JP4457673B2 - Plating cold-rolled steel sheet for high formability fuel tank excellent in secondary work brittleness resistance and plating adhesion and method for producing the same - Google Patents

Description

  The present invention has an excellent strength-r value balance with a tensile strength of 440 MPa or more and an r value of 2.0 or more, and also has a high formability fuel tank excellent in secondary work brittleness resistance and plating adhesion. The present invention relates to a plated cold steel sheet and a manufacturing method thereof.

Automotive fuel tanks are required to be free of perforated corrosion and corrosion products. For this reason, development has been carried out mainly for corrosion resistance as performance.
However, in recent years, in order to reduce the weight of automobiles, steel plates for fuel tanks have been required to be lightened by thinning and workability into complicated shapes.
In order to meet this requirement, it is necessary to increase the tensile strength of the base steel plate to 420 MPa class or higher and further improve the workability.

Conventionally, as a steel plate for a fuel tank, for example, Patent Document 1 proposes a technique for improving the corrosion resistance of a fuel tank by applying multilayer plating such as Ni plating, Zn rich plating and Sn rich plating on the surface of the steel plate. Has been.
However, although this steel plate is excellent in corrosion resistance, when the strength of the base steel plate is increased, the formability to the tank does not exceed the formability of the base steel plate, so it cannot be formed into a complicated tank shape. There was a problem.

In this respect, as an attempt to improve the workability of the base steel sheet, Patent Document 2 describes that ultra-low carbon steel is added with Ti and Nb to improve formability, and addition of B gives impact resistance. In order to further improve the plating adhesion, a rust-proof steel sheet for fuel tanks with addition of 0.5 mass% or less of Cu has been proposed for the purpose of preventing the diffusion of Fe into the plating film.
However, with this steel sheet, it is not possible to obtain the strength required for the fuel tank: 420 MPa class or higher, and the r value indicating deep drawability is about 1.5, which is not sufficient in terms of formability. . Moreover, this steel sheet has a problem that the plating adhesion is partially low because the crystal grains are large and the reaction between the plating and the base concentrates on the grain boundary of the base steel. In this way, if even a part of the plating adhesion is low, the plating is peeled off during pressing, and the necessary corrosion resistance cannot be maintained after tank processing.

Also, although not specifically intended for a fuel tank, Patent Document 3 describes steel in which Ti and Nb are added to ultra-low carbon steel and Cu is further added in an amount of 0.5 to 1.5 mass%, as having improved formability. Is manufactured and rolled at a temperature of ₃ points or less of Ar, and further subjected to Cu precipitation treatment, thereby manufacturing a hot rolled steel sheet having a strength of 440 MPa or more and excellent formability.
However, with this technology, the slab heating temperature must be reduced to an ultra-low temperature of about 950 ° C., and hot rolling at such a low temperature is actually impossible in view of the load on the rolling mill.

Further, Patent Document 4 discloses a technique in which Nb, Ti is added to ultra-low carbon steel, and steel added with Cu is lubricated and rolled at an Ar ₃ point or less and recrystallized and annealed.
However, the balance between the strength and the r value of the steel sheet obtained by this technique is about 490 MPa class with an r value of about 1.3, and it cannot be said that the steel sheet has sufficient deep drawability.

In addition, in Patent Document 5, Ti, Nb, and Cu are added to ultra-low carbon steel, and rolling is performed at 50% or more in a temperature range of 500 ° C to 750 ° C. After annealing, Cu is heated at a temperature of 450 to 650 ° C. A method for producing a hot-rolled steel sheet having excellent workability comprising precipitating steel is disclosed.
However, in this method, C at the grain boundary is fixedly removed by Ti and Nb, so the grain boundary strength is weak, and there remains a problem in terms of secondary work resistance. Moreover, for example, when hot-dip plating is performed, there is a problem that the plating penetrates into the grain boundary and causes liquid metal embrittlement. Therefore, even if hot-plating is applied to such a steel sheet, it is practically impossible to obtain a hot-formed steel sheet for high formability fuel tank excellent in secondary work brittleness resistance and plating adhesion.

