JP2005336511A - Steel sheet for fuel tank having excellent property of stripping coating with alkali, press workability and weldability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet for fuel tank having excellent property of stripping coating with an alkali, press formability and spot weldability, further having ≥440 MPa tensile strength and ≥2.0 r-value and also having an excellent balance between the strength and the r-value. <P>SOLUTION: A cold rolled steel sheet to be used has: a composition consisting of, by mass, ≤0.01% C, ≤0.5% Si, 0.15 to 0.5% Mn, ≤0.03% P, ≤0.02% S, ≤0.1% Al, ≤0.006% N, 0.5 to 2.0% Cu, 0.0002 to 0.0025% B, either or both of 0.01 to <0.1% Ti and ≤0.07% Nb and the balance Fe with inevitable impurities; and a steel structure composed of ferrite single phase and having ≤20 μm ferrite grain size. A plating layer consisting of at least one or more elements selected from among Zn, Ni, Sn, Al, Si, Fe, Co, Mg and Mn is formed at least on one side of the cold rolled steel sheet. Further, an alkali coating stripping type organic film containing 1 to 40 parts by mass of lubricant and 5 to 110 parts by mass of conductive auxiliary agent based on 100 parts by mass of organic resin containing a mixture or copolymer of a water-soluble copolymer (a) and a water-soluble copolymer (b) is formed on the above plating layer. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a steel plate for a fuel tank having a tensile strength of 440 MPa or more and an r value of 2.0 or more, and excellent in alkali film removal property, press workability and weldability, and in particular, an alkali film removal type organic film on the surface of a plating layer. This is intended to improve the press workability including powdering resistance and galling resistance, and further improve the weldability while maintaining good alkali film removal property (chemical conversion treatment property).

  Conventionally, zinc-based plated steel sheets (electro-plated steel sheets and alloyed hot-dip galvanized steel sheets) have been mainly applied to automotive structural members and parts that require a high degree of rust prevention. In recent years, from the viewpoint of cost reduction, the application of alloyed hot-dip galvanized steel sheets is expanding as an alternative material for parts employing electric galvanized steel sheets.

However, alloyed hot-dip galvanized steel sheet has a large dynamic friction coefficient and inferior press workability compared with electrical galvanized steel sheet, so when applying alloyed hot-dip galvanized steel sheet to the above fields, There was a serious problem.
(A) The fracture limit is lowered and the press workable range is narrowed.
(B) Further, even if no breakage occurs, peeling of the plating layer, that is, powdering occurs at the sliding portion between the mold and the steel plate.
(C) In a more severe case, mold galling occurs and the mold is contaminated and damaged, so that not only the frequency of repair of the mold increases, but also the quality of the processed parts decreases.
Therefore, in order to apply the alloyed hot-dip galvanized steel sheet to the above applications, it is necessary to improve press workability including powdering resistance and galling resistance.

  Further, it has been studied to further improve the press workability of electrogalvanized steel sheets and electro-Zn-Ni plated steel sheets by improving the lubrication characteristics (dynamic friction coefficient).

However, it is difficult to cope with this demand with rust preventive oil or press oil.
Thus, as a means for improving the press workability, a method using an organic film as a lubricating film has been proposed.

By the way, the metal plate coated with the organic film is usually subjected to chemical conversion treatment as a post-process after press working. Such a chemical conversion treatment is necessary in order to ensure the adhesion between the coating film coated on the plate surface (such as electrodeposition coating or electrostatic coating) and the metal plate. However, in order to form a chemical conversion treatment film, the above-mentioned lubricating film must be removed.
Therefore, it is necessary to remove such a lubricating film (alkaline defilming) by alkali cleaning (degreasing) after press processing at a consumer and before chemical conversion treatment.
In order to satisfy such characteristics, a method of coating an organic film of an alkaline film removal type on the surface of a steel sheet has been proposed conventionally.

For example, in Patent Document 1, Patent Document 2 and Patent Document 3, the press workability is improved by specifying the resin, and the acid value is further defined by the presence of a carboxyl group in the resin. Thus, a technique for ensuring alkali film removal properties has been proposed.
However, these techniques not only have an insufficient state of formation of the chemical conversion film crystal after alkali film removal, but also have problems in press workability, particularly warm press workability.

Patent Document 4 solves the above problem by forming an organic lubricating film containing two types of esterified products or salts of a specific styrene-maleic acid copolymer and an alkyl group or alkenike group. Trying to.
However, since this technique has a low molecular weight, the breaking strength of the film is lowered and the press workability is inferior. In particular, when the mold temperature rises, there is a problem that the performance is remarkably lowered.

Japanese Patent Publication No.53-37817 JP-A 62-84193 JP-A-3-203996 JP-A-5-194984

The problems to be solved by the present invention are as follows.
<Press workability (powdering resistance, galling resistance)>
It is said that the mold temperature rises to at least 40 ° C and in some cases to 100 ° C due to frictional heat and deformation resistance during hundreds to thousands of continuous presses and significant deep drawing by customers. . In order to mitigate this temperature rise, press working is generally performed while oil is applied to the mold. However, as the number of presses increases, the cooling effect by the oil decreases and the mold temperature rises. As a result, the above-described prior art has a problem that the organic resin film is softened, the lubrication effect is lowered, and press cracking, powdering, and mold galling occur.

<Weldability>
There is a technique of coating an organic resin film for the purpose of improving the press workability, but this method increases the electrical resistance of the steel sheet surface during welding, resulting in a problem of poor weldability.

<Alkali film removal properties (alkali film removal properties, chemical conversion properties)>
A consumer (fuel tank manufacturer) may perform chemical conversion treatment after pressing. By this chemical conversion treatment, by forming a chemical conversion treatment crystal, the adhesion between the coating film of the subsequent coating (such as electrodeposition coating or electrostatic coating) and the metal plate is improved. In order to form such a chemical conversion treatment film, it is necessary to remove the organic resin film required for improving the press workability. Therefore, it is necessary to ensure the alkali film-removing property that the organic resin is removed during the alkali cleaning (degreasing) before the chemical conversion treatment. In particular, in a galvanized steel sheet, the alkali film-removing property is remarkably lowered due to a crosslinking reaction between Zn in the plating layer and a carboxyl group contained in the organic resin, and the removal of the organic resin film is not completely completed. Therefore, even if there is no problem in water wettability after alkali cleaning (degreasing), when the chemical conversion treatment is actually carried out, the problem of the shape of the chemical conversion film crystals and the insufficient amount of adhesion occur.
The prior art described above has such a problem.

