JP2005314737A - Steel sheet for fuel tank having superior spot weldability, corrosion resistance and press workability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet for a fuel tank, which has a tensile strength of 440 MPa or higher and an r-value of 2.0 or larger, superior balance between the strength and the r-value, corrosion resistance for the inner and outer environments of the tank, and further superior spot weldability and press workability. <P>SOLUTION: This cold-rolled steel sheet comprises: a cold-rolled steel sheet having a composition comprising, by mass%, 0.01% or less C, 0.5% or less Si, 0.15 to 0.5% Mn, 0.03% or less P, 0.02% or less S, 0.1% or less Al, 0.006% or less N, 0.5 to 2.0% Cu, 0.0002 to 0.0025% B, one or more elements selected from 0.01 to 0.1% Ti and 0.07% or less Nb, and the balance Fe with unavoidable impurities, and having a steel structure of a ferrite single phase containing ferrite particles with sizes of 20 μm or smaller; a plated layer made of one or more elements selected from Zn, Ni, Sn, Al, Si, Fe, Co, Mg and Mn, formed on at least one surface of the cold-rolled steel sheet; a chemical conversion coated layer formed thereon; and further an organic coating formed thereon. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  This invention has a tensile strength of 440 MPa or more, an r value of 2.0 or more, excellent spot weldability, particularly spot weldability and press workability on the outer surface of the tank, and corrosion resistance, especially alcohol or gasoline mixed with formic acid. It is related with the steel plate for fuel tanks excellent in corrosion resistance to.

Steel plates for fuel tanks are required to be excellent in various performances such as corrosion resistance against fuel and external environment, weldability, and press workability.
First, the corrosion resistance to fuel will be described.

  In North America, Latin America, Europe, and other countries, an increasing number of countries have adopted a national policy to reduce the dependence on oil as an energy measure. Therefore, in these countries, alcohol (methanol, ethanol) itself as a new fuel for automobiles, or the so-called gasohol introduction ratio in which these are mixed with gasoline in a proportion of 5 to 20% tends to increase year by year.

However, these alcohol fuels
(A) easy to mix water,
(B) It is easy to separate the layers due to an increase in the amount of water mixing and a decrease in temperature.
(C) There is a possibility of producing an organic acid by oxidative degradation (for example, formic acid in the case of methanol, and acetic acid in the case of ethanol). In this case, alcohol and / or organic acid and water are used as a lower layer. A separation layer mainly composed of
(D) A mixture of alcohol and gasoline containing 40% or more of methanol is stronger than ordinary gasoline fuel, such as dissolving the plating layer of turn (Pb-Sn alloy) plated steel sheet, which is the mainstream of current tank materials. Corrosive.

  On the other hand, steel plates for fuel tanks of automobiles have no defects in the welds formed by seam welding or spot welding, the inner and outer surfaces of the tank do not corrode, and the fuel circulation system clogs the filter. It is required that no floating corrosion products are generated.

By the way, as a steel plate that has been put to practical use as a fuel tank steel plate for automobiles, for example, a Pb—Sn alloy hot-dip steel plate as shown in Patent Document 1 or an electric Zn as shown in Patent Document 2 is used. The thing which gave the thick chromate process to the plated steel plate is known.
Japanese Patent Publication No.57-61833 Japanese Patent Publication No.53-19981

However, the corrosion resistance of these materials to gasoline, alcohol or alcohol-mixed gasoline (hereinafter referred to as “internal corrosion resistance”) has the disadvantage that Pb-Sn alloys are extremely soluble in methanol and are practical for methanol-mixed gasoline. Is difficult.
On the other hand, the inner surface corrosion resistance of the material obtained by subjecting the electrical Zn-plated steel sheet to the thick chromate treatment has a slight rust prevention function due to the sacrificial anticorrosive action of zinc. However, since this material has a high zinc elution rate in alcohol or gasoline, a large amount of floating white precipitate is generated, and the filter is clogged in the fuel circulation system. Moreover, after zinc elution, red rust is generated on the base steel sheet, and this material is also insufficient as a fuel tank steel sheet.

  Therefore, it exhibits excellent internal corrosion resistance against alcohol alone or alcohol-mixed gasoline, especially gasoline with highly corrosive alcohol and formic acid, and the tank outer surface has excellent corrosion resistance against the external environment (hereinafter referred to as “ Development of steel plates for fuel tanks that have “outer surface corrosion resistance” and also excellent press workability and resistance weldability has been made.