JP 2003-268522 A Japanese Patent Laid-Open No. 2002-30384 JP 2001-131641 JP-A-6-65641 Japanese Patent Laid-Open No. 10-310843

  The present invention advantageously solves the above-mentioned problems, is excellent in secondary work brittleness resistance and plating adhesion, and has excellent tensile strength ≧ 440 MPa, r value ≧ 2.0, and a balance between strength and r value. The object is to propose a plated cold-rolled steel sheet for tanks together with its advantageous production method.

As a result of intensive studies to achieve the above 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 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. % 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: Contains one or two selected from 0.07% or less, the balance is Fe and inevitable impurities composition, has a steel structure with ferrite single phase and ferrite grain size of 20μm or less, tensile strength A plated cold-rolled steel sheet for fuel tanks with high formability and excellent secondary work brittleness resistance and plating adhesion, characterized by having a plating layer on the surface with a thickness of 440 MPa or more.

2. In the above 1, the steel composition is further mass%,
V: 0.1% or less and
Ni: High formability fuel tank plating excellent in secondary work brittleness resistance and plating adhesion, characterized by containing one or two selected from 0.2% or more and 1.0% or less Cold rolled steel sheet.

3. % 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% to 2.0% and B: 0.0002% to 0.0025%,
And including
Ti: 0.01% or more, less than 0.1% and
Nb: contain one or two species selected from among 0.07% or less, after the billet balance consisting Fe and inevitable impurities, exceeding 950 ° C., and heated to a temperature below 1100 ° C., finish rolling temperature : Rolling rate below 750 ° C and below 750 ° C: Hot rolled under conditions of 50% or higher, wound at a temperature of 550 ° C or lower, then pickled, then cold rolled at a rolling reduction of 50% or higher A high formability fuel tank with excellent secondary processing brittleness resistance and plating adhesion, which is then annealed and plated, and further subjected to Cu precipitation at temperatures of 450 ° C or higher and 650 ° C or lower. Method for plating cold-rolled steel sheet.

4). % 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% to 2.0% and B: 0.0002% to 0.0025%,
And including
Ti: 0.01% or more, less than 0.1% and
Nb: contain one or two species selected from among 0.07% or less, after the billet balance consisting Fe and inevitable impurities, exceeding 950 ° C., and heated to a temperature below 1100 ° C., finish rolling temperature : Rolling ratio below 750 ° C and below 750 ° C: Hot rolled under conditions of 50% or higher, wound at a temperature of 550 ° C or lower, annealed at a temperature of 600 ° C or higher, and then pickled. Rolling ratio: 50% or more of cold rolling, followed by annealing and plating, followed by Cu precipitation at a temperature of 450 ° C or higher and 650 ° C or lower. A method for producing a plated cold-rolled steel sheet for a highly formable fuel tank having excellent brittleness and plating adhesion.
5. In the above 3 or 4, the composition of the steel slab is further mass%,
V: 0.1% or less and
Ni: 0.2% or more, 1.0% or less
A method for producing a plated cold-rolled steel sheet for a high-formability fuel tank excellent in secondary work brittleness resistance and plating adhesion, wherein the composition contains one or two selected from the above.

  According to the present invention, an appropriate amount of Cu and B is added in combination, and the ferrite grain size is suppressed to 20 μm or less, thereby providing an excellent strength-r value balance with a tensile strength of 440 MPa or more and an r value of 2.0 or more. In addition, a plated cold-rolled steel sheet having excellent secondary work resistance and plating adhesion 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 formability, 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 formability. 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, it has the effect | action which prevents effectively the intergranular embrittlement by metal plating by making it contain 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 formability, 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) 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 a normal IF steel has a high r value, the ferrite grain size is as coarse as about 40 μm, and the secondary workability 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.