  The present invention advantageously solves the above-mentioned problems, and firstly proposes a steel plate for a fuel tank excellent in alkali film removal property, press workability (powdering resistance, galling resistance) and weldability. The purpose.

In recent years, in order to reduce the weight of automobiles, steel plates for fuel tanks have been required to be lightweight by thinning and press workability into complicated shapes. In order to meet this requirement, the tensile strength of the base steel sheet must be increased to 440 MPa or more, and press workability must be further improved.
The present invention satisfies the above requirements together, and is excellent in secondary work brittleness resistance and plating adhesion, and has excellent tensile strength ≧ 440 MPa, r value ≧ 2.0 and balance between strength and r value. The second object is to propose a steel plate for tanks.

As a result of intensive studies to achieve the first object, the inventors have obtained the following knowledge.
(1) By effectively coating the steel plate surface with one or more plating layers selected from Zn, Ni, Sn, Al, Si, Fe, Co, Mg and Mn, Can be anticorrosive.
(2) A mixture of one or more selected from maleic acid, fumaric acid, itaconic acid and monoalkyl esters thereof and a water-soluble copolymer (b) made of styrene, Alkali film removal property can be ensured by having an organic film containing coalescence.
(3) Furthermore, the organic coating contains a mixture or copolymer of a water-soluble copolymer (a) composed of a monomer having a methacrylic ester, styrene and a carboxyl group, and the organic coating contains the above-mentioned organic coating. Resin: Press workability and weldability can be advantageously improved by containing 1 to 40 parts by mass of a lubricant and 5 to 110 parts by mass of a conductive auxiliary agent with respect to 100 parts by mass.

Next, as a result of intensive studies to achieve the second object, the inventors have obtained the following knowledge.
(1) When Cu segregates at austenite grain boundaries, grain growth of austenite is suppressed, but grain growth is not suppressed at ferrite grain boundaries after ferrite transformation.
(2) When B is added simultaneously, grain boundary embrittlement due to plating is suppressed,
(3) Moreover, the subsequent Cu precipitation treatment can advantageously increase the strength of the steel,
(4) Further, by suppressing the ferrite grain size to 20 μm or less, good secondary work brittleness resistance is obtained,
(5) As a result, a plated cold-rolled steel sheet having excellent strength-r value balance of tensile strength: 440 MPa or more, r value: 2.0 or more, and excellent secondary work brittleness resistance and plating adhesion is obtained. It is done.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
(1) In mass%,
C: 0.01% or less,
Si: 0.5% or less,
Mn: 0.15% or more, 0.5% or less,
P: 0.03% or less,
S: 0.02% or less,
Al: 0.1% or less,
N: 0.006% or less,
Cu: 0.5% or more and 2.0% or less and B: 0.0002% or more and 0.0025% or less, and
Ti: 0.01% or more, less than 0.1% and
Nb: A cold rolled steel sheet containing one or two selected from 0.07% or less, the balance of which is Fe and inevitable impurities, and having a steel structure with a ferrite single phase and a ferrite grain size of 20 μm or less. At least one side has a plating layer composed of one or more selected from Zn, Ni, Sn, Al, Si, Fe, Co, Mg and Mn, and further, methacrylate, styrene And a water-soluble copolymer (a) comprising a monomer having a carboxyl group and a water-soluble copolymer (b) comprising at least one selected from maleic acid, fumaric acid, itaconic acid and their monoalkyl esters and styrene (b) ) Or an organic resin containing a copolymer: characterized by having an organic film containing 1 to 40 parts by mass of a lubricant and 5 to 110 parts by mass of a conductive auxiliary with respect to 100 parts by mass. Alkaline film removal, Scan processability and excellent steel sheet for fuel tank weldability.

(2) In the above (1), the cold-rolled steel sheet is further mass%,
V: 0.1% or less and
Ni: A steel plate for a fuel tank excellent in alkali film removal property, press workability and weldability, characterized in that it has a composition containing one or two selected from 0.2% or more and 1.0% or less.

  According to the present invention, a steel plate for a fuel tank that is excellent in alkali film removal properties, press workability and weldability, and has a tensile strength of 440 MPa or more, an r value of 2.0 or more, and an excellent strength-r value balance. Can be obtained.

Hereinafter, the present invention will be specifically described.
First, the reason why the composition of steel is limited to the above range in the present invention will be described. Unless otherwise specified, “%” in relation to ingredients means mass%.
C: 0.01% or less C is useful for increasing the strength of steel, but if it exceeds 0.01%, the r value, which is an index of press workability, is extremely lowered. Therefore, the upper limit of the C amount is set to 0.01%.

Si: 0.5% or less
Si is a useful element that can increase the strength of steel without accompanied by significant deterioration of elongation, but at the time of hot rolling, it produces Fe-Si oxide called red scale to deteriorate the surface properties. This deterioration of the surface property causes an increase in the coefficient of friction of the surface and deteriorates the press workability. Therefore, in the present invention, the upper limit of Si content is set to 0.5%.

Mn: 0.15% or more, 0.5% or less
Mn forms S and MnS in steel and prevents the occurrence of surface defects. Therefore, 0.15% or more is added in the present invention. On the other hand, if the Mn content is less than 0.15%, the transformation point becomes high and it becomes difficult to obtain fine particles. On the other hand, if added over 0.5%, a thin oxide is formed on the surface, resulting in a decrease in plating adhesion. Therefore, in the present invention, the amount of Mn is limited to the range of 0.15% or more and 0.5% or less.

P: 0.03% or less P is an element that contributes to solid solution strengthening, but if added in a large amount, it segregates at the grain boundary and deteriorates the secondary work brittleness resistance. Therefore, the upper limit of the P amount is set to 0.03%.

S: 0.02% or less S combines with Mn to form MnS. As described above, this MnS contributes effectively to the prevention of surface defects, but on the other hand, it becomes inclusions extended at the grain boundaries and reduces the local elongation of the steel. Therefore, it is preferable that the S content is low. Therefore, in the present invention, the upper limit of the amount of S is set to 0.02%.

Al: 0.1% or less
Since Al is used as a deoxidizer, it is contained in steel to some extent. If the Al content exceeds 0.1%, the steel becomes hard and the ductility is extremely reduced. Therefore, in the present invention, the Al content is limited to 0.1% or less. The deoxidizer preferably contains at least 0.005% Al.

N: 0.006% or less N dissolves in steel and lowers ductility. Moreover, it couple | bonds with Al and Ti and forms a precipitate. In particular, when the N content exceeds 0.006%, precipitation strengthening due to nitride becomes remarkable, and ductility decreases. Therefore, in the present invention, the upper limit of the N amount is set to 0.006%.

