chemical composition, so as to form a steel slab, a hot rolling step of hot rolling the steel slab to obtain a hot rolled steel sheet, a pickling step of pickling the hot rolled steel sheet, a brush grinding step of grinding the surface of the pickled steel sheet by a brush roll, a cold rolling step of cold rolling the brush ground hot rolled steel sheet to obtain a cold rolled steel sheet, an annealing step of annealing the cold rolled steel sheet under conditions of a -10°C or more and 20°C or less high dew point and a dwell time between 600 to 700°C of 12.5 seconds or less, a post-annealing pickling step of pickling the cold rolled steel sheet after annealing, and an optional chemical conversion step of dipping the steel sheet in a chemical conversion solution to form a conversion coating on the surface of the steel sheet.

[0113] In the method of production of the steel sheet of the present embodiment, the chemical composition of the molten steel is adjusted so that the chemical composition of the finally obtained steel sheet becomes a specific chemical composition containing, by mass%, Si: 0.30 to 3.00%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000% and satisfying Si / (Si+Mn)≥0.20. Due to this, as explained above, Mn oxides are made to form inside using the Si oxides as nuclei and formation of oxides of Mn at the surface of the steel sheet is suppressed and Sn is made easier to oxidize. In other words, the content of the remaining metal Sn is easily reduced.

[0114] Furthermore, in the method of production of the steel sheet of the present embodiment, the steel slab of the above-mentioned specific chemical composition is hot rolled and pickled, then the surface of the hot rolled steel sheet is brush ground to thereby remove at least part of the Sn difficult to oxidize during annealing and remaining after pickling.

[0115] Further, in the method of production of the steel sheet of the present embodiment, the steel sheet after the brush grinding is cold rolled. The obtained cold rolled steel sheet is annealed under conditions of a -10°C or more and 20°C or less high dew point and a dwell time between 600 to 700°C of 12.5 seconds or less, whereby the Sn is made to sufficiently oxidize and the content of the metal Sn remaining at the surface of the steel sheet, i.e., the solute Sn, is made to sufficiently decrease. It should be noted that, the temperature region of 600 to 700°C is a temperature region in which Sn can diffuse in the steel up to the surface, but since it is a temperature region in which Sn does not oxidize, it is necessary to shorten the time in which the temperature of the steel sheet remains in that temperature region. If the dwell time in such a temperature region becomes more than 12.5 seconds, even if oxidation of Sn occurs, the amount of solute Sn at the surface of the steel sheet cannot be sufficiently reduced.

[0116] Further, in the method of production of the steel sheet of the present embodiment, the annealed cold rolled steel sheet is pickled to remove the Sn oxide and Si-based oxides at the surface of the steel sheet.

[0117] According to such a method of production of a steel sheet of the present embodiment, even if the steel sheet contains the three elements of Ni, Cu, and Sn, it is possible to reduce the contents of solute Sn and Sn oxide at the surface of the steel sheet so that the intensity of Sn measured by X-ray photoelectronic spectroscopy (XPS) in the thickness direction from the surface of the steel sheet satisfies (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1 - 4 9 0 )≤0.5 and satisfies (depth 50 nm I 4 8 4 . 9 ) / (depth 50 nm I 4 8 6 . 5 )≤0.7, therefore it is possible to remarkably improve the chemical convertibility of the surface of the steel sheet and as a result possible to obtain excellent painted corrosion resistance.

[0118] Below, the preferable conditions etc. of the steps in the method of production of the steel sheet of the present embodiment will be explained in detail.[Casting Step]

[0119] In the method of production of a steel sheet of the present embodiment, the casting step is a step of casting molten steel adjusted in chemical composition so as to form a steel slab. The chemical composition has to be adjusted so that the chemical composition of the finally obtained steel sheet becomes a specific chemical composition containing, by mass%, Si: 0.30 to 3.00%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000% and satisfying Si / (Si+Mn)≥0.20.

[0120] For the conditions of the casting step other than the chemical composition, usual conditions known in the field may be employed. For example, the casting step need only comprise smelting by a blast furnace, electric arc furnace, etc. followed by various secondary refining, then the usual continuous casting, casting by the ingot method, etc.[Hot Rolling Step]

[0121] In the method of production of a steel sheet of the present embodiment, the hot rolling step is a step of hot rolling the steel slab to obtain a hot rolled steel sheet. The hot rolling step is performed by hot rolling the cast steel slab directly or after once cooling, then reheating. When reheating, the heating temperature of the steel slab is, for example, 1100 to 1250°C.

[0122] In the hot rolling step, usually rough rolling and finish rolling are performed. The temperatures and rolling reductions of the rolling operations can be suitably determined in accordance with the desired metallostructure and sheet thickness. For example, the end temperature of the finish rolling is 900 to 1050°C and the rolling reduction of the finish rolling is 10 to 50%.

[0123] The finish rolled hot rolled steel sheet is coiled up at a predetermined coiling temperature, then is supplied to the next pickling step. In the present embodiment, the hot rolled steel sheet is coiled at a 520°C or more coiling temperature. The coiling temperature may also be 550°C or more. Further, the coiling temperature may also be 600°C or less.[Pickling Step]

[0124] In the method of production of a steel sheet of the present embodiment, the pickling step is a step of pickling the coiled hot rolled steel sheet using a pickling solution. The pickling step may be performed using a generally used pickling solution, for example, a predetermined concentration hydrochloric acid solution containing an inhibitor for inhibiting corrosion of the steel sheet under conditions suitable for removing the external oxide layer and internal oxide layer of the hot rolled steel sheet. The pickling may be performed in one operation or may be performed divided into several operations so as to reliably remove the external oxide layer and internal oxide layer.[Brush Grinding Step]

[0125] In the method of production of a steel sheet of the present embodiment, the brush grinding step is a step of brush grinding the pickled hot rolled steel sheet under conditions of a grinding amount of 3.0 g / m 2< or more. By brush grinding under such conditions, it is possible to sufficiently remove the solute Sn difficult to oxidize during annealing and remaining after pickling. As a result, it is possible to sufficiently reduce the content of solute Sn at the surface of the steel sheet.

[0126] It should be noted that, in the brush grinding step, it is possible to remove not only the Sn at the surface of the steel sheet, but also at least one of the Ni and Cu.

[0127] The grinding amount by the brush grinding is preferably as large as possible from the viewpoint of more reliably removing the solute Sn at the surface of the steel sheet. For example, 4.0 g / m 2< or more is preferable while 5.0 g / m 2< or more is more preferable. The upper limit of the grinding amount by the brush grinding is, for example, 20.0 g / m 2< or less and may also be 15.0 g / m 2< or less.

[0128] The grinding amount by the brush grinding can be adjusted by any suitable method known to persons skilled in the art. For example, the grinding amount by the brush grinding can be adjusted by suitably selecting the type of the brush (for example, H115 made by Hotani etc.), the wire material, bristle length, rotational speed, density, brush reduction, coating solution used, etc.[Cold Rolling Step]

[0129] In the method of production of a steel sheet of the present embodiment, the cold rolling step is a step of cold rolling the brush ground hot rolled steel sheet to obtain a cold rolled steel sheet. The rolling reduction of the cold rolling can be suitably determined in accordance with the desired metallostructure, sheet thickness, etc. The rolling reduction of the cold rolling is, for example, 20 to 80%. After the cold rolling step, the steel sheet may, for example, be air cooled to cool it down to room temperature.[Annealing Step]

[0130] In the method of production of a steel sheet of the present embodiment, the annealing step is a step of annealing the cold rolled steel sheet after the cold rolling step under conditions of a high dew point of -10°C or more and 20°C or less and a dwell time between 600 to 700°C of 12.5 seconds or less. By annealing the cold rolled steel sheet under such specific conditions, it is possible to make the Sn sufficiently oxidize and sufficiently reduce the content of solute Sn remaining at the surface of the steel sheet.

[0131] It should be noted that the dew point in the annealing step is preferably 0°C or more, more preferably is 10°C or more. Further, the dwell time between 600 to 700°C in the annealing step is preferably 11.0 seconds or less, more preferably 10.0 seconds or less. It should also be noted that the dwell time between 600 to 700°C may also be, for example, 5.0 seconds or more or 6.0 seconds or more.

[0132] In the annealing step, the conditions other than the above specified conditions need only be conditions suitable for making the Sn sufficiently oxidize and annealing the cold rolled steel sheet. For example, the annealing step includes heating in an atmosphere with a dew point of -10 to 20°C to a 700 to 950°C temperature and holding for 0 to 300 seconds after reaching that temperature. However, when heating to the 700 to 950°C annealing temperature, it is necessary to make the dwell time between 600 to 700°C 12.5 seconds or less.

[0133] It should be noted that the annealing temperature is preferably 750°C or more, more preferably 780°C or more. Further, the annealing temperature is preferably 950°C or less, more preferably 900°C or less. The holding time at the annealing temperature is preferably 30 seconds or more, more preferably 50 seconds or more. Further, the holding time is preferably 200 seconds or less, more preferably 150 seconds or less.

[0134] The atmosphere in the annealing step may be a reducing atmosphere, more specifically a reducing atmosphere containing nitrogen and hydrogen, for example, a hydrogen concentration 1 to 10% reducing atmosphere (for example, hydrogen 3% and nitrogen balance).[Post-Annealing Pickling Step]

[0135] In the method of production of a steel sheet of the present embodiment, the post-annealing pickling step is a step of pickling the annealed cold rolled steel sheet so as to remove the Sn oxide and Si-based oxides at the surface of the steel sheet. The conditions of the post-annealing pickling step need only be conditions suitable for removing Sn oxide or Si-based oxides at the surface of the steel sheet using a generally used pickling solution, for example, a predetermined concentration hydrochloric acid solution containing an inhibitor for inhibiting corrosion of the steel sheet. For example, the post-annealing pickling step may be performed by dipping the annealed cold rolled steel sheet in a predetermined concentration hydrochloric acid solution for a predetermined time.

[0136] The pickling solution used in the post-annealing pickling step is, for example, a concentration 3 to 12% hydrochloric acid solution. Further, in the post-annealing pickling step, the temperature when dipping the cold rolled steel sheet in the pickling solution is, for example, 50 to 90°C. Furthermore, the time when dipping the cold rolled steel sheet in the pickling solution is, for example, 1 to 30 seconds.

[0137] The pickling may be performed at one time or may be performed divided into several times so as to reliably remove the Sn oxide and Si-based oxides.[Chemical Conversion Step]

[0138] In the method of production of a steel sheet of the present embodiment, the chemical conversion step is an optional step performed when producing a steel sheet having a conversion coating on its surface. It is a step of dipping the steel sheet after the post-annealing pickling step in a chemical conversion solution to form a conversion coating on the surface of the steel sheet. The chemical conversion step need only be performed under conditions suitable for forming a conversion coating on the surface of the steel sheet. For example, the chemical conversion step may comprise degreasing the steel sheet by a degreasing agent, rinsing it, then using a surface conditioner to treat the surface of the steel sheet before dipping the pickled steel sheet in a chemical conversion solution.

[0139] As the chemical conversion solution used for the chemical conversion step, for example, a known zinc phosphate-based chemical conversion solution or a zirconium-based chemical conversion solution may be mentioned.

[0140] The present invention is not limited to the above embodiments or the following examples etc. Suitable combinations or replacements, changes, etc. can be used to an extent not departing from the object or gist of the present invention.

[0141] Below, examples will be used to explain the present invention in more detail, but the following examples are just illustrations of the present invention. The present invention is not limited to these examples.EXAMPLES

[0142] In the following examples, steel sheets according to embodiments of the present invention were produced under various conditions and the features of the obtained steel sheets were investigated.

[0143] First, each molten steel was cast by the continuous casting method to form a steel slab having a chemical composition shown in Table 1. Further, the steel slab was cooled once, then was reheated to 1200°C and hot rolled and was coiled up at a 520°C or more coiling temperature. The hot rolling was performed by rough rolling and finish rolling. The end temperature of the finish rolling was 900 to 1050°C and the rolling reduction of the finish rolling was 30%.

[0144] Next, the obtained hot rolled steel sheet was pickled, then the surface of the steel sheet was brush ground by a 6.0 g / m 2< grinding amount using a grinding use brush roll (H115 made by Hotani).

[0145] Next, the brush ground hot rolled steel sheet was cold rolled by a rolling reduction of 50% to obtain a cold rolled steel sheet.

[0146] Further, this cold rolled steel sheet was annealed by heating to an 800°C temperature and holding for 100 seconds in an oxygen concentration 20 ppm or less furnace in an atmosphere of a dew point shown in Table 2 and hydrogen 3% (nitrogen balance). It should be noted that, when heating to the annealing temperature, the dwell time between 600 to 700°C was as shown in Table 2.