Plating layer Although the plating layer in the present invention is not particularly limited, for example, Al-based plating mainly composed of Al, Zn-based plating, Zn-Al-based plating, Zn-Sn-based plating and the like are advantageously adapted. . Both electroplating and hot dipping are compatible. Furthermore, a so-called alloyed plating layer obtained by alloying the plating layer and the base steel plate may be used.
The thickness of the plating layer is preferably about 5 to 10 μm.

Next, the manufacturing conditions of the present invention will be described.
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 was limited to a range exceeding 950 ° C. and less than 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. Therefore, finish rolling is completed at less than 750 ° C. and rolling is performed in the ferrite region, so that ferrite is recrystallized and a suitable texture is formed at the stage of hot rolling. For this reason, finish rolling was finished 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. did. 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 coiling temperature was limited to 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 does not recrystallize sufficiently, so the annealing temperature was 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, pickling is performed 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, when the cold rolling reduction is less than 50%, no significant change is observed in the texture. Therefore, the cold rolling reduction was set to 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.

Plating treatment The fuel tank is an important safety part and must not be pierced by corrosion. Therefore, plating is essential. In the present invention, it is necessary to perform plating suitable for the fuel tank. For example, Al-based plating mainly composed of Al, Zn-based plating, Zn-Al-based plating, Zn-Sn-based plating and the like are advantageously adapted. As the plating method, either electroplating or hot dipping may be used. In the case of electroplating, an annealing step is performed before plating, but the annealing temperature is preferably 600 ° C to 750 ° C. The annealing method may be box annealing or continuous annealing. In the case of hot dip plating, this annealing may be combined with the heat treatment before hot dip plating. The annealing temperature at this time is preferably 700 ° C. or higher and 850 ° C. or lower. Furthermore, a so-called alloying treatment in which the plating layer and the base steel sheet are alloyed after plating may be performed. Note that the plating may be performed on at least one side of the steel plate.

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 was limited to a range of 450 ° C. or higher and 650 ° C. or lower. In addition, this Cu precipitation process can also serve as the temperature maintenance by the plating tank immersion at the time of hot dipping, and the temperature maintenance by alloying.

  Steel slabs having the composition shown in Table 1 were hot-rolled to obtain hot-rolled steel sheets. The heating temperature during hot rolling was 1030 ° C, the finish rolling temperature was 690 ° C, the rolling reduction at 750 ° C or lower was 55%, and the coiling temperature was 480 ° C. Next, the obtained hot-rolled sheet was pickled, annealed at 830 ° C for a short time, and then Sn-Zn hot-dip plated. Further, a Cu precipitation treatment at 500 ° C. was then performed.

From the plated cold-rolled steel sheet thus obtained, 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 sheet strength was measured.
Further, JIS No. 5 test pieces were taken in parallel to the rolling direction, 45 ° direction, and perpendicular direction, respectively, 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
Furthermore, a disk-shaped blank was cut out from the obtained steel sheet and deep-drawn. Then, after trimming the ears of the deep-drawn product, the edge of the molded product was statically spread with a conical punch having an apex angle of 120 °. At this time, the molding temperature was gradually lowered from room temperature, and the temperature at which cracking occurred by molding was taken as the brittle transition temperature.
The ferrite grain size and structure of the steel sheet were cut from an optical microscope structure photograph (× 400 times) of the plate thickness cross section subjected to nital corrosion. As described above, the ferrite grain size was obtained by multiplying the ferrite grain segment length obtained by the cutting method (JIS G 0552) by 1.13.
In addition, after cutting the obtained steel sheet into a length of 100 mm and a width of 30 mm, it is tightly bent at the center in the longitudinal direction, and the cellophane tape is attached to the outside of the bent portion and peeled off, and then the plating peels off. Whether or not the plating did not peel off was evaluated as ○, and the peeled off was evaluated as ×.
The obtained results are shown in Table 2.
The examples in Table 2 based on the component systems in Table 1 were all ferrite single-phases regardless of invention examples and comparative examples.