Cu: 0.5% or more, 2.0% or less
Cu is the most important element in the present invention. Usually, the r value increases as the ferrite grains before cold rolling become finer. Further, at the time of recrystallization of the rolled ferrite, the r value increases as the ferrite grain growth is promoted. Ti and Nb are usually known as elements that suppress the austenite grain growth of low-carbon steel, but they suppress the grain boundary migration with fine precipitates. Suppresses growth. For this reason, for example, even if C is added in an amount of 0.01% or more necessary for suppressing grain growth to suppress austenite grain growth, improvement of the r value cannot be expected.
In contrast, Cu segregates at the austenite grain boundaries and suppresses austenite grain growth in the hot rolling heating and rough rolling processes, contributing to the refinement of crystal grains. Do not suppress. For this reason, the ferrite grains immediately after the γ → α transformation are made fine, and after the ferrite transformation is completed, grain growth during recrystallization of the processed ferrite is not hindered. For this reason, r value improves significantly compared with steel without Cu addition.
Moreover, since Cu precipitates finely in steel when it is subjected to a precipitation treatment, it effectively contributes to high strength.
Here, if the Cu content is less than 0.5%, the above-mentioned grain growth suppressing function is not sufficient, and the strength does not exceed the 440 MPa class. On the other hand, if it exceeds 2.0%, not only the grain growth of austenite is suppressed excessively and a grain-sized structure cannot be obtained, but also an increase in the amount of solid solution of Cu makes it easy to burn. Therefore, in this invention, Cu amount was limited to the range of 0.5% or more and 2.0% or less.

B: 0.0002% or more and 0.0025% or less B is a useful element that segregates at the ferrite grain boundary to strengthen the grain boundary and improve the secondary work brittleness resistance. Moreover, in this invention, there exists a function which prevents effectively the intergranular embrittlement by a plating layer containing component by containing together with above-described Cu. However, if the content is less than 0.0002%, the effect of addition is poor. On the other hand, if added over 0.0025%, baking tends to occur and it becomes difficult to obtain a sized structure, so the amount of B is 0.0002% or more, 0.0025 It was limited to the range of% or less.

Ti: 0.01% or more, less than 0.1%
Ti has the effect of lowering the amount of solid solution C and N that degrade ductility and r value by fixing C and N contained in a trace amount in steel as precipitates. In order to obtain an excellent r value, addition of 0.01% or more is necessary. However, if it is 0.1% or more, the rolling load increases more than necessary, and the surface properties of the rolled material deteriorate. Further, when the Ti amount is 0.1% or more, the amount of undissolved TiC increases at the time of hot rolling slab heating, the amount of solid solution Ti decreases, and the crystal grains of the hot rolled steel sheet are difficult to be refined. Therefore, the Ti content is limited to a range of 0.01% or more and less than 0.1%.

Nb: 0.07% or less
Nb, like Ti, has the function of fixing C as a precipitate and reducing the amount of dissolved C. Therefore, in this invention, it can add instead of Ti or with Ti. However, if the content exceeds 0.07%, the hot rolling load is remarkably increased, the surface properties of the steel sheet are deteriorated and the shape is not stable. Moreover, NbC does not melt | dissolve at the time of slab heating, and it becomes difficult to obtain a fine grain. Therefore, the Nb content is limited to 0.07% or less.

The basic components have been described above. However, in the present invention, other elements described below can be appropriately contained.
V: 0.1% or less V forms precipitates in steel with N or C. Also, the rolling load during hot rolling is not increased. For this reason, it can be added simultaneously with Ti for the purpose of sufficiently fixing C and N. However, if the V content exceeds 0.1%, fine VC and VN precipitate and the steel becomes low ductility, so the V content is limited to 0.1% or less.

Ni: 0.2% or more, 1.0% or less
Ni contributes effectively to reduce surface flaws caused by Cu addition. However, if the Ni content is less than 0.2%, the effect of addition is poor. On the other hand, if the Ni content exceeds 1.0%, a hardened structure tends to appear and the workability deteriorates. Therefore, the Ni content is limited to the range of 0.2% to 1.0%.

Although the preferred component composition range has been described above, in the present invention, it is not sufficient to limit the component composition to the above range, and adjustment of the steel structure is also important.
That is, in the present invention, it is important to make the steel structure a ferrite single phase structure and to regulate the ferrite grain size to 20 μm or less.
In the present invention, the reason why the steel structure is made of a ferrite single phase structure is that pearlite is not preferable because it lowers the deep drawability (r value) of the steel and deteriorates workability, and low temperature transformation such as martensite and bainite. This is because the inclusion of the phase also deteriorates the r value.
The ferrite single-phase structure in the present invention means that only ferrite grains are observed in addition to precipitates such as carbonitrides when a steel sheet cross-section subjected to nital etching is observed with a magnification of 400 times with an optical microscope. Say. The ferrite grain size of the present invention refers to the ASTM nominal grain size. ASTM nominal grain size is defined as the square root of the area per grain and is 1.13 times the cut length l. That is, the ferrite grain section length obtained according to the cutting method (JIS G 0552: 1998) is multiplied by 1.13 to obtain the ferrite grain size.

In the present invention, the reason for restricting the ferrite grain size to 20 μm or less is as follows.
That is, since the r value increases with an increase in the ferrite grain size, if the grain size is coarse, a good r value can be obtained, but if the ferrite grain size exceeds 20 μm in the cold-rolled sheet Further, the secondary work brittleness resistance is significantly deteriorated. For example, although normal IF steel has a high r value, the ferrite grain size is as coarse as about 40 μm, and the secondary work brittleness resistance is not good. Therefore, there has been a demand for a steel plate having a ferrite grain size of 20 μm or less and a high r value, but no such cold-rolled steel plate has been developed in the past.
In this regard, in the present invention, by using Cu, an excellent r value can be realized while finely retaining the ferrite grains immediately after the austenite-ferrite transformation and improving the secondary work brittleness resistance. At this time, since the ferrite grains immediately after transformation from austenite are fine, the ferrite grain size can be maintained at 20 μm or less even after the grains are sufficiently grown to secure the r value. Therefore, in the present invention, the upper limit of the ferrite grain size is set to 20 μm.
Moreover, since the cold rolled steel sheet has no element concentration on the surface, the reaction between the plating and the base is likely to proceed. In particular, when the ferrite grain size exceeds 20 μm, an alloying reaction that deteriorates plating adhesion proceeds. Also from this point, it is advantageous to make the ferrite grain size of the base steel sheet 20 μm or less.