[0147] Furthermore, the annealed steel sheet was rinsed by dipping it in a concentrate 5% hydrochloric acid solution at a 80°C temperature for 5.0 seconds to obtain various steel sheets having 1.6 mm thicknesses as examples and comparative examples. It should be noted that, when analyzing the chemical composition of the thus obtained steel sheets, they were similar to the chemical compositions of the steel slabs before hot rolling.[Table 1]

[0148] Table 1Steel typeChemical composition (mass%), balance: Fe and impuritiesSi / (Si+Mn)CSiMnAlNiCuSnPsNTiNbBMoCrA10.0020.381.480.0300.0100.0110.0030.0500.0050.0052------0.20A20.0020.401.490.0290.0450.0420.0050.0520.0050.00500.0430.0010.00010.0200.018-0.21A30.0020.391.480.0290.2350.3440.0200.0510.0050.00520.0450.0010.00010.0180.021-0.21A40.0020.401.520.0280.9830.9940.0970.0490.0050.00530.0470.0020.00020.0360.033-0.21A50.0020.391.440.0280.2310.3100.0210.0500.0050.00520.0430.0010.00010.0180.022V: 0.045, Hf: 0.0120.21A60.0020.381.380.0280.2310.3130.0220.0500.0050.00500.0430.0010.00010.0210.021Mg: 0.017, Zr: 0.1210.22A70.0020.381.440.0290.2270.3170.0210.0490.0050.00520.0440.0010.00010.0200.019W: 0.311, As: 0.0240.21A80.0020.411.410.0300.2250.3220.0230.0510.0050.00520.0430.0010.00010.0180.020Ca: 0.014, Ir: 0.2290.23A90.0020.411.380.0300.2210.3090.0210.0510.0050.00520.0430.0010.00010.0180.020Ce: 0.019, La: 0.0100.23B10.1170.441.220.0230.0110.0120.0030.0130.0030.0060------0.27B20.1180.431.280.0280.0400.0470.0050.0130.0030.00590.0280.0020.00150.0040.020-0.25B30.1220.461.250.0310.2300.3240.0220.0140.0040.00590.0220.0010.00140.0140.025-0.27B40.1220.451.230.0300.9941.0000.1000.0140.0030.00550.0390.0020.00150.0350.035-0.27C10.0830.572.030.0240.0100.0110.0030.0100.0020.0051------0.22C20.0850.592.030.0240.0440.0500.0050.0110.0020.00570.0290.0010.00150.0020.018-0.23C30.0850.582.020.0250.2310.3390.0230.0100.0020.00590.0290.0010.00150.0170.021-0.22C40.0840.602.000.0260.9980.9980.0950.0110.0020.00540.0220.0020.00160.0330.038-0.23D10.1300.992.210.0120.0110.0120.0030.0020.0020.0055------0.31D20.1280.992.200.0130.0400.0400.0050.0030.0020.00550.0310.0020.00160.0040.020-0.31D30.1281.012.200.0130.2310.3900.0250.0030.0020.00530.0220.0020.00160.0140.022-0.31D40.1311.022.180.0110.9950.9890.0940.0030.0020.00510.0330.0020.00150.0370.034-0.32E10.1811.832.580.0280.0110.0100.0030.0100.0010.0059------0.41E20.1811.822.600.0290.0440.0440.0050.0090.0010.00600.0200.0010.00150.0020.018-0.41E30.1791.842.620.0290.2500.3330.0220.0090.0010.00580.0210.0010.00160.0130.020-0.41E40.1821.832.620.0280.9880.9900.0930.0110.0010.00580.0210.0020.00150.0300.033-0.41F10.1420.422.390.0290.2470.3240.0230.0120.0020.00570.0200.0020.00140.0200.021-0.15F20.0020.130.330.0230.2340.3100.0200.0500.0040.00540.0200.0020.00010.0200.020-0.28F30.1420.181.450.0280.2350.3120.0210.0120.0020.00540.0210.0020.00130.0210.021-0.11 [Table 2]

[0149] Table 2NoClassSteel typeCompositionBrush grindingAnnealing conditionsPickling after annealingIntensity of Sn in XPS measurementVickers hardness (Hv)Type of chemical conversionPerformanceSi (mass%)Si / (Si+Mn)Dew point (°C)Dwell time between 600 to 700 °C (s)(Outermost surface I 481-490 ) / (depth 50 nm 1481-490)(Depth 50 nm I 484.9 ) / (depth 50 nm I 486.5 )Painted corrosion resistance1Ex.A10.380.20Yes-1012.2Yes0.50.7151Zn3P2A2Ex.A20.400.21Yes211.5Yes0.40.5149Zn3P2AA3Ex.A30.390.21Yes-99.5Yes0.30.7150Zn3P2AA4Ex.A40.400.21Yes39.1Yes0.20.5155Zn3P2AAA5Ex.B10.440.27Yes111.4Yes0.50.4190Zn3P2AA6Ex.B20.430.25Yes-49.1Yes0.20.7192Zn3P2AA7Ex.B30.460.27Yes109.8Yes0.30.5198Zn3P2AAA8Ex.B40.450.27Yes-411.8Yes0.50.7200Zn3P2A9Ex.C10.570.22Yes-29.2Yes0.30.6282Zn3P2AA10Ex.C20.590.23Yes09.7Yes0.20.5282Zn3P2AAA11Ex.C30.580.22Yes-910.9Yes0.50.7283Zn3P2A12Ex.C40.600.23Yes110.8Yes0.40.4288Zn3P2AA13Ex.D10.990.31Yes610.0Yes0.20.4332Zn3P2AAA14Ex.D20.990.31Yes-212.5Yes0.50.7332Zn3P2A15Ex.D31.010.31Yes1111.1Yes0.40.5339Zn3P2AA16Ex.D41.020.32Yes-68.9Yes0.30.6345Zn3P2AA17Ex.E11.830.41Yes-711.4Yes0.40.6398Zn3P2A18Ex.E21.820.41Yes311.0Yes0.40.4399Zn3P2AA19Ex.E31.840.41Yes-89.6Yes0.20.7401Zn3P2AA20Ex.E41.830.41Yes99.9Yes0.20.5409Zn3P2AAA21Ex.Al0.380.20Yes-1012.2Yes0.50.7151ZrA22Ex.A20.400.21Yes211.5Yes0.40.5149ZrAA23Ex.A30.390.21Yes-99.5Yes0.30.7150ZrAA24Ex.A40.400.21Yes39.1Yes0.20.5155ZrAAA25Ex.A50.390.21Yes-1012.0Yes0.50.6156Zn3P2A26Ex.A60.380.22Yes-811.7Yes0.40.6158Zn3P2A27Ex.A70.380.21Yes-911.2Yes0.40.6154Zn3P2A28Ex.A80.410.23Yes-1012.1Yes0.50.7155Zn3P2A29Ex.A90.410.23Yes-910.9Yes0.50.7156Zn3P2A30Comp. ex.F10.420.15Yes39.6Yes0.61.1333Zn3P2B31Comp. ex.F20.130.28Yes29.6Yes0.61.2131Zn3P2B32Comp. ex.F30.180.11Yes49.0Yes0.61.1200Zn3P2B33Comp. ex.A30.390.21No49.9Yes0.70.9150Zn3P2B34Comp. ex.D20.990.31Yes39.3No1.10.6332Zn3P2B35Comp. ex.B20.430.25Yes-129.7Yes0.51.2192Zn3P2B36Comp. ex.C40.600.23Yes315.9Yes0.70.7288Zn3P2B37Comp. ex.C30.580.22Yes239.6Yes0.71.2283Zn3P2B

[0150] Each of the various steel sheets obtained in the above way was measured by XPS and for Vickers hardness in accordance with the measurement methods explained below. Further, the painted corrosion resistance of each of the steel sheets was evaluated by the following method of evaluation. The results of these measurements and evaluations are shown in Table 2. It should be noted that the underlines given to the numerical values in Table 1 and Table 2 indicate values outside the scope of the present invention or not preferable production conditions.(Method of Measurement of XPS)

[0151] The steel sheet sample to be measured is measured for intensity at a predetermined binding energy (BE) down to a depth of 50 nm in the thickness direction from the surface of the sample using a photoelectronic spectrometer JPS-9200 made by JEOL. Specifically, it is measured in accordance with the next procedure.

[0152] First, the inside of the sample holder of the apparatus is rendered a vacuum. In particular, it should be noted that the pressure inside the holder at the time of measurement is, even at a maximum, 3.0×10 -6< Pa, while the pressure inside the holder at the time of sputtering is, even at a maximum, 1.0×10 -5< Pa.

[0153] Before measuring the sample, the time taken for cutting a 20 nm thickness SiO 2 wafer by 20 nm sputter is obtained.

[0154] Further, after measuring the outermost surface, the sample is sputtered over a time of 2.5 times the time obtained above and the intensity is measured. The sputtering is performed by 7.5×10 -2< Pa Ar gas. The thus obtained measured value is made the depth 50 nm measurement value.

[0155] For the intensity of the Sn, the BE values at 0.1 eV pitch are smoothed and the BE value at 484.9 eV of the obtained curve is employed.

[0156] Further, I 4 8 1 - 4 9 0 is obtained as the integrated value of the emitted photoelectronic intensity at the 481 eV≤BE≤490 eV range. Furthermore, I 4 8 4 . 9 is obtained as the emitted photoelectronic intensity at BE=484.9 eV and I 4 8 6 . 5 is obtained as the emitted photoelectronic intensity at BE=486.5 eV.

[0157] The specific measurement conditions of XPS are as follows: X-ray source: Mg ray source X-rag diameter: Φ1 mm X-ray output: 12 kV, 25 mA Acceleration voltage: 3 kV (Method of Measurement of Vickers Hardness)

[0158] The Vickers hardness is determined in the following way based on JIS Z 2244-1: 2024.

[0159] First, a test piece is cut out from any position from any position except the end parts of the steel sheet so that a cross-section vertical to the surface (sheet thickness cross-section) can be observed. The sheet thickness cross-section of the cut out test piece is polished using #600 to #1500 silicon carbon paper. Next, a liquid comprised of particle size 1 to 6 µm diamond powder dispersed in alcohol or other diluent or pure water is used to polish the sheet thickness cross-section of the test piece to a mirror finish. The sheet thickness cross-section is made the measured surface.

[0160] Next, a micro-Vickers hardness tester is used to measure the Vickers hardness by a load of 1 kgf at intervals of 3 times or more of the indentations. Specifically, a total of 20 points are randomly measured near the 1 / 4 position of sheet thickness of the test piece. The arithmetic average of these values is determined as the Vickers hardness of the steel sheet.[Evaluation of Painted Corrosion Resistance]

[0161] The painted corrosion resistance of the steel sheet was evaluated in the following way: First, a 50 mm×50 mm sample of the produced steel sheet to be evaluated was degreased under the following conditions.

[0162] Degreasing: A degreasing agent (Fine Cleaner E2083) was used. The sample was dipped in it at 40°C for 2 minutes, then was rinsed.

[0163] Next, the degreased sample of the steel sheet was treated by the following (i) zinc phosphate (Zn phosphate) or (ii) zirconium (Zr) as chemical conversion.(i) Zn phosphate treatment

[0164] Surface conditioning: the sample was dipped in a surface conditioner (Prepalene Z) at ordinary temperature for 30 seconds.

[0165] Chemical conversion: the sample was dipped in a zinc phosphate treatment agent (Palbond L3020 conversion agent) at 40°C for 2 minutes, then rinsed and dried.(ii) Zr treatment

[0166] Chemical conversion: the sample was dipped in a Zr-based treatment agent (PLC-2010) at 45°C for 2 minutes, then rinsed and dried.

[0167] Next, an L direction 50 mm × C direction 100 nm region of the sample of the steel sheet treated for chemical conversion was painted by electrodeposition under the following conditions: Electrodeposition solution: Powernics Excel 1200 (made by Nipponpaint Industrial Coatings Co., Ltd.) Electrodeposition temperature: 30°C Coating thickness: 18 µm Electrodeposition baking: 170°C for 30 minutes

[0168] Furthermore, an L direction 50 mm × C direction 100 nm region of the electrodeposition painted sample was scored by cuts and subjected to a JASO corrosion test "JASO M 609: 1991" for 30 cycles.

[0169] After the JASO corrosion test, the sample taken out was dried and the maximum value of the paint blisters of the sample was measured. From the maximum value of the paint blisters measured, the painted corrosion resistance of the sample was evaluated in accordance with the following criteria. AAA: maximum value of paint blister width of less than 0.9 mm AA: maximum value of paint blister width of 0.9 mm or more and less than 1.2 mm A: maximum value of paint blister width of 1.2 mm or more and less than 1.5 mm B: maximum value of paint blister width of 1.5 mm or more

[0170] A case where the painted corrosion resistance was evaluated as AAA, AA, and A was evaluated as a steel sheet excellent in painted corrosion resistance. On the other hand, a case where the painted corrosion resistance was evaluated as B was evaluated as a steel sheet poor in painted corrosion resistance.

[0171] As shown in Table 2, it was learned that the steel sheets of the Nos. 1 to 29 examples where the Si content was large, Si / (Si+Mn) was 0.20 or more, and the intensity of Sn measured by XPS in a thickness direction from the surface of the steel sheet satisfied (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1 - 4 9 0 )≤0.5 and satisfied (depth 50 nm I 4 8 4 . 9 ) / (depth 50 nm I 4 8 6 . 5 )≤0.7 were all steel sheets excellent in painted corrosion resistance.

[0172] In particular, it was learned that the steel sheets of the Nos. 2, 3, 5, 6, 9, 12, 15, 16, 18, 19, 22, and 23 examples where annealing was performed under conditions of a dew point in the annealing step of 0°C or more or a dwell time between 600 to 700°C of 10.0 seconds or less further satisfied (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1 - 4 9 0 )≤0.3 or satisfied (depth 50 nm I 4 8 4 . 9 ) / (depth 50 nm I 4 8 6 . 5 )≤0.5 and had more excellent painted corrosion resistance.

[0173] Furthermore, it was learned that the steel sheets of the Nos. 4, 7, 10, 13, 20, and 24 examples where annealing was performed under conditions of a dew point in the annealing step of 0°C or more and a dwell time between 600 to 700°C of 10.0 seconds or less further satisfied (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1 - 4 9 0 )≤0.3 and satisfied (depth 50 nm I 4 8 4 . 9 ) / (depth 50 nm I 4 8 6 . 5 )≤0.5 and had extremely excellent painted corrosion resistance.