In Tables 1 and 2, Nos. 1 to 9 and Nos. 12 to 14 are invention examples. In No. 3, Nb is not added, and in No. 4, Ti as a fixed element of C and N is not added. No. 5 and 8 are examples in which Ni is added, and No. 6 is an example in which Ni and V are added. On the other hand, No. 10 is a comparative example without addition of Cu.
No.11 to No.15 show the influence of Cu addition amount. In No. 11, the amount of Cu added is small, and in No. 15, the amount of Cu exceeds the upper limit of the present invention. No. 16 is an example in which Mn is below the lower limit, and Nos. 17 and 18 are examples in which Ti and Nb are added in excess of the upper limit.
No.1-9 and No.12-14 all have excellent mechanical properties such as tensile strength of 440 MPa or more, r value of 2.0 or more, brittle transition temperature of -30 ° C or less. In addition, there is no plating peeling and excellent plating adhesion.
On the other hand, No. 10 has no added Cu, so its tensile strength is well below 440 MPa and its brittle transition temperature is as high as -10 ° C. Moreover, since the ferrite particle size is large, the plating adhesion is also poor.
No. 11 had a tensile strength of less than 440 MPa due to the small amount of Cu added. Moreover, since the ferrite particle size is large, the plating adhesion is poor.
No. 15 has a low r value and a high brittle transition temperature of −10 ° C. because the Cu content exceeds the upper limit of the present invention.
Nos. 16, 17, and 18 all had a high ferrite grain size, and therefore had a high brittle transition temperature and poor plating adhesion.

C: 0.0019%, Si: 0.01%, Mn: 0.23%, P: 0.014%, S: 0.008%, Ti: 0.065%, Nb: 0.012%, Cu: 0.98%, N: 0.0020%, Al: 0.031%, A steel slab having a composition containing Ni: 0.55% and B: 0.0004% is subjected to hot rolling- (hot-rolled sheet annealing) -cold rolling-annealing-melting under the conditions shown in Nos. 1 to 9 in Table 3. Plating-Cu deposition treatment was performed.
C: 0.0020%, Si: 0.02%, Mn: 0.18%, P: 0.014%, S: 0.007%, Ti: 0.066%, Nb: 0.009%, Cu: 1.11%, N: 0.0018%, Al: 0.054 %, Ni: 0.51%, and B: 0.0007% of the steel slab with the conditions shown in Nos. 10 to 16 of Table 3, hot rolling-(hot rolled sheet annealing)-cold rolling-annealing -Hot dipping-Cu precipitation treatment was performed.
The results obtained by examining the ferrite grain size, tensile strength (TS), r value, brittle transition temperature and plating adhesion of the plated cold-rolled steel sheet thus obtained are also shown in Table 3.
The examples in Table 3 were all ferrite single phase regardless of invention examples and comparative examples.