For example, Patent Document 3 discloses a steel plate in which an organic resin film containing a metal powder is coated on a metal plating layer containing Pb—Sn alloy or Sn as a main component, and Patent Document 4 and Patent Document 5 include zinc. Each steel sheet in which an organic resin film containing metal powder is coated on a system plating layer is described.
Japanese Patent Publication No. 2-18981 Japanese Patent Publication No. 2-18982 Japanese Patent Publication No. 3-25349

In any of the organic resin films described in the three patent documents listed above, 40 to 90% of the organic resin film is occupied by phenoxy resin. Therefore, when a steel plate having these organic resin films is used as a material for a gasoline tank, the affinity between the hydroxyl group of the phenoxy resin and the metal powder is insufficient on the outer surface side. May desorb. Therefore, plating peeling occurs on the outer surface side, and press workability is deteriorated.
On the other hand, on the inner surface side of the gasoline tank, the inner surface corrosion resistance of the portion damaged by the detachment of the metal powder contained in the organic resin film or the peeling of the plating layer is deteriorated. Furthermore, since the corrosive liquid tends to stay between the resin and the metal powder in the film, the flat portion of the tank that is not damaged is also inferior in internal corrosion resistance.

  Moreover, since all the above-mentioned patent documents 3-5 coat | cover the organic resin film directly on the upper layer of a plating layer without interposing a chromate or a chemical conversion film, the adhesiveness of an organic resin film and a plating layer is good. Insufficient, the organic resin film peels off during press processing, and the shielding effect of organic acids and chloride ions by the organic resin film tends to deteriorate significantly. Therefore, these steel plates have not yet been put into practical use.

  Furthermore, all the steel plates proposed in the above-mentioned Patent Documents 3 to 5 contain a curing agent as an essential component of the organic resin film on the inner and outer surfaces of the tank. For this reason, when the degree of cure is high, it is difficult to melt by heat, and it becomes difficult to eliminate the film in the nugget generation process during welding with a current value in the proper range, and it may be necessary to weld at a high current value, In such a case, the electrode is significantly worn out, and it becomes difficult to continuously weld the electrode without care, and the productivity of the production line is significantly reduced. On the other hand, when the degree of cure is low and an unreacted curing agent is included, corrosion factors (acids, chloride ions, etc.) intrude due to the low cohesive strength of the part and the high hydrophilicity of the unreacted material. Therefore, the corrosion resistance of the inner and outer surfaces of the tank is lowered.

Patent Document 6 describes an epoxy-based weldable corrosion-resistant coating composition containing both a metal powder obtained by mixing aluminum, stainless steel, or an alloy thereof and a metal powder consisting essentially of nickel. When this composition is used on a gasoline tank material, the affinity between the epoxy resin, the phenoxy resin and the metal powder is insufficient as described above, so that the metal powder may be detached from the film during press working. is there. In addition, when this coating is applied to the inner and outer surfaces of the tank, the corrosion resistance is deteriorated due to the damage of the film and the accompanying damage of the plating. Furthermore, since the corrosive ions easily enter the interface between the resin and the metal powder having low affinity even in the flat portion of the tank that is not damaged, the corrosion resistance is inferior to the inner and outer surfaces of the tank.
JP-A-64-33173

  Further, when the above-described plated steel sheet or organic coated steel sheet is welded at a high current, a weld crack may occur. Since it is considered that the welded portion crack extends during actual driving of the automobile, it is necessary to prevent the occurrence of the crack. The weld crack does not occur within a proper range of welding current, but may occur when welding is performed at a high current value exceeding the proper range. In actual tank production, welding is performed within an appropriate range, but since a molded product having a complicated shape is welded, there is a possibility that locally high current density may be obtained depending on how the electrode and the steel plate are contacted. Therefore, it is necessary to design the material so that a weld crack does not occur even when welding is performed at a high current value.

In this regard, the inventors have newly developed a steel plate for a fuel tank that satisfies all of the resistance weldability, press workability, and corrosion resistance on the inner and outer surfaces of the tank as a solution to the above problems. Disclosed.
Japanese Patent Laid-Open No. 10-337805

  The fuel tank steel sheet disclosed in Patent Document 7 is formed by sequentially laminating a zinc-based plating layer and a chemical conversion film (for example, a chromate film) on both surfaces of the steel sheet, and further, Al and Ni metal powders and amines on one surface. Forming a metal powder-containing organic resin film containing a modified epoxy resin, and having at least one functional group selected from a hydroxyl group, an isocyanate group, a carboxyl group, a glycidyl group and an amino group on the other surface A silica-containing organic resin film containing one kind of organic resin, silica, and a lubricant is formed.