[0174] On the other hand, it was learned that the steel sheets of the Nos. 30 to 37 comparative examples where the content of Si was small, Si / (Si+Mn) was less than 0.20, (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1 - 4 9 0 )≤0.5 was not satisfied, or (depth 50 nm I 4 8 4 . 9 ) / (depth 50 nm I 4 8 6 . 5 )≤0.7 was not satisfied were all steel sheets poor in painted corrosion resistance.

[0175] Specifically, in the No. 30 steel sheet, Si / (Si+Mn) was low, therefore it is surmised that at the surface of the steel sheet, Mn oxides were mainly formed and oxidation of Sn was suppressed.

[0176] In the No. 31 steel sheet, the Si content was small and it is surmised the starting points of formation of nuclei for internal oxidation of Sn decreased and Sn oxidation was suppressed.

[0177] In the No. 32 steel sheet, the Si content was small and Si / (Si+Mn) was low, therefore it is surmised that oxidation of Sn was suppressed.

[0178] In the No. 33 steel sheet, brush grinding was not performed, therefore it is surmised that the Sn concentrated parts formed at the time of pickling were not completely oxidized at the time of annealing and remained in the metal state.

[0179] In the No. 34 steel sheet, the sheet was not pickled after annealing, therefore it is surmised that the Sn oxides at the surface were not removed.

[0180] In the No. 35 steel sheet, the dew point at the time of annealing was low and it is surmised that oxidation of Sn did not sufficiently occur.

[0181] In the No. 36 steel sheet, the dwell time between 600 to 700°C where Sn did not oxidize was long and it is surmised Sn was not sufficiently oxidized and was concentrated at the steel sheet surface layer in the metal state.

[0182] In the No. 37 steel sheet, the dew point at the time of annealing was high and it was surmised that mainly Fe oxides were formed at the surface of the steel sheet and oxidation of Sn was suppressed.

Examples

examples

[0142]In the following examples, steel sheets according to embodiments of the present invention were produced under various conditions and the features of the obtained steel sheets were investigated.

[0143]First, each molten steel was cast by the continuous casting method to form a steel slab having a chemical composition shown in Table 1. Further, the steel slab was cooled once, then was reheated to 1200°C and hot rolled and was coiled up at a 520°C or more coiling temperature. The hot rolling was performed by rough rolling and finish rolling. The end temperature of the finish rolling was 900 to 1050°C and the rolling reduction of the finish rolling was 30%.

[0144]Next, the obtained hot rolled steel sheet was pickled, then the surface of the steel sheet was brush ground by a 6.0 g / m 2< grinding amount using a grinding use brush roll (H115 made by Hotani).

[0145]Next, the brush ground hot rolled steel sheet was cold rolled by a rolling reduction of 50% to obtain a cold rolled steel sh...

FIELD

[0001] The present invention relates to a steel sheet and a part containing the same.BACKGROUND

[0002] To improve the corrosion resistance of a steel sheet, raising the chemical convertibility of the surface of the steel sheet and making a conversion coating uniformly form on the surface of that steel sheet is effective.

[0003] In relation to this, for example, PTL 1 discloses a method of production of a high strength cold rolled steel sheet comprising using a continuous annealing furnace with a cooling zone including part or all of a 600 to 250°C steel sheet temperature range following heating for recrystallization and with a cooling system of one of more of gas cooling, dissipative cooling, and cooling tube cooling, or a facility for both a cold rolled steel sheet / hot dip galvanized steel sheet having a continuous annealing furnace, to continuously anneal a high strength cold rolled steel sheet, during which exposing a surface of the steel sheet to an iron oxidizing atmosphere in that steel sheet temperature range, pickling it at an annealing furnace exit side, then iron or Ni plating it to 1 to 50 mg / m2. Furthermore, this PTL 1 teaches that while usually the steel sheet surroundings are rendered an ultralow concentration oxygen and / or ultralow dew point inert atmospheric gas to prevent oxidation of the steel sheet, conversely to the above, by deliberately exposing the sheet to an oxidizing atmosphere, making Si, Mn, and even the iron of the steel sheet oxidize, and, in the pickling when exiting the annealing furnace, removing the iron oxide film of the steel sheet along with the Si, Mn, and other oxide film by the pickling, a high strength cold rolled steel sheet free from "bald spots" even if the contents of Si, Mn, etc. are high and excellent in chemical convertibility is obtained.

[0004] Further, PTL 2 discloses a steel sheet for automotive use containing copper (Cu) in 0.10 mass% or more and 0.50 mass% or less, having a number of particles of residual scale at the surface of 160,000 / mm 2< or less, and having a maximum size of copper compound particles exposed at the surface of 2 µm or less. Furthermore, PTL 2 teaches that by this constitution, since the size of the copper compound particles of the steel sheet serving as cathode points in chemical conversion is made 2 µm or less and the residual scale is made a predetermined amount or less, it is possible to provide a steel sheet excellent in chemical convertibility.[CITATION LIST][PATENT LITERATURE]

[0005] [PTL 1] Japanese Unexamined Patent Publication No. 2008-190030 [PTL 2] Japanese Unexamined Patent Publication No. 2020-084238 SUMMARY[TECHNICAL PROBLEM]

[0006] The above PTL 2 teaches that, in addition to copper (Cu), nickel (Ni), tin (Sn), and other elements cause a drop in the strength, shapeability, and other mechanical properties of a steel sheet for automobile use plus the corrosion resistance and other chemical stability, in particular that the copper compounds present on the surface of the steel sheet cause a drop in the chemical convertibility for improving the corrosion resistance. Further, in general, if the chemical convertibility drops, sometimes regions called "bald spots" where no conversion coating is formed are formed. As a result, sometimes the painted corrosion resistance falls.

[0007] Therefore, the present invention has as its object the provision of a steel sheet containing Ni, Cu, and Sn and having excellent painted corrosion resistance and a part containing the same.[SOLUTION TO PROBLEM]

[0008] The present invention includes at least the following aspects:(Aspect 1)

[0009] A steel sheet, in which steel sheet, a chemical composition of the steel sheet contains, by mass%, Si: 0.30 to 3.00%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000% and satisfies Si / (Si+Mn)≥0.20, and an intensity of Sn measured by X-ray photoelectron spectroscopy in a thickness direction from a surface of the steel sheet satisfies (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1 - 4 9 0 )≤ 0.5 and satisfies (depth 50 nm I 4 8 4. 9 ) / (depth 50 nm I 4 8 6 . 5 )≤0.7. (Aspect 2)

[0010] The steel sheet according to aspect 1 wherein an intensity of Sn measured by X-ray photoelectron spectroscopy in a thickness direction from a surface of the steel sheet satisfies (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1 - 4 9 0 )≤0.3.(Aspect 3)

[0011] The steel sheet according to aspect 1 or 2 wherein an intensity of Sn measured by X-ray photoelectron spectroscopy in a thickness direction from a surface of the steel sheet satisfies (depth 50 nm I 4 8 4 . 9 ) / (depth 50 nm I 4 8 6. 5 )≤ 0.5.(Aspect 4)

[0012] The steel sheet according to any of the aspects 1 to 3, wherein the chemical composition of the steel sheet contains, by mass%, Si: 0.50 to 3.00%.(Aspect 5)

[0013] The steel sheet according to any of the aspects 1 to 4, wherein the chemical composition of the steel sheet contains, by mass%, Ni: 0.040 to 1.000%, Cu: 0.040 to 1.000%, and Sn: 0.004 to 1.000%. (Aspect 6)

[0014] The steel sheet according to any one of aspects 1 to 5, having a conversion coating on the surface of the steel sheet.(Aspect 7)

[0015] A part including a steel sheet according to any of aspects 1 to 6.[ADVANTAGEOUS EFFECTS OF INVENTION]

[0016] According to the present invention, it is possible to provide a steel sheet containing Ni, Cu, and Sn which has excellent painted corrosion resistance and a part containing the same.DESCRIPTION OF EMBODIMENTS

[0017] As explained above, in general, if a steel sheet drops in chemical convertibility, regions called "bald spots" where no conversion coating is formed sometimes occur. As a result, the corrosion resistance sometimes falls. For example, if Ni, Cu, Sn, and other elements are present dissolved in a steel sheet, the potential of the steel sheet becomes more precious compared with the potential in the state where these elements are not dissolved and, at the time of chemical conversion, sometimes the etchability of Fe falls. In particular, if a steel sheet simultaneously contains the three elements of Ni, Cu, and Sn, due to the Ni and Cu, the Sn easily concentrates at the surface of the steel sheet during the annealing step. The Sn concentrating at the surface of the steel sheet obstructs chemical conversion, therefore causes etching defects at the time of chemical conversion of the surface of the steel sheet and makes formation of a conversion coating difficult. For this reason, in such a case, the chemical convertibility falls and as a result the painted corrosion resistance ends up falling. Therefore, if a steel sheet simultaneously contains the three elements of Ni, Cu, and Sn, such a fall in the painted corrosion resistance particularly becomes a problem.

[0018] As the method for producing steel, two types of methods are generally known: the method of using the natural resource of iron ore as the main raw material for obtaining molten iron at a blast furnace, then refining this at a converter etc. to produce molten st eel and the method of using the recycled resource of scrap metal as the main raw mater ial for producing molten steel at an electric furnace. The steel produced by the former method, i.e., the blast furnace material, can contain Ni, Cu, Sn, and other elements as a dded elements, therefore it is necessary to suitably deal with this issue if these elements are contained. On the other hand, in the steel produced by the latter method, i.e., the el ectric furnace material, since as explained above, scrap metal is used as the main raw m aterial, Ni, Cu, Sn, and other scrap derived elements (so-called "tramp elements") are co ntained in relatively large amounts. In addition, the three elements of Ni, Cu, and Sn ar e easily simultaneously contained. Therefore, in the electric furnace material, this issue p articularly becomes prominent.

[0019] Therefore, the inventors engaged in studies focusing in particular on the distribution of elements at the steel sheet surface layer so as to provide a steel sheet having excellent painted corrosion resistance even when the steel sheet simultaneously contains the three elements of Ni, Cu, and Sn. As a result, they learned that even among the three elements of Ni, Cu, and Sn, Sn in particular easily concentrates at the surface of a steel sheet in an annealing step due to the presence of Ni and Cu and the content of metal Sn at the surface of the steel sheet in the dissolved state (below, sometimes referred to as the "solute Sn") easily increases. Solute Sn present at the surface of a steel sheet obstructs chemical conversion, therefore causes etching defects at the time of chemical conversion of the surface of a steel sheet and becomes a cause of deterioration of the painted corrosion resistance.

[0020] The inventors discovered that by oxidizing the Sn in a steel sheet surface layer to form tin oxide (below, sometimes referred to as "Sn oxide"), it is possible to reduce the content of solute Sn at the surface of a steel sheet and that, further, by removing the Sn oxide at the surface of a steel sheet, it is possible to remarkably improve the chemical convertibility of the surface of a steel sheet and as a result obtain an excellent painted corrosion resistance. Specifically, the inventors discovered that even in a steel sheet containing the three elements of Ni, Cu, and Sn, by adopting the specific technique of making the chemical composition of the obtained steel sheet contain Si in 0.30 to 3.00% and satisfy Si / (Si+Mn)≥0.20, pickling, then brush grinding the hot rolled steel sheet, performing an annealing step at a high dew point, making the dwell time at the range of 600 to 700°C in the annealing step 12.5 seconds or less, and, furthermore, performing the pickling after the annealing step, it is possible to reduce the contents of solute Sn and Sn oxide at the surface of the steel sheet so that the intensity of Sn measured by X-ray photoelectron spectroscopy in the thickness direction from the surface of the steel sheet satisfies (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1 - 4 9 0 )≤0.5 and (depth 50 nm I 4 8 4 .9 ) / (depth 50 nm I 4 8 6 . 5 )≤0.7. Furthermore, they discovered that due to this, it is possible to remarkably improve the chemical convertibility of the surface of a steel sheet and as a result obtain an excellent painted corrosion resistance.

[0021] The present invention was completed based on the above such discoveries and includes the following aspects of an embodiment:

[0022] Below, a preferred embodiment of the steel sheet of the present invention will be explained in detail.<Steel Sheet>

[0023] A steel sheet according to one embodiment of the present invention has a specific chemical composition containing, by mass%, Si: 0.30 to 3.00%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000% and satisfying Si / (Si+Mn)≥0.20.

[0024] Further, the steel sheet of the present embodiment is provided with a distinctive constitution having an intensity of Sn measured by X-ray photoelectronic spectroscopy in a thickness direction from the surface of the steel sheet satisfying (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1 - 4 9 0 )≤0.5 and satisfying (depth 50 nm I 4 8 4 . 9 ) / (depth 50 nm I 4 8 6 . 5 )≤0.7.

[0025] In general, in chemical conversion, anodic dissolution of Fe (etching) occurs, whereupon electrons are generated. On the other hand, the electrons generated by the anodic dissolution of Fe cause a cathodic reaction (2H +< +2e -< →H 2 , 10H +< +NO 3 -< +8e -< →NH 4 +< +3H 2 O) to occur. In turn, the pH of the chemical conversion solution near the surface of the steel sheet rises. Along with this, zinc phosphate crystals and other compounds precipitate forming a conversion coating at the surface of the steel sheet.