Next, the preferable manufacturing conditions for the cold-rolled steel sheet will be described.
In the present invention, the steel slab adjusted to the above preferred component composition is heated to a temperature of more than 950 ° C. and less than 1100 ° C., and then the finish rolling temperature is less than 750 ° C. and the rolling reduction is 750 ° C. After rolling at a temperature of 550 ℃ or less, if necessary, annealing at a temperature of 600 ℃ or more, then pickling, and after cold rolling with a reduction ratio of 50% or more, A cold-rolled steel sheet having desired characteristics can be obtained by performing an annealing treatment and further performing a Cu precipitation treatment at a temperature of 450 ° C. or higher and 650 ° C. or lower.
The reason why the manufacturing conditions of the steel sheet are as described above is as follows.
Heating temperature: more than 950 ° C. and less than 1100 ° C. The heating temperature is an important production condition in the present invention. When this heating temperature is 1100 ° C. or higher, the austenite grains of the slab become extremely coarse, the effect of suppressing the austenite grain growth by Cu is weakened, and the structure is mixed. On the other hand, if the heating temperature is less than 950 ° C., the rolling load becomes too high and rolling may not be possible, and the surface properties are significantly deteriorated. For this reason, the slab heating temperature during hot rolling is preferably about 950 ° C. and less than about 1100 ° C.

Finishing rolling temperature: less than 750 ° C. The present invention achieves a good r value with a cold-rolled steel sheet. The texture of ferrite transformed from austenite is close to random, and the r value is 1.0 or less. The higher the r value of the steel sheet before cold rolling, the higher the r value after cold rolling and annealing. For this reason, finish rolling is completed at less than 750 ° C. and rolling is performed in the ferrite region, whereby ferrite is recrystallized and a suitable texture is formed at the stage of hot rolling. For this reason, it is preferable to finish the finish rolling at less than 750 ° C. If the finish rolling temperature is below 500 ° C., the surface properties deteriorate and the flat steel plate shape cannot be maintained. Therefore, the finish rolling temperature is preferably 500 ° C. or higher. Desirably, it is 650 ° C or higher.

Rolling rate at 750 ° C. or lower: 50% or more In the present invention, the rolling rate at 750 ° C. or lower is extremely important. Since ferrite is easy to recover, the strain energy necessary for recrystallization cannot be accumulated simply by hot rolling in the ferrite region. Therefore, it is necessary to perform rolling at 750 ° C or less, which is slow to recover. If the rolling reduction at 750 ° C or lower is less than 50%, the strain energy necessary for recrystallization is not accumulated when annealing a hot-rolled sheet, so the rolling reduction at 750 ° C or lower is 50% or higher. It is preferable to do. When the cold rolling is performed as it is, the strain of the hot rolled sheet is added to the strain of the cold rolling, the actual cold rolling reduction ratio is increased, and the r value is increased. However, if the rolling reduction at 750 ° C. or lower is less than 50%, the accumulated amount of strain energy is still insufficient.

Winding temperature: 550 ° C or less Strain energy is released by recovery in the ferrite region. Therefore, when the coiling temperature exceeds 550 ° C., the release of strain energy proceeds and it becomes impossible to recrystallize with the strain energy remaining in the hot rolled sheet. In the case of direct cold rolling, the substantial accumulated strain energy is reduced. For this reason, the winding temperature is preferably 550 ° C. or lower.

Hot-rolled sheet annealing temperature: 600 ° C. or higher When the hot-rolled sheet is annealed before cold rolling and recrystallized once, and then cold-rolled, the r value is further improved. Therefore, in this invention, this hot-rolled sheet annealing can be implemented as needed. However, when the hot-rolled sheet annealing temperature is less than 600 ° C, the hot-rolled sheet is not sufficiently recrystallized, and therefore the annealing temperature is preferably set to 600 ° C or higher.
In this annealing, there is no problem even if the hot-rolled sheet is pickled and then annealed or pickled after annealing.

Pickling process In this invention, after removing the scale produced | generated by hot rolling, cold rolling is performed. When cold rolling is performed without removing the scale, the scale is pushed into the surface of the steel sheet, and the surface appearance and plating adhesion deteriorate. For this reason, it is preferable to perform pickling before cold rolling. When hot-rolled sheet annealing is performed, it may be pickled before or after annealing, but when peeling occurs when the scale is annealed, surface picking may be performed before annealing. preferable.

Cold reduction ratio: 50% or more In the present invention, strain energy is accumulated during hot finish rolling, and formation of a texture suitable for improving the r value is promoted, but after hot rolling or after hot-rolled sheet annealing. By performing cold rolling at 50%, it is possible to develop a rolling texture that is further favorable for improving the r value. However, if the cold rolling reduction is less than 50%, no significant change is observed in the texture. Therefore, the cold rolling reduction is preferably 50% or more. Moreover, although this cold rolling may be performed without annealing after hot rolling, the r value is further improved when it is performed after hot rolling sheet annealing.

Cold-rolled sheet annealing treatment Cold-rolled sheets have many dislocations introduced by cold-rolling, so they are hard and have low ductility and are not suitable for pressing. Therefore, the ductility is also recovered by annealing and softening the cold-rolled sheet. The annealing temperature is not particularly limited, but is preferably about 600 ° C or higher and 900 ° C or lower. Particularly preferred is a range of 700 ° C. or more and 900 ° C. or less.

Cu precipitation treatment: 450 ° C. or more, 650 ° C. or less In the present invention, after producing a steel plate having a good r value by the above method, Cu precipitation treatment is performed, and the steel plate strength is maintained while maintaining the r value high. Higher strength than MPa. Here, when the treatment temperature in the Cu precipitation treatment is less than 450 ° C., Cu becomes fine, but a sufficient precipitation amount cannot be obtained, and a strength of 440 MPa or more cannot be secured. On the other hand, if it exceeds 650 ° C, Cu becomes coarse or re-solidifies, making it difficult to obtain a tensile strength of 440 MPa or more. Therefore, the Cu precipitation treatment temperature is preferably in the range of 450 ° C. or higher and 650 ° C. or lower.
In addition, in the case of hot dipping, this Cu precipitation treatment can be combined with temperature holding by dipping bath immersion and temperature holding by alloying heating.

