JP2005200751A - Plated hot rolled steel sheet for fuel tank having high formability and excellent secondary working brittleness resistance and plating adhesion, and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a plated hot rolled steel sheet for a fuel tank having excellent secondary working brittleness resistance and plating adhesion and also having ≥420 MPa tensile strength and ≥1.6 r-value and excellent in a balance between strength and r-value. <P>SOLUTION: As for a steel sheet as a basis material, this steel sheet has a composition consisting of, by mass, ≤0.01% C, ≤0.5% Si, 0.15 to 0.5% Mn, ≤0.03% P, ≤0.02% S, ≤0.1% Al, ≤0.006% N, 0.5 to 2.0% Cu, 0.0002 to 0.0025% B, either or both of 0.01 to <0.1% Ti and ≤0.07% Nb, and the balance Fe with inevitable impurities and also has a steel structure composed of a ferrite single phase and having ≤25μm ferrite grain size, and further, tensile strength is made to ≥420 MPa. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention has an excellent strength-r value balance with a tensile strength of 420 MPa or more and an r value of 1.6 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 hot rolled steel sheet and a method for producing the same.

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 that the grain boundary strength is weak and a problem remains in the secondary work embrittlement 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, has excellent secondary work brittleness resistance and plating adhesion, and has excellent tensile strength ≧ 420 MPa, r value ≧ 1.6, and a balance between strength and r value. The object is to propose a hot-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) Furthermore, by suppressing the ferrite grain size to 25 μm or less, good secondary work brittleness resistance can be obtained,
(5) As a result, a plated hot rolled steel sheet having excellent strength-r value balance of tensile strength: 420 MPa or more, r value: 1.6 or more, and excellent secondary work brittleness resistance and plating adhesion can be obtained. .
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, has a steel single-phase ferrite grain size of 25μm or less, and has a tensile strength Highly formable hot-plated steel sheet for fuel tanks with excellent secondary work brittleness resistance and plating adhesion, characterized by having a thickness of 420 MPa or more and a plating layer on the surface.

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 Hot 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: A steel slab with a composition containing one or two selected from 0.07% or less is heated to a temperature exceeding 950 ° C and less than 1100 ° C, and then finish rolling temperature: less than 750 ° C and 750 ° C or less Rolling ratio: Hot rolling under conditions of 50% or higher, winding at a temperature of 550 ° C or lower, then pickling, plating, and then Cu precipitation at temperatures of 450 ° C or higher and 650 ° C or lower A method for producing a plated hot-rolled steel sheet for a highly formable fuel tank excellent in secondary work brittleness resistance and plating adhesion.

  According to the present invention, by adding a proper amount of Cu and B and suppressing the ferrite grain size to 25 μm or less, the tensile strength is 420 MPa or more and the r value is 1.6 or more. Moreover, it is possible to obtain a plated hot-rolled steel sheet having excellent secondary work brittleness resistance and plating adhesion.

Hereinafter, the present invention will be specifically described.
First, the reason why the composition of steel is limited to the above range in the present invention will be described. Unless otherwise specified, “%” in relation to ingredients means mass%.
C: 0.01% or less C is useful for increasing the strength of steel, but if it exceeds 0.01%, the r value, which is an index of press 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 420 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 25 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 25 μm or less is as follows.
That is, since the r value increases as the ferrite particle size increases, a good r value can be obtained if the particle size is coarse, but if the ferrite particle size exceeds 25 μm in the hot 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 25 μm or less and a high r value, but such a hot-rolled steel plate has not been developed conventionally.
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 25 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 25 μm.
Further, the adhesion of plating strongly depends on the ferrite grain size of the base steel plate. Therefore, by setting the ferrite grain size to 25 μm or less, the crystal of the plating can be refined, and the surface property of the plating is also improved.

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.

Finish rolling temperature: less than 750 ° C. The present invention achieves a good r value with a hot-rolled steel sheet. The texture of ferrite transformed from austenite is close to random, and the r value is 1.0 or less. Therefore, finish rolling is completed at less than 750 ° C. and rolling is performed in the ferrite region, whereby the ferrite is recrystallized to form a suitable texture. 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 required for recrystallization is not accumulated. Therefore, the rolling reduction at 750 ° C. or lower is set to 50% or higher. When annealing a hot-rolled sheet, strain energy required for recrystallization is not accumulated, so the rolling reduction at 750 ° C. or lower was set to 50% or more.

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. For this reason, the coiling temperature was limited to 550 ° C or lower.

Pickling treatment In the present invention, a steel plate having a hot-rolled scale is pickled, and after removing the scale, plating is performed. It is also possible to perform hot dip galvanizing directly without removing the scale by pickling, but in this case, the surface appearance deteriorates, and further, the reduction of the plating adhesion due to the scale remaining on the interface between the plating layer and the steel plate. Deteriorates. For this reason, pickling is performed before plating.

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 may be performed in which the plating layer and the base steel plate are alloyed after plating. Note that the plating may be performed on at least one side of the steel plate.