[0026] In this regard, in a steel sheet with Ni, Cu, and Sn present dissolved in the steel sheet, compared with one where these elements are not dissolved, the potential of the steel sheet becomes precious and sometimes, at the time of chemical conversion, the etchability of Fe falls. Further, among the three elements of Ni, Cu, and Sn as well, Sn in particular easily concentrates at the surface of the steel sheet during the annealing step due to the presence of Ni and Cu, therefore the concentration of Sn at the surface of the steel sheet in the dissolved state easily increases. The solute Sn present at the surface of the steel sheet, as explained above, obstructs the chemical conversion of the surface of the steel sheet, therefore causes etching defects at the time of chemical conversion and becomes a cause of a drop in the painted corrosion resistance.

[0027] In the steel sheet of the present embodiment, even if a steel sheet containing the three elements of Ni, Cu, and Sn, due to the technique explained later, the contents of solute Sn and Sn oxide at the surface of the steel sheet are reduced so that the intensity of Sn measured by X-ray photoelectronic spectroscopy (XPS) in the thickness direction from the surface of the steel sheet satisfies (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1 - 4 9 0 )≤0.5 and satisfies (depth 50 nm I 4 8 4 . 9 ) / (depth 50 nm I 4 8 6 . 5 )≤0.7. Due to this, in the steel sheet of the present embodiment, the chemical convertibility of the surface of the steel sheet can be remarkably improved and as a result an excellent painted corrosion resistance can be obtained.

[0028] It should be noted that, the steel sheet of the present embodiment covers not only electric furnace materials unavoidably containing Ni, Cu, and Sn as tramp elements, but also blast furnace materials containing Ni, Cu, and Sn as essential elements or optional elements. Further, according to the steel sheet of the present embodiment, compared with a conventional steel sheet simultaneously containing the three elements of Ni, Cu, and Sn, it is possible to obtain excellent chemical convertibility and in turn obtain excellent painted corrosion resistance. Therefore, the steel sheet of the present embodiment is particularly useful in the field of automobiles where excellent painted corrosion resistance is demanded.

[0029] Below, the constituent elements of the steel sheet of the present embodiment will be explained in detail.[Chemical Composition]

[0030] In the present embodiment, the steel sheet has a specific chemical composition containing, by mass%, Si: 0.30 to 3.00%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000% and satisfying Si / (Si+Mn)≥0.20. The present invention has as its object, as explained above, the provision of a steel sheet having excellent painted corrosion resistance even if a steel sheet containing Ni, Cu, and Sn and achieves that object by making the contents of solute Sn and Sn oxide at the surface of the steel sheet decrease so that the intensity of Sn measured by XPS in the thickness direction from the surface of the steel sheet satisfies the above specific relationship to thereby achieve the above object.

[0031] Therefore, the chemical composition of the steel sheet is not particularly limited so long as containing, by mass%, Si: 0.30 to 3.00%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000% and satisfying Si / (Si+Mn)≥0.20.

[0032] It should be noted that, regarding Ni, Cu, and Sn, from the viewpoints of the chemical convertibility, strength, and corrosion resistance of the steel sheet, Ni is preferably, by mass%, 0.040% or more. Further, Ni is preferably, by mass%, 1.000% or less. Similarly, Cu is preferably, by mass%, 0.040% or more. Further, Cu is preferably, by mass%, 1.000% or less. Similarly, Sn is preferably, by mass%, 0.004% or more. Further, Sn is preferably, by mass%, 1.000% or less. Also, it should be noted that, the preferable contents etc. of these elements will be explained later.

[0033] In particular, in the present embodiment, the chemical composition of the steel sheet preferably contains, by mass%, Ni: 0.040 to 1.000%, Cu: 0.040 to 1.000%, and Sn: 0.004 to 1.000%.

[0034] The chemical composition of the steel sheet of the present embodiment can contain, in addition to Ni, Cu, and Sn, any alloying elements generally added in the technical field of the present invention in amounts within suitable ranges.

[0035] Below, the chemical composition able to be employed in the steel sheet of the present embodiment will be explained in detail. The following explanation is intended to simply illustrate the chemical composition of a steel sheet for application as a steel sheet for automobile use and is not intended to limit the present invention to a steel sheet having such a specific chemical composition.

[0036] For example, the steel sheet of the present embodiment may have a chemical composition containing, by mass%, C: 0.001 to 0.500%, Si: 0.30 to 3.00%, Mn: 0.10 to 3.00%, Al: 0.001 to 2.000%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, Sn: 0.003 to 1.000%, P: 0.100% or less, S: 0.100% or less, N: 0.0150% or less, O: 0.0100% or less, Ti: 0 to 0.150%, Nb: 0 to 0.150%, B: 0 to 0.0100%, Mo: 0 to 1.000%, Cr: 0 to 1.000%, V: 0 to 0.150%, W: 0 to 1.000%, Hf: 0 to 0.050%, Mg: 0 to 0.050%, Zr: 0 to 0.500%, Ca: 0 to 0.050%, REM: 0 to 0.100%, As: 0 to 0.100%, Ir: 0 to 1.000%, and a balance: Fe and impurities. It should be noted that this chemical composition satisfies Si / (Si+Mn)≥0.20.

[0037] Below, these elements will be explained in further detail.[C: 0.001 to 0.500%]

[0038] C is an element inexpensively making the strength increase and is an important element for controlling the strength of the steel. To sufficiently obtain such an effect, the C content is preferably 0.001% or more. The C content may also be 0.005% or more, 0.010% or more, 0.030% or more, 0.040% or more, 0.070% or more, 0.100% or more, 0.150% or more, or 0.200% or more. On the other hand, if excessively containing C, sometimes a drop in elongation is invited. For this reason, the C content is preferably 0.500% or less. The C content may also be 0.450% or less, 0.400% or less, 0.350% or less, 0.300% or less, or 0.250% or less.[Si: 0.30 to 3.00%]

[0039] Si is an element effective for raising strength as a solution strengthening element. Further, Si is formed surrounding Si oxides as nuclei. It is also an element contributing to the formation of oxides of Mn. To sufficiently obtain these effects, the Si content is 0.30% or more. The Si content may also be 0.35% or more, 0.40% or more, 0.45% or more, 0.50% or more, 0.55% or more, or 0.60% or more. On the other hand, if excessively containing Si, sometimes, along with an increase in steel strength, a drop in elongation is invited. For this reason, the Si content is 3.00% or less. The Si content may also be 2.50% or less, 2.00% or less, 1.50% or less, or 1.20% or less. In particular, the Si content is preferably 0.50 to 3.00%. It should be noted that the Si content, as explained later, has to satisfy the relationship of Si / (Si+Mn)≥0.20.[Mn: 0.10 to 3.00%]

[0040] Mn is an element raising the quenchability of the steel and is an element effective for raising the strength. The Mn content may also be 0%, but to sufficiently obtain such an effect, the Mn content is preferably 0.10% or more. The Mn content may also be 0.50% or more, 1.00% or more, 1.30% or more, 1.50% or more, or 1.80% or more. On the other hand, if excessively containing Mn, sometimes, along with an increase in steel strength, a drop in elongation is invited. Further, if excessively containing Mn, sometimes a large number of oxides of Mn are formed on the surface of the steel. In this case, a large amount of oxygen is consumed for formation of oxides of Mn at the surface of the steel, whereupon there is a possibility that the above-mentioned Sn oxides will not be sufficiently formed and the solute Sn will not be sufficiently decreased. Therefore, the Mn content is preferably 3.00% or less. The Mn content may also be 2.80% or less, 2.50% or less, or 2.00% or less. It should be noted that the Mn content, as explained later, has to satisfy the relationship of Si / (Si+Mn)≥0.20.[Al: 0.001 to 2.000%]

[0041] Al is an element acting as a deoxidizer of the steel and acting to make the steel sounder. To sufficiently obtain such an effect, the Al content is preferably 0.001% or more. The Al content may also be 0.005% or more, 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, if excessively containing Al, sometimes coarse Al oxides are formed and the elongation of steel sheet falls. For this reason, the Al content is preferably 2.000% or less. The Al content may also be 1.500% or less, 1.000% or less, 0.500% or less, 0.100% or less, or 0.050% or less.[Ni: 0.010 to 1.000%][Cu: 0.010 to 1.000%]

[0042] Ni and Cu are elements contributing to improvement of precipitation strengthening or solution strengthening. To sufficiently obtain such an effect, the Ni and Cu content are respectively 0.010% or more. The Ni and Cu contents may also be respectively 0.020% or more, 0.030% or more, 0.040% or more, 0.050% or more, 0.080% or more, 0.100% or more, 0.150% or more, or 0.200% or more. On the other hand, if excessively containing these elements, sometimes formation of oxides at the surface of the steel sheet, in particular Si-based surface oxides or iron oxides, is excessively promoted. Therefore, the Ni and Cu contents are respectively 1.000% or less. The Ni and Cu contents may also be respectively 0.800% or less, 0.600% or less, 0.400% or less, or 0.300% or less.[Sn: 0.003 to 1.000%]

[0043] Sn is an element effective for raising the corrosion resistance. To sufficiently obtain such an effect, the Sn content is 0.003% or more. The Sn content may also be 0.004% or more, 0.008% or more, 0.010% or more, 0.020% or more, 0.030% or more, 0.040% or more, 0.050% or more, 0.080% or more, or 0.100% or more. On the other hand, if excessively containing Sn, sometimes formation of oxides at the surface of the steel sheet, in particular Si-based surface oxides or iron oxides, is excessively promoted. Therefore, the Sn content is 1.000% or less. The Sn content may also be 0.800% or less, 0.600% or less, 0.400% or less, 0.300% or less, or 0.200% or less.[P: 0.100% or Less]

[0044] P is an element segregating at the grain boundaries and promoting embrittlement of the steel. The smaller the P content, the more preferable. Ideally, it is 0%. However, excessive reduction of the P content sometimes invites a great increase in the production costs. For this reason, the P content may also be 0.0001% or more and may also be 0.001% or more or 0.005% or more. On the other hand, if excessively containing P, as explained above, sometimes embrittlement of the steel is invited due to grain boundary segregation. Therefore, the P content is preferably 0.100% or less. The P content may also be 0.050% or less, 0.030% or less, 0.020% or less, or 0.010% or less.[S: 0.100% or Less]

[0045] S is an element forming MnS and other nonmetallic inclusions in the steel and inviting a drop in ductility of the steel material part. The smaller the S content, the more preferable. Ideally it is 0%. However, excessive reduction of the S content sometimes invites a large increase in the production costs. For this reason, the S content may be 0.0001% or more and may be 0.0005% or more, 0.001% or more, or 0.002% or more. On the other hand, if excessively containing S, sometimes occurrence of cracks starting from the nonmetallic inclusions is invited at the time of cold working. Therefore, the S content is preferably 0.100% or less. The S content may also be 0.050% or less, 0.020% or less, or 0.010% or less[N: 0.0150% or Less]

[0046] N is an element forming coarse nitrides in the steel sheet and causing the workability of the steel sheet to fall. The smaller than N content, the more preferable. Ideally it is 0%. However, excessive reduction of the N content sometimes invites a large increase in the production costs. For this reason, the N content may be 0.0001% or more and may be 0.0005% or more or 0.0010% or more. On the other hand, if excessively containing N, as explained above, sometimes coarse nitrides are formed and the workability of the steel sheet is made to fall. Therefore, the N content is preferably 0.0150% or less. The N content may also be 0.0080% or less, 0.0050% or less, or 0.0030% or less.[O: 0.0100% or Less]

[0047] O is an element entering in the production process to form coarse inclusions and causes the workability of the steel sheet to fall. The smaller the O content, the more preferable. Ideally it is 0%. However, excessive reduction of the O content sometimes invites a large increase in the production costs. For this reason, the O content may be 0.0001% or more and may be 0.0005% or more or 0.0010% or more. On the other hand, if excessively containing O, as explained above, sometimes coarse inclusions are formed and the workability of the steel sheet is made to decrease. Therefore, the O content is preferably 0.0100% or less. The O content may also be 0.0080% or less, 0.0060% or less, or 0.0040% or less.

[0048] The preferable basic chemical composition of the steel sheet of the present embodiment is as explained above. Furthermore, the steel sheet of the present embodiment may, in accordance with need, contain at least one of the following elements in place of part of the balance of Fe.

[0049] [Ti: 0 to 0.150%] [Nb: 0 to 0.150%] [V: 0 to 0.150%]

[0050] Ti, Nb, and V form carbonitrides in the steel to give the effect of improving the strength of the steel sheet by precipitation strengthening. The Ti, Nb, and V contents may also be 0%, but to sufficiently obtain such effects, the Ti, Nb, and V contents are preferably respectively 0.001% or more. The Ti, Nb, and V contents may also be respectively 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, even if containing much more of these elements, the effect becomes saturated. Inclusion of more than necessary in the steel material invites a rise in production costs. Therefore, the Ti, Nb, and V contents are preferably respectively 0.150% or less. The Ti, Nb, and V contents may also be respectively 0.120% or less, 0.100% or less, 0.080% or less, 0.050% or less, 0.020% or less, or 0.015% or less.[B: 0 to 0.0100%]

[0051] B segregates at the grain boundaries to raise the grain boundary strength and thereby has the effect of improving the low temperature toughness. The B content may also be 0%, but to sufficiently obtain the above such effect, the B content is preferably 0.0001% or more. The B content may also be 0.0002% or more, 0.0005% or more, or 0.0010% or more. On the other hand, even if B is contained much more, the effect becomes saturated and a rise in production costs is liable to be invited. Therefore, the B content is preferably 0.0100% or less. The B content may also be 0.0050% or less, 0.0030% or less, 0.0020% or less, or 0.0015% or less.