Furthermore, the inventors further examined the steel plate for the fuel tank, and found that the steel plate for the fuel tank had a resistance weldability, in particular, the state where the steel plates were stacked so that the electrode was in direct contact with the silica-containing organic resin film. The weldability when continuously resistance-welding is made by including particles having conductivity in the silica-containing organic resin film, and further by optimizing the composition components of the steel plate before applying the zinc-based plating, The improvement was found and disclosed in Patent Document 8.
JP 2001-341228 A

As described above, the technique disclosed in Patent Document 8 can provide a fuel tank steel plate that is excellent in resistance weldability, press workability, and corrosion resistance on the inner and outer surfaces of the tank.
However, 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 to 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, and is excellent in spot weldability, press workability and corrosion resistance, and has a tensile strength ≧ 440 MPa, r value ≧ 2.0 and a balance between strength and r value. The purpose is to propose a steel plate.

As a result of intensive studies to achieve the above object, the inventors have obtained the following knowledge.
(1) Corrosion protection is applied to the steel sheet by coating the steel plate surface with one or more plating layers selected from Zn, Ni, Sn, Al, Si, Fe, Co, Mg and Mn. be able to.
(2) Furthermore, by providing a chemical conversion treatment layer such as a Cr (VI) -containing chromate treatment layer, a Cr (VI) -free Cr (III) chromate treatment layer or a chromate-free treatment layer on the plating layer. The adhesion between the lower plating layer and the upper organic resin layer can be improved, and the corrosion resistance of the inner and outer surfaces of the tank can be improved.
(3) Furthermore, an organic film, preferably an organic film made of an organic resin having a functional group such as a hydroxyl group, an isocyanate group, a carboxyl group, a glycidyl group and an amino group, is formed on the chemical conversion treatment layer. It is possible to prevent corrosion factors from entering the tank and improve the corrosion resistance of the inner and outer surfaces of the tank. Moreover, when an organic film is provided on the outer surface of the tank, press workability can be remarkably improved.

Next, as a result of intensive studies to achieve balanced strength and r value, 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 a chemical conversion treatment layer is further formed thereon. A steel plate for a fuel tank, which has an organic film on the surface and is excellent in spot weldability, press workability and corrosion resistance.

(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 spot weldability, press workability and corrosion resistance, characterized by having a composition containing one or two selected from 0.2% or more and 1.0% or less.

(3) In the above (1) or (2), the main component of the organic film is an organic resin having at least one functional group selected from a hydroxyl group, an isocyanate group, a carboxyl group, a glycidyl group and an amino group. Characteristic steel plate for fuel tanks with excellent spot weldability, press workability and corrosion resistance.

(4) In any one of the above (1) to (3), in the organic film on at least one side of the cold-rolled steel sheet, the lubricant is added at a ratio of 1 to 40 parts by mass with respect to 100 parts by mass of the organic resin. A steel plate for a fuel tank that is excellent in spot weldability, press workability and corrosion resistance.

(5) In any one of the above (1) to (4), at least one type of conductive material selected from Al, Ni, Sn, stainless steel, Cu and C is present in the organic film on at least one side of the cold-rolled steel sheet. A steel plate for a fuel tank excellent in spot weldability, press workability, and corrosion resistance, characterized by containing a property aid in a ratio of 5 to 110 parts by mass with respect to 100 parts by mass of the organic resin.

  According to the present invention, the steel sheet has excellent inner and outer surface corrosion resistance, is excellent in spot weldability and press workability, 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. A steel plate for a fuel tank can be obtained.

The present invention will be specifically described below.
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.

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 reduces the deep drawability (r value) of the steel and deteriorates the press workability, and low temperatures such as martensite and bainite. This is because the inclusion of the transformation phase also degrades 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 ° C or lower, annealing at a temperature of 600 ° C or higher as necessary, then pickling and cold rolling at a reduction ratio of 50% or higher. 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, it is preferable that the slab heating temperature at the time of hot rolling is 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 it is not only uneconomical but also may deteriorate the weldability. More preferably in the range of 20 to 100 g / m ^2.