Next, the surface coating layer will be described.
In the present invention, on both surfaces of the cold-rolled annealed plate obtained as described above, first, one or more selected from Zn, Ni, Sn, Al, Si, Fe, Co, Mg and Mn are formed. A plating layer is formed. The plating layer forming means may be electroplating, hot dipping, or alloying hot dipping.
Examples of electroplating include electrogalvanization, electrozinc-nickel alloy plating, electrozinc-cobalt alloy plating, electrozinc-iron alloy plating, and hot-dip plating. Zinc-magnesium plating, hot-dip zinc-aluminum-magnesium plating, and the like, and alloyed hot-dip galvanizing and the like are advantageously suitable as the alloying hot-dip plating. Of course, multi-layer plating in which these layers are laminated may be used.

These plating layers preferably have an adhesion amount per side of about 10 to 200 g / m ² . This is because if it is less than 10 g / m ² , the coating effect on the surface of the steel sheet is insufficient, and the corrosion resistance required for the plating layer is insufficient, while if it exceeds 200 g / m ² , the effect of improving corrosion resistance cannot be expected. This is because not only is it uneconomical, but also the weldability may deteriorate. More preferably, it is the range of 20-100 g / m < ² >.

Next, an organic film is formed on the plating layer.
As the resin used as the base of this organic film, in order to ensure alkali film-removability and improve press workability, the water-soluble copolymer (a) and water-soluble copolymer (b) described below are used. It must be a mixture or copolymer.
The water-soluble copolymer (a) is advantageous for improving press workability (powdering resistance and galling resistance), and the water-soluble copolymer (b) is advantageous for improving alkali film-removing property. Is.
Moreover, in this invention, in order to neutralize each water-soluble copolymer, neutralizing agents, such as ammonia and amines, are contained.

Water-soluble copolymer (a)
As a result of studying acrylic resins from the viewpoint of press workability, the rupture energy of the resin film is increased to a high level by copolymerizing methacrylic ester, styrene, and a monomer having a carboxyl group, not just a blend. It has been found that this is effective for improving the press workability.
The methacrylic acid ester of the present invention is ethyl methacrylate (abbreviation EMA: glass transition point Tg65 ° C), isopropyl methacrylate (abbreviation iso-PMA: Tg81 ° C), normal butyl methacrylate (abbreviation n-BMA: Tg20 ° C), And isobutyl methacrylate (abbreviation: iso-BMA: Tg67 ° C.).

  As described above, it is preferable that the ester terminal carbon number is 2 or more. This is because in the case of a methyl group having 1 carbon, the rust prevention property is extremely lowered. This is presumably because the polarization of the ester group is increased by the methyl group, and as a result, the interaction with water is increased. The alcohol having 2 or more carbon atoms to be esterified with methacrylic acid is preferably an aliphatic monohydric alcohol practically. In the case of an acrylic ester, the Tg is usually less than 0 ° C., and the press workability tends to be inferior. In the present invention, a methacrylate ester is used. Therefore, the above-mentioned methacrylic acid ester has a Tg of 0 ° C. or more and is excellent in press workability. In general, in actual press work performed by customers, the temperature of a metal plate or mold reaches 100 ° C due to frictional heat or heat due to deformation resistance of the metal plate because it is formed continuously and at high speed. . Therefore, when a monomer having a Tg of less than 0 ° C. is added as a polymer, the soft component is detached from the metal plate or melted by heat, thereby reducing the lubrication effect of the entire film. Tg is preferably in the range of 10 to 90 ° C.

Next, styrene which is the second copolymer component of the water-soluble copolymer (a) will be described below.
Styrene has a high moisture resistance and a high Tg, so it is a useful component for achieving both corrosion resistance and press workability. The copolymerization ratio of the methacrylic acid ester and styrene is preferably 10:90 to 90:10 as a mass ratio of methacrylic acid ester: styrene. If the ratio of the methacrylic acid ester is less than 10% by mass in the total amount of both, since the content of styrene increases, the press workability and corrosion resistance are improved. Tends to decrease, and the storage stability of the paint tends to decrease. When it exceeds 90% by mass, the breaking strength of the film is lowered, the press workability is lowered, and at the same time the hydrophobicity of the film is lowered, so that the corrosion resistance is lowered. A more preferable mass ratio of the copolymerization ratio of methacrylic ester: styrene is 10:90 to 50:50.

The third polymerization component is a monomer having a carboxyl group, and examples thereof include acrylic acid, methacrylic acid, maleic acid, and itaconic acid. As a result, the copolymer is imparted with a carboxyl group and can be water-soluble by neutralization with ammonia or amines, for example, so that a water-soluble copolymer is obtained. That is, it is necessary to make it water-soluble in order to impart alkali film-removing property, which is one of the important performances of this type of paint, and it is necessary to contain a monomer having a carboxyl group.
This polymerization ratio is preferably adjusted so that the acid value of the copolymer (a) is 20 to 300 mg-KOH / g. When the acid value is less than 20 mg-KOH / g, the alkali film removal performance of the film carried out with an alkali degreasing solution is extremely lowered. In addition, it becomes difficult to form a coating by water-solubilizing the copolymer itself. On the other hand, if it exceeds 300mg-KOH / g, the corrosion resistance is extremely lowered, and the effect of rust and other added rust preventives is not achieved. A more preferable acid value is 25 to 230 mg-KOH / g, still more preferably 100 to 200 mg-KOH / g.
In order to ensure the above-mentioned preferable acid value, for example, in the case of methacrylic acid, the water-soluble copolymer (a) of methacrylic acid and styrene and methacrylic acid ester (a) 100 parts by mass of methacrylic acid is 4 to 35 parts by mass. It is preferable to do.

  The preferred molecular weight of the water-soluble copolymer (a) is 10000 to 60000 in terms of weight average molecular weight. Within this range, the lubricating effect of the film is advantageously expressed, which is preferable for press workability. Furthermore, in the water-soluble copolymer (a) of the present invention, the carboxyl group is water-solubilized with a neutralizing agent such as ammonia or amine.

  The water-soluble copolymer (a) in the present invention can be obtained by known solution copolymerization in a water-soluble solvent. In a water-soluble solvent adjusted to 80 to 140 ° C. (for example, butyl cellosolve), methacrylic acid ester: styrene is preferably adjusted at a mass ratio of 10:90 to 90:10 in a nitrogen atmosphere, and further contains a carboxyl group The monomer to be polymerized is preferably added dropwise with the polymerization initiator over 4 to 5 hours so that the acid value is 20 to 300 mg-KOH / g, and further heated and stirred at the same temperature for 2 to 8 hours for polymerization. To complete. Then, after cooling to 55 to 60 ° C., neutralization and water-solubilization is performed with a base such as ammonia or amine to produce a predetermined water-soluble copolymer (a).