[0052] [Mo: 0 to 1.000%] [Cr: 0 to 1.000%] [W: 0 to 1.000%]

[0053] Mo, Cr, and W are elements raising the quenchability of the steel and contributing to improvement of the strength. The Mo, Cr, and W contents may also be 0%, but to sufficiently obtain such effects, the Mo, Cr, and W contents are preferably respectively 0.001% or more. The Mo, Cr, and W contents may also be respectively 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, even if these elements are contained much more, the effects become saturated. Inclusion of more than necessary in the steel material invites a rise in production costs. Therefore, the Mo, Cr, and W contents are preferably respectively 1.000% or less. The Mo, Cr, and W contents may also be respectively 0.500% or less, 0.100% or less, 0.050% or less, or 0.040% or less.

[0054] [Hf: 0 to 0.050%] [Mg: 0 to 0.050%] [Zr: 0 to 0.500%] [Ca: 0 to 0.050%] [REM: 0 to 0.100%]

[0055] Hf, Mg, Zr, Ca, and REM are elements enabling control of the form of the nonmetallic inclusions. The Hf, Mg, Zr, Ca, and REM contents may also be 0%, but to sufficiently obtain such an effect, the Hf, Mg, Zr, Ca, and REM contents are preferably respectively 0.0001% or more. The Hf, Mg, Zr, Ca, and REM contents may also be respectively 0.0005% or more or 0.001% or more. On the other hand, even if these elements are contained much more, the effect becomes saturated. Inclusion of more than necessary in the steel material invites a rise in production costs. Therefore, the Hf and Mg contents are preferably respectively 0.050% or less, while the Zr content is preferably 0.500% or less. The Hf, Mg, and Zr contents may also be respectively 0.010% or less, 0.005% or less, or 0.003% or less. Similarly, the Ca content is preferably 0.050% or less and the REM content is preferably 0.100% or less. The Ca and REM contents may also be respectively 0.010% or less, 0.005% or less, or 0.003% or less.

[0056] It should be noted that "REM" is the general name of the 17 elements of atomic number 21 scandium (Sc), atomic number 39 yttrium (Y), and the lanthanoid atomic number 57 lanthanum (La) to atomic number 71 lutetium (Lu). The REM content is the total content of these elements.[As: 0 to 0.100%]

[0057] As is an element effective for improvement of the corrosion resistance. The As content may also be 0%, but to sufficiently obtain such an effect, the As content is preferably 0.001% or more. The As content may also be 0.002% or more or 0.003% or more. On the other hand, even if As is contained much more, the effect becomes saturated. Inclusion of more than necessary in the steel material invites a rise in production costs. Therefore, the As content is preferably 0.100% or less. The As content may also be 0.008% or less or 0.005% or less.[Ir: 0 to 1.000%]

[0058] Ir is an element segregating at the former austenite grain boundaries and making the strength of the grain boundaries rise. The Ir content may also be 0%, but to sufficiently obtain such an effect, the Ir content is preferably 0.001% or more. The Ir content may also be 0.003% or more, 0.005% or more, or 0.010% or more. On the other hand, even if containing much more Ir, the effect becomes saturated. Inclusion of more than necessary in the steel material invites a rise in production costs. Therefore, the Ir content is preferably 1.000% or less. The Ir content may also be 0.500% or less, 0.100% or less, 0.030% or less, or 0.015% or less.

[0059] In the steel sheet, the balance other than the above elements is comprised of Fe and impurities. The "impurities" are constituents entering due to various factors in the production process such as ore, scrap, and other starting materials when industrially producing the steel sheet.[Si / (Si+Mn)≥0.20]

[0060] As explained above, in the steel sheet of the present embodiment, the contents of Si and Mn have to satisfy the relationship of Si / (Si+Mn)≥0.20. It should be noted that, in this relationship, "Si" means the Si content in mass% units. "Mn" means the Mn content in mass% units.

[0061] It is known that the Si and Mn in a steel sheet form oxides during annealing. Further, at the time of high temperature annealing, internal oxides comprised of Si oxides as nuclei around which Mn oxides are formed tend to be formed. If the Si content is small, many oxides of Mn are formed at the surface of the steel sheet. In this case, much oxygen is consumed for formation of oxides of Mn at the surface of the steel sheet, therefore the above-mentioned Sn oxides are not sufficiently formed and there is a possibility the remaining content of metal Sn cannot be sufficiently reduced.

[0062] Therefore, in the steel sheet of the present embodiment, by sufficiently increasing the Si content to the extent that Si / (Si+Mn) becomes 0.20 or more, it is possible to suppress the formation of oxides of Mn at the surface of the steel sheet and sufficiently form the above-mentioned Sn oxides, i.e., it is possible to sufficiently reduce the remaining content of metal Sn.

[0063] It should be noted that Si / (Si+Mn) may also be 0.21 or more, 0.22 or more, 0.23 or more, 0.24 or more, or 0.25 or more. Further, the upper limit of Si / (Si+Mn) may also be 1.00, 0.80, 0.60, or 0.50.

[0064] The chemical composition of the steel sheet may be measured by a general analysis method. For example, the chemical composition of the steel sheet may be measured using ICP-AES (inductively coupled plasma-atomic emission spectrometry) on cutting scraps based on JIS G 1201: 2022. Specifically, for example, a 35 mm square test piece is taken from near the sheet thickness 1 / 4 position of the steel sheet and identified by measurement by an ICPS-8100 made by Shimadzu etc. (measurement device) based on a calibration curve prepared in advance. Of the elements which cannot be measured by ICP-AES, C and S can be measured using the combustion-infrared absorption method, and N using the inert gas melting-thermal conductivity method.[Intensity of Sn Measured by X-Ray Photoelectronic Spectroscopy in Thickness Direction From Surface of Steel Sheet]

[0065] Outermost surface I 481 − 490 / depth 50 nm I 481 − 490 ≤ 0.5 Depth 50 nm I 484.9 / depth 50 nm I 486.5 ≤ 0.7

[0066] The steel sheet of the present embodiment, as explained above, is provided with a characterizing configuration of the intensity of Sn measured by X-ray photoelectronic spectroscopy (XPS) in the thickness direction from the surface of the steel sheet satisfying (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1 - 4 9 0 )≤0.5 and satisfying (depth 50 nm I 4 8 4 . 9 ) / (depth 50 nm I 4 8 6 . 5 )≤0.7.

[0067] Here, in this Description, the "surface of the steel sheet" means the outermost surface of the steel sheet. If the steel sheet has a conversion coating, it means the interface of the conversion coating and the back iron. It should be noted that, in this Description, the "steel sheet surface layer" is the region near the surface of the steel sheet. Specifically, it means the region between the 0.1 µm depth position in the thickness direction from the surface of the steel sheet and the 2.0 µm depth position in the thickness direction from the surface of the steel sheet. It should also be noted that, in this Description, the "steel sheet surface layer" will sometimes be simply called the "surface layer".

[0068] Further, regarding the intensity of Sn measured by XPS in the thickness direction from the surface of the steel sheet, "satisfying (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1 - 4 9 0 )≤0.5" means the Sn content at the surface of the steel sheet, specifically the total content of the solute Sn and Sn oxide, is 0.5 time or less of the Sn content at the depth 50 nm position in the thickness direction from the surface of the steel sheet (i.e., the total content of the solute Sn and Sn oxide). In other words, this means the Sn content of the surface of the steel sheet is an amount low to the extent of becoming half or less of the Sn content of the inside of the steel sheet (steel sheet surface layer).

[0069] It should further be noted that, in this Description, I 4 8 1 - 4 9 0 is the integrated value of emitted photoelectronic intensity in the range of a binding energy of 481 to 490 eV and the integrated value of the emitted photoelectronic intensity derived from solute Sn and Sn oxide.

[0070] Similarly, I 4 84 . 9 is the emitted photoelectronic intensity appearing at a binding energy of 484.9 eV and the emitted photoelectronic intensity derived from solute Sn.

[0071] Further, I 4 8 6 . 5 is the emitted photoelectronic intensity appearing at a binding energy of 486.5 eV and the emitted photoelectronic intensity derived from Sn 2 +< , i.e., Sn oxide. It should be noted that, the method of measuring these will be explained later.

[0072] Furthermore, regarding the intensity of Sn measured by XPS in the thickness direction from the surface of the steel sheet, "satisfying (depth 50 nm I 4 8 4 . 9 ) / (depth 50 nm I 4 8 6 . 5 )≤0.7" means the content of solute Sn at a depth position of 50 nm in the thickness direction from the surface of the steel sheet is 0.7 time or less of the Sn 2 +< of the same position, i.e., 0.7 time or less of the content of Sn oxide. In other words, this means that due to the large amount of Sn oxide formed at the steel sheet surface layer, the content of the solute Sn in the steel sheet surface layer is a relatively small amount.

[0073] These relationships relating to the intensity of Sn at the steel sheet surface layer can be realized by the specific technique of making the chemical composition of the obtained steel sheet contain Si in 0.30 to 3.00% and satisfy Si / (Si+Mn)≥0.20, pickling, then brush grinding the hot rolled steel sheet, and performing an annealing step at a high dew point, making a dwell time between 600 to 700°C in the annealing step 12.5 seconds or less, and further pickling after the annealing step. It should be noted that the specific method of production of the steel sheet of the present embodiment will be explained later.

[0074] As explained above, if the contents of solute Sn and Sn oxide at the surface of the steel sheet are reduced so that the intensity of Sn measured by XPS in the thickness direction from the surface of the steel sheet satisfies (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1 - 4 9 0 )≤0.5 and satisfies (depth 50 nm I 4 8 4 . 9 ) / (depth 50 nm I 4 8 6 . 5 )≤0.7, it is possible to remarkably improve the chemical convertibility of the surface of the steel sheet and as a result obtain an excellent painted corrosion resistance.

[0075] In the present embodiment, from the viewpoint of a more excellent painted corrosion resistance being obtained, the intensity of Sn measured by XPS in the thickness direction from the surface of the steel sheet preferably satisfies (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1 - 4 9 0 )≤0.3. It should be noted that (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1-4 9 0 ) may also be 0.2 or less.

[0076] Further, in the present embodiment, from the viewpoint of a more excellent painted corrosion resistance being obtained, the intensity of Sn measured by XPS in the thickness direction from the surface of the steel sheet preferably satisfies (depth 50 nm I 4 8 4 . 9 ) / (depth 50 nm I 4 8 6 . 5 )≤0.5. It should be noted that (depth 50 nm I 4 8 4 . 9 ) / (depth 50 nm I 4 8 6 . 5 ) may also be 0.4 or less.(Method of Measurement of XPS)

[0077] The XPS may be measured in the following way.

[0078] First, a sample for measuring the intensity of Sn is prepared. Here, the sample is obtained by cutting out the steel sheet to be measured from the product covered. The sample is cleaned of oil and dirt on its surface using a solvent etc. It should be noted that if the steel sheet has paint or a conversion coating or if the steel sheet is a steel sheet included in an auto part, the paint or conversion coating is removed by a later explained paint removal step or conversion coating removal step.

[0079] The sample prepared in the above way is, for example, measured for intensity at a predetermined binding energy (BE) down to a depth of 50 nm in the thickness direction from the surface of the sample using a photoelectronic spectrometer JPS-9200 made by JEOL. Specifically, it is measured in accordance with the next procedure.

[0080] First, the inside of the sample holder of the apparatus is rendered a vacuum. It should be noted that the pressure inside the holder at the time of measurement is, even at a maximum, 3.0×10 - 6< Pa, while the pressure inside the holder at the time of sputtering is, even at a maximum, 1.0×10 -5< Pa.

[0081] Before measuring the sample, the time taken for cutting by 20 nm sputter using a 20 nm thickness SiO 2 wafer is obtained.

[0082] Further, after measuring the outermost surface, the sample is sputtered over a time of 2.5 times the time obtained above and the intensity is measured. The sputtering is performed by 7.5×10 -2< Pa Ar gas. The thus obtained measured value is made the depth 50 nm measurement value.

[0083] For the intensity of the Sn, the BE values at 0.1 eV pitch are smoothed and the BE value at 484.9 eV of the obtained curve is employed.

[0084] Further, the I 4 8 1 - 4 9 0 at XPS measurement is made the integrated value of the emitted photoelectronic intensity at the 481 eV≤BE≤490 eV range.

[0085] It should be noted that "I 4 8 4 . 9 " is the emitted photoelectronic intensity at BE=484.9 eV. Similarly, "I 4 8 6 . 5 is the emitted photoelectronic intensity at BE=486.5 eV.

[0086] The specific measurement conditions of XPS are as follows: X-ray source: Mg ray source X-rag diameter: Φ1 mm X-ray output: 12 kV, 25 mA Acceleration voltage: 3 kV [Conversion coating]

[0087] As explained above, the steel sheet of the present embodiment may have a conversion coating on its surface. The steel sheet of the present embodiment is decreased in solute Sn and Sn oxides at the surface of the steel sheet and has excellent chemical convertibility, therefore can be formed with a close knit high structural uniformity conversion coating at the surface of the steel sheet. As a result, excellent painted corrosion resistance can be exhibited.

[0088] It should be noted that, as the chemical conversion used for formation of a conversion coating, for example, a known zinc phosphate-based chemical conversion solution or a zirconium-based chemical conversion solution may be mentioned.(Thickness of Steel Sheet)

[0089] In the present embodiment, the thickness of the steel sheet may be a general sheet thickness. As such a sheet thickness, for example, a 0.2 to 8.0 mm sheet thickness may be mentioned. Furthermore, the thickness of the steel sheet may also be 0.3 mm or more, 0.6 mm or more, 1.0 mm or more, 1.6 mm or more, or 2.0 mm or more. Similarly, the thickness of the steel sheet may also be 7.0 mm or less, 6.0 mm or less, 5.0 mm or less, or 4.0 mm or less.