Cu precipitation treatment: 450 ° C. or more and 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 strength of the steel plate is increased 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 420 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 420 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 1050 ° C, the finish rolling temperature was 700 ° C, the rolling reduction at 750 ° C or less was 65%, and the winding temperature was 500 ° C. Subsequently, the obtained hot-rolled sheet was pickled, annealed at 780 ° C. for a short time, and then Sn—Zn hot-dip plated. Further, Cu deposition treatment at 520 ° C. was then performed.

From the plated hot-rolled steel sheet thus obtained, a JIS No. 5 test piece was sampled so that the tensile direction was perpendicular to the rolling direction, a tensile test was performed, 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, bending it 90 ° at the center in the longitudinal direction, attaching cellophane tape to the outside of the bent part, and then peeling off the plating It was observed whether or not the plating was performed, and visual evaluation was performed by assuming that the plating did not peel off as ◯ and the peeling off as x.
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 8 and Nos. 12 to 14 are invention examples. In No. 3, Nb is not added, and in No. 4, Ti is not added. No. 5 and No. 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. 9 is a comparative example in which B which is an essential element of the present invention is not added, and No. 10 is a comparative example in which Cu and B are not added.
No.11 to No.15 show the effect 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. Nos. 16, 17, and 18 are examples in which Mn is the lower limit, Ti is the upper limit, and Nb is out of the upper limit.
Inventive examples No. 1-8 and No. 12-14 all have excellent mechanical properties such as a tensile strength of 420 MPa or more, an r value of 1.6 or more, and a brittle transition temperature of −30 ° C. or less. In addition, there is no plating peeling and excellent plating adhesion.
On the other hand, No. 9 has a brittle transition temperature as high as 0 ° C. because B is not added. Since the particle size is also large, the plating adhesion is low.
In No. 10, Cu and B are not added, the tensile strength is well below 420 MPa, and the brittle transition temperature is also high at 0 ° C. Further, since the particle size is large, the plating adhesion is low.
No. 11 had a low Cu content, so the tensile strength was below 420 MPa. Furthermore, since the particle size is large, the plating adhesion is low.
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 have a high r value and a high brittle transition temperature because the hot-rolled sheet has large ferrite grains. Plating peeling was also observed.

C: 0.0022%, Si: 0.11%, Mn: 0.16%, P: 0.020%, S: 0.005%, Ti: 0.071%, Nb: 0.017%, Cu: 1.04%, N: 0.0013%, Al: 0.032%, A steel slab having a composition containing Ni: 0.75% and B: 0.0004% was hot-rolled under the conditions shown in Table 3, and subjected to plating treatment and Cu precipitation treatment under the conditions shown in Table 3 as well.
The results obtained by examining the ferrite grain size, tensile strength (TS), r value, brittle transition temperature and plating adhesion of the plated hot-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 3, Nos. 7 to 9, and Nos. 12 to 13 are invention examples. No. 4 is a comparative example with a high finish rolling temperature, No. 5 is a comparative example with a low rolling reduction, and No. 6 is a comparative example with a high heating temperature. No. 10 is a comparative example with a high coiling temperature, and No. 11 is a comparative example with a low Cu precipitation temperature. No. 14 is a comparative example with a high Cu precipitation treatment temperature.
Inventive examples No. 1 to 3, No. 7 to 9, and No. 12 to 13 have excellent tensile strength of 420 MPa or more, r value of 1.6 or more, and brittle transition temperature of −30 ° C. or less. It has excellent properties and has excellent plating adhesion.
On the other hand, No. 4 had a high finish rolling temperature, so the ferrite grains became coarse and the r value, brittle transition temperature, and plating adhesion deteriorated.
Since No. 5 had a low rolling reduction, the tensile strength was low, and the r value and brittle transition temperature were also inferior.
In No. 6, since the heating temperature was high, the ferrite grains became coarse, the r value was low, and the brittle transition temperature was also high. Moreover, the plating adhesion was also inferior.
No. 10 had a low r value due to the high winding temperature.
No. 11 has low tensile strength due to its low Cu precipitation treatment temperature.
No. 14 had a high Cu precipitation treatment temperature, so the tensile strength was low, the brittle transition temperature was high, and the plating adhesion was poor.

Claims (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% 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, has a steel single-phase ferrite grain size of 25μm or less, and has a tensile strength Highly formable hot-plated steel sheet for fuel tanks with excellent secondary work brittleness resistance and plating adhesion, characterized by having a thickness of 420 MPa or more and a plating layer on the surface. 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 Hot 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: A steel slab with a composition containing one or two selected from 0.07% or less is heated to a temperature exceeding 950 ° C and less than 1100 ° C, and then finish rolling temperature: less than 750 ° C and 750 ° C or less Rolling ratio: Hot rolling under conditions of 50% or higher, winding at a temperature of 550 ° C or lower, then pickling, plating, and then Cu precipitation at temperatures of 450 ° C or higher and 650 ° C or lower A method for producing a plated hot-rolled steel sheet for a highly formable fuel tank excellent in secondary work brittleness resistance and plating adhesion.