Preferred neutralizing agent Further, when the metal plate to be coated is an alloyed hot-dip galvanized steel plate, as described above, the alkali film removal tends to be inferior due to the interaction between zinc and carboxyl groups in the plating layer. is there. This can be further improved by specifying a neutralizing agent and adding a water-soluble copolymer (b) described later. From the viewpoint of improving corrosion resistance and press workability, the resin is required to improve the breaking strength of the resin film. For this purpose, it is necessary to improve the Tg and molecular weight of the resin. However, the alkali removal property of the film is lowered. Moreover, the alkaline degreasing conditions in the consumer are often performed at a low temperature and in a relatively short time. For this reason, even when a large amount of -C00H groups are present in the resin film after film formation, if the alkali degreasing time is short at a temperature equal to or lower than the resin Tg, the alkali film removal property may be deteriorated. For this reason, in order to improve the alkali film-removing property, a hydrophilic substance is contained in the film by neutralizing a part of the -C00H group in the film with an amine which is difficult to evaporate during film formation, that is, an amine having a high boiling point. It is effective to leave

  Thereby, in alkali degreasing, the penetration of alkali into the inside of the film is helped, and processing at a low temperature and in a short time is enabled. That is, it is preferable to use a mixture of ammonia and an amine having a boiling point of 30 ° C. or higher when neutralizing the amine of the water-soluble copolymer (a). The reason why the boiling point is defined as the amines is that when the temperature is lower than 30 ° C., the neutralized amine volatilizes even during drying at room temperature, and the alkali film-removing property deteriorates. Preferably, the boiling point is 100 ° C or higher. More preferably, the temperature is set to 160 ° C. or higher. Examples of amines having a boiling point of 30 ° C or higher that can be applied in the present invention include isopropylamine (boiling point 32 ° C), diethylamine (boiling point 56 ° C), N, N-dimethylethanolamine (boiling point 130 ° C), monoethanolamine (boiling point 170 ° C). ), Triisopropanolamine (boiling point 305 ° C.), triethanolamine (boiling point 360 ° C.), and the like. Sodium hydroxide and potassium hydroxide can also be used, but a preferred neutralizing agent is an alkanolamine or alkylamine having a boiling point of 30 ° C. or higher.

  The addition amount of ammonia and amines having a boiling point of 30 ° C. or higher is preferably 0.5 to 1.5 equivalents of ammonia and 0.1 to 0.5 equivalents of amines having a boiling point of 30 ° C. or higher with respect to 1 equivalent of acid contained in the resin. The reason for this is that if the amount of amines having a boiling point of 30 ° C. or higher is less than 0.1 equivalent, it is difficult to ensure alkali film removal under alkaline degreasing conditions in a short time, and conversely if the amount exceeds 0.5 equivalents. This is because the breaking strength of the film is remarkably lowered, the press workability is also lowered, and the productivity is lowered. Furthermore, it is because blocking resistance also falls. A more preferable addition amount is 0.2 to 0.4 equivalent. On the other hand, if the amount of ammonia added is less than 0.5 equivalent, water-solubilization by neutralization becomes difficult. Conversely, if it exceeds 1.5 equivalents, the breaking strength of the film tends to deteriorate and lubricity tends to be impaired.

Water-soluble copolymer (b)
Furthermore, in order to impart a high degree of alkali film-removing property and film drying property, in addition to the water-soluble copolymer (a), at least one selected from maleic acid, fumaric acid, itaconic acid and monoalkyl esters thereof And a water-soluble copolymer (b) of styrene. This is because the COOH group of the water-soluble copolymer (b) functions as a residual site such as amine contained as a neutralizing agent. Thereby, alkali film removal property is securable. The molar ratio of styrene: maleic acid is preferably 75:25 to 1:99. The maleic acid is at least one selected from maleic acid, fumaric acid, itaconic acid and monoalkyl esters thereof. Moreover, it is preferable that a monoalkyl ester is practically based on a C1-C4 alcohol. The reason why the molar ratio of styrene: maleic acid is preferably 75:25 to 1:99 is that when the molar ratio of styrene is less than 1%, the film breaking strength decreases because there is little styrene with high film breaking strength. The press workability, particularly the press workability and powdering resistance when the mold temperature rises deteriorates. On the other hand, when it exceeds 75%, the COOH group is remarkably reduced, and the alkali film removal property is deteriorated. More preferably, the molar ratio of styrene: maleic acid is 75:25 to 20:80.

  In the water-soluble copolymer (b), it is preferable that at least one C00H group of maleic acid remains in one molecular chain. In particular, it is preferable to leave 1-7. This is because alkali film-removing properties can be advantageously ensured. The molecular weight is preferably 500 to 10,000 as a weight average molecular weight. When the molecular weight is less than 500, the breaking strength of the film is remarkably lowered, which adversely affects the press workability. On the other hand, if it exceeds 10,000, the effect of improving the alkali film removal property cannot be expected. The water-soluble copolymer (b) in the present invention can be obtained by known solution copolymerization, bulk polymerization, emulsion polymerization, and gas phase polymerization in a water-soluble solvent.

  The water-soluble copolymer (b) is preferably neutralized with ammonia to make it water-soluble. The addition amount of the neutralizing agent is preferably in the range of 0.6 to 4 equivalents per 1 equivalent of acid in the resin solid content of the water-soluble copolymer (b). This is because if the added amount of the neutralizing agent is less than 0.6 equivalent, the coating stability is lowered, and if it exceeds 4 equivalents, the breaking strength of the film is lowered and the press workability is lowered. The more preferable addition amount of the neutralizing agent is 0.8 to 2.5 equivalents.

  In the present invention, the content ratio (mass ratio) of the water-soluble copolymer (b) and the water-soluble copolymer (a) to be contained is preferably about 1:99 to 50:50. This is because when the water-soluble copolymer (b) is less than 1% by mass, the alkali film-removing property, the film drying property and the blocking resistance are deteriorated. On the other hand, when it exceeds 50% by mass, the press workability tends to deteriorate. A more preferable content ratio of water-soluble copolymer (b): water-soluble copolymer (a) is 10:90 to 50:50 by mass ratio. In the present invention, the water-soluble copolymer (a) and the water-soluble copolymer (b) may be simply mixed or copolymerized. For example, the copolymerization is carried out by the following method. In a water-soluble solvent adjusted to 80 to 140 ° C. by solution polymerization, for example, butyl cellosolve, in a nitrogen atmosphere, styrene, a methacrylic acid ester, and a monomer having a methacrylic acid group together with a polymerization initiator are used for 4 to 5 hours. The copolymer is added while being added, and a styrene-maleic acid copolymer which is a water-soluble copolymer (b) is added, and the mixture is heated and stirred for 2 to 3 hours for copolymerization. Then, it cools to 55-60 degreeC and neutralizes with bases, such as ammonia, and manufactures water-soluble acrylic resin.