[0090] It should be noted that if the steel sheet has a conversion coating, the overall thickness of the steel sheet is comprised of the above-mentioned sheet thickness plus the thickness of the conversion coating. The thickness of the conversion coating may be, for example, 1 µm or more, 5 µm or more, 10 µm or more, 15 µm or more, or 20 µm or more. Further, the thickness of the conversion coating may also be, for example, 60 µm or less, 55 µm or less, 50 µm or less, 45 µm or less, or 40 µm or less.(Mechanical Properties)[Vickers Hardness]

[0091] The steel sheet of the present embodiment may also have, as strength, for example, a 90 HV or more Vickers hardness. The Vickers hardness of the steel sheet may also be 150 HV or more, 190 HV or more, 200 HV or more, 250 HV or more, 300 HV or more, 350 HV or more, 400 HV or more, or 450 HV or more. Further, the Vickers hardness of the steel sheet may also be 650 HV or less, 600 HV or less, 550 HV or less, or 500 HV or less.

[0092] It should be noted that the Vickers hardness is determined as follows based on JIS Z 2244-1: 2024.

[0093] First, a test piece is cut out from any position of the steel sheet except the end parts so that a cross-section vertical to the surface (sheet thickness cross-section) can be observed. The sheet thickness cross-section of the test piece cut out is polished using #600 to #1500 silicon carbide paper.

[0094] Next, a solution comprised of particle size 1 to 6 µm diamond powder dispersed in alcohol or other diluent or pure water is used to polish the sheet thickness cross-section of the test piece to a mirror finish. The sheet thickness cross-section is made the measured surface.

[0095] Next, a micro-Vickers hardness tester is used to measure the Vickers hardness by a load of 1 kgf at intervals of 3 times or more of the indentations. Specifically, a total of 20 points are randomly measured near the 1 / 4 position of sheet thickness of the test piece. The arithmetic average of these values is determined as the Vickers hardness of the steel sheet.<Part>

[0096] The steel sheet of the present embodiment, as explained above, can realize excellent chemical convertibility compared with a conventional steel sheet simultaneously containing the three elements of Ni, Cu, and Sn and in turn can realize excellent painted corrosion resistance. Therefore, the steel sheet of the present embodiment is useful for use in parts etc. in technical fields in which excellent painted corrosion resistance is demanded. In particular, the steel sheet of the present embodiment is useful in parts etc. in the automobile industry.

[0097] In a preferred embodiment, an auto part including the steel sheet according to an embodiment of the present invention is provided. As examples of auto parts, frame parts, bumpers, and other structural parts and reinforcement parts etc. where strength is required may be mentioned. Further, as other examples of auto parts, roofs, hoods, fenders, doors, and other external panel parts where high beauty is demanded may be mentioned. These parts need only contain a steel sheet according to an embodiment of the present invention at least in part. For this reason, these parts at least partially satisfy the features of the steel sheet of the above embodiment. At portions of the steel sheet which do not directly contact the dies in press-forming and other shaping or which directly contact the dies, but where the degree of work is relatively low, the features of the steel sheet do not particularly change before and after shaping. For example, in a part including a steel sheet according to an embodiment of the present invention, the portion of the location where the sample was taken, explained later (i.e., a portion avoiding the locations of (i) to (iv) explained later) can be recognized as a nonworked portion. This portion maintains the features of the steel sheet of the above-mentioned embodiment before and after shaping as a part.

[0098] The steel sheet according to an embodiment of the present invention, for example, can be employed for the various auto parts explained earlier after being formed with any conversion coating or paint on the surface. Whether an auto part having paint or a conversion coating contains a steel sheet according to an embodiment of the present invention can be judged by removing the paint or conversion coating of the sample obtained from that auto part. The location from which the sample is taken in this case, the paint removal step, and the conversion coating removal step are as follows:(Location From Which Sample is Taken)

[0099] The sample used when performing the various measurements and analyses is taken from the auto part avoiding the locations of the following (i) to (iv). (i) Welded parts: locations within 20 mm from toes of spot welded parts and locations within 20 mm from toes of arc / laser welded parts. (ii) Worked parts: worked parts with radii of curvature of less than 15 mm and locations within 5 mm from the worked parts. (iii) End parts: end parts within 5 mm from cut end parts of a part. (iv) Red rust: locations within 5 mm from locations at which red rust is visually observed. (Paint Removal Step)

[0100] The surface of the sample cut out from the auto part or steel sheet is coated with a paint remover (Neorever ®< #160, made by Sansaikako) at room temperature and allowed to stand for 5 minutes. After that, the surface of the sample coated with the paint remover is rubbed using a hard sponge (for example, Kanefil ®< , made by Aion Co., Ltd.) to peel off the paint from the surface of the sample.

[0101] Next, the surface of the sample after peeling off the paint is rinsed and dried. At this time, the residual state of the paint is confirmed by measuring the surface of the rinsed and dried sample (100 µm square, 5 fields) by SEM-EPMA.

[0102] In the elemental distribution map obtained by EPMA, the regions where the C concentration becomes 10 mass% or more are identified and if the area ratio of those regions is 5% or more, it is judged that the paint has insufficiently been removed.

[0103] The area ratio of the regions where the C concentration becomes 10 mass% or more is measured by first obtaining an elemental distribution map of C in EPMA setting the C concentration range to 10 to 30%. The specific measurement conditions of the EPMA are as follows: Apparatus: JXA-8230 electron probe microanalyzer made by JEOL Acceleration voltage: 15 kV Emitted current: 0.05 µA Surface analysis: WDS Interval of analysis: 300 µm or more Area ratio: average value of 5 fields

[0104] Next, the obtained elemental distribution map of C is image processed to measure the area ratios. For the image processing, the image analysis software "ImageJ" is used. Specifically, the elemental distribution map of C is read into ImageJ, then the function "Make Binary" in "Binary" of "Process" is used to binarize the map so that regions with a C concentration of 10 mass% or more are displayed as black and regions with a C concentration of less than 10 mass% are displayed as white. After binarization, the function "Measure" of "Analyze" is used to read the numerical value of the "Area fraction" in the "Results". The read value is determined as the area ratio of the regions with a C concentration of 10 mass% or more.

[0105] If the paint is insufficiently peeled off, the paint is repeatedly removed until the area ratio of the regions with a C concentration of 10 mass% or more becomes less than 5%.(Conversion coating Removal Step)

[0106] A sample cut out from an auto part or steel sheet and stripped of paint is stripped of its conversion coating by a suitable method when removal of the conversion coating is necessary for XPS and other various measurements. For example, when the conversion coating is a zinc phosphate coating, the method based on JIS K 3151: 1996 is performed so as to remove the conversion coating from the surface of the sample. Specifically, the sample after removal of the paint is dipped in a 5% chromic acid aqueous solution heated to 75°C for 15 minutes to thereby remove the conversion coating from the surface of the sample.

[0107] Next, the surface of the sample after removal of the conversion coating is rinsed and dried. At this time, the residual state of the chemical conversion crystals is confirmed by measuring the surface of the rinsed and dried sample (100 µm square, 5 fields) by SEM-EPMA.

[0108] In the elemental distribution map obtained by EPMA, the regions where the P concentration becomes 5 mass% or more are identified and if the area ratio of those regions is 5% or more, it is judged that the conversion coating has insufficiently been peeled off.

[0109] The area ratio of the regions where the P concentration becomes 5 mass% or more is measured by first obtaining an elemental distribution map of P in EPMA setting the P concentration range to 5 to 10%. Next, the obtained elemental distribution map of P is image processed to measure the area ratios. For the image processing, the image analysis software "ImageJ" is used. Specifically, the elemental distribution map of P is read into ImageJ, then the function "Make Binary" in "Binary" of "Process" is used to binarize the map so that regions with a P concentration of 5 mass% or more are displayed as black and regions with a P concentration of less than 5 mass% are displayed as white. After binarization, the function "Measure" of "Analyze" is used to read the numerical value of the "Area fraction" in the "Results". The read value is determined as the area ratio of the regions with a P concentration of 5 mass% or more.

[0110] If the conversion coating is insufficiently peeled off, the conversion coating is repeatedly removed until the area ratio of the regions with a P concentration of 5 mass% or more becomes less than 5%.<Method of Production of Steel Sheet>

[0111] Next, a preferable method of production of a steel sheet according to one embodiment of the present invention will be explained. The following explanation is intended to illustrate the characteristic method for producing a steel sheet according to one embodiment of the present invention and is not intended to limit the steel sheet to one produced by the method of production such as explained below.

[0112] The steel sheet of the present embodiment, for example, can be produced by a method of production including a casting step of casting molten steel, adjusted so that the chemical composition of the finally obtained steel sheet becomes the specific chemical composition, so as to form a steel slab, a hot rolling step of hot rolling the steel slab to obtain a hot rolled steel sheet, a pickling step of pickling the hot rolled steel sheet, a brush grinding step of grinding the surface of the pickled steel sheet by a brush roll, a cold rolling step of cold rolling the brush ground hot rolled steel sheet to obtain a cold rolled steel sheet, an annealing step of annealing the cold rolled steel sheet under conditions of a -10°C or more and 20°C or less high dew point and a dwell time between 600 to 700°C of 12.5 seconds or less, a post-annealing pickling step of pickling the cold rolled steel sheet after annealing, and an optional chemical conversion step of dipping the steel sheet in a chemical conversion solution to form a conversion coating on the surface of the steel sheet.

[0113] In the method of production of the steel sheet of the present embodiment, the chemical composition of the molten steel is adjusted so that the chemical composition of the finally obtained steel sheet becomes a specific chemical composition containing, by mass%, Si: 0.30 to 3.00%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000% and satisfying Si / (Si+Mn)≥0.20. Due to this, as explained above, Mn oxides are made to form inside using the Si oxides as nuclei and formation of oxides of Mn at the surface of the steel sheet is suppressed and Sn is made easier to oxidize. In other words, the content of the remaining metal Sn is easily reduced.

[0114] Furthermore, in the method of production of the steel sheet of the present embodiment, the steel slab of the above-mentioned specific chemical composition is hot rolled and pickled, then the surface of the hot rolled steel sheet is brush ground to thereby remove at least part of the Sn difficult to oxidize during annealing and remaining after pickling.

[0115] Further, in the method of production of the steel sheet of the present embodiment, the steel sheet after the brush grinding is cold rolled. The obtained cold rolled steel sheet is annealed under conditions of a -10°C or more and 20°C or less high dew point and a dwell time between 600 to 700°C of 12.5 seconds or less, whereby the Sn is made to sufficiently oxidize and the content of the metal Sn remaining at the surface of the steel sheet, i.e., the solute Sn, is made to sufficiently decrease. It should be noted that, the temperature region of 600 to 700°C is a temperature region in which Sn can diffuse in the steel up to the surface, but since it is a temperature region in which Sn does not oxidize, it is necessary to shorten the time in which the temperature of the steel sheet remains in that temperature region. If the dwell time in such a temperature region becomes more than 12.5 seconds, even if oxidation of Sn occurs, the amount of solute Sn at the surface of the steel sheet cannot be sufficiently reduced.

[0116] Further, in the method of production of the steel sheet of the present embodiment, the annealed cold rolled steel sheet is pickled to remove the Sn oxide and Si-based oxides at the surface of the steel sheet.

[0117] According to such a method of production of a steel sheet of the present embodiment, even if the steel sheet contains the three elements of Ni, Cu, and Sn, it is possible to reduce the contents of solute Sn and Sn oxide at the surface of the steel sheet so that the intensity of Sn measured by X-ray photoelectronic spectroscopy (XPS) in the thickness direction from the surface of the steel sheet satisfies (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1 - 4 9 0 )≤0.5 and satisfies (depth 50 nm I 4 8 4 . 9 ) / (depth 50 nm I 4 8 6 . 5 )≤0.7, therefore it is possible to remarkably improve the chemical convertibility of the surface of the steel sheet and as a result possible to obtain excellent painted corrosion resistance.

[0118] Below, the preferable conditions etc. of the steps in the method of production of the steel sheet of the present embodiment will be explained in detail.[Casting Step]

[0119] In the method of production of a steel sheet of the present embodiment, the casting step is a step of casting molten steel adjusted in chemical composition so as to form a steel slab. The chemical composition has to be adjusted so that the chemical composition of the finally obtained steel sheet becomes a specific chemical composition containing, by mass%, Si: 0.30 to 3.00%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000% and satisfying Si / (Si+Mn)≥0.20.

[0120] For the conditions of the casting step other than the chemical composition, usual conditions known in the field may be employed. For example, the casting step need only comprise smelting by a blast furnace, electric arc furnace, etc. followed by various secondary refining, then the usual continuous casting, casting by the ingot method, etc.[Hot Rolling Step]

[0121] In the method of production of a steel sheet of the present embodiment, the hot rolling step is a step of hot rolling the steel slab to obtain a hot rolled steel sheet. The hot rolling step is performed by hot rolling the cast steel slab directly or after once cooling, then reheating. When reheating, the heating temperature of the steel slab is, for example, 1100 to 1250°C.

[0122] In the hot rolling step, usually rough rolling and finish rolling are performed. The temperatures and rolling reductions of the rolling operations can be suitably determined in accordance with the desired metallostructure and sheet thickness. For example, the end temperature of the finish rolling is 900 to 1050°C and the rolling reduction of the finish rolling is 10 to 50%.