Next, a chemical conversion treatment is performed on the surface of the plating layer. This chemical conversion treatment is necessary for ensuring sufficient adhesion between an organic film to be described later formed thereon and a plating layer as a lower layer, in addition to the effect of improving the corrosion resistance.
As such a chemical conversion treatment film, a chromium (VI) -containing chromate treatment layer is suitable, but of course, a Cr (VI) -free Cr (III) chromate treatment layer or a chromate-free treatment layer may be used. Absent.

When the chromate layer is formed, the amount of adhesion is preferably about 5 to 200 mg / m ² per side in terms of metallic chromium. Because it is less than the amount deposited is 5 mg / m ^2, due to the lack of adhesion between the organic coating thereon, corrosion resistance and press workability is not sufficient, whereas 200 mg / m ² is more than the chromate layer itself This is because it becomes very brittle and the chromate layer is peeled off at the sliding portion of press working, and the organic film is peeled off along with this. More preferably in the range of 10-100 mg / m ^2.

  The formation of the chromate layer can be performed according to a normal treatment method and is not particularly limited. For example, a chromate layer can be formed as a trivalent chromium compound film by performing immersion chromate treatment or electrolytic chromate treatment using a treatment liquid mainly composed of chromic acid, chromate, dichromate, etc. Alternatively, a chromate layer may be formed as a film containing a hexavalent chromium compound by performing a coating-type chromate treatment in which a treatment liquid in which colloidal silica or the like is mixed with the treatment liquid is applied onto a plated steel sheet. .

Moreover, although the said processing liquid mainly contains hexavalent chromium, in this invention, what is called a trivalent chromate processing liquid which does not contain hexavalent chromium other than the said processing liquid can be used as a chromate processing liquid. This trivalent chromate treatment solution does not contain hexavalent chromium, and therefore is preferable from the viewpoint of the environment. Trivalent chromate can be obtained by a method in which chromic acid (CrO ₃ ) is used as a starting material and is changed to Cr ³⁺ using a reducing agent.
Here, as the reducing agent, polysaccharides such as starch, fructose and sucrose, organic acids such as succinic acid and formic acid, or inorganic compounds such as phenols, hydrogen peroxide, phosphorous acid and hypophosphorous acid Can be used. Furthermore, in addition to the method described above, a trivalent chromium compound can also be used.

In addition, as a chromate-free treatment in which the chemical conversion treatment layer does not contain Cr at all, a publicly disclosed technique can be used. For example, phosphate treatment systems (JP 2000-309880, JP 2000-129460), silicate treatment systems (JP 11-335862, JP 9-53192), organic film treatment systems Techniques such as (JP-A 2000-519, JP-A 10-110093) and organic / inorganic composite coating treatment system (JP-A 2001-131773, JP-A 2001-316845) are suitable.
In addition, when forming this chromate-free process layer, it is suitable that the adhesion amount shall be about 0.1-1.0 g / m < ² >. By setting the adhesion amount within this range, sufficient corrosion resistance and adhesion can be obtained.

Next, an organic film is formed on the surface of the chemical conversion treatment layer.
The organic resin that is the base of the organic film is not particularly limited in its type, but has an organic resin having at least one functional group selected from a hydroxyl group, an isocyanate group, a carboxyl group, a glycidyl group, and an amino group. Advantageously fit.
Moreover, the thing whose glass transition temperature (Tg) is about 0-90 degreeC is preferable.
Examples of such organic resins include epoxy resins, alkyd resins, acrylic resins, urethane resins, polyvinyl butyral resins, phenol resins, and melamine resins.

  The thickness of the organic film is preferably about 0.5 to 10 μm. This is because when the thickness is less than 0.5 μm, the organic film cannot cover the unevenness of the steel sheet, so it cannot be expected to improve the corrosion resistance and press workability, while when it exceeds 10 μm, the corrosion resistance and press workability are improved. This is because not only the effect is saturated but also spot welding becomes difficult.

  Since the coating obtained as described above is excellent in corrosion resistance, press workability and spot weldability on the inner and outer surfaces of the tank, according to the present invention, the surface of the steel sheet is selected according to various properties required for the inner and outer surfaces of the fuel tank. There is an advantage that it is not necessary to make a film on the back side.