In the present invention, a lubricant is added to the above organic film for the purpose of further improving the press workability.
The type of the lubricant is not particularly limited, but a lubricant having a melting point (mp) of about 70 to 150 ° C. is preferable.
Among such lubricants, examples of the solid lubricant include graphite, molybdenum disulfide, talc, Teflon (registered trademark), boron nitride, calcium carbonate, melamine, and isocyanuric acid adduct. Examples of waxes include polyethylene wax, petroleum paraffin wax, animal and vegetable oils and fats, higher fatty acids, higher alcohols, esters of higher fatty acids and higher alcohols, higher fatty acid amides, and amine salts of higher fatty acids. Therefore, it is preferable to use one having a melting point of 70 ° C. or higher. In order to increase the melting point, there is a method of eliminating double bonds by hydrogenation.

The blending ratio of the lubricant contained in the organic film needs to be about 1 to 40 parts by mass with respect to 100 parts by mass of the organic resin. This is because if the blending ratio of the lubricant is less than 1 part by mass, sufficient improvement in lubricity cannot be expected. On the other hand, if it exceeds 40 parts by mass, the strength of the formed film decreases, and the lubricity also decreases. It is because there is a possibility of doing. More preferably, it is the range of 5-30 mass parts.
The average particle size of the lubricant is preferably about 1 to 7 μm. If the average particle size is less than 1 μm, the amount of lubricant protruding from the film may be small, and sufficient press workability may not be obtained. On the other hand, if it exceeds 7 μm, the film becomes too brittle and the film is resistant to powder. This is because the ring property is lowered and the press workability tends to deteriorate.

Furthermore, in the present invention, for the purpose of further improving the weldability such as spot welding and seam welding performed when the fuel tank is manufactured, a conductive auxiliary agent is added to the organic film.
As such a conductive auxiliary agent, those having a high specific resistance and a large calorific value are preferable, and specific examples include Al, Ni, Sn, stainless steel, Cu and C (graphite). Among these, Ni is the most useful metal because of its excellent corrosion resistance to methanol and high specific resistance. In addition, Al has a lower specific resistance and melting point than Ni and is not necessarily optimal for welding, but by adding Al in the form of scales (flakes) into the film, corrosion of formic acid aqueous solution, etc. It is a metal that can suppress the permeation of ionic ions and is also useful for improving the inner surface corrosion resistance.

The compounding ratio of the conductive auxiliary agent contained in the organic film is in the range of 5 to 110 parts by mass with respect to 100 parts by mass of the organic resin. This is because if the blending ratio of the conductive auxiliary agent is less than 5 parts by mass, sufficient improvement in weldability cannot be expected. On the other hand, if it exceeds 110 parts by mass, the organic film becomes brittle and is resistant to powder during press working. It is because ring property falls. More preferably, it is the range of 45-110 mass parts.
In addition, it is preferable that the average particle diameter of this conductive support agent shall be about 0.01-3 micrometers. If the average particle size is less than 0.01μm, the energization point may be insufficient and the weldability may be inferior. On the other hand, if it exceeds 3μm, the conductive aid may be in contact with the mold and press workability may be reduced. is there.

  In the present invention, in addition to the above components, if necessary, a solvent such as hydrocarbon, a colorant, a pigment, a dye or the like may be added as an additive, and an oily agent, extreme pressure agent, Additives that are added to ordinary coating compositions such as rust inhibitors can also be added. Preferred additives include metal soaps that are salts with alkaline earth metals such as calcium, barium and the like, and salts of phosphoric acid and molybdic acid with zinc, calcium, or ammonium, such as lauric acid, palmitic acid, stearic acid, and oleic acid. It is. These preferable addition amount is 3-30 mass parts in total with respect to 100 mass parts of organic resins, Preferably it is 5-20 mass parts. Within this range, the effect of each additive can be exhibited.

Moreover, when depositing such an organic film, the amount of adhesion is preferably about 0.5 to 5 g / m ² by dry weight per side. The reason is that if it is less than 0.5 g / m ² , the unevenness of the surface of the metal plate cannot be filled, and the effect of this film on the press workability and corrosion resistance is not expressed, whereas if it exceeds 5 g / m ² , This is because there is a great possibility that powdering will occur during press working. A more preferable range is 0.5 to 3 g / m ² .
The film can be formed by a known method such as roll coating, spray coating, dip coating or brush coating. The drying conditions are a temperature of 40 to 200 ° C. and a drying time of about 1 to 90 seconds.

Steel slabs having the composition shown in Table 1 were hot-rolled to obtain hot-rolled steel sheets. The steel slab heating temperature during hot rolling was 1030 ° C, the finish rolling temperature was 690 ° C, the rolling reduction below 750 ° C was 55%, and the coiling temperature was 480 ° C. Next, the obtained hot-rolled sheet was pickled, cold-rolled at a reduction ratio of 70%, annealed at 830 ° C for a short time, and then subjected to Cu precipitation treatment at 500 ° C.
The ferrite grain size, tensile strength (TS) and r value of the cold-rolled annealed plate thus obtained were investigated.

Next, plating treatment and organic coating treatment were performed on both surfaces of the obtained cold-rolled annealed plate to form plating layers and organic coatings shown in Table 2.
The composition of the resin in the above organic film coating treatment is as shown in Table 3, and the type of the conductive auxiliary used is as shown in Table 4. In addition, the organic film coating treatment was performed by roll coating and dried so that the ultimate plate temperature after 10 seconds was 60 ° C.
The thus obtained fuel tank steel sheet was examined for alkali film removal, limit drawing ratio, powdering resistance (C count ratio), galling resistance, secondary work brittleness and weldability.
The results obtained are summarized and shown in Table 5.