[0123] The finish rolled hot rolled steel sheet is coiled up at a predetermined coiling temperature, then is supplied to the next pickling step. In the present embodiment, the hot rolled steel sheet is coiled at a 520°C or more coiling temperature. The coiling temperature may also be 550°C or more. Further, the coiling temperature may also be 600°C or less.[Pickling Step]

[0124] In the method of production of a steel sheet of the present embodiment, the pickling step is a step of pickling the coiled hot rolled steel sheet using a pickling solution. The pickling step may be performed using a generally used pickling solution, for example, a predetermined concentration hydrochloric acid solution containing an inhibitor for inhibiting corrosion of the steel sheet under conditions suitable for removing the external oxide layer and internal oxide layer of the hot rolled steel sheet. The pickling may be performed in one operation or may be performed divided into several operations so as to reliably remove the external oxide layer and internal oxide layer.[Brush Grinding Step]

[0125] In the method of production of a steel sheet of the present embodiment, the brush grinding step is a step of brush grinding the pickled hot rolled steel sheet under conditions of a grinding amount of 3.0 g / m 2< or more. By brush grinding under such conditions, it is possible to sufficiently remove the solute Sn difficult to oxidize during annealing and remaining after pickling. As a result, it is possible to sufficiently reduce the content of solute Sn at the surface of the steel sheet.

[0126] It should be noted that, in the brush grinding step, it is possible to remove not only the Sn at the surface of the steel sheet, but also at least one of the Ni and Cu.

[0127] The grinding amount by the brush grinding is preferably as large as possible from the viewpoint of more reliably removing the solute Sn at the surface of the steel sheet. For example, 4.0 g / m 2< or more is preferable while 5.0 g / m 2< or more is more preferable. The upper limit of the grinding amount by the brush grinding is, for example, 20.0 g / m 2< or less and may also be 15.0 g / m 2< or less.

[0128] The grinding amount by the brush grinding can be adjusted by any suitable method known to persons skilled in the art. For example, the grinding amount by the brush grinding can be adjusted by suitably selecting the type of the brush (for example, H115 made by Hotani etc.), the wire material, bristle length, rotational speed, density, brush reduction, coating solution used, etc.[Cold Rolling Step]

[0129] In the method of production of a steel sheet of the present embodiment, the cold rolling step is a step of cold rolling the brush ground hot rolled steel sheet to obtain a cold rolled steel sheet. The rolling reduction of the cold rolling can be suitably determined in accordance with the desired metallostructure, sheet thickness, etc. The rolling reduction of the cold rolling is, for example, 20 to 80%. After the cold rolling step, the steel sheet may, for example, be air cooled to cool it down to room temperature.[Annealing Step]

[0130] In the method of production of a steel sheet of the present embodiment, the annealing step is a step of annealing the cold rolled steel sheet after the cold rolling step under conditions of a high dew point of -10°C or more and 20°C or less and a dwell time between 600 to 700°C of 12.5 seconds or less. By annealing the cold rolled steel sheet under such specific conditions, it is possible to make the Sn sufficiently oxidize and sufficiently reduce the content of solute Sn remaining at the surface of the steel sheet.

[0131] It should be noted that the dew point in the annealing step is preferably 0°C or more, more preferably is 10°C or more. Further, the dwell time between 600 to 700°C in the annealing step is preferably 11.0 seconds or less, more preferably 10.0 seconds or less. It should also be noted that the dwell time between 600 to 700°C may also be, for example, 5.0 seconds or more or 6.0 seconds or more.

[0132] In the annealing step, the conditions other than the above specified conditions need only be conditions suitable for making the Sn sufficiently oxidize and annealing the cold rolled steel sheet. For example, the annealing step includes heating in an atmosphere with a dew point of -10 to 20°C to a 700 to 950°C temperature and holding for 0 to 300 seconds after reaching that temperature. However, when heating to the 700 to 950°C annealing temperature, it is necessary to make the dwell time between 600 to 700°C 12.5 seconds or less.

[0133] It should be noted that the annealing temperature is preferably 750°C or more, more preferably 780°C or more. Further, the annealing temperature is preferably 950°C or less, more preferably 900°C or less. The holding time at the annealing temperature is preferably 30 seconds or more, more preferably 50 seconds or more. Further, the holding time is preferably 200 seconds or less, more preferably 150 seconds or less.

[0134] The atmosphere in the annealing step may be a reducing atmosphere, more specifically a reducing atmosphere containing nitrogen and hydrogen, for example, a hydrogen concentration 1 to 10% reducing atmosphere (for example, hydrogen 3% and nitrogen balance).[Post-Annealing Pickling Step]

[0135] In the method of production of a steel sheet of the present embodiment, the post-annealing pickling step is a step of pickling the annealed cold rolled steel sheet so as to remove the Sn oxide and Si-based oxides at the surface of the steel sheet. The conditions of the post-annealing pickling step need only be conditions suitable for removing Sn oxide or Si-based oxides at the surface of the steel sheet using a generally used pickling solution, for example, a predetermined concentration hydrochloric acid solution containing an inhibitor for inhibiting corrosion of the steel sheet. For example, the post-annealing pickling step may be performed by dipping the annealed cold rolled steel sheet in a predetermined concentration hydrochloric acid solution for a predetermined time.

[0136] The pickling solution used in the post-annealing pickling step is, for example, a concentration 3 to 12% hydrochloric acid solution. Further, in the post-annealing pickling step, the temperature when dipping the cold rolled steel sheet in the pickling solution is, for example, 50 to 90°C. Furthermore, the time when dipping the cold rolled steel sheet in the pickling solution is, for example, 1 to 30 seconds.

[0137] The pickling may be performed at one time or may be performed divided into several times so as to reliably remove the Sn oxide and Si-based oxides.[Chemical Conversion Step]

[0138] In the method of production of a steel sheet of the present embodiment, the chemical conversion step is an optional step performed when producing a steel sheet having a conversion coating on its surface. It is a step of dipping the steel sheet after the post-annealing pickling step in a chemical conversion solution to form a conversion coating on the surface of the steel sheet. The chemical conversion step need only be performed under conditions suitable for forming a conversion coating on the surface of the steel sheet. For example, the chemical conversion step may comprise degreasing the steel sheet by a degreasing agent, rinsing it, then using a surface conditioner to treat the surface of the steel sheet before dipping the pickled steel sheet in a chemical conversion solution.

[0139] As the chemical conversion solution used for the chemical conversion step, for example, a known zinc phosphate-based chemical conversion solution or a zirconium-based chemical conversion solution may be mentioned.

[0140] The present invention is not limited to the above embodiments or the following examples etc. Suitable combinations or replacements, changes, etc. can be used to an extent not departing from the object or gist of the present invention.

[0141] Below, examples will be used to explain the present invention in more detail, but the following examples are just illustrations of the present invention. The present invention is not limited to these examples.EXAMPLES

[0142] In the following examples, steel sheets according to embodiments of the present invention were produced under various conditions and the features of the obtained steel sheets were investigated.

[0143] First, each molten steel was cast by the continuous casting method to form a steel slab having a chemical composition shown in Table 1. Further, the steel slab was cooled once, then was reheated to 1200°C and hot rolled and was coiled up at a 520°C or more coiling temperature. The hot rolling was performed by rough rolling and finish rolling. The end temperature of the finish rolling was 900 to 1050°C and the rolling reduction of the finish rolling was 30%.

[0144] Next, the obtained hot rolled steel sheet was pickled, then the surface of the steel sheet was brush ground by a 6.0 g / m 2< grinding amount using a grinding use brush roll (H115 made by Hotani).

[0145] Next, the brush ground hot rolled steel sheet was cold rolled by a rolling reduction of 50% to obtain a cold rolled steel sheet.

[0146] Further, this cold rolled steel sheet was annealed by heating to an 800°C temperature and holding for 100 seconds in an oxygen concentration 20 ppm or less furnace in an atmosphere of a dew point shown in Table 2 and hydrogen 3% (nitrogen balance). It should be noted that, when heating to the annealing temperature, the dwell time between 600 to 700°C was as shown in Table 2.

[0147] Furthermore, the annealed steel sheet was rinsed by dipping it in a concentrate 5% hydrochloric acid solution at a 80°C temperature for 5.0 seconds to obtain various steel sheets having 1.6 mm thicknesses as examples and comparative examples. It should be noted that, when analyzing the chemical composition of the thus obtained steel sheets, they were similar to the chemical compositions of the steel slabs before hot rolling.[Table 1]

[0148] Table 1Steel typeChemical composition (mass%), balance: Fe and impuritiesSi / (Si+Mn)CSiMnAlNiCuSnPsNTiNbBMoCrA10.0020.381.480.0300.0100.0110.0030.0500.0050.0052------0.20A20.0020.401.490.0290.0450.0420.0050.0520.0050.00500.0430.0010.00010.0200.018-0.21A30.0020.391.480.0290.2350.3440.0200.0510.0050.00520.0450.0010.00010.0180.021-0.21A40.0020.401.520.0280.9830.9940.0970.0490.0050.00530.0470.0020.00020.0360.033-0.21A50.0020.391.440.0280.2310.3100.0210.0500.0050.00520.0430.0010.00010.0180.022V: 0.045, Hf: 0.0120.21A60.0020.381.380.0280.2310.3130.0220.0500.0050.00500.0430.0010.00010.0210.021Mg: 0.017, Zr: 0.1210.22A70.0020.381.440.0290.2270.3170.0210.0490.0050.00520.0440.0010.00010.0200.019W: 0.311, As: 0.0240.21A80.0020.411.410.0300.2250.3220.0230.0510.0050.00520.0430.0010.00010.0180.020Ca: 0.014, Ir: 0.2290.23A90.0020.411.380.0300.2210.3090.0210.0510.0050.00520.0430.0010.00010.0180.020Ce: 0.019, La: 0.0100.23B10.1170.441.220.0230.0110.0120.0030.0130.0030.0060------0.27B20.1180.431.280.0280.0400.0470.0050.0130.0030.00590.0280.0020.00150.0040.020-0.25B30.1220.461.250.0310.2300.3240.0220.0140.0040.00590.0220.0010.00140.0140.025-0.27B40.1220.451.230.0300.9941.0000.1000.0140.0030.00550.0390.0020.00150.0350.035-0.27C10.0830.572.030.0240.0100.0110.0030.0100.0020.0051------0.22C20.0850.592.030.0240.0440.0500.0050.0110.0020.00570.0290.0010.00150.0020.018-0.23C30.0850.582.020.0250.2310.3390.0230.0100.0020.00590.0290.0010.00150.0170.021-0.22C40.0840.602.000.0260.9980.9980.0950.0110.0020.00540.0220.0020.00160.0330.038-0.23D10.1300.992.210.0120.0110.0120.0030.0020.0020.0055------0.31D20.1280.992.200.0130.0400.0400.0050.0030.0020.00550.0310.0020.00160.0040.020-0.31D30.1281.012.200.0130.2310.3900.0250.0030.0020.00530.0220.0020.00160.0140.022-0.31D40.1311.022.180.0110.9950.9890.0940.0030.0020.00510.0330.0020.00150.0370.034-0.32E10.1811.832.580.0280.0110.0100.0030.0100.0010.0059------0.41E20.1811.822.600.0290.0440.0440.0050.0090.0010.00600.0200.0010.00150.0020.018-0.41E30.1791.842.620.0290.2500.3330.0220.0090.0010.00580.0210.0010.00160.0130.020-0.41E40.1821.832.620.0280.9880.9900.0930.0110.0010.00580.0210.0020.00150.0300.033-0.41F10.1420.422.390.0290.2470.3240.0230.0120.0020.00570.0200.0020.00140.0200.021-0.15F20.0020.130.330.0230.2340.3100.0200.0500.0040.00540.0200.0020.00010.0200.020-0.28F30.1420.181.450.0280.2350.3120.0210.0120.0020.00540.0210.0020.00130.0210.021-0.11 [Table 2]