Further, in the present invention, for the purpose of further improving the press workability, a lubricant can be added to the above-described organic film, particularly to the organic film on the outer surface side (that is, the side in contact with the outside world) of the fuel tank. .
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.
As such a lubricant, a polyolefin wax, for example, a wax made of a polymer of an olefinic hydrocarbon such as polyethylene, polypropylene, polybutylene or the like is advantageously suitable, and a combination thereof may be used. Further, a lubricant containing fluorine may be used. These lubricants can improve the press workability by forming a lubrication layer between the coating layer and the mold at the time of press work.

The blending ratio of the lubricant contained in the organic film is preferably 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 not 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 has a powder resistance. This is because the ring property is lowered and the press workability tends to deteriorate.

Furthermore, in this invention, a conductive support agent can also be added in the above-mentioned organic membrane | film | coat for the purpose of the improvement of spot weldability further.
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 blending ratio of the conductive auxiliary agent contained in the organic film is preferably about 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, a sufficient improvement in spot weldability cannot be expected. On the other hand, if it exceeds 110 parts by mass, the organic film becomes fragile, and the resistance to powder during pressing is reduced. This is because the ring property is lowered and the inner surface corrosion resistance is deteriorated. More preferably, it is the range of 45-110 mass parts.
In addition, it is preferable that the average particle diameter of this electroconductive auxiliary agent is as follows according to a film thickness.
That is, when the film thickness is less than 3 μm, the particle diameter is preferably about 0.01 to 3 μm. The reason is that when the particle size is less than 0.01 μm, the energization point is insufficient and the effect of improving spot weldability is poor. On the other hand, when the particle size exceeds 3 μm, the conductive particles are in contact with the mold and the pressability is deteriorated. This is because the corrosion resistance of the pressed portion may deteriorate. Further, when the film thickness exceeds 3 μm, the particle diameter is preferably about 1 to 9 μm. This is because when the particle size is less than 1 μm, the current-carrying point is insufficient and the effect of improving spot weldability is poor, while when it exceeds 9 μm, the coating tends to be porous, and the internal corrosion resistance and press workability are reduced. Because there is a possibility of doing.

In the present invention, in addition to the above components, additives such as a coupling agent, a pigment, a thixotropic agent, and a dispersing agent can be added as necessary.
In addition, it is not preferable to add general additives such as reaction accelerators, stabilizers, and dispersants to the organic resin that is the base of the organic film as long as the gist of the present invention is not impaired. .

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, the obtained cold-rolled annealed plate was subjected to a plating treatment and a chemical conversion treatment to form a plating layer and a chemical conversion treatment layer shown in Table 2, and then, as shown in Table 3, the tank outer surface side on one side The organic film on the inner surface of the tank was formed on the other surface.
The composition of the treatment liquid and the treatment conditions in the chemical conversion treatment are as shown in Table 4, and the type of conductive auxiliary used is as shown in Table 5.
The limit drawing ratio, powdering resistance (C residual ratio), secondary work brittleness resistance, spot weldability, and corrosion resistance of the steel sheet front and back powder surfaces corresponding to the inner and outer surfaces of the tank were investigated.
The obtained results are summarized and shown in Table 6.

In addition, the evaluation method of each characteristic is as follows.
・ Ferrite diameter The cold-rolled annealed plate after plating treatment was subjected to nital corrosion, and the section length determined by the cutting method (JIS G 0552: 1998) from the optical micrograph of the plate thickness cross section (400 times) Obtained by multiplying by 1.13.
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 taken 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

Cylindrical processing was performed on the obtained fuel tank steel sheet under the following conditions, and press workability was evaluated by investigating the limit drawing ratio and 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: 19.6 kN (2tf)
・ Aperture speed: 60mm / sec

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 (the maximum value of the blank diameter / punch diameter of the drawn sample) is determined for each sample. The lubricity was evaluated according to the standard.
A: 2.1 ≦ limit aperture ratio B: 1.9 ≦ limit aperture ratio <2.1
C: Limit drawing ratio <1.9

Further, 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 with the blank diameter of 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

-Secondary processing brittleness The cold-rolled annealed plate after plating treatment was subjected to cylindrical processing under the above press processing conditions with a blank diameter of 60 mm. 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 during molding was taken as the brittle transition temperature.