Nos. 1 to 4 are invention examples. Nos. 1 and 2 are examples in which hot-rolled sheet annealing is not performed, and Nos. 3 and 4 are examples in which hot-rolled sheet annealing is performed. Although both are examples of the present invention, Nos. 3 and 4 subjected to hot-rolled sheet annealing were superior in r value than Nos. 1 and 2 not subjected to hot-rolled sheet annealing. No. 5 was a comparative example in which cold rolling was not performed, and the r value was less than 2.0.
No. 6 is an example having a high finish rolling temperature (FT). Ferrite grains were coarsened, the r value was low, the brittle transition temperature was high, and the plating adhesion was inferior.
No. 7 is an example of a low rolling reduction at 750 ° C. or lower during hot rolling. Ferrite grains were coarsened, the r value was low, the brittle transition temperature was high, and the plating adhesion was poor.
No. 8 is an example in which the heating temperature is high, and since the ferrite particle diameter exceeded 20 μm, the r value was low, the brittle transition temperature was high, and the plating adhesion was inferior.
No. 9 is an example having a high coiling temperature. Since the strain energy in the hot-rolled sheet was insufficient, the r value was low, the ferrite grains were coarsened, and the brittle transition temperature was high. Moreover, it was inferior also to plating adhesiveness.
Nos. 10 and 11 are invention examples, and No. 11 subjected to hot-rolled sheet annealing also had an r value superior to No. 10 not subjected to hot-rolled sheet annealing.
No. 12 is an example with a high coiling temperature. Since the strain energy in the hot-rolled sheet was insufficient, the r value was low, the ferrite grains were coarsened, and the brittle transition temperature was high. Moreover, it was inferior also to plating adhesiveness.
Nos. 13 to 16 show changes in workability and secondary work embrittlement resistance due to changes in the Cu precipitation temperature. No. 13 treated at low temperature had a tensile strength of less than 440 MPa. Since Nos. 14 and 15 were within the scope of the present invention, the tensile strength was 440 MPa or more, the r value was 2.0 or more, and the brittle transition temperature was −30 ° C. or less, showing good values. The plating adhesion was also good. No. 16 was treated at a high temperature, so the Cu particles were coarsened, the tensile strength was low, the ferrite grains were also coarsened, and the brittle transition temperature was low. Moreover, it was inferior also to plating adhesiveness.

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: Contains one or two selected from 0.07% or less, the balance is Fe and inevitable impurities composition, has a steel structure with ferrite single phase and ferrite grain size of 20μm or less, tensile strength A plated cold-rolled steel sheet for fuel tanks with high formability and excellent secondary work brittleness resistance and plating adhesion, characterized by having a plating layer on the surface with a thickness of 440 MPa or more. In claim 1, the steel composition is further in mass%,
V: 0.1% or less and
Ni: High formability fuel tank plating excellent in secondary work brittleness resistance and plating adhesion, characterized by containing one or two selected from 0.2% or more and 1.0% or less Cold rolled steel sheet. % 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% to 2.0% and B: 0.0002% to 0.0025%,
And including
Ti: 0.01% or more, less than 0.1% and
Nb: contain one or two species selected from among 0.07% or less, after the billet balance consisting Fe and inevitable impurities, exceeding 950 ° C., and heated to a temperature below 1100 ° C., finish rolling temperature : Rolling rate below 750 ° C and below 750 ° C: Hot rolled under conditions of 50% or higher, wound at a temperature of 550 ° C or lower, then pickled, then cold rolled at a rolling reduction of 50% or higher A high formability fuel tank with excellent secondary processing brittleness resistance and plating adhesion, which is then annealed and plated, and further subjected to Cu precipitation at temperatures of 450 ° C or higher and 650 ° C or lower. Method for plating cold-rolled steel sheet. % 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% to 2.0% and B: 0.0002% to 0.0025%,
And including
Ti: 0.01% or more, less than 0.1% and
Nb: contain one or two species selected from among 0.07% or less, after the billet balance consisting Fe and inevitable impurities, exceeding 950 ° C., and heated to a temperature below 1100 ° C., finish rolling temperature : Rolling ratio below 750 ° C and below 750 ° C: Hot rolled under conditions of 50% or higher, wound at a temperature of 550 ° C or lower, annealed at a temperature of 600 ° C or higher, and then pickled. Rolling ratio: 50% or more of cold rolling, followed by annealing and plating, followed by Cu precipitation at a temperature of 450 ° C or higher and 650 ° C or lower. A method for producing a plated cold-rolled steel sheet for a highly formable fuel tank having excellent brittleness and plating adhesion. In Claim 3 or 4, the composition of a steel slab is further mass%,
V: 0.1% or less and
Ni: 0.2% or more, 1.0% or less
A method for producing a plated cold-rolled steel sheet for a high-formability fuel tank excellent in secondary work brittleness resistance and plating adhesion, wherein the composition contains one or two selected from the above.