In addition, the evaluation method of each characteristic is as follows.
<Ferrite particle size>
The obtained cold-rolled annealed plate was subjected to 1.13 times the section length determined by the cutting method (JIS G 0552: 1998) from the optical micrograph (400 times) of the thickness cross section of the nital corroded plate. Asked.
The examples in Table 6 based on the component systems in Table 1 were all ferrite single-phases regardless of invention examples and comparative examples.
<Tensile strength (TS)>
From the obtained cold-rolled annealed plate, a JIS No. 5 test piece was sampled and subjected to a tensile test so that the tensile direction was perpendicular to the rolling direction, and the steel plate strength was measured.
<R value>
From the obtained cold-rolled annealed plate, JIS No. 5 test pieces were sampled in parallel to the rolling direction, 45 ° direction, and perpendicular direction, and the r value was measured. Evaluation of r value was performed by the average value (bar r) of r values in each direction. When r value parallel to the rolling direction is r ₀ , the 45 ° direction is r ₄₅ , and the perpendicular direction is r ₉₀ , the bar r is expressed by the following formula: bar r = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4
It is represented by

<Alkali film removal properties>
The obtained fuel tank steel plate is immersed in an alkaline membrane removal liquid (CLN-364S (Nippon Parker) 20 g / l 60 ° C) for 10 seconds, and calculated from the following formula from the C count before and after immersion (measured by FX). The residual ratio of C to be used is defined as the residual film ratio, and is determined according to the following criteria.
C remaining rate (film remaining rate) (%) = (C count after film removal / C count before film removal) × 100
A: Film remaining ratio <5%
B: 5% ≦ film remaining rate <10
C: Film remaining rate ≧ 10%

Cylindrical processing was performed on the fuel tank steel sheet obtained under the following conditions, and the press workability was evaluated by investigating the limit drawing ratio and the powdering resistance.
<Pressing conditions>
・ Oil coating: 1 g / m ^{2 of} anti-rust oil Z5 (made by Idemitsu Oil Co., Ltd.) ・ Punch diameter and shape: 33 mmφ flat bottom cylinder ・ Clearing: 1 mm
・ Blank diameter: Various changes ・ Wrinkle presser load: 19.6kN (2tf)
・ Aperture speed: 60mm / sec

<Limit drawing ratio>
Under the above conditions, cylinder processing is performed with the outer surface side of the tank as the die side and the inner surface of the tank as the punch side, and the limit drawing ratio of each sample (the maximum value of the blank diameter / punch diameter of the drawn sample) is obtained. The lubricity was evaluated according to the following criteria.
A: 2.1 ≦ limit aperture ratio B: 1.9 ≦ limit aperture ratio <2.1
C: Limit drawing ratio <1.9

<Powdering resistance>
In addition, the powdering resistance was evaluated by investigating the degree of powdering of the resin film on the side wall of the cup when the blanking was performed at 60 mm under the above pressing conditions. That is, the C count ratio before and after processing (spot count of C after processing / spot count of C before processing) was measured by an electron beam microanalyzer (EPMA), and powdering resistance was evaluated according to the following criteria.
A: 0.8 ≦ C count ratio B: 0.2 ≦ C count ratio <0.8
C: C count ratio <0.2

<Scratch resistance>
A press working test was performed on the fuel tank steel plate obtained under the following conditions, the cup outer surface side wall after the test was visually observed, and galling resistance was evaluated according to the following criteria.
・ Punch diameter: 33mmφ (with cylindrical bead)
・ Drawing die shoulder curvature: 2mmR
・ Blank diameter: 68mmφ
・ Aperture speed: 200mm / sec
・ Wrinkle holding load: 19.6kN (2tf)
○: Mold galling does not occur △: Mold galling slightly occurs ×: Mold galling occurs remarkably

<Secondary processing brittleness resistance>
The cold-rolled annealed plate after the plating treatment was subjected to cylindrical processing under the press processing conditions in which the blank diameter was 60 mm and the critical drawing ratio was evaluated. And after trimming the ear | edge of a processed product, the shaping | molding which spreads the edge of a processed product statically with the conical punch of apex angle: 120 degrees was performed. At this time, the molding temperature was lowered from room temperature, and the temperature at which cracking occurred by molding was taken as the brittle transition temperature.

<Weldability>
Electrode: Chrome-copper alloy, DR type (16mmD-8mmr) and CF type (16mmD-11mmd)
Board assembly: Double stack Energizing conditions: Shown in Table 6 Cooling: Internal water cooling

Under the conditions shown in Table 6, a plurality of test pieces having a size of 100 × 200 mm were used and overlapped so that one surface was in contact with the DR type electrode and the other surface was in contact with the CF type electrode. The welding state during the process was confirmed using a 20 mm × 80 mm test piece every 20 strike points. That is, the welded part of a 20 mm × 80 mm weld test piece was peeled off, the major and minor axes of the nugget formed by spot welding were measured, and the maximum striking point where the minor axis was maintained at 4√t or more was determined. Based on the score, the following evaluation criteria were used.
A: More than 600 RBIs B: More than 300 RBIs: Less than 600 RBIs C: Less than 300 RBIs

  As is apparent from Table 6, the surface-coated steel sheets for fuel tanks obtained according to the present invention not only have a tensile strength of 440 MPa or more, an r value of 2.0 or more, and an excellent strength-r value balance, It was excellent in alkali film removal, press workability and weldability.

Claims (2)

% By mass
C: 0.01% or less,
Si: 0.5% or less,
Mn: 0.15% or more, 0.5% or less,
P: 0.03% or less,
S: 0.02% or less,
Al: 0.1% or less,
N: 0.006% or less,
Cu: 0.5% or more and 2.0% or less and B: 0.0002% or more and 0.0025% or less, and
Ti: 0.01% or more, less than 0.1% and
Nb: A cold rolled steel sheet containing one or two selected from 0.07% or less, the balance of which is Fe and inevitable impurities, and having a steel structure with a ferrite single phase and a ferrite grain size of 20 μm or less. At least one side has a plating layer composed of one or more selected from Zn, Ni, Sn, Al, Si, Fe, Co, Mg and Mn, and further, methacrylate, styrene And a water-soluble copolymer (a) comprising a monomer having a carboxyl group and a water-soluble copolymer (b) comprising at least one selected from maleic acid, fumaric acid, itaconic acid and their monoalkyl esters and styrene (b) ) Or an organic resin containing a copolymer: characterized by having an organic film containing 1 to 40 parts by mass of a lubricant and 5 to 110 parts by mass of a conductive auxiliary with respect to 100 parts by mass. Alkaline film removal, Scan processability and excellent steel sheet for fuel tank weldability. In claim 1, the cold-rolled steel sheet is further in mass%,
V: 0.1% or less and
Ni: A steel plate for a fuel tank excellent in alkali film removal property, press workability and weldability, characterized in that it has a composition containing one or two selected from 0.2% or more and 1.0% or less.