・ Spot weldability Electrode: Chromium-copper alloy, DR type (16mmD-8mmr) and CF type (16mmD-11mmd)
Board assembly: Two sheets,
DR type (tank inner surface organic film contacts)
CF type (tank outer surface organic film contacts)
Energization conditions: shown in Table 7.
Cooling: Internal water cooling

Under the conditions shown in Table 7, using a plurality of test pieces with a fuel tank steel plate size of 100 × 200 mm, the tank inner surface organic film is formed on the DR electrode and the tank outer surface organic film is formed on the CF electrode. The layers were overlapped so as to be in contact with each other, continuous welding was performed, and a welding state in the middle was confirmed using a test piece of 20 mm × 80 mm every 20 points. That is, the welded part of a 20 mm × 80 mm weld test piece was peeled off, and the major axis and minor axis of the nugget formed by spot welding were measured, and the maximum spot where the minor axis: 4√t or more was maintained was obtained. Evaluation was performed according to the following evaluation criteria based on the obtained number of hit points.
A: More than 600 RBIs B: More than 300 RBIs: Less than 600 RBIs C: Less than 300 RBIs

Evaluation method of outer surface corrosion resistance The outer surface of the cup processed under pressing condition A {blank diameter: 98 mmφ, punch diameter: 50 mmφ, die shoulder: 3 mmR, wrinkle retainer: 4.9 kN (500 kgf)} was evaluated.
The above sample was subjected to the JASO-M610 method conditions {salt spray (5 mass% NaCl, 35 ° C., 2 h) → dry (60 ° C., relative humidity: 20-30%, 4 h) → wet (50 ° C., relative humidity: 98%, 2h) is taken as 1 cycle} and subjected to 240 cycles, and the thickness reduction values at 10 locations for each were measured to determine the average value. The outer surface corrosion resistance was evaluated according to the following criteria based on the obtained average value. In actual tank production, it is common to apply a top coat to the upper layer of the film, but in order to evaluate the performance of the outer film, the performance was evaluated here without the top coat. .
A: 0.3-0.5mm
B: 0.6-0.8mm
C: Perforated

-Evaluation method for inner surface corrosion resistance Similarly, the inner surface of a cup processed under pressing condition A {blank diameter: 98 mmφ, punch diameter: 50 mmφ, die shoulder: 3 mmR, wrinkle retainer: 4.9 kN (500 kgf)} was evaluated.
About 80% of the volume of sour gasoline was put into the cup and allowed to stand at 40 ° C. for 10 days, and then the total amount of metal eluted in the sour gasoline was measured. Sour gasoline was produced by adding oxygen to regular gasoline at a pressure of 0.7 MPa and oxidizing at 100 ° C for 24 hours. Sour gasoline contained formic acid: 1500 mg / l, acetic acid: 2900 mg / l, methanol: 200 mg / l, ethanol: 230 mg / l. The inner surface corrosion resistance was evaluated according to the following criteria based on the measured total metal amount.
A: 0 to 10 g / m ² or less B: Over 10 g / m ² , 20 g / m ² or less C: Over 20 g / m ²

  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, Excellent corrosion resistance on the inner and outer surfaces, as well as spot weldability and press workability.

Claims (5)

% 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 a chemical conversion treatment layer is further formed thereon. A steel plate for a fuel tank, which has an organic film on the surface and is excellent in spot weldability, press workability and corrosion resistance. 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 spot weldability, press workability and corrosion resistance, characterized by having a composition containing one or two selected from 0.2% or more and 1.0% or less.   3. The spot welding according to claim 1, wherein the organic film is an organic resin having at least one functional group selected from a hydroxyl group, an isocyanate group, a carboxyl group, a glycidyl group and an amino group. Steel sheet for fuel tanks with excellent heat resistance, press workability and corrosion resistance.   In any 1 item | term of Claims 1-3, the lubricant is included in the ratio of 1-40 mass parts with respect to 100 mass parts of said organic resin in the organic membrane | film | coat of the at least single side | surface of the said cold-rolled steel plate. A steel plate for fuel tanks with excellent spot weldability, press workability and corrosion resistance.   In any 1 item | term of Claims 1-4, the at least 1 sort (s) of conductive support agent chosen from Al, Ni, Sn, stainless steel, Cu, and C in the organic film of the said cold-rolled steel plate at least on one side. The organic resin: A steel plate for a fuel tank excellent in spot weldability, press workability, and corrosion resistance, characterized by containing 5 to 110 parts by mass with respect to 100 parts by mass.