[0149] Table 2NoClassSteel typeCompositionBrush grindingAnnealing conditionsPickling after annealingIntensity of Sn in XPS measurementVickers hardness (Hv)Type of chemical conversionPerformanceSi (mass%)Si / (Si+Mn)Dew point (°C)Dwell time between 600 to 700 °C (s)(Outermost surface I 481-490 ) / (depth 50 nm 1481-490)(Depth 50 nm I 484.9 ) / (depth 50 nm I 486.5 )Painted corrosion resistance1Ex.A10.380.20Yes-1012.2Yes0.50.7151Zn3P2A2Ex.A20.400.21Yes211.5Yes0.40.5149Zn3P2AA3Ex.A30.390.21Yes-99.5Yes0.30.7150Zn3P2AA4Ex.A40.400.21Yes39.1Yes0.20.5155Zn3P2AAA5Ex.B10.440.27Yes111.4Yes0.50.4190Zn3P2AA6Ex.B20.430.25Yes-49.1Yes0.20.7192Zn3P2AA7Ex.B30.460.27Yes109.8Yes0.30.5198Zn3P2AAA8Ex.B40.450.27Yes-411.8Yes0.50.7200Zn3P2A9Ex.C10.570.22Yes-29.2Yes0.30.6282Zn3P2AA10Ex.C20.590.23Yes09.7Yes0.20.5282Zn3P2AAA11Ex.C30.580.22Yes-910.9Yes0.50.7283Zn3P2A12Ex.C40.600.23Yes110.8Yes0.40.4288Zn3P2AA13Ex.D10.990.31Yes610.0Yes0.20.4332Zn3P2AAA14Ex.D20.990.31Yes-212.5Yes0.50.7332Zn3P2A15Ex.D31.010.31Yes1111.1Yes0.40.5339Zn3P2AA16Ex.D41.020.32Yes-68.9Yes0.30.6345Zn3P2AA17Ex.E11.830.41Yes-711.4Yes0.40.6398Zn3P2A18Ex.E21.820.41Yes311.0Yes0.40.4399Zn3P2AA19Ex.E31.840.41Yes-89.6Yes0.20.7401Zn3P2AA20Ex.E41.830.41Yes99.9Yes0.20.5409Zn3P2AAA21Ex.Al0.380.20Yes-1012.2Yes0.50.7151ZrA22Ex.A20.400.21Yes211.5Yes0.40.5149ZrAA23Ex.A30.390.21Yes-99.5Yes0.30.7150ZrAA24Ex.A40.400.21Yes39.1Yes0.20.5155ZrAAA25Ex.A50.390.21Yes-1012.0Yes0.50.6156Zn3P2A26Ex.A60.380.22Yes-811.7Yes0.40.6158Zn3P2A27Ex.A70.380.21Yes-911.2Yes0.40.6154Zn3P2A28Ex.A80.410.23Yes-1012.1Yes0.50.7155Zn3P2A29Ex.A90.410.23Yes-910.9Yes0.50.7156Zn3P2A30Comp. ex.F10.420.15Yes39.6Yes0.61.1333Zn3P2B31Comp. ex.F20.130.28Yes29.6Yes0.61.2131Zn3P2B32Comp. ex.F30.180.11Yes49.0Yes0.61.1200Zn3P2B33Comp. ex.A30.390.21No49.9Yes0.70.9150Zn3P2B34Comp. ex.D20.990.31Yes39.3No1.10.6332Zn3P2B35Comp. ex.B20.430.25Yes-129.7Yes0.51.2192Zn3P2B36Comp. ex.C40.600.23Yes315.9Yes0.70.7288Zn3P2B37Comp. ex.C30.580.22Yes239.6Yes0.71.2283Zn3P2B

[0150] Each of the various steel sheets obtained in the above way was measured by XPS and for Vickers hardness in accordance with the measurement methods explained below. Further, the painted corrosion resistance of each of the steel sheets was evaluated by the following method of evaluation. The results of these measurements and evaluations are shown in Table 2. It should be noted that the underlines given to the numerical values in Table 1 and Table 2 indicate values outside the scope of the present invention or not preferable production conditions.(Method of Measurement of XPS)

[0151] The steel sheet sample to be measured is measured for intensity at a predetermined binding energy (BE) down to a depth of 50 nm in the thickness direction from the surface of the sample using a photoelectronic spectrometer JPS-9200 made by JEOL. Specifically, it is measured in accordance with the next procedure.

[0152] First, the inside of the sample holder of the apparatus is rendered a vacuum. In particular, it should be noted that the pressure inside the holder at the time of measurement is, even at a maximum, 3.0×10 -6< Pa, while the pressure inside the holder at the time of sputtering is, even at a maximum, 1.0×10 -5< Pa.

[0153] Before measuring the sample, the time taken for cutting a 20 nm thickness SiO 2 wafer by 20 nm sputter is obtained.

[0154] Further, after measuring the outermost surface, the sample is sputtered over a time of 2.5 times the time obtained above and the intensity is measured. The sputtering is performed by 7.5×10 -2< Pa Ar gas. The thus obtained measured value is made the depth 50 nm measurement value.

[0155] For the intensity of the Sn, the BE values at 0.1 eV pitch are smoothed and the BE value at 484.9 eV of the obtained curve is employed.

[0156] Further, I 4 8 1 - 4 9 0 is obtained as the integrated value of the emitted photoelectronic intensity at the 481 eV≤BE≤490 eV range. Furthermore, I 4 8 4 . 9 is obtained as the emitted photoelectronic intensity at BE=484.9 eV and I 4 8 6 . 5 is obtained as the emitted photoelectronic intensity at BE=486.5 eV.

[0157] The specific measurement conditions of XPS are as follows: X-ray source: Mg ray source X-rag diameter: Φ1 mm X-ray output: 12 kV, 25 mA Acceleration voltage: 3 kV (Method of Measurement of Vickers Hardness)

[0158] The Vickers hardness is determined in the following way based on JIS Z 2244-1: 2024.

[0159] First, a test piece is cut out from any position from any position except the end parts of the steel sheet so that a cross-section vertical to the surface (sheet thickness cross-section) can be observed. The sheet thickness cross-section of the cut out test piece is polished using #600 to #1500 silicon carbon paper. Next, a liquid comprised of particle size 1 to 6 µm diamond powder dispersed in alcohol or other diluent or pure water is used to polish the sheet thickness cross-section of the test piece to a mirror finish. The sheet thickness cross-section is made the measured surface.

[0160] Next, a micro-Vickers hardness tester is used to measure the Vickers hardness by a load of 1 kgf at intervals of 3 times or more of the indentations. Specifically, a total of 20 points are randomly measured near the 1 / 4 position of sheet thickness of the test piece. The arithmetic average of these values is determined as the Vickers hardness of the steel sheet.[Evaluation of Painted Corrosion Resistance]

[0161] The painted corrosion resistance of the steel sheet was evaluated in the following way: First, a 50 mm×50 mm sample of the produced steel sheet to be evaluated was degreased under the following conditions.

[0162] Degreasing: A degreasing agent (Fine Cleaner E2083) was used. The sample was dipped in it at 40°C for 2 minutes, then was rinsed.

[0163] Next, the degreased sample of the steel sheet was treated by the following (i) zinc phosphate (Zn phosphate) or (ii) zirconium (Zr) as chemical conversion.(i) Zn phosphate treatment

[0164] Surface conditioning: the sample was dipped in a surface conditioner (Prepalene Z) at ordinary temperature for 30 seconds.

[0165] Chemical conversion: the sample was dipped in a zinc phosphate treatment agent (Palbond L3020 conversion agent) at 40°C for 2 minutes, then rinsed and dried.(ii) Zr treatment

[0166] Chemical conversion: the sample was dipped in a Zr-based treatment agent (PLC-2010) at 45°C for 2 minutes, then rinsed and dried.

[0167] Next, an L direction 50 mm × C direction 100 nm region of the sample of the steel sheet treated for chemical conversion was painted by electrodeposition under the following conditions: Electrodeposition solution: Powernics Excel 1200 (made by Nipponpaint Industrial Coatings Co., Ltd.) Electrodeposition temperature: 30°C Coating thickness: 18 µm Electrodeposition baking: 170°C for 30 minutes

[0168] Furthermore, an L direction 50 mm × C direction 100 nm region of the electrodeposition painted sample was scored by cuts and subjected to a JASO corrosion test "JASO M 609: 1991" for 30 cycles.

[0169] After the JASO corrosion test, the sample taken out was dried and the maximum value of the paint blisters of the sample was measured. From the maximum value of the paint blisters measured, the painted corrosion resistance of the sample was evaluated in accordance with the following criteria. AAA: maximum value of paint blister width of less than 0.9 mm AA: maximum value of paint blister width of 0.9 mm or more and less than 1.2 mm A: maximum value of paint blister width of 1.2 mm or more and less than 1.5 mm B: maximum value of paint blister width of 1.5 mm or more

[0170] A case where the painted corrosion resistance was evaluated as AAA, AA, and A was evaluated as a steel sheet excellent in painted corrosion resistance. On the other hand, a case where the painted corrosion resistance was evaluated as B was evaluated as a steel sheet poor in painted corrosion resistance.

[0171] As shown in Table 2, it was learned that the steel sheets of the Nos. 1 to 29 examples where the Si content was large, Si / (Si+Mn) was 0.20 or more, and the intensity of Sn measured by XPS in a thickness direction from the surface of the steel sheet satisfied (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1 - 4 9 0 )≤0.5 and satisfied (depth 50 nm I 4 8 4 . 9 ) / (depth 50 nm I 4 8 6 . 5 )≤0.7 were all steel sheets excellent in painted corrosion resistance.

[0172] In particular, it was learned that the steel sheets of the Nos. 2, 3, 5, 6, 9, 12, 15, 16, 18, 19, 22, and 23 examples where annealing was performed under conditions of a dew point in the annealing step of 0°C or more or a dwell time between 600 to 700°C of 10.0 seconds or less further satisfied (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1 - 4 9 0 )≤0.3 or satisfied (depth 50 nm I 4 8 4 . 9 ) / (depth 50 nm I 4 8 6 . 5 )≤0.5 and had more excellent painted corrosion resistance.

[0173] Furthermore, it was learned that the steel sheets of the Nos. 4, 7, 10, 13, 20, and 24 examples where annealing was performed under conditions of a dew point in the annealing step of 0°C or more and a dwell time between 600 to 700°C of 10.0 seconds or less further satisfied (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1 - 4 9 0 )≤0.3 and satisfied (depth 50 nm I 4 8 4 . 9 ) / (depth 50 nm I 4 8 6 . 5 )≤0.5 and had extremely excellent painted corrosion resistance.

[0174] On the other hand, it was learned that the steel sheets of the Nos. 30 to 37 comparative examples where the content of Si was small, Si / (Si+Mn) was less than 0.20, (outermost surface I 4 8 1 - 4 9 0 ) / (depth 50 nm I 4 8 1 - 4 9 0 )≤0.5 was not satisfied, or (depth 50 nm I 4 8 4 . 9 ) / (depth 50 nm I 4 8 6 . 5 )≤0.7 was not satisfied were all steel sheets poor in painted corrosion resistance.

[0175] Specifically, in the No. 30 steel sheet, Si / (Si+Mn) was low, therefore it is surmised that at the surface of the steel sheet, Mn oxides were mainly formed and oxidation of Sn was suppressed.

[0176] In the No. 31 steel sheet, the Si content was small and it is surmised the starting points of formation of nuclei for internal oxidation of Sn decreased and Sn oxidation was suppressed.

[0177] In the No. 32 steel sheet, the Si content was small and Si / (Si+Mn) was low, therefore it is surmised that oxidation of Sn was suppressed.

[0178] In the No. 33 steel sheet, brush grinding was not performed, therefore it is surmised that the Sn concentrated parts formed at the time of pickling were not completely oxidized at the time of annealing and remained in the metal state.

[0179] In the No. 34 steel sheet, the sheet was not pickled after annealing, therefore it is surmised that the Sn oxides at the surface were not removed.

[0180] In the No. 35 steel sheet, the dew point at the time of annealing was low and it is surmised that oxidation of Sn did not sufficiently occur.

[0181] In the No. 36 steel sheet, the dwell time between 600 to 700°C where Sn did not oxidize was long and it is surmised Sn was not sufficiently oxidized and was concentrated at the steel sheet surface layer in the metal state.

[0182] In the No. 37 steel sheet, the dew point at the time of annealing was high and it was surmised that mainly Fe oxides were formed at the surface of the steel sheet and oxidation of Sn was suppressed.

Examples

examples

[0142]In the following examples, steel sheets according to embodiments of the present invention were produced under various conditions and the features of the obtained steel sheets were investigated.

[0143]First, each molten steel was cast by the continuous casting method to form a steel slab having a chemical composition shown in Table 1. Further, the steel slab was cooled once, then was reheated to 1200°C and hot rolled and was coiled up at a 520°C or more coiling temperature. The hot rolling was performed by rough rolling and finish rolling. The end temperature of the finish rolling was 900 to 1050°C and the rolling reduction of the finish rolling was 30%.

[0144]Next, the obtained hot rolled steel sheet was pickled, then the surface of the steel sheet was brush ground by a 6.0 g / m 2< grinding amount using a grinding use brush roll (H115 made by Hotani).

[0145]Next, the brush ground hot rolled steel sheet was cold rolled by a rolling reduction of 50% to obtain a cold rolled steel sh...

Claims

1. A steel sheet, in which steel sheet, a chemical composition of the steel sheet contains, by mass%, Si: 0.30 to 3.00%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000% and satisfies Si / (Si+Mn)≥0.20, and an intensity of Sn measured by X-ray photoelectron spectroscopy in a thickness direction from a surface of the steel sheet satisfies (outermost surface I4 8 1 - 4 9 0 ) / (depth 50 nm I4 8 1 - 4 9 0 )≤ 0.5 and satisfies (depth 50 nm I4 8 4 . 9 ) / (depth 50 nm I4 8 6 . 5 )≤0.7.

2. The steel sheet according to claim 1 wherein an intensity of Sn measured by X-ray photoelectron spectroscopy in a thickness direction from a surface of the steel sheet satisfies (outermost surface I4 8 1 - 4 9 0 ) / (depth 50 nm I4 8 1 - 4 9 0 )≤ 0.3.

3. The steel sheet according to claim 1 or 2 wherein an intensity of Sn measured by X-ray photoelectron spectroscopy in a thickness direction from a surface of the steel sheet satisfies (depth 50 nm I4 8 4 . 9 ) / (depth 50 nm I4 8 6 . 5 )≤0.5.

4. The steel sheet according to any one of claims 1 to 3, wherein the chemical composition of the steel sheet contains, by mass%, Si: 0.50 to 3.00%.

5. The steel sheet according to any one of claims 1 to 4, wherein the chemical composition of the steel sheet contains, by mass%, Ni: 0.040 to 1.000%, Cu: 0.040 to 1.000%, and Sn: 0.004 to 1.000%.

6. The steel sheet according to any one of claims 1 to 5, having a conversion coating on the surface of the steel sheet.

7. A part including a steel sheet according to any one of claims 1 to 6.

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